THE BIOLOGY OF LEARNING OR 'PSYCHOBIOLOGY'
For children growing up in
the complex world of the 'global village' an effective philosophical framework
or 'paradigm' for educational theory is one based on the knowledge of learning as a natural process.
Insights on the
physiological mechanisms which underlie the process of natural learning i.e.
'biology of learning' provide evidence which validates the paradigm of
freedom in education... education
for the person as a whole i.e. 'holistic
education'. Holistic education is the practice of freedom for learning, for creativity and
productivity or 'work'. Meaningful work is a function of
the natural functioning of the human brain. ("It is now essential to develop educational
systems that are truly appropriate for our times." (Gerald Karnow.
"Educating the Whole Person for the Whole of Life". Holistic Education Review
vol. 5 no. 1 Spring, 1992. 59-64)
IMPACT OF
BRAIN RESEARCH ON LEARNING THEORY...
Education as 'schooling' and traditional teaching methods
which emphasize facts and
outcomes or 'behavioural objectives' are not compatible with the natural
functioning of the brain. They
are antagonistic to natural brain functioning or 'brain-antagonistic'. Brain-antagonistic teaching methods can actually prevent real understanding of meaningful learning
and are therefore ineffective in the development of
the human potential for creative intelligence
required for adaptability.
Development of intelligence depends on a
learning environment characterised by respect for the individual's
'freedom' and their instinctive responsibility for thir own growth and
development and capacity for 'self-evaluation'. Brain-antagonistic teaching
ignores the role of the unconscious or 'emotion' in the learning process
which is meaningful because it engages
personal initiative
based on instinctive motivations or 'emotional drives' i.e.
'intrinsic
motivation'.
An individual's type of motivation - 'motivational type' - is determined
by the level of psychological development which they have reached ('sociocognitive stage').
Human development is a function of fulfillment of a range of human motives for learning or
'human needs'. These include the the basic
psychological 'ego needs' for security and self-esteem and the so-called 'higher needs'... 'spiritual
needs' or 'metaneeds' for self-actualisation and 'ego-transcendance'.
Intrinsically motivated learning which engages personality development is a
function of natural functioning of the brain.
(See
http://www.comfsm.fm/socscie/biolearn.htm)
A
fundamental shift is taking place in the philosophical framework or
'paradigm' of education making it possible to conceptualize a teaching methodology which
is appropriate for human adaptability to the complexities of the modern world.
The attention of educators is being drawn away from the traditional paradigm of
the behavioural sciences or 'behaviourism' and shifted towards the
new paradigm of systems theory of the science
of interconnectedness or wholeness i.e. 'holistic science'. From the perspective of systems theory, it is possible
to view the complexity of the learning process as a natural product of brain
functioning. Findings of brain research or
'neuroscience' provide new information about the learning process as a function
of natural brain processes... the biological basis of the human potential for
learning is a natural function of the
'brain/mind'...
cognitive functions of the brain learning,
remembering etc.
can be analysed in terms of physiological mechanisms
'brain functions' involving the
propagation and
transmission of
electrochemical signals or nerve impulses
along nerve cells or 'neurons' and across their interconnections... contact
points...'synaptic connections' or synapses. This continuity of information
or 'information flow' in the brain is the physiological basis of learning...
The focal point of the biology of learning is
activity at the synapse. Depending on molecular events at the
synapse... whether there is an excitatory input which exceeds the inhibitory
input... existing synapses can be strengthened, allowing for transmission of
electrical impulses from one neuron to the next; they can be weakened,
preventing transmission of electrical impulses from one neuron to the next; and
they can be created ('synaptogenesis').
However the synapse is
modified, the end result is formation 'neural pathways' ('neural
circuits' or 'neural networks'). The brain's capacity to change with
learning - 'brain plasticity' or 'neuroplasticity' is largely influenced by
environment. Brain development and development of 'intelligence' as
'creative intelligence' depends
on environmental conditions which are complex and therefore stimulating because the individual is
intrinsically motivated. Intrinsic
motivation enhances learning because it enhances the strengthening of
existing synaptic connections as well as the formation of new ones. The source
of
'intrinsic motivation' for learning is natural
curiosity which requires freedom in education.
"...It is in fact nothing short of a miracle that the modern
methods of instruction have not yet entirely strangled the holy curiosity of
inquiry; for this delicate little plant, aside from stimulation, stands
mainly in need of freedom; without this it goes to wrack and ruin without fail."
(Albert Einstein)
Findings in brain research have
significant implications for the improvement of education. They support those
principles of learning which are based on learner interest i.e.
'brain-based learning'. Brain-based
learning is facilitated by teaching methods which are compatible with brain
funcioning i.e. 'brain compatible' pedagogy. (See
http://www.21learn.org/arch/articles/caine_principles.html)
"What is needed is a framework for a more complex form of learning
that makes it possible for us to organize and make sense of what we already know
about educational theory and methods... Such a framework has to have a 'bottom
line' integrity; for us that means it must integrate human behaviour and
perception, emotions and physiology. To make our point, we borrow heavily
from cognitive psychology, education, philosophy, sociology, science and
technology, the new physics, and physiological responses to stress, as well as
the neurosciences... The new framework can be created in the link between
education and the neurosciences." (Renate Nummela Caine and Geoffrey Caine.
Making Connections: Teaching and the Human Brain. page viii)
"Learning methods based on the natural
functioning of the brain enhance learning because they enhance the formation of
synaptic connections between nerve cells. Learning occurs as a result of
changing the effectiveness of synapses so that their influence on other neurons
also changes. Learning is a physiological function of the brain involving the
transmission of signals along nerve cells and across their junctional
connections. It is a function of the effectiveness of synapses to propagate
signals and initiate or 'fire' new signals along neighboring neurons."
(Geoffrey Hinton, "How Neural Networks Learn from Experience," Scientific
American, 267:3, September 1992, 145).
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Subsections
homepage
founder of neuroscience Ramon y Cajal... Cajal's study of the
architecture of neurons: 'neuron doctrine'...
biology of the mind or 'psychobiology'...
neuron...
neurons are specialised for the function
of communication: morphology of the neuron...
structural
components of the neuron....
neurons 'at rest'... the 'resting cell': properties of the
'semipermeable' cell membrane
threshold..
nerve impulse...
role of sodium atms
in the generation of
a nerve impulse...
propagation of nerve imlpulse...
The biology of learning is based on the molecular
events involved in the propagation of nerve impulses along neurons and their
transmission across the points of connection i.e. 'synapses'... not all signals are transmitted...
they must be sufficiently intense intense.
the problem of continuity of information in the brain
or 'information flow'...
nerve impulses do not jump from
one neuron to the next.
the
synapse plays a critical role in information flow...
structure of the synapse ...
synaptic
theory of transmission of nerve impulses
from one neuron to the next
across the
synapse...
molecular events at the synapse...
synapse function - binding of
neurotransmitters..
synaptic excitation... synaptic
inhibition...
continued transmission of impulses across the
synapse depends
on the effect of binding.
Learning and the synapse
synapse modification or 'neuroplasticty'...
retention of learning or 'memory'...
notion of localisation of
memory as memory trace
or 'engram'
history of
'search for the engram'
Karl Lashley and his search for the
neural basis of memory or 'search for the engram'
Donald Hebb inspired research
in the development of a new field - psychobiology ('biology of learning')
molecular activity at synapse as
'memory trace'...
learning from
experience or 'experiential learning' involves
memory as the 'process of
remembering' a cognitive
process...
Memory as the process of remembering involves the
interaction of conscious or 'explicit' memory and unconscious or 'implicit'
memory.
learning
(and memory) involves emotions..
implications for education
knowledge of the role of stimulation on
synapse modification is directly related to educational methodology...
..role of emotions
so-called 'brain-antagonistic methods of teaching' inhibit experiential learning
because they inhibit the formation of new synaptic connections in the cortex of
the brain.
'brain-based learning'... 'optimalearning'.
so-called 'brain-compatible' methods of teaching
facilitate the learning process
'holistic education'
References
Founder of
NEURAL SCIENCE... neuroscience: Ramon y Cajal
and the neuron doctrine. (Cajal The Integrative Action of the Nervous System, published 1906) Cajal is considered as the founder of modern brain research. At the end of the nineteenth century
it was generally believed that the brain is made up of a continuous net of nerve
tissue... 'reticular network' or 'syncitium'. About 100 years
ago in Santiago, Spain the great Spanish neuroanatomist Ramon
y Cajal Cajal was the first to suggest that the functions of the brain
could be understood by analyzing the functional architecture or 'wiring diagram'
of the nervous system. Cajal was the first to study the
architecture of the nerve cell or 'neuron'. Cajal carried out
his morphological studies with both adult and embryonic neurons. He applied Golgi's silver staining technique to the morphological study of adult and
embryonic nerve tissue and observed that only some cells were stained in their
entirety. With this technique, Cajal observed a large variety of nerve cells or 'neurons' - those with short axons communicating with neighbouring
cells, those with long axons projecting to other regions of the brain, those
with spindle shaped bodies, those with rounded shape cell bodies and those with
short branched extensions called 'dendrites'... 'dendritic
branching' or 'arbors'.
(We know today that it is the diverse
nature of the neurons which accounts for the brain's complexity.)
Cajal's findings led to the formulation of his
'neuron
theory' or 'neuron doctrine' which states that the nervous system is
made up of discrete units... isolated unjoined nerve cells or
'neurons' each one independently living
its own biological life... with its independent biological existence rather than a continuous net of nerve tissue
or 'syncytium' linked up
with hypothetical 'protoplasmic bridges' as was generally believed at the
time. It is now
known that neurons differ from other cell types as a result of their specialization
for neural functions. Cajal described the neuron as an electrically charged
or 'polarized' cell with a polarized membrane which produces electrochemical reactions in the form of 'electrochemical signals'
or 'electrochemical pulses'
or 'nerve impulses'.
A
nerve impulse is a spike of
electrical activity also known as an 'action
potential'. Cajal integrated his knowledge of the action
potential into his neuron doctrine. He recognized that the properties of the
neuron could be explained in terms of the transduction, conduction and
transmission of electrical signals or 'impulses'. As a
polarized cell the neuron has the functions of a transducer,
conductor and transmitter of electrical impulses
all at the same time... converting
energy from one form to another. As a transducer the neuron converts or 'transduces'
the stimulus energy from the outside world the environmental stimulus energy -
into electrical impulses. As a conductor the neuron
propagates or 'conducts' the transduced signals from the dendrites to the
cell body and then down long unbranched
extensions the axons the axon. As a transmitter, the neuron converts the
conducted electrical signals into chemical messages and then conveys or 'transmits'
them from one neuron to to a neighboring neuron. He deduced that as a polarized cell, the neuron
receives 'input signals' on the
'dendrites'... and sends
the 'output signals' on long unbranched extensions - the 'axons'. Nerve impulses would travel from the dendrites of a neuron to
its cell body and then along its long unbranched extensions - the 'axons'
-
to the dendrites of the neighboring neuron. This flow of information would be a finite process. With this suggestion Cajal set the
stage for a cellular analysis of the 'reflex arc of the spinal cord. English physiologist Charles Sherrington
(1861-1952) worked out the details of the reflex arc in the spinal cord of
mammals. He described the specialized contact points between connecting neurons and in 1897 introduced the
term 'synapse' derived
from the Greek word 'synapto' meaning 'to clasp tightly'.. (The Integrative Action of the Nervous
System published 1906). (
'synapse' ). More than a
thousand billion synapses connect the neurons to form 'nerve circuits'
or 'neural pathways'.
Since the time of Cajal neuroscience has expanded to include
various areas of inquiry... neuroanatomy, neurology, cognitive psychology, psychobiology,
neurochemistry....
Although great strides have been made in the
neurosciences the question which still remains to be
answered is the following: What exactly are the structural and functional changes in the brain
that form the basis of learning and the 'retention of learning' or 'memory'?
The
biological basis of the natural learning function of the brain... biology of the mind...or
'psychobiology'. Over the past several decades, the science of the
brain or 'neurobiology' or 'neurology' has merged with the science of
the 'mind' - 'cognitive psychology' to produce the new field of 'psychobiology'
(also known as 'physiological psychology')...
bridge between cognitive psychology and molecular
biology. Psychobiology is the study of the brain as the
physical basis of the 'mind' or 'brain/mind'... biological substrates of mental functions,of learning or 'cognition'... biology of learning as 'global learning'... a natural function of the brain.
The brain
is the biological organ of learning with an innate predisposition to search for how things make sense,
to search for some meaning of experience. The brain responds to stimuli in the
field of focused attention and at the same time absorbs information and signals
which are peripheral to the field of focused attention. Many signals perceived
peripherally interact with the brain and are processed at the subconscious level
The aim of
research
in psychobiology... psychobiological research... the basic problem of psychobiology with the application of biological
techniques.. is to to find an explanation to describe the biological
or 'neural' mechanisms which underlie functioning of the mind...the basis of mental processes
which underlie mental functioning as 'thought', or 'mind'... the neural
basis of the biological basis
of mental phenomena such as urges, desires, conscious
and subconscious forms of learning, remembering, emotional states and affective
states, conscious thought, imagination and creativity... 'neural information'... the mental
functions of learning or 'cognition' and the process of acquiring and
retaining new knowledge i.e. 'memory'.
The
thinking patterns of the mind consist of interrelated
processes of remembering and comparing mental data i.e. 'analysis' and organizing, integrating, evaluating, detecting relationships and making
connections i.e. 'synthesis'. Analysis and synthesis are based on the structural patterns of interconnections 'nerve cells' or
'neurons'. (The number of neurons in the brain - perhaps about 100 billion -
is fixed at birth.) Neurons are connected in patterns of
'nerve circuits'... 'neural circuits' ...'neural networks'
...'neural pathways' throughout the cortex of the brain. The formation of neural pathways results
from neuron activity - propagation of electrical stimuli or
'nerve impulses' along the neurons and the
'two process mechanism' of
transmission across their connections between them - specialized points of
contact - the 'junctional connections' or 'neural junctions' i.e.
'synapses'. The functions of learning and memory are analysed in terms of cellular and
molecular mechanisms... physiological mechanisms or 'neural mechanisms'
involving the 'nerve cells' or 'neurons' and their interconnections - the
'synapses'. There are more than a thousand billion
synapses in the brain. The most
useful strategy for analysis is the so-called 'cellular connection approach' based on the
assumption that both the transmission and transformation of neural information and its storage as memory involve only neurons and their
interconnections the 'synapses'. The synapse is considered as the focal point of
learning. provides a new paradigm for the understanding of the learning process in terms
of the biological substrates underlying the mental functions psychobiology lends a new framework for the study of
learning.... this framework is based on the study of biological substrates of mental functions
of the brain.. of
the process of acquiring new knowledge or 'learning' and the process of retaining new knowledge or 'memory'... are analysed in terms of cellular and
molecular mechanisms involving the neurons and their interconnections the
synapses... Basic problem of psychobiology: to describe the neural mechanisms which
underlie learning and memory. As the study of the learning process, it
provides a new framework
for educational theory. significant implications for education because they
challenge a number of assumptions promoted by the
'worldview' or
'paradigm' of traditional education as
'schooling'. They challenge the notion that the emotional or 'affective'
aspect of the learning process is located in different areas of the brain from
the processing of information i.e. thinking or 'cognition'. Consequently
they challenge the traditionally
held assumption that learning does not involve emotions...
but is a
matter of 'conditioning'.
Conditioned learning depends on forced learning of memorization or
'rote learning'... 'unnatural learning'. Emphasis is on 'content' ...
in the form of fragmented information or data which must be learned in order to
meet given requirements or 'learning outcomes'. Successful retention of
learning is rewarded and failure is punished... with the use of an evaluation
system of points or 'grades'. The grading system fosters motivation by mechanisms which are
external or 'extrinsic' to the individual's capacity for personal
decision-making i.e. 'extrinsic
motivation'. The traditional methods are formulated within the paradigm of
'behavioural science' or ''behaviourism.
The 'behavioural paradigm' is being seriously
questioned today.
"Introducing a new framework for the study of learning, psychobiology
is the study of the biological substrates of mental functions. Its aim is to
describe the functioning of the mind and the learning process. Its aim is to
describe the 'neural' basis of mental phenomena such as urges, desires,
conscious and subconscious forms of learning, remembering, emotional states and
affective states as well as conscious thought, imagination and creativity. A reconceptualization of the teaching and learning process must be based on the
knowledge of brain functioning. Progress in psychobiology has resulted from the
application of biological techniques to the study of the biological mechanisms
which underlie the mental processes of learning and memory or 'neural
information.' They are analysed in terms of mechanisms involving the nerve
cells, or 'neural' mechanisms. The most useful strategy for analysis has been
the so called 'cellular connection approach.' This approach is based on the
assumption that the transmission of neural information and its storage as memory
involve only nerve cells and their interconnections". ( Eric Kandel, "Nerve Cells
and Behavior", Scientific American 223: 1 July 1970, 281.)
In the new 'cognitive
paradigm' new concepts of brain functioning are being applied to teaching
techniques which are confluent with the brain's rules for learning or
'brain-based learning'... i.e. 'brain-compatible pedagogies'. The implications
for teaching and the role of the teacher... traditional praradigm: 'teacher as
instructor'... cognitive paradigm: 'teacher as facilitator'.
Since the 1940s techniques have been
developed for studying individual neurons. The new techniques have
revolutionized the sciences of the brain 'neuroscience'
making it possible to analyze neural processes of increasing complexity. Technology of the 'electron
microscope' has made it possible to study individual neurons and to magnify
them enough to 'see' how they are connected. There is no evidence of
'protoplasmic bridges' connecting neurons to each other. Instead there are gaps
which separate them. Discovery of the gaps has produced a picture of the brain
as a network of single and separate neurons which propagate nerve impulses travelling at varying rates. The problem for neuroscience has been to provide an
explanation for information flow along successive neurons when there are gaps
which separate them. Investigations with the electron microscope reveal
gaps between the neurons... The gaps
are components of the intimate contact points of connection between
interconnecting neurons - the 'synapses'. This
raised the problem... if there are gaps between the neurons then what explains
the flow of information in the brain? This is the problem of
continuity of information.
The
nerve cell or 'neuron' The
brain is made up of single and separate nerve cells or
'neurons'. Neurons are specialized for the function of propagating
electrochemical signals or 'nerve
impulses' along their unbranched extensions or 'axons'. The
diameter of an axon is equivalent to hundredths or thousandths of a hair strand. Nerve
impulses are collected from neighbouring neurons through branched extensions or
'dendrites'. They enter the neuronal cell body for processing and are
then propagated along... travel at definite rates along axons which split or
'bifurcate' into thousands of branches which terminate as 'axon terminals'
also known as 'synaptic knobs' and 'synaptic buttons'. Axon
terminals connect with dendrites of neigbouring neurons at specialized points
of contact known as 'neural junctions', 'synaptic junctions' or 'synapses'.
The term 'synapse' was introduced in 1897 by C.S. Sherrington and is
derived from the Greek word 'synapto' meaning 'to clasp tightly'.
The number of nerve cells in the brain
is fixed at birth and no new nerve cells grow and develop. Learning and
experience change the structure of the neural networks. Learning is a function
of stimuli strong enough to influence the effectiveness of synapses in the
transmission of signals from one neuron to another across the synaptic clefts.
It is a function of the effectiveness of synapses to propagate signals and
initiate or 'fire' new signals along neighboring neurons. Those teaching and
learning methods which inhibit learning inhibit the formation of connections
between nerve cells, the 'synapses'.
The characteristic features of neurons are
specifically adapted for cell communication and the transmission of electrical impulses
or 'nerve
impulses' which travel along their axons. The transmission of
nerve impulses from one neuron to another involves the binding
of receptor molecules with neurotransmitter molecules on the membrane .
Neurons are specialised for the function
of communication: morphology of the neuron The
central bulb of the neuron - the 'cell body' or 'soma' - contains
the nucleus and its enclosed genetic material. Branched extensions of the
soma are the structural pathways for incoming nerve impulses... 'dendrites'. Projections along the
dendritic branches - the 'dendritic
spines' - are outgrowths through which nerve impulses are transmitted from
neigboring cells. They carry the 'input signal' into the cell body. The
structural pathway for the transmission of the 'output signal' is the long
unbranched extension from the soma - the 'axon'. Axons extend from
the soma over varying distances and then bifurcate several times giving rise to
smaller branches and their expanded terminals - the 'axon terminals' or 'synaptic knobs'
or 'synaptic buttons'. These make contact with neighboring cells at
specialised contact points which function in the transmission of nerve impulses
- the 'synapses'. Each neuron has hundreds or even
thousands of synapses on its surface.
Structural components of the neuron.
Neurons have the same basic
structural components as other cell types: The 'nucleus' contains genetic material DNA (deoxyribonucleic
acid) which directs the cell's activities. The 'cytoplasm' outside
the nucleus is made up of a structural framework - 'endoplasmic reticulum'
- within which are located a variety of functional units or 'organelles'.
The 'mitochondria' - the 'powerhouses' of the cell contain enzymes
and substrates for the breakdown of glucose molecules in the production of
energy rich ATP molecules (adenosinetriphosphate). The stored
energy is released in the 'ribosomes' - organelles located throughout the
interior of the cell. Ribosomes contain the enzymes and
substrates for the synthesis of macromolecules or 'protein synthesis'.
Cell specific proteins are produced and packaged into 'vessicles' in the Golgi bodies and then travel to
the functional boundary of the cell or 'cell membrane'. The membrane
surrounds the entire cell including the soma, the axon with its branches and
terminals, and the dendrites with their branches and spines. The membrane is
described as 'semipermeable' because it is permeable to some substances
and not to others thus regulating the exchange of molecules between the interior
of the cell and the exterior. The property of semipermeability is a function of
the activity of pores or 'channels' which can open and close their 'gates'
to allow the passage of certain atoms, ions and molecules. Chemical 'receptor
molecules' which project from the outer surface of the cell membrane
recognize and bind to specific chemical substances approaching the cell membrane
such as 'neurotransmitters' involved in the transmission of nerve
impulses from one neuron to the next i.e. 'information flow'.
"There are perhaps about one hundred billion
neurons, or nerve cells, in the brain, and in a single human brain the number of
possible inter-connections between these cells is greater than the number of
atoms in the universe." (Robert Ornstein and Richard Thompson, The Amazing
Brain. Boston: Houghton Mifflin Company. 1984, 21)
T
All neurons transmit information in the form of
electrochemical reactions which travel along their axons i.e. electrochemical
pulses or 'nerve impulses'.
Neurons 'at rest'...
the 'resting cell': properties of the 'semipermeable'
cell membrane which is permeable to some substances and not to others.the 'semipermeable' cell membrane inhibits the easy access of sodium
ions to the cell interior. A nerve cell or 'neuron' which is not carrying a nerve impulse is said to be in the 'resting
state'... 'at rest'. It is a 'resting cell'. A resting cell is characterised by a specific difference in chemical composition. This difference
is manifest as difference in electric charge on either side of the membrane - positive on the outside and negative on the inside
- resulting from the chemical properties
of 'sodium atoms' which play a key role in the generation of an
electrochemical reaction known as the action potential or nerve impulse.
The telectrically neutral sodium atoms (Na) containing the same number of
negatively charged electrons as positively charged protons. They
function as positively
charged 'sodium ions' (Na+) which are prevented from moving across the
membrane of the resting as a result of its semipermeability. An active
sodium pump (active because it requires
energy - ATP molecules) keeps them from moving through the special pores... 'sodium
channels' or sodium gates' across the membrane from the outside to the inside with the
'gradient'. the 'sodium gates' are closed. Positive potassium ions are kept on the outside of the
membrane by
way of a 'potassium pump'. In addition negative chloride ions (derived from the salt
molecule) are pumped to the inside. As a result of the inhibition of easy access of sodium ions to the
cell interior the intracellular cytoplasm of the resting state remains negatively charged compared to the tissue fluid
outside the cell, the 'extracellular fluid.' The extracellular concentration of
sodium ions is
ten times greater than the intracellular concentration. The result is a difference in voltage across the membrane, a
'differential voltage' which measures 70 millivolts, nearly a tenth of a volt.
This
differential voltage or 'gradient' of -70 millivolts is called the 'resting
membrane potential' which is maintained through the constant action of the sodium and
potassium pumps. Despite the gradient, positive sodium ions are prevented
from moving towards the cell interior.
The nerve impulse as electrochemical pulse...
initiation or generation of 'action potential'...spike of electrical
activity... and
conduction... propagation of nerve impulse along the neuron
Role of sodium atoms in the generation
of a nerve impulse The energy source and the signal source for each neuron is
the electrially charged sodium ion - a derivative of the
electrically neutral sodium atom. In the sodium atom, the number of negatively
charged electrons orbiting the nucleus is equal to the number of positively
charged protons within the nucleus. The sodium atom gains positive electric
charge when combined with chlorine atoms in salt crystals. Chloride ions (Cl-) are negatively charged. Sodium ions (Na+) are positively
charged. Potassium ions (K+) are positively charged. In the resting cell, the concentration of
positively charged sodium ions in the tissue fluid (outside the cell membrane ('extracellular' concentration)
is ten times greater than the concentration of sodium ions in the cytoplasm on
the inside of the cell ('intracellular' concentration). On the inside of the nerve fibre in its resting state there are more negatively charged chloride ions (Cl-)
than positively charged sodium (Na+) and potassium ions (K +). Consequently the
inside of the nerve fiber is 
negatively charged. The concentration of chloride
ions inside the cell - 'intracellular concentration - is greater than the
concentration of chloride ions on the outside of the cell - extracellular
concentration. On the outside of the nerve fibre in its resting state there are more positively charged sodium (Na+) and
potassium (K+) ions than negatively charged chloride ions (Cl-). Consequently
the outside of the nerve fiber is positively charged. As a result, the intracellular cytoplasm is negatively charged
compared to the tissue fluid outside the cell, the extracellular fluid.
The negative voltage... difference in voltage across the membrane... The
inside is negative relative to the outside. The result is a difference in
voltage across the membrane, a 'differential voltage.' Measuring 70 millivolts,
nearly a tenth of a volt, this differential voltage or ''voltage
gradient' of -70 millivolts is called the 'resting membrane potential.' Despite
the gradient, positive sodium ions are prevented from moving towards the cell
interior as a result of the 'semipermeability' of the cell membrane which
inhibits the easy access of sodium ions to the cell interior. Special pores for
the passage of sodium ions across the membrane, the 'sodium gates,' are closed
in the resting cell. This semipermeability of the cell membrane prevents sodium
ions from moving across the membrane to the negatively charged cytoplasm of the
resting cell. The inside of the nerve fibre in its resting state remains
negatively charged.
The 'semipermeable membrane' of the neuron is
acted upon by the three forces: first the tendency of ions to move from areas of
high charge or 'voltage' to areas of low voltage... 'electrical
gradient'; second the tendency of ions to move from areas of high
concentration to areas of low concentration... 'concentration gradient';
and third the 'sodium-potassium pump' which prevents the sodium ions from
moving across the membrane to the cell interior by closing the special pores or
'sodium gates'. In this way the intracellular cytoplasm remains
negatively charged.The special properties of the cell membrane give rise to
electrochemical reactions described as 'electrochemical pulses' or
'electrical impulses'.
The electric charge on one nerve
impulse is about one tenth of one volt (has an amplitude of 100 millivolts) and
a duration of one millisecond (thousandth of a second) during which time the
influx of sodium ions ceases. At a
critical point called the 'threshold,' the inside of the cell becomes positive
with respect to the outside. Sodium ions cease to move across the membrane
In about one millisecond, ta
new ionic balance is created and the 'differential voltage returns to the resting membrane potential value of -70 millivolts. A new ionic balance is created and the
'differential voltage' returns to the resting state value of -70 millivolts. In this
way, nerve impulses or 'signals' are transmitted along
nerve cells throughout the brain.
Nerve impulses are initiated by incoming
chemical stimuli or 'excitatory signals' which decrease the 'membrane
potential', ('voltage gradient') and depolarize the membrane creating an 'action potential'.
The depolarization of the membrane increases its permeability to the positively
charged sodium ions on the outside of the membrane. The sodium gates open
creating a flux of ions across the membrane ...the positively charged
sodium ions rush inwards across the membrane into the cytoplasm of the interior
of the cell while potassium ions rush outwards to the exterior of the cell.
when the interior of the cell becomes positive with respect to the exterior
...
a spike of electrical activity... the 'action
potential'... marks the generation of a nerve impulse. With the
influx of sodium ions and the efflux of potassium ions the voltage gradient is
decreased and the membrane is further depolarized. The influx of sodium ions and
the efflux of potassium ions decreases the voltage gradient further depolarizing
the membrane. As a result, the neighboring sodium gates open increasing the
permeability to sodium ions. They rush across the cell membrane into the
interior cytoplasm of the neuron. The resulting depolarization of the membrane
increases its permability to other sodium ions and so on in a sequential fashion
throughout the length of the axon. The sequential inrush of sodium ions from one
point of the membrane to the next is the process which constitutes transmission
of the nerve impulse along the axon.
When
an incoming physical or
chemical stimulus or 'excitatory signal' reaches a nerve cell, the
voltage gradient or 'membrane potential'
is decreased and the membrane is
depolarized. With the
depolarization of the membrane, there is an increase in its permeability to
sodium ions on the outside of the membrane. The sodium gates open and the
positively charged sodium ions rush into the cytoplasm of the interior of the cell(influx). At
the same time potassium
ions rush outwards to the exterior of the cell(efflux). This marks the generation
of a nerve impulse. The influx of sodium ions and the efflux of potassium ions decreases
the voltage gradient further depolarizing the membrane and incresing its
permeabiity to other sodium ions.The neighboring sodium gates are opened resulting in increased
permeability to sodium ions. They rush into the cell interior, depolarizing the
membrane and increasing its permability to other sodium ions and so on in a
sequential fashion throughout the length of the axon. The
sequential inrush of sodium ions into the interior of the neuron from one point
of the membrane to the next is the process which constitutes action
potential or nerve impulse.
Physical
manifestation of the nerve impulse... electrical impulse
action potential refers to the flux of ions
across the membranes of nerve cells- is the result of an electrochemical
reaction which is a function of the special properties of the cell membrane
sodium ions rush inwards across the membrane to the interior of the cell -
influx - and potassium ions rush outwards to the exterior of the cell- efflux.
The membrane potential is rapidly changed and a new ionic balance is created.
The change in membrane potential is the basis of the action potential. .
The transmission of nerve impulses from one neuron to another occurs at the
junction between neurons, the specialized contact points known as 'synapses.'
Their transmission is primarily involved with the movement of sodium ions
into the interior of the cell. Incoming signals are
collected from other neurons through dendrites. They enter the cell body for
processing and are sent outwards through the axon which bifurcates at many
points giving rise to numerous 'axon terminals.' At the synaptic junction
between neurons, signals pass from the synaptic knob of one neuron and are
propagated across the synaptic cleft to the contact point on the connecting
neuron. Signals reach the synapse as bursts of electrochemical pulses - so many
per second. These do not jump from one neuron to the next. The electrical code
of transmission is first changed into the chemical code of transmission. The
arrival of electrochemical pulses at the synapse triggers the release of
specialized neural transmitter molecules, known as 'neurotransmitters.' These
are contained in small vesicles located in the synaptic knob. They are releasedthrough the presynaptic membrane and are propagated across
the synaptic cleft. They attach to special receptor binding sites on the
postsynaptic membrane. Their binding triggers a change in the membrane
permeability of the connecting neuron. The binding is excitatory if it causes
the movement of electrically charged ions and results in the depolarization of
the membrane. A new action potential is generated and the nerve impulse is transmitted.The binding is inhibitory if it results in the further polarization
of the membrane. The generation of a new action potential is inhibited and the
nerve impulse is not ransmitted.
The voltage of an action potential is
measured in thousandths of volts - 'millivolts'. A small pocket flashlight
battery could energize five thousand action potentials... amplitude and speed of the action
potential is related to the length and the diameter of the conducting
nerve. Small fibers conduct impulses slowly. Large fibers conduct impulses
rapidly. Action potentials of small fibers are smaller and wider than action
potentials of large fibers.
The nerve impulse is
an electrochemical pulse or 'signal'
... a region of
charge reversal travelling along the
neron... involvies ionized atoms of chlorine, sodium and potassium -
negatively charged 'chloride ions' (Cl-), positively charged sodium ions
(Na+) and positively charged potassium ions (Ka+)... travelling along the nerve
fiber or 'axon'.
Communication between one
neuron and the next takes place in the form of
transmission of nerve impulses across the
synapse Messages are sent from one neuron
to another by way of electrochemical pulses - a few at a time or in bursts of up
to a thousand per second. Nerve impulses are collected from other neurons
through the dendrites. The impulses are sent through the axons which split into
thousands of branches. At the end of each branch is the point of connection with
dendrites of another neuron - the
'synapse'. At the synapse the signals are transmitted from one neuron to the
next across their interconnections. First the electrical activity from the axon
is converted to chemical activity which either inhibits or excites activity in
the connecting neuron, depending on the intensity of the signal. An electrical
impulse from one axon is transmitted to the dendrites of the next neuron if the
excitatory input is greater than the inhibitory input. The transmission of nerve
impulses to neigbouring neurons is the result of a 'biophysical process'
which takes place at the synapse... More than a
thousand billion synapses connect the neurons to form 'nerve circuits'
or 'neural pathways'.
Recent brain research findings indicate that the synapse or neural junction
is involved in the transmission of nerve impulses from one neuron to another.
The two processes, synaptic inhibition and synaptic excitation are instrumental
in the formation of nerve circuits and networks in the brain. The functioning of
the brain involves this 'two process mechanism' as well as the mechanism of
transmission of nerve impulses along nerve cells.Learning is a natural function
of the brain. It involves the transmission of signals along nerve cells and in
addition the two process mechanism of signal transmission across their junctional connections, the synapses. The networks of the brain are a function
of the activity of nerve cells and their functional junctions, the synapses. No
new nerve cells develop in the human brain after birth. Modification of brain
tissue occurs in the synapses. Synapse is a neural junction. "The unit of
analysis for brain function has classically been the neuron." The neural
junction is involved in the transmission of nerve impulses. Called the 'junctional
mechanism' or 'two process mechanism' involving both neurons and their
junctions. Neural modifiability and memory mechanisms: Search for the engram:
The most basic property of the nervous system is its 'time-binding' function -
capacity for learning and memory. "It is in the junctional mechanism that long
lasting modifications of brain tissues must take place. Although adult nerve
cells do not divide, a mechanism of permanent modification of brain tissue does
display many of the properties of the mechanism of differentiation of embryonic
tissue. Experientially initiated guided growth of new nerve fibers does take
place and alters the spatial pattern of junctional relationships among neurons.
Long term memory therefore becomes more a function of junctional structure than
of strictly neural (nerve impulse generating) processes." (47) New synapses grow
and develop. Experience can cause new synapses to grow. learning can cause new
synapses to form and grow. Learning involves a change in the effectiveness of a
synapse which causes a change in the influence of one neuron on another.
Learning is a function of the point of connection between two neurons.
Until the recent findings of brain research, the unit of analysis for
the functioning of the brain has been the neuron. Freud called the cell-to-cell
junctions between the nerve cells 'contact barriers'. Recent findings in brain
research indicate that the neural junction is involved in the transmission of
nerve impulses from one neuron to another. The transmission of nerve impulses
from one neuron to another occurs at the junction between neurons, the
specialized contact points known as 'synapses.' (John Carew Eccles, The
Physiology of Nerve Cells, Baltimore: Johns Hopkins Press 1957)
Nerve impulses are transmitted from one neuron to another at specialized
contact points known as 'synapses.' The synapse is the point of junction between
two neurons. Synapse is a neural junction. More than a thousand billion synapses
in the brain.
Nerve impulses do not jump from
one neuron to the next.... role of neurotransmitters...
Their transmission across the synapse is not simply a matter of jumping
across the synaptic cleft.
They are first
translated into... converted into... corresponding chemical messages. The arrival of a
electrochemical pulses nerve impulse at the
synapse triggers an increase in permeability of the
membrane to calcium ions which rush into the interior of the neuron and combine
with molecules of the protein 'calmodulin'. The resulting calcium-calmodulin complex
produces a specific physiological response... powerful metabolic
activity which activates the release of large numbers of specialized chemical
transmitter molecules - 'neural transmitter molecules' or 'neurotransmitters'
(acetylcholine, endorphin, enkephalin, epinephrine, norepinephrine, glutamate, glycine, serotonin, oxytocin, histamine,
vasopressin,dopamine, dynorphin, aspartate, gaba). Neurotransmitters are synthesized, packaged and stored in
small 'synaptic vesicles' located in the synaptic knob of the
pre-synaptic neuron. When the appropriate electric signal arrives, the transmitter molecules are
released through the presynaptic membrane... travel from the
synaptic vesicles in the 'synaptic knob' and are released through the presynaptic
membrane of the axon terminal into the fluid-filled 'synaptic space' or 'synaptic
cleft'. They are propagated across the synaptic cleft to the post-synaptic
membrane of the connecting neuron. They attach or 'bind' to special receptor
binding sites on the postsynaptic membrane and their binding triggers a change
in the membrane permeability of the connecting neuron.
Not all signals are transmitted from one neuron to
an adjacent neuron. Whether or not they generate a new
impulse depends on the nature of their binding to the membrane of the connecting
neuron.
Nerve impulses which reach the synapse are subjected to
one of two synaptic processes: synaptic excitation
or synaptic inhibition. Depending on the process occurring at the synapse, an incoming signal is
transmitted to the connecting neuron and fired or not fired. A signal crossing the synapse can have
one of two effects: it can have an excitatory effect by lowering the threshold
for firing the signal; it can have an inhibitory effect by raising the threshold
for firing the signal. The stronger the stimulus, the more pulses are generated.
Depending on the number of pulses generated, they initiate or inhibit the
'firing' of new signals along the connecting neuron. Depending on the specific
properties of the signal, its electrical effects either inhibit or excite
activity in the connecting neuron. The continued transmission of the signal can
be either inhibited or enhanced. If the inhibitory input exceeds the excitatory
input then a connection is not made between one neuron and the next. If the
excitatory input exceeds the inhibitory input then transmission is enhanced and a connection is made between
one neuron and the next.
The problem of continuity of information in the brain
or 'information flow'. The term information flow' refers to the
propagation of electrical signals or 'nerve impulses'
along nerve cells or
'neurons' and their transmission across their interconnnections or
'synapses'.
Recent findings in brain research suggest that it is
possible to understand the functioning of the brain once there is sufficient
explanation for the specific functions of individual neurons in the generation
and propagation of nerve impulses and their transmission across the synapses.
Communication between one neuron and the next takes place in the form of
transmission of nerve impulses at the synapse.
Nerve impulses are electrical messages collected from neighbouring neurons through the dendrites. They
enter the neuronal cell body for processing and are then transmitted along the
axons which split or bifurcate into thousands of branches giving rise to 'axon
terminals'. Axon terminals connect with dendrites of neigbouring neurons at the
points of contact - the 'synapse'. At the synaptic junction
between neurons, signals pass from the synaptic knob of one neuron and are
propagated across the synaptic cleft to the contact point on the connecting neuron.The
synapse plays a critical role in the transmission of impulses from one neuron to
the next. Not all signals are transmitted from one neuron to
an adjacent neuron.
At the synapse, activity from the
axon is converted to electrical stimuli that inhibit or excite activity in the
connecting neuron. An electrical impulse from one axon is transmitted to the
dendrites of the next neuron if the excitatory input is greater than the
inhibitory input. As a result of ...depending on... synaptic activity
(transmission of nerve impulses)... molecular events at the synapse... lead to
the formation of 'neural circuits'... 'neural networks'...
'neural pathways'. Neural pathways can be altered with the creation of new
synapses and with the strengthening or weakening 'modification' of
existing synapses in a process of
'synapse modification'. Synapse modification is the focal point of
investigation into the physiological process of continuity of information or
'information flow' ias the basis for 'learning' and the 'retention
of learning' or 'memory'.
For a long time it
was believed that information flow was a purely electrical phenomenon because the transmission
of impulses from one neuron to another is so fast. Up until the 1940s the problem of information flow
was approached in terms of analysis of the neuron because it was believed to be
simply a function neural processes involved in the generation and
propagation of nerve impulses along the neurons. Neurons would be connected to
each other by junctions or bridges of protoplasm... the hypothetical 'protoplasmic bridges'
which would solve the problem of continuity and the unimpeded flow of
information in the brain.
Each of the millions of single neurons in the brain has a separate
biological existence. The separate neurons are connected to each other by way of
more than a thousand billion synapses to form the various nervecircuits or 'neural
pathways'. Neural pathways can be altered with the creation of new synapses
and with the strengthening or weakening of existing synapses in a process of
'synapse modification'. Synapse modification is the biological basis
for the process of forming neural networks or 'learning'. Learning involves the
generation of nerve impulses at the synapse, their propagation at definite rates
along the axons of neurons and their transmission across the synaptic clefts. It
explains the mechanism... series of molecular events involved in the...
transmission... transformation of neural information as
it travels from one neuron to a connecting neuron across the synaptic cleft. The
hypothesis is known as the 'junctional mechanism', the 'two process
mechanism', the 'junctional two process mechanism' and the 'synaptic
theory of transmission'. According to the synaptic theory, nerve impulses
are transmitted from one neuron to another as follows: nerve impulses arrive at
the synapse at varying intensities. If they are sufficiently intense they
trigger the release of specialized molecules contained in small vesicles in the
synaptic knob - 'neural transmitter molecules' or 'neurotransmitters'.
The neurotransmitters are released through the synaptic membrane of the synaptic
knob - the presynaptic membrane - and propagated across the synaptic
cleft. When they reach post synaptic membrane of the neighbouring neuron
they attach to protein receptor molecules i.e. 'binding
sites'. The binding sites are specialized for the attraction
and binding of neurotransmitter molecules. The binding of neurotransmitter
molecules to the receptor molecule forms a 'transmitter-receptor complex'.
The formation of the transmitter-receptor complex triggers a change in the
membrane permeability of the connecting neuron.The nature of the transmitter-receptor
complex alters the postsynaptic membrane in one of two ways:the effect is 'excitatory'... the effect is 'inhibitory'. The binding is excitatory if it lowers the threshold for
initiating or 'firing' the signal and
produces movement of electrically charged ions which results in the
depolarization of the membrane and generation of a new nerve impulse... 'excitatory
binding'.The binding is inhibitory... it raises the
threshold for firing the signal and produces movement of electrically
charged ions which results in the further polarization of the membrane and the
failure to generate a new nerve impulse i.e. 'inhibitory binding'.
Nerve
impulses are transmitted across the synaptic clefts only if excitatory binding
exceeds inhibitory binding. As a result of excitatory binding, existing
synaptic connections are strengthened and new ones are created... leading to
reinforcement of existing neural pathways and the creation of new ones...
'neuroplasticity'. The neuron discharges impulses only when the
synaptic excitation is much stronger than inhibition. The resulting
'synapse modification' or 'neuroplasticity' is the basis for learning.
Each neuron has a threshold for
transmission across the synapse... Each neuron has a critical threshold for
generating a new impulse in a connecting neuron.
The 'engram' refers to the physical changes in the
brain that accompany learning. Search for the engram is the search for evidence
that experience produces permanent changes in the nervous system,
An impulse is transmitted only if it is sufficiently intense
and its strength exceeds a critical point when
the interior of the cell becomes positive with respect to the exterior i.e. 'critical threshold'
The
threshold is the minimum number of electrochemical pulses per second required to
trigger a response and generate or 'fire' a new signal. Each neuron has a critical threshold for
transmission across the synapse... generating
a new impulse in a connecting neuron. The transmission of impulses
from one neuron to the next depends on the number of electrochemical pulses
reaching the synapse. If this number exceeds the critical threshold required for
triggering a response in the connecting neuron, then the signal is fired. An
impulse is transmitted from one neuron to the next if there are enough pulses to
trigger a response and 'fire' the signal. Nerve
impulses as electrical signals are first converted to chemical messages before
they are transmitted across the synaptic clefts. There is a new working
hypothesis which provides an explanation for the problem of information flow
across the gaps which separate the neurons. The hypothesis is known as the 'junctional
mechanism', the 'two process mechanism', the 'junctional two
process mechanism' and the 'synaptic theory of transmission'.
According to the 'synaptic theory', the transmission of nerve impulses
from one neuron to another occurs as a result of the interplay of two separate
processes -'synaptic excitation' and 'synaptic inhibition'.
Synaptic inhibition and synaptic excitation are instrumental in the formation of
nerve circuits and networks or 'neural pathways' which are functional in
learning and the 'retention of learning' or
'memory'. The transformation of neural information and its storage as
memory
involve only nerve cells and their interconnections.
Information flow in the brain depends on the
strength of incoming stimuli. These must be strong enough to produce nerve impulses of
sufficient intensity to ensure their transmission across the synaptic clefts and
their propagation along connecting neurons. This is of particular significance
to learning theory.
('brain-based learning').
The synapse and the
transmission of nerve impules via neurotransmitters... synaptic function:
interaction or 'binding' between neurotransmitter molecules and protein
receptors
Anatomy or structure of the synapse The
synapse is a structure which is specialised for the function of transmitting
nerve impulses across connecting neurons... for the transmission of impulses
from one neuron to the next. Its structure can be described in terms of three
main components: first, the specialized point
of contact... connection on the presynaptic
or the limiting membrane of the terminating axon 'terminal button' of the axon
terminal -the 'synaptic knob' 'synaptic button,'derived from the multiple axon branches resulting from the
bifurcation of the axon at many points along its length; second, specialized
point of contact... the
point of connection on the membrane of the
postsynaptic neuron... 'postsynaptic membrane' 'or subsynaptic membrane of
a dendrite of the connecting neuron; third, the gap between the
two points of contact the 'synaptic gap'
or 'synaptic cleft' . The gap separates the two communicating neurons
by a millionth of an inch... two one
thousandths of a millimeter (0.000002mm) or twenty nanometers.
John Carew
The
synapse plays a critical role in
information flow which is the basis for learning Nerve impulse do not jump from
one neuron to the next. At the
synapse, nerve impulses are converted to chemical processes which excite or
inhibit activity in the connecting neuron depending on their strength
(intensity) which determines the molecular events at the synapse (synaptic
activity). Depending on the occurrence of excitation or inhibition synapses are
either strengthened or weakened. New synapses can be created. Established
synapses can deteriorate. As a result of this process of 'synapse
modification' neural pathways are altered. Alteration of neural pathways
involves the physiological process of continuity of information from one part of
the brain to another i.e.
'information flow'. Information flow is a function of the propagation of
nerve impulses along the neurons and their transmission from one neuron to
another across the gaps which separate them, the 'synaptic clefts'. The
effectiveness of synapses to initiate or 'fire' the propagation of new
signals on connecting neurons involving a series of complex molecular events and
depends on nerve impulses which are strong enough to increase the strength or 'potency'
of other synapses so that their influence enhances the creation of new synapses
and new pathways. When stimuli are not strong enough then synapses regress.
Formation of new synapses depends on stimulation through activity. Changes in
neural pathways and the creation of new ones constitute the biological basis for
'learning' and the retention of learning or 'memory'. Synapse modification is the 'trace of learning'
or 'memory trace' or 'engram'.
The discovery of the synaptic cleft modified the picture of the brain as a
'syncitium' of neurons and raised a major problem in brain biology. If each
of the millions of single neurons in the brain has a separate biological
existence then how are nerve impulses transmitted from one neuron to a
connecting neuron if there is a gap which separates them?
In other words what
mechanism provides for the continuity of information in the brain i.e.
'information flow'? The answer is in the functioning of the
synapse.
The function of the synaptic cleft is explained in
a working hypothesis known as the 'synaptic theory of transmission of
impulses' which provides an explanation for the problem of information
flow from one neuron to the next when there are gaps which separate them.
Generation
and propagation of nerve impulses: molecular events at the synapse ... synaptic theory of transmission of impulses
as a 'working hypothesis'. synaptic transmission involves a series of complex molecular events.
Nerve impulses are propagated to the
synapse in the form of packets of electrochemical pulses or 'signals'. They
arrive at the synapse in
bursts of a few at a time toa thousand per second. If the signals are sufficiently intense they are
transmitted to connecting neurons and propagated along their axons.
A nerve impulse as an electrochemical pulse triggers the influx of calcium
ions into the dendrites. The calcium combines with the
protein 'calmodulin' to form the calciumcalmodulin complex which
has a powerful metabolic action resulting in the manufacture of 'macromolecules'
such as proteins which are instrumental in increasing the potency of the synapses and in increasing their numbers.
... Not all are transmitted across the synaptic cleft.
Each neuron has a a critical
point or critical threshold for firing an incoming
nerve impulse and generating a new impulse in the connecting
neuron. If the threshold is lowered by the impulse crossing
the synapse then the effect is 'excitatory'. If the
threshold is raised then the effect is 'inhibitory'. Activity in the postsynaptic membrane is either
inhibited or excited, correspondingly preventing or enhancing the propagation of
new impulses along the connecting neuron.
The generation and propagation of a new
impulse depends on the specific properties of the nerve impulses - the intensity
of the signal and therefore on the strength of the stimulus... determines
whether or not a nerve impulse excites or inhibits activity in the connecting
neuron and whether a new signal is fired. An impulse is transmitted only if it
is sufficiently intense and its strength exceeds the critical threshold required
to generate or 'fire' a new signal and its continued propagation along
the axon of the connecting neuron.. when the synapse is excited above the
threshold. The threshold is the minimum number of electrochemical pulses per
second required to trigger a response. The stronger the stimulus, the more
intense the impulse and the greater the likelihood it will be transmitted across
the synaptic cleft. If the stimulus is not intense enough, connections
are not made and transmission does not occur.
SYNAPTIC TRANSMISSION: SYNAPTIC THEORY EVENTS AT THE SYNAPTIC JUNCTION:
SYNAPTIC ACTION Nerve impulses in the form of electrochemical pulses do
not jump from one neuron to the next. They are first changed into chemical
messages. The arrival of electrochemical pulses at the synapse triggers the
release of specialized neural transmitter molecules, known as
'neurotransmitters.' These are synthesized in the pre-synaptic cell and stored
in small vesicles located in the synaptic knob. When the appropriate electric
signal arrives, they are released through the presynaptic membrane and
propagated across the synaptic cleft. They attach to special receptor binding
sites on the postsynaptic membrane. Their binding triggers a change in the
membrane permeability of the connecting neuron. Receptor molecules are proteins
in the cell membrane of the connecting neuron. They attract and bind the
neurotransmitter molecules which are released across the synaptic cleft. The
resulting transmitter-receptor complex alters the postsynaptic membrane in one
of two ways: it has an 'excitatory' effect or an 'inhibitory' effect, depending
on its effect on the small pores which allow passage of ions across the
membrane. Opening of the pores makes the membrane more permeable to the ions anmd results in the depolarization of the membrane. The cell is more easily
excited. The binding of neurotransmitter molecules to receptor molecules on the
postsynaptic membrane excites the cell if it enhances the movement of
electrically charged ions across the membrane. In this way the binding is
'excitatory' if it results in the depolarization of the membrane. A new action
potential is generated in the connecting neuron. In this way the nerve impulse
is transmitted from one neuron to a connecting neuron. Closing of the pores
makes the membrane less permeable to the ions and results in further
polarization of the membrane. The cell is less easily excited. The binding of
neurotransmitter molecules to receptor molecules on the postsynaptic membrane
inhibits the cell if it prevents the movement of electrically charged ions
across the membrane. In this way the binding is 'inhibitory' if it results in
the further polarization of the membrane. The generation of a new action
potential is inhibited and the nerve impulse is not transmitted to the
connecting neuron. SYNAPSE The transmission of nerve impulses from one neuron to
another up to a thousand per second. SYNAPSE-PHYSICAL "TRACE'
OF LEARNING
"Learning occurs as a result of changing the effectiveness of
synapses ... At the synaptic junction between neurons, signals pass from the
synaptic knob of one neuron and are propagated across the synaptic cleft to the neigboring neuron. Signals are transmitted to the synapse in the form of
electrochemical pulses. When they reach the synaptic knob, it emits
neurotransmitter molecules. These are propagated across the synaptic cleft.
Depending on the nature of the synapse, they initiate or inhibit the 'firing' of
new signals along the axon of the neighboring neuron. Signals reach the synapse
as bursts of electrochemical pulses. They are collected from other neurons
through the dendrites. They enter the cell body for processing and are sent
outwards through the axon. They enter the cell body for processing and are sent
outwards through the axon which bifurcates at many points giving rise to
numerous 'axon terminals.' Bifurcating at many points, the axon gives rise to
numerous 'axon terminals.' At the synaptic junction between neurons, signals
pass from the synaptic knob of one neuron and are propagated across the synaptic
cleft to the contact point on the connecting neuron. The signal crosses the
synaptic cleft (about one millionth of an inch wide) Signals arrive at the
synapse -so many per second in the form of bursts of electrical pulses. They are
converted into a given number of chemical packets. If this number exceeds the
critical threshold required for triggering a response in the neighboring neuron,
then the signal is fired and the the electrical code is changed into the
chemical code for transmission along the axon of the neighboring neuron Incoming
signals are collected from other neurons through dendrites. At the synaptic
junction between neurons, signals pass from the synaptic knob of one neuron and
are propagated across the synaptic cleft to the contact point on the connecting
neuron. Signals reach the synapse as bursts of electrochemical pulses - so many
per second. These do not jump from one neuron to the next. The electrical code
of transmission is first changed into the chemical code of transmission. The
arrival of electrochemical pulses at the synapse triggers the release of
specialized neural transmitter molecules, known as 'neurotransmitters.' These
are contained in small vesicles located in the synaptic knob. They are released
through the presynaptic membrane and are propagated across the synaptic cleft.
They attach to special receptor binding sites on the postsynaptic membrane.
Their binding triggers a change in the membrane permeability of the connecting
neuron. The binding is excitatory if it causes the movement of electrically
charged ions and results in the depolarization of the membrane. A new action
potential is generated and the nerve impulse is transmitted. The binding is
inhibitory if it results in the further polarization of the membrane. The
generation of a new action potential is inhibited and the nerve impulse is not
transmitted.
SYNAPTIC TRANSMISSION: IMPULSES ARE TRANSMITTED FROM ONE NEURON TO ANOTHER
WHEN THE SYNAPSE IS EXCITED ABOVE THE THRESHOLD LEVEL "Each neuron has hundreds
or even thousands of synapses on its surface and it discharges impulses only
when the synaptic excitation is much stronger than inhibition." (Popper K. and
John Eccles. The Self and Its Brain. New York, London: Springer International,
1979 232) How does the presynaptic impulse evoke depolarization of the
postsynaptic membrane? The nerve impulse causes transmitter molecules to be
released from the synaptic vesicles...
A signal crossing the synapse can have one
of two effects: it can have an excitatory effect by lowering the threshold for
firing the signal; it can have an inhibitory effect by raising the threshold for
firing the signal. Nerve impulses which reach the synapse are subjected to one
of two synaptic processes: synaptic excitation or synaptic inhibition... (File
ORGAN p. 8) Depending on the process occurring at the synapse, an incoming
signal is transmitted to the connecting neuron and fired or not fired. Each
neuron has a particular threshold for firing an incoming signal. The
transmission of impulses from one neuron to the next depends on the number of
electrochemical pulses reaching the synapse. If this number exceeds the critical
threshold required for triggering a response in the connecting neuron, then the
signal is fired. An impulse is transmitted from one neuron to the next if there
are enough pulses to trigger a response and 'fire' the signal. A signal crossing
the synapse can have one of two effects: it can have an excitatory effect by
lowering the threshold for firing the signal; it can have an inhibitory effect
by raising the threshold for firing the signal. The stronger the stimulus, the
more pulses are generated. Depending on the number of pulses generated, they
initiate or inhibit the 'firing' of new signals along the connecting neuron.
Depending on the specific properties of the signal, its electrical effects
either inhibit or excite activity in the connecting neuron. The continued
transmission of the signal can be either inhibited or enhanced. If the
inhibitory input exceeds the excitatory input then a connection is not made
between one neuron and the next. If the excitatory input exceeds the inhibitory
input then a connection is made between one neuron and the next. The two
processes, synaptic inhibition and synaptic excitation are instrumental in the
formation of nerve circuits and networks in the brain. The functioning of the
brain involves this 'two process mechanism' as well as the mechanism of
transmission of nerve impulses along nerve cells. Called the 'junctional
mechanism' or 'two process mechanism', synaptic inhibition and synaptic
excitation involves both neurons and their junctions. "..the synaptic excitation
of all nerve cells evokes a depolarization of the postsynaptic membrane (the EPSP) which generates an impulse if it is sufficiently intense." The impulse of
the presynaptic neuron must be sufficiently intense (above a certain threshold)
in order to evoke a depolarization of the postsynaptic membrane which will
generate an impulse in the postsynaptic neuron. (John Carew Eccles 1957. "The
Physiology of Nerve Cells" Johns Hopkins Press Baltimore page 94) "..inhibitory
action on a nerve cell is brought about by a depression of the EPSP below the threhold level for initiation of an impulse." (ibid. page 132)
1. Impulses or 'signals' in their electric
form are first converted to their chemical form: 'neurotransmitters' When
nerve impulses arrive at the synapse they are first converted from electrical
messages or 'signals' into corresponding chemical messages in the
following way. The arrrival of nerve impulses at the synapse triggers the
release of specialized molecules which are manufactured, packaged and stored in
small vesicles located in the synaptic knob of the pre-synaptic neuron i.e. 'neural
transmitter molecules' or 'neurotransmitters'. The neurotransmitters
are released through the presynaptic membrane of the synaptic knob and
transmitted across the synaptic cleft. When they reach the postsynaptic membrane
of the connecting neuron they attach to specialized protein receptor molecules
or 'receptor binding sites'. Receptor molecules are
proteins in the cell membrane of the connecting neuron. They attract and bind
the neurotransmitter molecules which are released across the synaptic cleft. The
resulting transmitter-receptor complex alters the postsynaptic membrane in one
of two ways: it has an 'excitatory' effect or an 'inhibitory' effect, depending
on its effect on the small pores which allow passage of ions across the
membrane. The binding sites are specialized for the
attraction and binding of neurotransmitter molecules. The binding of
neurotransmitter molecules to the receptor molecule forms a 'transmitter-receptor
complex'. The formation of the transmitter-receptor complex triggers a
change in the membrane permeability of the connecting neuron. Depending on the nature of the complex and how it
affects the permeability of the postsynaptic membrane the effect of the
transmitter-receptor complex can be excitatory or inhibitory
Opening of the
pores makes the membrane more permeable to the ions and results in the
depolarization of the membrane. The cell is more easily excited. The binding of
neurotransmitter molecules to receptor molecules on the postsynaptic membrane
excites the cell if it enhances the movement of electrically charged ions across
the membrane. In this way the binding is 'excitatory' if it results in the
depolarization of the membrane. A new action potential is generated in the
connecting neuron. In this way the nerve impulse is transmitted from one neuron
to a connecting neuron. Closing of the pores makes the membrane less permeable
to the ions and results in further polarization of the membrane. The cell is
less easily excited.
3.
Synapse excitation
Membrane
permeability is increased if the pores are opened allowing for passage of
electrically charged ions and decreasing polarization (or increasing
depolarization) thus lowering the threshold and exciting the neuron to generate
a new signal. Binding which results in the depolarization of the membrane and
the generation of a new impulse is 'excitatory binding'. Membrane
permeability is decreased if the pores are closed preventing the passage of
electrically charged ions and increasing polarization (or decreasing
depolarization) thus raising the threshold and inhibiting the neuron from
generating a new signal. The effect of the binding is an excitatory one if it
lowers the threshold required for generating a new signal. If the nerve impulse
reaching the synapse is sufficiently intense... above the threshold required
for depolarizing the postsynaptic membrane, then the effect of the binding is
excitatory. The formation of the neurotransmitter-receptor complex opens the
pores in the postsynaptic membrane so that it is made permeable to the passage
of electrically charged ions resulting in decreased, polarization bringing it
closer to the threshold required for generating the movement of electrically
charged ions across the membrane ('action potential') which initiates a
new impulse. In the destabilized, or 'excited state', a new nerve
impulse is generated and the signal is propagated along the connecting neuron.
Dans une synapse
excitatrice les sites récepteurs du neurotransmetteur sont situés sur les
canaux au sodium de la membrane postsynaptique.
Ainsi, l'interaction du neurotransmetteur avec son récepteur provoque une
ouverture des canaux aux sodium entraînant une augmentation de la
perméabilité aux ions sodium.
L'entrée des ions
sodium à l'intérieur du neurone postsynaptique conduit alors à une
dépolarisation de la membrane postsynaptique. Le
ppse ne dure que quelques millisecondes.
Si cette
dépolarisation atteint le seuil d'excitation du neurone, un influx nerveux
partira le long du neurone postsynaptique.On appelle
potentiel postsynaptique excitateur (ppse) cette dépolarisation qui résulte de l'action du
neurotransmetteur sur la membrane postsynaptique.
4. Synapse inhibition The
effect of the binding is inhibitory if it raises the threshold required for
generating a new signal. If the nerve impulse reaching the synapse is not
sufficiently
intense... below the threshold required for depolarizing the postsynaptic
membrane, then the effect of the binding is inhibitory. The formation of the
neurotransmitter-receptor complex closes the pores in the postsynaptic membrane
so that it is made less permeable to the passage of electrically charged ions
across the postsynaptic membrane resulting in increased polarization making it
further stabilized near its resting value and further from the threshold
required for generating the movement of electrically charged ions across the
membrane which initiates a new impulse. In this stabilized or 'non-excited'
state the generation of a new nerve impulse or action potential is prevented and
the nerve impulse is not propagated along the connecting neuron. Binding which results in the further polarization
of the membrane and does not generate a new impulse is 'inhibitory binding'. Binding
is inhibitory if it results in the further polarization of the membrane
and does not generate a new impulse.
In inhibitory synapse the receptor sites of the
neurotransmitter are situated on the potassium channels so that the
interaction between neurotransmitter with its receptor provokes increase in
permeability to potassium ions. As the exterior of the postsynaptic neuron is
positive, the efflux of potassium ions results in even higher increase of
positive charge on the exterior and decrease of positive charge in the interior.
The neuron is said to be hyperpolarised and the threshold for transmission is
made higher. making it more difficult to stimulate
5. Continued transmission of impulses depends
on the effect of binding. When they reach the postsynaptic membrane of the connecting neuron, the
neurotransmitter molecules interact with specific protein receptor molecules known
as 'transmitter receptors' or
'receptors'. Receptors are protein molecules specialized for the
function of attracting and binding the neurotransmitter molecules. The receptor protein molecules are
embedded in the semi-fluid matrix of the cell membrane with pieces sticking out
above and below the surface. Many receptors have two functional components...
the first is
a region on the surface of the receptor protein specialized for the
binding of the transmitter molecule i.e. 'receptor binding site'
which is precisely tailored to match the
shape and configuration of the so that the latter fits into
the transmitter molecule as specifically as a lock and key.The second is a
'selectively
permeable pore' which is permeable to some ions and not to others. Binding between transmitter and receptor creates a 'transmitter-receptor complex'
which triggers a change in the three-dimensional
shape of the receptor protein and opens the pore component. This allows
for the passage of ions inside and outside the cell membrane across their
concentration gradients. Depending on the nature of the ions which move and the
direction of their movement, the transmitter-receptor binding initiates a
sequence of events which either inhibit or enhance the generation and
propagation of new signals along the axon of the connecting neuron.
Binding of neurotransmitter molecules to the
receptor sites of receptor
molecules can have either an excitatory effect or an
inhibitory effect depending on the nature of the complex and how it affects the
permeability of the membrane i.e. on the nature of the ions which move and the
direction of their movement. The continued transmission of the nerve impulse depends on the
effect of binding on the pores which allow for movement of ions across the
membrane... how activity at the synapse affects the permeability of the
membrane and hence the small pores which allow passage of electrically charged
ions through the membrane... Binding is inhibitory if it increases polarisation. The
binding of neurotransmitter molecules to receptor molecules on the postsynaptic
membrane is inhibitory if it closes the pores. Closing of the pores makes the membrane less
permeable and prevents the passage of electrically charged ions.across the
membrane This result is
increased polarisation of the membrane. If the membrane is further polarized, it
becomes stabilized near its resting value. This raises the threshold for the the
generation of a new impulse and inhibits the continued transmission of the impulse to the connecting neuron. Binding
is excitatory if it decreases polarisation. The
binding of neurotransmitter molecules to receptor molecules
on the postsynaptic membrane is excitatory if it opens the pores. Opening of the pores makes
the membrane more permeable to the passage of electrically
charged ions. This
results in increased depolarization of the membrane. It is further destabilized and closer to the threshold for
generating a new impulse and a new nerve impulse is initiated in the connecting
neuron.
If the inhibitory input exceeds the excitatory input then
a connection is not made between one neuron and the next. If
the excitatory input exceeds the inhibitory input then a
connection is made between one neuron and the next. Propagation
along the connecting cell occurs only if excitation of the
synapse exceeds inhibition. Whether inhibitory or
excitatory, binding of neurotransmitter molecules to their protein receptor
sites is precisely controlled... control of transmission is maintained by a mechanism
which rapidly inactivates the transmitter molecule once it is bound to its
receptor molecule. If excitatory binding exceeds inhibitory binding then
existing synaptic connections are strengthened and new ones are created leading
to creation and reinforcement of new neural pathways. Each neuron discharges
impulses only when the total synaptic excitation is much stronger than total
inhibition. Electrical impulses from one axon are transmitted to the dendrites
of connecting neurons if the excitatory input is greater than the inhibitory
input. If the inhibitory input exceeds the excitatory input then a connection is
not made between one neuron and the next.
Connections are made between
neurons only if the total synaptic excitation exceeds the total synaptic
inhibition.Propagation along the connecting
cell occurs ONLY if excitation of the synapse exceeds inhibition.
The transmission of impulses from one
neuron to the next depends on the specific properties of the 'presynaptic impulse'
- the number of electrochemical pulses reaching the synapse (the stronger the
stimulus, the more pulses are generated) as well as its effect on the 'postsynaptic
membrane' (whether polarization is increased or decreased).
Experiential
learning and the synapse... Learning from experience -
'experiential learning' - stimulates the growth of new synapses... new
synaptic connections.
Experiential learning is 'integrated learning'
or
'holistic learning'. Holistic learning
the basis for learning from experience
involves capacity for remembering
memory cognitive faculty...
the ability to store
mental representations of experience
important because it forms ...required for adaptation and survival. The allows
for guidance of behavior
on the basis of learning from
experience... Memory
as the ability to
remember
and reflect on experience accounts for
the human capacity for 'contemplation' or 'inner
dialogue' the basis
for human dignity or
'freedom'.
Education for freedom is based on recognition and respect for stages of human
growth and development -
'sociocognitive stages'.
"The brain naturally learns and remembers the moment-to-moment events that
constitute life experience. In order to make sense of new experience, the brain
attempts to categorize and pattern new information with the information which is
already stored in memory. The brain's mechanism of 'patterning' allows for the
rapid processing of complex stimuli. At a very high rate of speed, the brain
processes new experiential information in the context of previous patterns.
Creating spatial maps and patterns, the brain naturally thrives on complexity.
In its attempt to process new information from complex sensory input, the brain
automatically recalls previously stored programs and formulates new programs. It
formulates 'programs' which provide it with crucial information about the
surroundings. Allowing for the instant memory of experiences, new information is
rapidly processed in the 'spatial memory system' located in the brain's
hippocampus. Necessary for survival, the spatial memory system drives the
brain's innate search for meaning and is constantly monitoring and comparing the
present with past surroundings and experiences. Learning and memory are most
effective when facts and skills are 'embedded' in the natural spatial memory and
in the context of real life experiences. New learning experiences are naturally
'embedded' in previous learning experiences. With continued learning and
experience, the spatial memory system is enriched over time". (Renate
Nummela Caine and Geoffrey Caine The Brain: Making Connections Alexandria,
Va.: ASCD, 1991, 40-
"Learning
occurs as a result of changing the effectiveness of synapses so that their
influence on other neurons also changes... Learning is a function of the
effectiveness of synapses to propagate signals and initiate new signals along
connecting neurons. Learning and experience change the structure of the neural
networks." (Geoffrey Hinton, "How Neural Networks Learn from Experience,"
Scientific American, 267:3, September 1992, 145.)
Learning is a function of transmission of
signals along nerve cells or 'neurons' and across their interconnections or
'synapses'. All neurons transmit information in the form of electrochemical
reactions which travel along their axons i.e. electrochemical pulses or 'nerve
impulses'. Nerve impulses are 'electrochemical pulses,' regions of charge
reversal travelling along the nerve fiber, also known as nerve 'signals'...
'action potential'... The structural unit of psychology.
Experiential learning involves stimuli
which are strong enough to establish new synaptic connections between neurons in
the cortex Nerve impulses are transmitted to the connecting neuron
and propagated along its axon only if they are sufficiently
intense. The intensity of the impulse in the presynaptic neuron
must be above the required threshold to depolarize the
postsynaptic membrane. If the number of incoming electrochemical
pulses exceeds the critical threshold, then the incoming impulse
is transmitted and propagated along the connecting neuron.
Learning is a function of stimuli strong enough to influence
the effectiveness of synapses in the transmission of signals
from one neuron to another across the synaptic clefts. Intrinsically motivated
learning produces stronger stimuli than extrinsically motivated learning...
Learning is a natural function of the
'brain' ...based on brain functioning...
'brain
functions'. 'Brain
based learning' or 'natural learning' is based on the optimal
functioning of the brain i.e. 'optimal learning' or 'optimalearning'Optimalearning is natural learning which is compatible with
brain functioning. So-called 'brain-compatible' methods of teaching
facilitate the learning process. Facilitative teaching or 'thematic
teaching' engages the learner's intrinsic motives for learning or 'human
needs'. Motivation by human needs involves the psychological value of
creativity and productivity or 'work' i.e 'intrinsic
motivation'.
Knowledge of the role of stimulation on
synapse modification is directly related to educational methodology.The
effectiveness of synapses is modified or altered by experience. In experiences
of learning the stimulation of nerve impulses at the synapse enhances the
influence of neurons on each other and causes new synapses to form and grow...
'experiential learn
