link: nerve impulse

                          BIOLOGY OF LEARNING: THE NERVE IMPULSE OR 'ACTION POTENTIAL'

theme: 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.  

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neuron at rest...   role of sodium ions...   propagation of nerve imlpulse...   threshold...

implications for education...

Neurons 'at rest': properties of the cell membrane  

This difference is manifest as a difference in electric charge on either side of the membrane. The difference in electric charge results from the chemical properties of the sodium atoms involved in the electrochemical reaction of the nerve impulse. Electrically neutral sodium atoms contain the same number of negatively charged electrons as positively charged protons. Playing a key role in the generation of an action potential, the sodium atoms function as negatively charged sodium ions. In the resting cell, the concentration of sodium ions outside the membrane is ten times greater than the concentration of sodium ions on the inside of the membrane and inside the cell. The extracellular concentration of sodiumm ions is ten times greater than the intracellular concentration. Consequently the intracellular cytoplasm is negatively charged compared to the tissue fluid outside the cell, the 'extracellular fluid.' The inside is negative relative to the outside

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 in chemical composition... electric charge on either side of the membrane - positive on the outside and negative on the inside. The difference in electric charge results from the chemical properties of the 'sodium atoms' which play a key role in the generation of an  electrochemical reaction known as the action potential or nerve impulse.

 

Techniques have been developed for studying individual nerve cells... 

the 'action potential' the electrical impulse is the result of an electrochemical reaction which is a function of the special properties of the cell membrane. An electrochemical impulse involves the movement of electrically charged chemical ions - ionized atoms of chlorine, sodium and potassium.

Chloride ions (Cl-) are negatively charged. Sodium ions (Na+) are positively charged. Potassium ions (K+) are positively charged. 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. The intracellular concentration of chloride ions is greater than the extracellular concentration of chloride ions. 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. The extracellular concentration of positively charged sodium ions is ten times the intracellular concentration. As a result, the intracellular cytoplasm is negatively charged compared to the extracellular fluid. In the resting cell, the concentration of sodium ions outside the membrane is ten times greater than the concentration of sodium ions on the inside of the membrane and inside the cell. The extracellular concentration of sodiumm ions is ten times greater than the intracellular concentration. Consequently the intracellular cytoplasm is negatively charged compared to the tissue fluid outside the cell, the 'extracellular fluid.' 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 'gradient' of -70 millivolts is called the 'resting membrane potential.'

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. The sodium ion is 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. Sodium atoms function as positively charged sodium ions in their key role in the generation of an action potential. In the 'resting cell', positive sodium ions are kept outside the membrane of the nerve cell. An 'active pump'(active because it requires energy - ATP molecules) keeps sodium ions from moving through the 'sodium channels' across the membrane from the outside to the inside with the 'gradient'. Positive potassium ions are kept on the outside of the nerve cell by way of the 'potassium pump'. Negative chloride ions (also derived from the salt molecule) are pumped to the inside.(?) For each nerve cell, a concentration gradient of electrically charged particles creates an energy tension called the 'membrane potential'. The energy tension - on the order of a one tenth of a volt (90 millivolts) - is maintained through the constant action of the sodium and potassium pumps. Techniques have been developed for studying individual nerve cells 'at rest'. A nerve cell which is not carrying a nerve impulse is said to be 'at rest.' In this state, the cell membrane is characterized by a specific difference in chemical composition. This difference is manifest as a difference in electric charge on either side of the membrane. The difference in electric charge results from the chemical properties of the sodium atoms involved in the electrochemical reaction of the nerve impulse. In the resting cell, the concentration of sodium ions outside the membrane is ten times greater than the concentration of sodium ions on the inside of the membrane and inside the cell. The extracellular concentration of sodiumm ions is ten times greater than the intracellular concentration. Consequently the intracellular cytoplasm is negatively charged compared to the tissue fluid outside the cell, the 'extracellular fluid.' The inside is negative relative to the outside.

 Electrically neutral sodium atoms (Na) contain the same number of negatively charged electrons as positively charged protons. The sodium atoms function as positively charged 'sodium ions' (Na+). The concentration of 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). Consequently the 'intracellular cytoplasm' is negatively charged compared to the 'extracellular fluid'. The inside of the cell is -70 millivolts compared to the outside. This negative voltage... difference in voltage across the membrane... measuring 70 millivolts (nearly a tenth of a volt)... 'differential voltage' or 'voltage gradient' 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 nerve impulse or 'signal' is a region of charge reversal involving 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'.

Propagation of nerve impulse along the neuron Physical manifestation of the nerve impulse...

Permeable to some substances and not to others, the 'semipermeable' cell membrane 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 membrane is depolarized, increasing its permeability to sodium ions. These move across the membrane into the interior of the cell. The membrane is further depolarized and its permeabiity to other sodium ions is increased. The nerve impulse results from the movement of positively charged sodium ions across the cell membrane into the interior of the cell. It has an amplitude of 100 millivolts (a tenth of a volt) and a duration of one millisecond (one thousandth of a second.) The electric charge on one impulse is about one tenth of one volt and each impulse lasts about one thousandth of a second. At a critical point called the 'threshold,' the inside of the cell becomes positive with respect to the outside. The movement of sodium ions across the membrane ceases. In about one millisecond, the membrane potential returns to its resting value of -70 millivolts.(Hobson)

HYPOTHESIS FOR ACTION POTENTIAL GENERATION When an excitatory signal reaches a nerve cell, 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 electrical impulse or 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.

Their transmission is primarily involved with the movement of sodium ions into the interior of the cell. They are initiated by incoming physical or chemical stimuli which decrease the voltage gradient and depolarize the membrane. With the depolarization of the membrane, there is an increase which decrease the voltage gradient and depolarize the membrane. 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, further depolarizing the membrane. 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 inrush of sodium ions into the interior of the cell from one point of the membrane to the next constitutes the nerve impulse. The electric charge on one impulse is about one tenth of one volt and each impulse lasts about one thousandth of a second. At a critical point calledthe 'threshold,' the inside of the cell becomes positive with respect to the outside. Sodium ions cease to move across the membrane and the differential voltage returns to the resting membrane potential value of -70 millivolts. In this fashion, 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. With depolarization of the membrane there is an increase in its permeability to sodium ions. 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. This spike of electrical activity... the 'action potential'... 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. As a result, the neighboring sodium gates are opened increasing the permeability to sodium ions which move across the cell membrane into the cytoplasm. 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 into the interior of the neuron from one point of the membrane to the next is the process which constitutes transmission of the nerve impulse. 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. A new ionic balance is created and the 'differential voltage' returns to the resting state value of -70 millivolts.

 Measurements of the electrical signals awaited the technical developments in electronics. The diameter of an axon is equivalent to hundredth or thousandths of a hair strand. The voltage of an action potential is measured in thousandths of volts - 'millivolts'. A small pocket flashlight battery could energize five thousand action potentials. In 1903, Dutch physiologist William Einthoven (1860-1927), developed a low mass voltmeter which could measure the millivolt signals of the heartbeat. The 'string galvanometer', precursor of the modern electrocardiogram, was designed to record the electrical activity of muscle tissue. Nervous signals were made to travel along a very fine string. In the early 1930s Edgar Adrian and Brian Matthews of Cambridge University developed the precursor of the standard recording device for electrophysiology - the 'oscilloscope'. This instrument allowed for measurement of electrical signals by way of their projection on a luminescent screen. Essentially weightless electron beams of the nervous signals were applied to a magnetic field. In the magnetic field, the electron beam was made visible by its intereaction with the phosphorescent material on the screen. Electric signals of small voltages caused vertical deflections of the electron beams.  "The development of the oscilloscope led to an explosion of neurobiological studies." With the use of microelectrodes -fine tipped electrodes-the action potentials of individual nerves fibers could be recorded. It was discovered that the amplitudes and the speeds of the action potential were related to the lengths and the diameters of the conducting nerves. 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.

An understanding of the actual mechanism of the action potential was made possible with the development of a technique which enabled scientists to record the voltages on the inside of the semipermeable membrane of the nerve fiber, as well as the outside. Hodgkin, Huxley and Katz devised the technique of 'intracellular recording' which enabled them to demosnstrate that the mechanism of the action potential involves the rapid influx and efflux of elemental ions across a semipermeable membrane.

For summary of research developments: J. Allan Hobson, The Dreaming Brain. London: Penguin Books, 1988.

Each neuron has a threshold for transmission across the synapse  Nerve impulses are collected from neighbouring neurons through the dendrites. They enter the neuronal cell body for processing and are then transmitted along the axons which bifurcate split into thousands of branches giving rise to axon terminals. Axon terminals connect with dendrites of neigbouring neurons at the points of contact - the 'synapse'. 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. An impulse is transmitted only if it is sufficiently intense and its strength exceeds a critical point or 'critical threshold' when the interior of the cell becomes positive with respect to the exterior. 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 generating a new impulse in a connecting neuron.

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... existing synapses are modified - strengthened or weakened - ('synapse modification')  ...new synapses are created... leading to the formation of 'neural circuits'... 'neural pathways'... 'neural networks'...

Synapse modification is the focal point of investigation into the physiological process of continuity of information or 'information flow' in 'learning'.

Implications for education   Learning is a natural function of the 'brain'. 'Brain based learning' or 'natural learning' based on the optimal functioning of the brain i.e. 'optimal learning' or 'optimalearning'. Optimalearning is a function of natural 'brain functions'. 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'. Meaningful work engages the human potential for self-fulfilment i.e. 'mature growth' or 'self-actualisation'. Education for self-actualisation is 'holistic education'. In the holistic paradigm the teacher's role is 'facilitator of learning'.

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 References  J. Allan Hobson, The Dreaming Brain. London: Penguin Books, 1988, 100-133

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