In this article we will discuss about:- 1. Classification of Nerve Fiber 2. Properties of Nerve Fiber 3. Role of Calcium in Nerve Function.
Classification of Nerve Fiber:
Based on the presence or absence of myelin sheath, they are classified into myelinated or non-myelinated nerve fibers. In the peripheral nervous system, the Schwann cells are responsible for the formation of the myelin sheath.
In the central nervous system, the oligodendroglial cells are responsible for the formation of the myelin sheath. This sheath is broken at regular intervals known as the nodes of Ranvier. Here only the neurilemmal membrane will separate the interior and exterior of the nerve fiber.
Nerve fibers can be classified based on different criteria:
1. Histologically, as myelinated or non-myelinated (Fig. 2.2).
2. Functionally, as afferent (sensory) or efferent (motor).
3. Based on diameter and conduction velocity which is known as Gasser and Erlanger’s classification.
4. Based on the type of neurotransmitter released from their terminals as adrenergic, cholinergic, dopaminergic, etc.
In myelinated nerve fiber, the wrapping of the axon by the myelin sheath provided Schwann cell occurs. Whereas in non-myelinated nerve fiber, the Schwann cell just covers the nerve fiber without wrapping. In myelinated nerve fibers, at certain places the myelin sheath is absent. These areas are called as nodes of Ranvier. The protoplasm present in the axon region is known as axoplasm.
The transport of materials within the axoplasm can occur in either direction (from the cell body towards the axon and vice versa). When the substance is transported from the cell body towards the axon terminals, it is known as anterograde transport (fastest—400 mm/day). This type of movement of axoplasm helps for transport of neurotransmitter, proteins, etc. from the cell body to the end of the nerve terminals.
When the transport of substance occurs in opposite direction (that is from the nerve terminals towards the cell body), it is known as retrograde transport (slowest—200 mm/day). Many of the viruses (rabies virus, polio), bacteria (tetanus), etc. reach the cell bodies in the nervous system because of the retrograde transport.
Myelinogenesis is the process by which myelination of the nerve fiber takes place. In the peripheral nervous system, the myelinogenesis is contributed by the Schwann cells whereas in the CNS it is being contributed by the oligodendroglial cells. The sheath of Schwann cell wraps the axon by about 80-100 times. The cell membrane lipids form the myelin sheath.
Functions of the Myelin Sheath:
1. In myelinated nerve fibers, the velocity of impulse transmission is faster because the process of depolarization occurs only at the nodes of Ranveir and, therefore, it appears as if the impulses are jumping from one node to the successive node (Fig. 2.3).
This type of impulse transmission is known as saltatory or leaping type of conduction. Because of this type of impulse transmission, the energy required for conduction is markedly reduced.
2. It acts as a protective sheath minimizing injury to the nerve fiber.
3. It acts as an insulator and prevents cross transmission of impulses from one fiber to the other in a mixed nerve.
Based on the diameter and velocity of impulse conduction, the nerve fibers are also classified into A, B and C types. And A is further divided in to A alpha, A beta, A gamma and A delta fibers. This classification is known as Erlanger and Gasser classification. Diameter and conduction velocity have direction relationship.
A and B group fibers are myelinated whereas C group fibers are non-myelinated (Table 2.1).
Properties of Nerve Fiber:
When a stimulus is applied, the nerve fiber demonstrates a change in its electrical activity from its resting state.
It is the ability of the nerve fiber to transmit impulses all along the whole length of axon without any change in the amplitude of the action potential. This type of conduction is termed as decrementless conduction.
iii. Refractory period (Fig. 2.13):
It is the duration after an effective stimulus, when a second stimulus is applied, there will be no response for the second stimulus.
a. From the time of the application of the stimulus till the initial one-third of the repolarization phase, the nerve fiber excitability will be zero and is completely refractory for the second stimulus. This duration is known as absolute refractory period.
b. Relative refractory period is the duration after an effective stimulus, when a second stimulus, which is slightly above threshold, is applied there will be response for the second stimulus as well.
iv. All or none law:
It states that, when the tissue is stimulated with threshold or more than threshold strength, the amplitude of response will remain the same but for a stimulus of less than threshold strength, there will not be any response.
All or none is obeyed by:
a. A single nerve fiber.
b. A single skeletal muscle fiber.
c. A motor unit.
d. Whole of cardiac muscle.
e. A single fiber of multi-unit smooth muscle.
f. Whole of visceral smooth muscle.
Role of Calcium in Nerve Function:
Unlike sodium and potassium ions of ECF, which are directly involved in the development of action potential, the calcium’s role is an indirect one. It does affect the excitability of the tissue but in an indirect way.
The details are:
i. At rest, calcium ions keep sodium channels closed. So when calcium ion in ECF is low, the number of sodium channels remaining in the closed state will be less.
ii. This leads to more sodium entry from ECF to ICF.
iii. This increases the excitability of the tissues.
iv. It is for this reason, when the ECF calcium ion concentration is reduced, the person has tendency to develop tetany (sustained state of contraction of muscle).