THE BIOLOGY OF LEARNING: THE SYNAPSE AND THE TRANSMISSION OF NERVE IMPULSES VIA NEUROTRANSMITTERS

 

theme: 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'. There are hundreds and even thousands of synapses on the surface of each neuron.

 "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).  

                                                                                                                                                  home

neurons and nerve impulses...  

structure of the synapse... 

 problem of information flow...  synaptic theory of transmission of impulses...   molecular events at the synapse... 

implications for education...   references...

Neurons and nerve impulses  The brain is made up of single and separate nerve cells or 'neurons' (see Cajal) Neurons are specialized for the function of propagating electrochemical signals or 'nerve impulses' along their unbranched extensions or 'axons'. 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'. More than a thousand billion synapses connect the neurons to form  'nerve circuits' or 'neural pathways'.

 "Sherrington called the specalized contct points between neurons 'synapses' from the Greek word 'synapto' which means to clasp tightly. (Popper K. and John Eccles. The Self and Its Brain New Yiork, London: Springer International, 1979 230)

The synapse plays a critical role in the transmission of impulses from one neuron to the next. 

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. 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' as the basis for 'learning' and the 'retention of learning' or 'memory'.

Structure of the synapse  The synapse is a structure which is specialised for the function of transmitting nerve impulses across connecting neurons. Its structure can be described in terms of three main components: first, the 'presynaptic membrane' is a specialized point of contact on the 'terminal button' of the axon terminal, second, the  'postsynaptic membrane' is the specialized point of contact on the dendrite of the connecting neuron; third, the 'synaptic gap' or 'synaptic cleft' separates the contact points by two one thousandths of a millimeter (twenty nanometers).

The function of the synaptic cleft is explained in a working hypothesis known as the 'synaptic theory of transmission of impulses'.                                                  

Problem of information flow 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. 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'.

"It is generally conjectured that the physical evidence for learning and memory is in the increase in potent synaptic connections. New synapses grow and develop. The modifiable synapses are the 'spine synapses' on the dendrites of neurons in the cerebral cortex and the hippocampus. Modifications are manifest in the growth and ramification of new synapses and in the hypertrophy of existing synapses. Regression of synapses occurs with forgetting. There is evidence for this function-dependent formation of synapses in the cortex" Eccles.

 Generation and propagation of nerve impulses: synaptic theory of transmission of impulses Nerve impulses are 'signals' in the form of electrochemical pulses occurring in bursts of a few to a thousand per second... reach the synapse at varying intensities. Not all are transmitted across the synaptic cleft. Each neuron has a a critical point or 'critical threshold' for generating a new impulse in a 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. ("...the synaptic excitation of all nerve cells evokes a depolarization of the postsynaptic membrane which generates an impulse if it is sufficiently intense... A new action potential is generated and the nerve impulse is transmitted". Eccles)

If the stimulus is not intense enough, connections are not made and transmission does not occur.

Molecular events at the synapse  Current research is based on the following 'working hypothesis': synaptic transmission involves a series of complex molecular events. In the form of an electrochemical pulse, a nerve impulse triggers the influx of calcium ions into the dendrites. The calcium combines with a recently discovered protein called 'calmodulin'. The calciumcalmodulin complex has a powerful metabolic action which results in the manufacture of proteins and other 'macromolecules' (neurotransmitters?). These are instrumental in increasing the potency of the synapses and in increasing their numbers.

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'. 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

2.Transmitter-receptor complex can have excitatory or inhibitory effect depending on the threshold of the neuron and properties of the impulse - intensity of the signal and strength of the stimulus The complex can have one of two effects on the postsynaptic membrane - 'excitatory' or 'inhibitory'.

An excitatory effect initiates the generation of new signals along the axon of the connecting neuron and an inhibitory effect prevents it.

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.

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'.

5. Connections are made between neurons only if the total synaptic excitation exceeds the total synaptic inhibition:  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.

Propagation along the connecting cell occurs ONLY if excitation of the synapse exceeds inhibition.

 

Learning and the synapse... Learning is a physiological function of the brain involving the transmission of signals along nerve cells and across their junctional connection. 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'. Methods based on the natural funtioning 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". (Geoffrey Hinton, "How Neural Networks Learn from Experience," Scientific American, 267:3, September 1992, 145)

 

Implications for education The continuity of information in the brain is a function of stimuli which are intense ..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. Learning is a natural function of the 'brain' ...based on brain functioning... 'brain functions'.  'Brain based learning' or 'natural learning' is a function of 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... optimal brain functioning or 'optimal learning' or 'optimalearning'. Optimalearning is a function of the creation and enhancement of synaptic transmission ...facilitated by teaching to the brain's potential for holistic learning with the use of teaching methods which are compatible with brain functioning or 'brain-compatible' i.e. 'facilitative teaching'. Facilitative teaching engages the learner's intrinsic motives for learning or 'human needs' i.e. 'intrinsic motivation'. Intrinsic motivation enhances learning... involves the psychological value of the human capacity for creativity and productivity or 'work'. Meaningful work engages the human potential for self-development ... development of 'moral consciousness' or 'conscience' i.e. 'self-actualisation'. Education for self-actualisation is 'holistic education'.

 Those teaching and learning methods which inhibit learning inhibit the formation of connections between nerve cells, the 'synapses'. Methods based on the natural funtioning of the brain enhance learning because they enhance the formation of synaptic connections between nerve cells.

 Top of page  / Introduction  /  Homepage

references:

1. Eccles and Robinson The Wonder of Being Human: Our Brain and Our Mind

 2. Eccles, J.C. The Physiology of Nerve Cells. Baltimore, MD: Johns Hopkins Press 1957

 3. Eccles, J.C. The Physiology of Synapses. Berlin: Springer Verlag, 1964

 4. Eccles, J.C. "The Neurophysiological Basis of Mind: The Principles of Neurophysiology Oxford: Clarendon Press 1952 Chapter VI Prolonged Functional Changes (Plasticity) in the Nervous System p. 193-227 Section C . Synaptic Activity and Plasticity p. 203-211p.

 5. Eccles, J.C. The Understanding of the Brain

 6. Hinton, G. How Neural Networks Learn from Experience Scientific American, 267:3, September 1992, 145

Thomas Leahey and Richard Harris, Human Learning. New Jersey: Prentice Hall, 1989 

Eric Kandel and Robert Hawkins, The Biological Basis of Learning and Individuality Scientific American, 267: 3, Sept 1992

 7. Popper K. and Eccles J.C. The Self and Its Brain. New York, London: Springer International, 1979

 8. Ornstein R. and R.Thompson The Amazing Brain

 9. Hobson. The Dreaming Brain

_________________________________________________________________________

notes:

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.

THEMES 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.

 The word 'synapse' is from the Greek 'synapsis' meaning 'contact'. (Hobson The Dreaming Brain 105)'. The term 'synapse' was introduced in 1897. It was introduced by C.S. Sherrington ("The Central Nervous System" vol. 3 In A Textbook of Physiology, 7th ed., Ed. M. Foster. London, Macmillan, 1897) "Sherrington called the specalized contact points between neurons 'synapses' from the Greek word 'synapto' which means to clasp tightly." The term 'synapse' is derived from the Greek word meaning to 'clasp.' (Popper K. and John Eccles. The Self and Its Brain. New York, London: Springer International, 1979, 230)

ANATOMY OF THE SYNAPSE The synapse consists of the 'synaptic knob' (portion of a terminating axon at the synapse), 'subsynaptic membrane' (portion of the postsynaptic neuron at the synapse) and the gap between them, the 'synaptic cleft." (John Carew Eccles The Physiology of Synapses Springer Verlag Berlin 1964) The synapse has three main components: the point of connection on the 'presynaptic membrane' of an axon terminal, also known as the 'synaptic knob' or 'synaptic button,' the point of connection on the 'postsynaptic membrane' of a dendrite of the connecting neuron and a gap which separates them, known as the 'synaptic cleft.' The gap which separates two communicating neurons is a millionth of an inch wide. Nerve impulses or signals are transmitted in the form of electrochemical pulses - a few at a time or in bursts of up to a thousand per second.

 SYNAPTIC TRANSMISSION: SYNAPTIC THEORY EVENTS AT THE SYNAPTIC JUNCTION: SYNAPTIC ACTION For account of synaptic action see Eccles, J. The Understanding of the Brain. 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...

SYNAPTIC EXCITATION AND INHIBITION 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)

 EHANCEMENT OF SYNAPTIC FUNCTION BY USAGE 209-211 Discussion on use and disuse "Residual potentiation would be a special illustration of the enhancement of synaptic function by usage." "It can be envisaged that the normally occurring bursts of impulses in the presynaptic fibers bring about frequent minor distensions of the synaptic knobs, which are hence maintained at a normal level of size and effectiveness. With disuse...the absence of these repeated distensions results in a gradual shrinkage of synaptic knob size and hence leads to defectiveness of function....." "..direct experimental evidence has been obtained in support of the hypothesis that usage leads to increased functional efficiency of synapses and disuse to defective function." This is the 'plastic change' of neurons. (John Carew Eccles "The Neurophysiological Basis of Mind: The Principles of Neurophysiology" Oxford Clarendon Press 1952 Chapter VI "Prolonged Functional Changes (Plasticity) in the Nervous System" p. 193-227 Section C. Synaptic Activity and Plasticity p. 203-211p.)

SYNAPTIC TRANSMISSION 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 (File ORGAN p.6).. up to a thousand per second. SYNAPSE-PHYSICAL "TRACE' OF LEARNING "Learning occurs as a result of changing the effectiveness of synapses ...

 LEARNING AND THE SYNAPSE Learning occurs as a result of changing the effectiveness of synapses so that their influence on other neurons also changes. (Geoffrey Hinton, "How Neural Networks Learn from Experience," Scientific American, 267:3, September 1992, 145)

Learning is a physiological function of the brain involving the transmission of signals along nerve cells and across their junctional connections. 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. The number of nerve cells in the brain is fixed at birth and no new nerve cells grow and develop. 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

"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."

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.

Nerve impulses which reach the synapse are subjected to one of two synaptic processes: synaptic excitation or synaptic inhibition...

 The basic activity of synapses comprises two processes which determine the outcome of nerve impulses reaching the synapse: synaptic excitation and synaptic inhibition. 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.

Mediation of psychological states is through the synapse  

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),

The synapse has three main components: the point of connection on the 'presynaptic' membrane of an axon terminal, the 'synaptic knob' or 'synaptic button,' the point of connection on the 'postsynaptic' or 'subsymaptic' membrane of a dendrite of the connecting neuron, and a gap which separates them, known as the 'synaptic cleft.' The gap which separates two communicating neurons is a millionth of an inch wide. Nerve impulses or signals are transmitted in the form of electrochemical pulses - a few at a time or in bursts of up to a thousand per second. 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. 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. 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. 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. 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 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),  Introduced in 1897, the term 'synapse' is derived from the Greek word meaning 'to clasp.'  The synapse has three main components: the point of connection on the 'presynaptic' membrane of an axon terminal, the 'synaptic knob' or 'synaptic button,' the point of connection on the 'postsynaptic' or 'subsymaptic' membrane of a dendrite of the connecting neuron, and a gap which separates them, known as the 'synaptic cleft.' The gap which separates two communicating neurons is a millionth of an inch wide. Nerve impulses or signals are transmitted in the form of electrochemical pulses - a few at a time or in bursts of up to a thousand per second. 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 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 ransmitted. 

   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. 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.