Structure of Neuron and Nerve impulses

The nervous system is one of the most important systems of the body, responsible for receiving stimuli, transmitting nerve impulses, coordinating different activities, and helping the body respond to changes in the internal and external environment. The basic structural and functional unit of nervous tissue is the neuron.

Understanding the structure of a neuron, nerve impulse, resting membrane potential, action potential, saltatory conduction, and synaptic transmission is essential for understanding how the nervous system functions. This article explains the structure and functions of neurons, resting membrane potential, propagation of nerve impulses, saltatory conduction, and synaptic transmission.

Table of Contents

1.1 Nerve

The nervous tissue is a specialized tissue for receiving and transmitting stimuli. Four types of tissues are found in the body which are epithelial, muscular, connective and nervous. The nervous tissue is composed of nerve cells or neurons.

The system which is responsible for co-ordination of various activities of different parts of the body amongst themselves and in response to stimuli of various kinds is called nervous system.

Some functions of nervous system are as follows:

It receives the changes in the external and internal environment by receptors such as eyes, ears, tongue, nose and skin; and after interpreting and analysing sends the messages to the effectors.
It is the link between the receptors and effectors and serves the dual functions of transmission and integration.
It controls and regulates muscular contraction, glandular secretion, metabolism etc.
It receives and co-ordinates the activities of internal organs in the body.
It also unites the organism by interacting with the changes of the external and internal environment.
It produces sensations like vision, hearing, smell, taste, pleasure and pain with the help of special sense organs attached to them.
It makes the animal aware of its environment and help the body in adjusting to that environment.

1.1.1. Structure of a Neuron

Neurons are the most highly specialized cells and structural units of nervous tissue. Each neuron consists of three parts i.e. (i) cell body, (ii) axon and (iii) dendrite.

Cell body

The part of the neuron that contains the nucleus is called the cell-body or centron or perikaryon. It contains mitochondria, Golgi body, melanin, lipofuscin pigments etc., but lacking the centrosome, because of centromere it cannot divide. So, number is same in adult as in infants; only size of the nerve cells increases in adult. The cytoplasm of the nerve cells is called as neuroplasm containing dark staining ribosome granules called Nisslis granules.

Axon

An axon is that which radiates out from the cell body and is highly elongated process. A typical neuron has a single axon. It is surrounded by its whole length by a series of sheath cells (Schwann cells). The sheath cells may have myelin (medullated) sheath containing alternate layers of protein and fat molecules or not. So, depending on the presence or absence of myelin sheath, the nerve fiber is called myelinated or medullated and non-myelinated or non-medullated nerve fiber. All axons of the peripheral nervous system are covered by cells arranged end to end forming a sheath knowns as neurolemma and the cells of the neurolemma are also called Schwann cells. But the axolemma is the plasma membrane of nerve cells.

The medullary sheath insulates and prevents the nerve impulses to spread to other adjacent fibers. As it is not continuous and is interrupted at intervals, the nerve fibers are constricted at certain points. These are called nodes of Ranvier which are the site for ion exchange between axoplasm and extracellular fluid.

Dendron

Dendrons are relatively short processes which branch repeatedly and are given off from the cell body. Due to profuse branching the thickness of the dendrons decrease and these thin branches are known as dendrites.

Dendrons are found in brain and spinal cord. Its function is to conduct the nerve impulses induced by stimuli to the neuron. The axons are mostly found in white matters of central nervous system and peripheral nervous system.

There are three types of neurons: (i) motor (ii) sensory (iii) connector. The sensory neurons transmit the nerve impulses toward the central nervous system. The motor neurons transmit them away from the central nervous system. Connector serves as the immediate links between numerous sensory and motor neurons. The connector or associated neuron consists of a cell body, one axon and numerous dendrites which type of neuron occur exclusively in the central nervous system.

Addition to it, neurons can also be classified in three types on the basis of number of cell processes emerge from the cell body. They are: (a) unipolar (b) bipolar and (c) multipolar.

Unipolar neurons possess a single process which divides not far from the cell body into two branches. One of these proceeds to some peripheral structure and the other enters in the central nervous system. Majority of this type neurons are found in the cerebrospinal ganglia and are prevalent in invertebrates and lower vertebrates.

Bipolar neurons have two processes one of which is dendron and the other one is axon. These neurons have a spindle shaped cell body and are found in the spinal and vestibular ganglia of the eight cranial nerve and retinal ganglia.

Multipolar neurons have many processes arise from the multipolar cyton, but only two are of structural types. One type is the axon which is long and possesses only a few processes. The other type is short but possesses numerous branches. The multipolar neurons are found mostly in the central nervous system and automatic ganglia.

1.1.2. Resting Membrane Potential

Resting membrane potential is the membrane potential of nerve fibers when an impulse or signal is not transmitting through it. The membrane potential at resting state of large nerve fibers is about-90 mv which means the potential inside the fiber is 90 mv more negative than the potential outside the fiber in the interstitial fluid. All cell membranes of the body have a powerful Na-K pump which pumps three sodium ions to the outside of the fiber and two potassium ions to the inside. Thus, for each cycle of the pump the inside of the fiber loses one positive charge. It will lead to an excess of positive charges on the outside. Na-K pump is an example of primary active transport in which ATP provides the required energy.

The sodium and potassium ions can leak through a channel type of transport protein in the cell membrane which are called 'potassium-sodium leak' channels. These channels are about 100 times more permeable to potassium than to sodium. As the high ratio of potassium ions inside to the potassium ions outside, the diffusion potential caused by the potassium would be -86 mv. So, the normal resting membrane potential is established by two important factors. The potassium and sodium diffusion gives the membrane potential of approximately -86 mv. It is mainly determined by the potassium diffusion. The Na-K pump contributes the additional -4 mv to the membrane potential giving a net resting membrane potential of -90 mv.

So, resting membrane potential is the potential before the action potential occurs. During the stage the potentiality of membrane is highly negative and the membrane is said to be 'polarized'.

1.1.3. Origin of a Potential and its Propagation in Nerve Fiber

Nerve signals are propagated by the action potential which is also called as nerve action potential. An action potential is the changes of the potentiality of nerve fiber from negativity to positivity and again to negativity. Normally, the membrane potential of a nerve fiber is negative when there is no propagation of a stimulus or signal; and it is called resting potential. Each action potential starts with a sudden change from the normal resting negative potential to a positive membrane potential and ends with an almost equally rapid change back again to the negative potential. The sodium and potassium diffusion and Na-K pump contribute in the establishment of the normal resting membrane potential. More positive ions are present outside the membrane in comparison to inside the membrane causing the nerve fiber negatively charged.

The successive stages of the action potential are as follows:

Polarized state or resting state
Depolarized state
Repolarized state
Reorientation state

Polarized state means when the nerve fiber is negatively charged and no impulse is propagated i.e., resting membrane potential before the action potential occurs. There are more numbers of Na+ ions outside the nerve fiber membrane i.e., in the interstitial fluid in comparison to inside of the nerve fiber membrane. Generally, in large nerve fiber, the membrane potential is-90 mv.

Depolarized state is that when an impulse is put on the nerve fiber, the potentiality is changed from negative to positive. Actually, as soon as any impulse is put on the nerve fiber, that part of the nerve fiber membrane becomes highly permeable to sodium ions (Na+) and a tremendous number of Nat ions enter inside the membrane i.e., interior of the axon. In large nerve fiber the negative potential of the polarized state of -90mv becomes slightly positive (about + 35mv). In smaller nerve fibers as well as many central nervous system neurons, it only approaches the zero level. Ultimately, inside the membrane more Na+ ions enter making it more positive in comparison to the outside the membrane.

Once generated the wave of depolarization travels all over the axon and is called as nerve impulse. This characteristic nature of the nerve impulse is known as the all-or-none principle. Depolarization propagating as an impulse is followed by subsequent repolarization.

In repolarized state, the entry of Na' ions to inside the membrane is stopped and the exit of K ions start to the outside i.e., interstitial fluid within a few 10,000 th of a second. It again causes the initial negative resting membrane potential.

In reorientation stage, the exchange of Na' ions and K' ions occur as it was at the most initial stage- the Na ions are more outside the nerve fiber membrane and not the K ions. The Na ions exit and K ions enter the membrane, forming the original state of the nerve fiber membrane.

Two other stages are occurred in between these, i.e., the negative after potential, positive after potential.

The negative after-potential: Sometimes the membrane potential fails to return all the way to its resting level for a few milli seconds at the termination of a spike potential. This may occur after a series of rapidly repeated action potentials and it is called negative after potential.

The positive after-potential: The membrane potential when returned to resting value, it becomes more negative than the normal values which is called positive after potential i.e.. actually a misnomer. It is a fraction of a milli volt to a few milli volt more negative than the normal resting potential, but it can last from 50 millisecond to as long as many seconds. The first part of this positive after potential is caused by the excess permeability of nerve membrane to potassium ions of the end of the spike potential. The prolonged continuance of this potential is caused principally by the electrogenic pumping of excess sodium outward through the nerve fiber membrane.

1.1.4. Propagation of an Impulse Through a Non-myelinated Nerve Fiber

Propagation of an impulse through a non-myelinated nerve fiber has been explained by 'Local circuit theory'.

When the nerve fiber membrane is stimulated at a point, the polarized resting membrane becomes depolarized. At resting or polarized state i.e. when an impulse is not passing over the neuron, the nerve fiber membrane is negative having a greater number of positive ions of sodium outside the membrane in comparison to inside the membrane.

At depolarized state the membrane part which is simulated becomes highly permeable to Na+ ions and entry of a huge number of the ions causes the membrane potential to approach the sodium equilibrium potential.

Ultimately, a potential difference occurs between the local region (stimulated) and an adjacent resting portion of the membrane and that potential difference generates a local current. It flows from the active region to the adjacent, resting inactive region through the cellular fluid to discharge the membrane. The return current flows back to the active region through the extracellular fluid and membrane. The local circular current flow reduces membrane charge of the resting region in the direction of the propagation of the nerve impulse.

To conduct another impulse, the repolarization must occur. The time taken for restoration of the normal resting membrane potential is called refractory period as during this, another impulse cannot be generated by the membrane. It is also significant because impulses travel in the axon only in forward direction- unidirectional impulse conduction. As the refractory period is about one milli second, a nerve fiber can transmit one thousand impulses in a second.

1.1.5. Saltatory Conduction of Nerve Impulse in the Myelinated Nerve Fiber

Depolarization occurs at the nodes of Ranvier only when an impulse travels along a myelinated neuron. The condition of nerve impulse in a myelinated nerve fiber is exceeding rapid as compared to non-myelinated nerve fiber an average of five to seven-fold, because it comes in contact with interstitial fluid or extra cellular fluid at the nodes of Ranvier only.

The distance between the two nodes of Ranvier is more in neurons of larger diameter than the neurons of smaller diameter. Hence, the action current jumps from node to node instead of a linear conduction and that mode of conduction of nerve impulse in myelinated nerve fiber is called as saltatory conduction.

The saltatory conduction of an impulse in a myelinated nerve fiber is of more important. First, it causes faster rate of transmission as the impulse jumps faster from node to node in more speed. Secondly, as only nodal regions are repolarized, the energy required to re-establish transmembrane potential is saved. But in case of non-myelinated, depolarization occurs throughout the length of the axon. Therefore, in myelinated one of the advantages is metabolic economy.

1.1.6. Synaptic Transmission

An impulse is transmitted from one neuron to another through synapse. Synapse is the junction between two neurons where one neuron ends and another one starts. According to the neuron of connection, it can be classified as (1) axo-somatic synapse, (ii) axo-dendritic synapse (iii) axoaxonic synapse.

When the presynaptic terminal of the axon ends in the body of the neuron, the synapse is called axosomatic. When the presynaptic fibers of any axon end in the dendrites of the post synaptic cell, the synapse is called axo-dendritic; when presynaptic fiber so any axon end in the axon of the postsynaptic cell, it is called axoaxonic synapse.

Hence, in synaptic transmission nerve impulse is transmitted from presynaptic axonal endings to either axon or dendrite or cell body of the post synaptic cell.

The gap between presynaptic and post synaptic membrane is called synaptic cleft. The portion of presynaptic neuron membrane over synaptic region is called as presynaptic membrane and the membrane of post synaptic neuron lying below the presynaptic membrane is called as post synaptic membrane.

The presynaptic terminal exhibits numerous vesicles containing special chemical substance known as neurotransmitter substance or neurohumor, for example acetylcholine. When an impulse moves or a nerve action potential reaches at the presynaptic terminal, the vesicles burst upon and the neurotransmitter substances are released in the synaptic cleft.

The energy required for it is supplied by ATP produced by the mitochondria present in the presynaptic terminals. The transmitter substance thus released, are combined with the receptor factors from the postsynaptic neuron. Now, the neurotransmitter-receptor complexes are formed which cause the permeability of the post synaptic membrane to sodium ions. As a result of entry of sodium ions inside the post synaptic membrane, the depolarization and generation of action potential in the postsynaptic membrane occurs.

Once depolarization reaches the threshold level, it generates either an excitatory postsynaptic potential (EPSP) or an inhibitory postsynaptic potential (IPSP). The neurotransmitter substances after making the post synaptic membrane permeable to sodium ions are lost immediately from the synaptic cleft.

In case of cholinergic synapse, acetylcholine is hydrolyzed by an enzyme, acetylcholine esterase, present at the synapse in high concentration. As a result of hydrolysis, acetic acid and choline are released which are reabsorbed into the synaptic node which are resynthesized into acetylcholine by the energy ATP.

Neurotransmitters

Neurotransmitters or neurohumors are the chemical substances which involve in the process of synaptic transmission. They are present in the vesicles at the terminals of the presynaptic neurons. They are of two types excitatory and inhibitory which are found in the nervous system of vertebrates and invertebrates.

The chemical substances release from presynaptic membrane diffuse through the synaptic cleft and react with specific receptors on the sub-synaptic membrane. The neurohumor and receptor complex triggers the permeability changes in the postsynaptic membrane causing generation or inhibition of the action potential in the post synaptic effector. Some most common neurohumors are, acetylcholine, adrenaline, noradrenaline, dopamine, 5-hydroxytriptamine and many more.

Important Questions and Solutions

1. What is a nerve? Explain the functions of the nervous system.

Ans: The nervous tissue is a specialized tissue for receiving and transmitting stimuli. The nervous tissue is composed of nerve cells or neurons. The system which is responsible for co-ordination of various activities of different parts of the body amongst themselves and in response to stimuli of various kinds is called nervous system.

Some functions of nervous system are as follows:

It receives the changes in the external and internal environment by receptors such as eyes, ears, tongue, nose and skin; and after interpreting and analysing sends the messages to the effectors.
It is the link between the receptors and effectors and serves the dual functions of transmission and integration.
It controls and regulates muscular contraction, glandular secretion, metabolism etc.
It receives and co-ordinates the activities of internal organs in the body.
It also unites the organism by interacting with the changes of the external and internal environment.
It produces sensations like vision, hearing, smell, taste, pleasure and pain with the help of special sense organs attached to them.
It makes the animal aware of its environment and help the body in adjusting to that environment.

2. Describe the structure of a neuron with its different parts.

Ans: Neurons are the most highly specialized cells and structural units of nervous tissue. Each neuron consists of three parts i.e. (i) cell body, (ii) axon and (iii) dendrite.

Cell body

Axon

Dendron

3. Explain resting membrane potential and its establishment.

Ans: Resting membrane potential is the membrane potential of nerve fibers when an impulse or signal is not transmitting through it. The membrane potential at resting state of large nerve fibers is about-90 mv which means the potential inside the fiber is 90 mv more negative than the potential outside the fiber in the interstitial fluid.

All cell membranes of the body have a powerful Na-K pump which pumps three sodium ions to the outside of the fiber and two potassium ions to the inside. Thus, for each cycle of the pump the inside of the fiber loses one positive charge. It will lead to an excess of positive charges on the outside. Na-K pump is an example of primary active transport in which ATP provides the required energy.

The potassium and sodium diffusion gives the membrane potential of approximately -86 mv. The Na-K pump contributes the additional -4 mv to the membrane potential giving a net resting membrane potential of -90 mv.

So, resting membrane potential is the potential before the action potential occurs. During the stage the potentiality of membrane is highly negative and the membrane is said to be 'polarized'.

4. Describe the origin and propagation of an action potential in a nerve fiber.

Ans: Nerve signals are propagated by the action potential which is also called as nerve action potential. An action potential is the changes of the potentiality of nerve fiber from negativity to positivity and again to negativity.

The successive stages of the action potential are as follows:

Polarized state or resting state
Depolarized state
Repolarized state
Reorientation state

Polarized state

Polarized state means when the nerve fiber is negatively charged and no impulse is propagated i.e., resting membrane potential before the action potential occurs.

Depolarized state

Depolarized state is that when an impulse is put on the nerve fiber, the potentiality is changed from negative to positive. Actually, as soon as any impulse is put on the nerve fiber, that part of the nerve fiber membrane becomes highly permeable to sodium ions (Na+) and a tremendous number of Nat ions enter inside the membrane i.e., interior of the axon.

Once generated the wave of depolarization travels all over the axon and is called as nerve impulse. This characteristic nature of the nerve impulse is known as the all-or-none principle.

Repolarized state

In repolarized state, the entry of Na' ions to inside the membrane is stopped and the exit of K ions start to the outside i.e., interstitial fluid within a few 10,000 th of a second. It again causes the initial negative resting membrane potential.

Reorientation stage

In reorientation stage, the exchange of Na' ions and K' ions occur as it was at the most initial stage- the Na ions are more outside the nerve fiber membrane and not the K ions. The Na ions exit and K ions enter the membrane, forming the original state of the nerve fiber membrane.

5. Explain saltatory conduction and synaptic transmission.

Ans:

Saltatory conduction

Hence, the action current jumps from node to node instead of a linear conduction and that mode of conduction of nerve impulse in myelinated nerve fiber is called as saltatory conduction.

Synaptic transmission

An impulse is transmitted from one neuron to another through synapse. Synapse is the junction between two neurons where one neuron ends and another one starts.

The transmitter substance thus released, are combined with the receptor factors from the postsynaptic neuron. Now, the neurotransmitter-receptor complexes are formed which cause the permeability of the post synaptic membrane to sodium ions. As a result of entry of sodium ions inside the post synaptic membrane, the depolarization and generation of action potential in the postsynaptic membrane occurs.

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