With an estimate of approximately 100 billion the neurons form the most basic units of the nervous system and are also known as the nerve cell. The major characteristics which define a nerve cell are – Excitability and Conductivity. Excitability refers to the ability of responding to any external stimulus by the neuron. Conductivity however is the transmission of the external stimulus through the cell and onwards. Neurons are also sub-divided into various categories based upon their functions and also their basic structure. [1]
Neuron Structure
Any external stimulus sensed by the nervous system is transmitted by means of electrical signals by the individual neurons. The neuron consists of three principal substructures the dendrites, cell body (soma) and axon. The figure below shows these substructures.
Fig1: Neuron and its substructures. [5]
Dendrite – Is responsible for the increase in surface area of the neuron and reception of the nervous signals by means of specialised molecules called receptors that detect the neurotransmitters from the previous neuron, sensory part, muscle, etc. They appear as branch-like projections on the top of the cell body. Dendritic Spines are bag like structures present on the dendritic surface of some neurons. [4]
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Cell Body – Also known as the Soma it appears as a star shaped structure with a diameter of 20μm into which many dendrites end and a single axon filament emerges. It consists of a large nucleus with a prominent nucleolus containing the genetic material and is the centre of control of the nerve cell. [4] Ribosomes in the cell body located on the rough endoplasmic reticulum (rER) produce proteins which are stored in vesicles by the Golgi body after sorting. The rER can be seen by an ordinary light microscope after staining with a dye and is called Nissls substance. [3] The vesicles formed by the Golgi apparatus contain the precursors for neurotransmitters. Cellular respiration in the cell body is carried out by the mitochondria, which is the site for the production of the ATP – the primary source of energy for the cell. Intercellular transport of proteins and nutrients is carried out by means of tube like structures which run the entire length of the cell and are known as Neurofilaments or Neurotubules. [1]
Axon – Also known as the nerve fibre is responsible for carrying electrical impulse away from the cell body. The region of the axon just below the cell body is known as the “initial part” or “trigger zone” is the site where the electrical signal is generated for its transmission to the next neuron. [2] The length of the axon varies from a few millimetres to as long as from the head to the toes. In some neurons the length of the axons might be insulated by means of a fatty, membranous sheath also known as the Myelin sheath. The myelin sheath is formed from the surrounding cells – oligodendrocytes in the Central Nervous System (CNS), Schwann cells in the Peripheral Nervous System (PNS). These accessory cells (glial) wrap themselves around the axon fibre and thereby help in increasing the speed of signal transmission and also provide a good degree of electrical insulation to the axon. The different layers of the glial cells are known as Neuurolemmocyte while the outer layer of this multilayered sheath is known as the Neurilemma (Schwann sheath).[1] The myelin sheath is formed in segments rather than one continuous layer; these segments are visible along the length of the axon and are known as the Nodes of Ranvier (neurofinral nodes). [1,2] The figure below shows the multi-layered sheath which is formed around the axon by these cells.
Fig2: Myelin sheath and its formation [6]
The axon is devoid of any protein synthesising structures since the neurotransmitters and proteins are replenished by the structures present within the soma. [3] The terminal part of the axon branches (telodendria) into many different swollen bulb-like terminals (boutons or synaptic knobs) which contain neurotransmitters for the transmission of the neural signal onwards hence a single neuron can trigger a larger area of cells or muscle fibres at the site of action. [1, 2, 3]
Neuron Classification
Neurons can be classified into several different groups based on their functional as well as structural characteristics. When classified on the basis of functional aspects the major distinctions which can be given is with respect to the direction in which the neuron carries the signal.
Afferent Neurons – These neurons are considered as the sensory neurons since they are responsible for the conveying of information to the central nervous system. They might be “general somatic afferent neurons” or “general visceral afferent neurons”. Somatic afferent neurons carry the sensory stimulus information from the skin, voluntary muscles, joint and other connective tissues to the central nervous system. However the visceral afferent neurons carry the sensory stimulus information from the various internal organs (visceral organs) to the brain. [1] They possess an extremely long axon owing to the signal transmission length to the central nervous system.[2]
Efferent Neurons – Neural cells carrying information from the autonomic nervous system to the effecter muscles and are also known as motor neurons. Like the afferent neurons these are also classified as “general somatic efferent neurons” or “general visceral afferent neurons”. The somatic efferent neurons send the impulses to the voluntary skeletal muscles thereby causing muscle contraction. The visceral efferent neurons however send the information to the involuntary smooth muscles of the various organs and glands. [1] Like the afferent neurons efferent neurons also possess an extremely long axon owing to the signal transmission length from the autonomic nervous system to the effecter muscle fibres.[2]
Interneurons – Accounting for almost 99 percent of all neurons and are called association neurons, connector neurons, or internuncial neurons. [1] They function is to convey the information from the sensory neurons to the motor neurons after processing the sensory information. The length of the axons in the interneurons differ, if they are short and branching then they are known as “local circuit neurons” while if they are long then they are known as “relay neurons”. Local circuit neurons are concerned with the transmission of information over a short distance while the relay neurons transmit the information over long distances. Interneurons are mostly found in the Central nervous system. [1, 2]
Neurons can be further classified into three separate categories based on their structural characteristics.
Multi-polar neurons- neurons with several dendrites and a single axon are known as multi-polar neurons. These neurons are mostly located in the brain and spinal cord.
Bipolar neurons – such type of neurons have only a single axon and dendrite emerging from the cell body. They are located in a few places in the body such as the olfactory nerves in the upper nasal cavity and the retina of the eye.
Unipolar neurons – these are the most common type of nerve cells found in the peripheral nervous system. They consist of only a single nerve process which splits into two with one brain leading into the brain while the other leads into the spinal cord.[1]
The figure below shows the different structural characteristics displayed by the neurons.
Fig3: Structural characteristics of Neurons. [7]
Neuron Functions
The primary function of the nerve cell is the transmission of the neural signals from the nervous system to the various parts of the body. This is done by means of electrical signals which travel through the nerve cells in bands of excitation with the movement of ions in and out of the cell membrane of the nerve cell along the axon length and by means of neurotransmitters (acetylcholine {Ach}) at the junction of one neuron to another or target site (synaptic cleft).
Electrical Impulse Conduction along the Axon
The neuron is said to be in a resting phase when it is not conducting any neural impulse, during such a phase the neuron is charged or polarized due to a concentration gradient across its plasma membrane. The concentration gradient is due to the differential amount of positively and negatively charged (sodium {Na+}, potassium {K+} and chloride {Cl-}) ions present on either side of the plasma membrane. Due to this difference an electrical potential is developed across the plasma membrane known as the resting membrane potential. [2]
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The resting membrane potential is typically about -70mV and is present due to the difference in the ionic concentrations of Na+, K+ and Cl-. In the normal resting phase the extracellular fluid of the neuron is more positively charged than the interior of the neuron since there is higher concentration of Na+ ions on the outside than the inside of the neuron. In the same manner there is a higher concentration of K+ ions and negatively charged protein molecules on the inside than the outside.
The ions are able to freely pass across the plasma membrane from a region of higher to lower concentration but the relative concentration of the ions remains constant across the membranes. This is possible due to the homeostatic functions of the energy driven (ATP), self regulating transport mechanism known as the sodium potassium pump. The pump’s activity increases as more sodium diffuses into the plasma membrane and it pumps three sodium ions outside the plasma membrane while pumping only two potassium molecules into the cell. Thus the pump helps in maintaining the overall resting membrane potential by keeping the interior of the neuron negatively charged when compared to the exterior. Apart from the sodium potassium pumps voltage gated open ion channels also help in maintaining the charged state of the neuron by allowing the ions to pass through the membrane when the concentration of any one of the ions is very high on either side of the membrane.
The conduction mechanism is very similar in both the myelinated and unmyelinated nerve cells however there are differences due to the presence of the myelin sheath. In the unmyelinated nerve cells the change in resting potential is brought about due to the presence of an impulse which might be known as the threshold stimulus. This threshold stimulus causes the change in the permeability of the sodium ions in the plasma membrane of the axon. Hence a greater number of sodium ions rush into the neural cell due to the opening of the sodium ion channels and cause the depolarisation of the neural cell. The depolarisation causes the interior of the neural cell to become positively charged and makes the exterior of the cell negatively charged for a period of half a millisecond and raises the potential from -70mV in the resting phase to about +30mV in the action phase.
The region of the plasma membrane which has been depolarised due to massive influx of the sodium ions causes the flow of current which further depolarises the neighbouring region by opening the voltage gated sodium ion channels and allowing the inflow of sodium ions.
The sodium channels quickly deactivate shortly after the membrane has been depolarised and the inflow of sodium into the plasma membrane is stopped. The potassium ions try to escape out into the extracellular fluid in order to balance the potential outside and bring the membrane potential back to its original resting membrane potential. Once the membrane potential has been restored to the original resting potential it is said to be repolarised. Thus when it is observed the wave of polarisation and depolarisation travels as a band along the length of the axon in the neuron.
After each firing of the neuron there is an interval of approximately one to one and half milliseconds before it is possible for the neuron to generate the appropriate action potential for the next depolarisation, this time interval is called as the refractory period. During this period the plasma membrane is being repolarised after the wave of depolarisation has passed through it. The graph showing the various levels of the electrical potential across the plasma membrane during the wave of polarisation and repolarisation is given below.
Fig4: Action Potential during polarisation and depolarisation of the neuron. [8]
The conduction mechanism in mylienated nerve fibres is called as salutatory conduction, since the action potential appears to jump between the successive Nodes of Ranvier present along the myelin sheath. It is at the Nodes of Ranvier that the voltage gated sodium gates are highly concentrated and exposed to the extracellular fluid and hence it is observed that the electrical potential jumps to these sites. [1,2]
The figure below illustrates this process.
Fig5: Salutatory Conduction [9]
Electrical Conduction at the Synapses
Once the action potential reaches the terminating branches of the axon after travelling through the entire length of the axon, the action potential is transmitted to the next neuron or the target muscle or organ, etc. across the synaptic cleft (the junction between neurons or the target organs, muscles, glands, etc). The the nerve cell getting the action potential towards the synapse is known as the presynaptic neuron while the one carrying the action potential away from the synapse is known as the postsynaptic neuron. [1,2]
The transmission of neural impulse across the synapse is done by neurotransmitters (acetylcholine). These neurotransmitters can bring a change in the resting potential of the postsynaptic cells. As the action potential reaches the synaptic endings it causes the depolarisation of the presynaptic plasma membrane which causes the diffusion of the calcium ions into the presynaptic terminal. These calcium ions cause the neurotransmitter storage vesicles to fuse with the plasma membrane and release the neurotransmitters into the extracellular region of the synapse by means of exocytosis. The flow of neurotransmitter occurs only in a single direction since the presynaptic neurons only contain the neurotransmitter vesicles and no receptor sites while the post synaptic neurons contain the exact opposite. The neurotransmitters take a short interval of time in traversing the distance between the synapse and this interval is known as the synaptic delay. Upon the reception of the neurotransmitter in the receptor sites of the postsynaptic region changes start to occur in the membrane potential and a new wave of depolarisation begins in the next neuron due to change in the membrane permeability for the ions. After the reception of the neurotransmitter (acetylcholine) in the receptor site it is broken down into acetate and choline by the action of the enzyme acetylcholinestrase. The remainder of the neurotransmitters which were unable to reach the receptor sites and are stuck in the synaptic cleft are broken down by the action of proteins and enzymes. Over a period of time the acetylcholine is restored back to the presynaptic region. [1] The figure below shows the conduction of the neural impulse across the synapses by neurotransmitters.
Fig6: Neural transmission at synapses [10]
Conclusion
The neurons thus form the basic functional unit of the nervous system transmitting and receiving the neural impulses to and from the various parts of the body to the central nervous system. They are classified into different categories based upon their structure and direction of conduction of the nerve signal. The nerve cells are surrounded by many accessory cells which provide insulation, support and nutrition to the nerve cells. Through the essay the basic structure of the neuron was explained showing the different substructures present within the neuron and also the mechanism of nerve impulse transmission through the neuron was explained.
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