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                   ARCHITECTURE OF THE NERVOUS SYSTEM: THE NERVE CELL OR 'NEURON'

theme: Until the recent findings of brain research, it was believed that the functioning of the brain could be explained in terms of its functional unit - the nerve cell or 'neuron'. The neuron is specialised for the conduction of electrochemical signals or 'nerve impulses'.

 "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)

FOUNDER OF NEUROSCIENCE CAJAL STUDIED THE ARCHITECTURE OF NEURONS Modern research of the brain began at the end of the nineteenth century in Santiago, Spain with the studies of the great neuroanatomist Ramon y Cajal. Cajal proposed that it would be possible to understand the functions of the brain with an analysis of the functional architecture of the nervous system. He carried out the first morphological studies and began with both adult and embryonic nerve tissue. When he applied 'Golgi's silver staining' technique, he observed that some cells were stained entirely. Amongst these was a large variety of cells: cells with short axons communicating with neighbouring cells, cells with long axons projecting to other regions of the brain, cells with spindle shaped bodies, cells with round shaped cell bodies, cells with extensive branching and so on. Cajal demonstrated that the brain consists not of a continuous net of nerve tissue or 'syncitium' as was originally thought, but of discrete units - the 'nerve cells' or 'neurons'. As a result of his findings Cajal formulated his 'neuron doctrine' according to which  he described neurons as independent cells without 'protoplasmic bridges' connecting them. He 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 'electrical impulses' - spikes of electrical activity also known as the 'action potential'.

The characteristic features of neurons are specifically adapted for cell communication and the transmission of nerve impulses.

NEURON IS A TRANDUCER, CONDUCTOR AND TRANSMITTER OF NERVE IMPULSES Cajal recognized that the neuron had the properties of a transducer, a conductor, and a transmitter of electrical impulses ...converting energy from one form to another. As a transducer the neuron converts or 'transduces' the stimulus energy from the outside world into electrical signals. As a conductor the neuron propagates or 'conducts' the transduced signals from the dendrites to the cell body and then down the axon. As a transmitter, the neuron converts the electrical signals into chemical messages and 'transmits' them from one neuron to a neighboring neuron. The neuron receives the 'input signals' on short branched extensions of their cell bodies - 'dendrites' and sends the 'output signals' on long unbranched extensions - the 'axons'. The flow of information takes place from the dendrites to the cell body and then along the axon to the dendrites of the next cell. With this suggestion he set the stage for a cellular analysis of the 'reflex arc'. (English physiologist Charles Sherrington (1861-1952) worked out the details of the reflex arc in the spinal cord of mammals. The Integrative Action of the Nervous System, published 1906)  Since the 1940s techniques have been developed for studying individual neurons. The new techniques have revolutionized the sciences of the brain 'neural sciences' or 'neuroscience' making it possible to analyze neural processes of increasing complexity.

 All neurons transmit information in the form of electrical impulses which travel along their axons i.e. 'electrochemical pulses' or 'nerve impulses' .

BASIC STRUCTURAL COMPONENTS OF THE NEURON As a result of their specialization for neural functions, nerve cells differ from other cell types though they have the same basic structural components. 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' are the powerhouses of the cell. They 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' which are located throughout the interior of the cell. Ribosomes contain the enzymes and substrates for 'protein synthesis'. Cell specific proteins are produced in the 'Golgi bodies' where they are packaged into 'vesicles' which travel to the functional boundry 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 its branches and spines. The membrane is described as 'semipermeable' because it is permeable to some substances nd not to others... allows only certain molecules to pass through it... regulates 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 or 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'.

The binding of receptor molecules with neurotransmitter molecules on the membrane plays a key role in the transmission of nerve impulses from one neuron to another.

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 'dendrites'. Projections along the dendritic branches - the 'dendritic spines' - are outgrowths through which nerve impulses are transmitted from neigboring cells. The dendrites which carry the 'input signal' into the cell body are the structural pathways for incoming nerve impulses. The long unbranched extension from the soma - the 'axon' - is the structural pathway for the transmission of the 'output signal'. Extending from the soma over varying distances, axons bifurcate several times giving rise to smaller branches and their 'axon terminals'. At the end of each branch are expanded terminals - the 'axon terminals' or 'synaptic knobs' or 'synaptic buttons'. They make contact with neighboring cells at specialised points of contact - the 'synapses'. As the point of connection between one neuron and the next the synapse functions in the transmission of nerve impulses across the 'synaptic gaps'.

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

 NEURONS 'AT REST' PROPERTIES OF THE CELL MEMBRANE A nerve cell or 'neuron' which is not carrying a nerve impulse is said to be in the 'resting state'... 'at rest' ... is a 'resting cell'. In the resting state there is a specific difference in 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 action potential... are involved in the electrochemical reaction of the nerve impulse. 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.

A 'sodium-potassium pump'...sodium gates' are special pores for the passage of sodium ions across the membrane... are closed preventing the sodium ions from moving across the membrane and the negatively charged intracellular cytoplasm remains negatively charged.

CONDUCTION OF NERVE IMPULSE ALONG THE NEURON Nerve impulses are initiated by incoming  chemical stimuli or 'excitatory signals' which decrease the 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. Sodium ions rush into the cytoplasm of the neuron and potassium ions rush outwards to the exterior of the cell. With the influx of sodium ions and the efflux of potassium ions the voltage gradient is decreased and the membrane is further depolarized. As a result, the neighboring sodium gates open increasing the permeability to sodium ions. They rush across the cell membrane into the interior 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.

Communication between one neuron and the next takes place in the form of transmission of nerve impulses at the synapse ...when the interior of the cell becomes positive with respect to the exterior

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...

The resulting 'synapse modification' is the basis for learning..

IMPLICATIONS FOR EDUCATION 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 nerve cells 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'... based on brain functioning... 'brain functions'. Synapse modification becomes the focal point of investigation into the physiological process of continuity of information or 'information flow' in 'learning'. 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 facilitated ('facilitative teaching') with the use of teaching methods which are compatible with brain functioning or 'brain-compatible' i.e. 'thematic teaching'. Thematic teaching engages the learner's intrinsic motives for learning or 'human needs' i.e. 'intrinsic motivation' . Intrinsic motivation involves the psychological value of the human capacity for creativity and productivity or 'work'. Meaningful work engages the human potential for self-fulfilment or 'self-actualisation'.

 Education for self-actualisation is 'holistic education

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 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

The Biological Neuron. The brain is a collection of about 10 billion interconnected neurons. Each neuron is a cell [right] that uses biochemical reactions to receive, process and transmit information.  A neuron's dendritic tree is connected to a thousand neighbouring neurons. When one of those neurons fire, a positive or negative charge is received by one of the dendrites. The strengths of all the received charges are added together through the processes of spatial and temporal summation. Spatial summation occurs when several weak signals are converted into a single large one, while temporal summation converts a rapid series of weak pulses from one source into one large signal. The aggregate input is then passed to the  soma one (cell body). The soma and the enclosed nucleus don't play a significant role in the processing of incoming and outgoing data. Their primary function is to perform the continuous maintenance required to keep the neuron functional. The part of the soma  that does concern itself with the signal is the axon hillock. If the aggregate input is greater than the axon hillock's threshold value, then the neuron  and an output signal is transmitted down the axon. The strength of the output is constant, regardless of whether the input was just above the threshold, or a hundred times as great. The output strength is unaffected by the many divisions in the axon; it reaches each terminal button with the same intensity it had at the axon hillock. This uniformity is critical in an analogue device such as a brain where small errors can snowball, and where error correction is more difficult than in a digital system.

Each terminal button is connected to other neurons across a small gap called a synapse [left]. The physical and neurochemical characteristics of each synapse determines the strength and polarity of the new input signal. This is where the brain is the most flexible, and the most vulnerable. Changing the constitution of various neuro- transmitter chemicals can increase or decrease the amount of stimulation that the firing axon imparts on the neighbouring dendrite. Altering the neurotransmitters can also change whether the stimulation is excitatory or inhibitory. Many drugs such as alcohol and LSD have dramatic effects on the production or destruction of these critical chemicals. The infamous nerve gas sarin can kill because it neutralizes a chemical (acetylcholinesterase) that is normally responsible for the destruction of a neurotransmitter (acetylcholine). This means that once a neuron fires, it keeps on triggering all the neurons in the vicinity. One no longer has control over muscles, and suffocation ensues.

 

The nerve cell or neuron has the same basic structural components as other types of cell. A nucleus contains the genetic material, DNA (deoxyribonucleic acid), which directs the cell's activities. The contents of the cell outside the nucleus, the 'cytoplasm,' include a structural framework within which various other functional units are located. 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.) Ribosomes, organelles located throughout the interior of the cell, contain the enzymes and substrates for the synthesis of protein molecules. Golgi bodies, organelles involved in the production of vesicles, function in the collecting and packaging of cell specific protein molecules. The cell membrane, functional boundary of the cell, regulates the exchange of chemicals between the interior and exterior of the cell. Allowing only certain molecules to pass through it, the membrane is 'semipermeable.' The property of semipermeability is the result of the activity of holes or 'channels' in the membrane which can open and close their 'gates' to allow passage of certain atoms, ions and molecules. The membrane surrounds the entire cell including all its branches. Projecting from the outer surface of the cell membrane are chemical receptor molecules which 'recognize' and bind to specific chemical substances approaching the cell membrane. The binding of receptor molecules on the membrane plays a key role in the transmission of nerve impulses from one cell to another. As a result of their specialization for neural functions, nerve cells differ from other cell types. The characteristic features of neurons are specifically adapted for cell communication and the transmission of nerve impulses. A central cell body, the 'soma,' contains the nucleus and its enclosed genetic material. Extensions of the soma, the 'dendrites,' are the structural pathways for incoming nerve impulses. Projections along the dendritic branches, the 'dendritic spines,' are outgrowths through which nerve impulses are transmitted from neigboring cells. A long thin fiber extending from the soma, the 'axon,' is the structural pathway for outgoing nerve impulses. Extending from the soma over varying distances, axons bifurcate several times giving rise to smaller branches and their 'axon terminals.' Expanded terminals which make contact with neighboring cells, 'synaptic knobs' or 'synaptic buttons,' are functional in the transmission of nerve impulses from one neuron to another. A cell membrane surrounds the entire cell, including the soma, the axon with its branches and terminals, and the dendrites with its branches and spines. All nerve cells transmit information in the form of electrical impulses or 'electrochemical pulses.' Also known as 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. 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. 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. 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.' Despite the gradient, positive sodium ions are prevented from moving towards the cell interior. 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. Nerve impulses are 'electrochemical pulses,' regions of charge reversal travelling along the nerve fiber, also known as nerve 'signals.' 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 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 called the '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 transmitted from one neuron to another at specialized contact points known as 'synapses.' The synapse is the point of junction between two neurons and 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. 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 transmitted. 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. Recent findings in brain research indicate that the junction between neurons, the synapse, 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' in addition to the mechanism of transmission of nerve impulses along nerve cells. Until the recent findings of brain research, the unit of analysis for the functioning of the brain has been the neuron.