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Essay on Nervous Tissue of Human Body
Nervous tissue is a highly specialised tissue for reception, discharge of stimuli and transmission. It is made up of nerve cells and their processes, called the nerve fibres (Fig. 1.74).
Receptive processes are known as dendrons or dendrites and the discharging process is known as axon (Fig. 1.75).
A nerve cell body or perikaryon with all its processes is called a neuron (neurone). Besides these, there are cells neuroglial cells which support the nerve cells. The cells have been described under nervous system. Here only the nerve fibres will be described. There are two types of nerve fibres medullated and non-medullated.
Essay # 1. Medullated (Myelinated) Fibres (Fig. 1.76A & B):
It is composed of three elements-axis cylinder, myelin sheath or medullary sheath and neurolemma (neurilemma). The axis cylinder is central and is the direct continuation of the protoplasm of the nerve cells. It remains covered by a thin tubular sheath the axolemma (axon membrance), which is possibly the modified surface membrane, of the corresponding nerve cell.
The neurofibrils of the nerve cells are continued into the axis cylinder, where they remain embedded in a clear liquid ground substance-the axoplasm. So long as the nerve fibre remains within grey matter, it remains uncovered but after entering white matter, it receives a white covering called the myelin sheath or medullary sheath (white substance of Schwann).
This sheath is characteristic of myelinated fibres. Electron microscopic study indicates that the myelin sheath is an integral part of the Schwann cell. It is composed of layers of mixed lipids arranged concentrically alternating with the layers of neurokeratogenic protein (neurokeratin). The axon is at first enveloped by the Schwann cell.
Gradually this cell encircles the axon and forms many turns and thus the axon is surrounded by multilayered membrane-myelin sheath. Each layer of the membrane consists of the lipid molecules which extend radially and sandwich between tangentially arranged layers of proteins. Most nerve fibres which come out of and enter the central nervous system-somatic or autonomic, possess this sheath.
The functions of the medullary sheath is to insulate the nerve fibres and thus to prevent the spread of the nerve impulse to other adjacent fibres. All nerve fibres, somatic or autonomic, outside the central nervous system, receive another homogeneous nucleated covering the neurolemma (sheath of Schwann). Myelinated nerve fibres which are found in the brain and spinal cord differ from those of the peripheral nerve fibres in that the neurolemma is not present. It is of ectodermal origin.
Functions of Neurolemma:
1. To protect the nerve fibre.
2. To supply its nutrition partly.
3. To play an essential role in the regeneration of damaged peripheral nerves.
At regular intervals the peripheral nerve fibre is to possess constrictions (some 80-200 µ apart), known as the nodes of Ranvier (Fig. 1.77). These interruptions of the myelin sheath at these nodes permit sites for ion exchange between axoplasm and extracellular fluid. At these nodes the medullary sheath is broken down and the outer neurolemma comes into contact with the central axis cylinder.
Branching of the nerve fibres takes place at these nodes only. That portion of the fibre which lies between two adjacent nodes, is called the inter node. Each internode is found to possess a neurolemmal cell (lemmal cell or Schwann’s cell) with an oval nucleus just under the neurolemma. These cells link up with each other and form the tube-like neurolemmal sheath.
The myelin sheath in the internode may show a variable number of oblique clefts (myelin clefts or clefts of Schmidt- Lantermann), which divide the sheath into small segments-the segments of Lantermann (Schwann’s segment).
Probably these clefts are artifacts being produced during histological preparation. Medullated fibres vary in size. The thickest ones are about 30 µ or more in diameter, the intermediate ones 4-10 µ and the thinnest 1-3 µ. The rate of propagation of nerve impulse varies directly as the thickness of the fibre.
Essay # 2. Non-Medullated (Amyelinated) Fibres (Fibres of Remark) (Fig. 1.76C):
The postganglionic fibres of the autonomic nervous system belong to this type. These are composed of two elements only-the central axis cylinder and the neurolemma. Non-melinated nerve fibres differ from myelinated nerve fibres in great reduction or absence of the myelin sheath, the fibre being directly invested with the neurolemma.
In the peripheral nerve trunks the fibres are grouped into separate bundles. The individual nerve fibres are held together by loose connective tissue, called the endoneurium. Several nerve fibres are collected into a bundle and surrounded by a sheath-the perineurium.
Another tough sheath, epineurium, encloses the whole nerve trunk. These sheaths support and protect the nerve fibres and also support blood vessels and lymphatics to the nerve trunk branches, each branch becomes covered by a sheath-Henlel’s sheath, which is a prolongation from the epineurium and perineurium.
The blood supply to the nerve trunk is scanty but is sufficient for its low metabolic needs. The nerve trunks also receive nerve fibres (nervi nervorum) which ramify and terminate in end plates situated in the perineurium.