The neural system is composed of nerve cells or neuron, surrounded by a delicate web of connective tissue called neuroglia. Come and have a look of Anatomy Of Nervous System : The Neuron | Zoology Notes. Neuron or neurone is the structural as well as functional unit of neural system. The unit of nervous tissue are nerve cells or neurons of ectodermal origin, a neuron has a cell body or cyton from while arise two types of fibers. According to the ‘ neuron theory’ , each neuron is a distinct anatomical unit, having no protoplasmic continuity with other neurons. It also physiologically distinct, so that damage or destruction of a neuron may not effect adjacent neuron. The neuron, rather than the nerve, transmits the nerve impulse.
Structure of neurons
Neurons are of different shape, but consist of of an irregular cytoplasmic cell body called cyton, with a number of branching cell processes or fibers.
A cyton has nucleus and numerous baso philic Nissi’s granules, these granules are made up of ribonuceleic acid. They produce new cytoplasm in neurons and are also concerned with protein synthesis in nerve cell. The cell body has fine thread or neurofibrils forming a network. A group or mass of cell bodies within the grey matter of brain or spinal cord is called a nucleus, while outside the central neural system, it is called a Ganglion.
A Neuron has a number of processes or fibers projecting from the cell body, two types of fibers are differentiated on the basis of the direction of the nerve impulse conducted by them.
These are shorter, usually several, much branched, with Nissl granules carry impulses towards or into the cell body.
It is longer, usually single, without branches and Nissl’s granules, and normally conduct impulses away from the cell body. A nerve fiber consists of a central thin cytoplasmic strand, called axis cylinder, which is continuous with the cell body. Axon or axis cylinder is a single process which is generally long and has terminal branches called axon endings which have small swelling at their ends. Axon are different because impulses are set out through them.
All nerve fibers outside brain and spinal cord, are covered by the thin delicate membrane of protein, the Schwan sheath OR NEURILEMMA. Each sheath call has a thin layer of cytoplasm with a flat nucleus. In nerve fibers there is a second sheath called myelin sheath or medullary sheath between the axon and its neurilemma. Such fivers are termed as myelinated or medullated sheath. Myelin is a shining white substance made of bodies of sheath cells wrapped around the axon many times, it has alternate layer of protein and fats. Myelin is not continuous uniformly but becomes interrupted at intervals by circular constrictions termed NODES OF RANVIER. Part of nerve fibers between two adjacent nodes is called an internode.
Nerve fibers having thin layer which lack the fatty sheath of a medullary sheath are called non myelinate or non medullated and are gray in appearance. Nerve fibers having thick layer of a medullary sheath are medullated fibers and they appear white in colour. Medullated fibers conduct impulses much faster than non medullated ones. Just below neurilemma is a thin cytoplasmic layer with scattered slat nuclei, forming sheath cells or Schwann cell. Collateral branches may arise at right angles from long fibers or axons.
Nerve ending are relayed from the aoxn ending to one neuron to the dendrites of another, they are arranged in such a way that the axon endings of one neuron rest on the dendrites or even on the cyton of the next neuron, this linking of neuron is called a synapses or synapsis. Neuron from pathways for conduction of nerve impulses, but cytoplasm of one neuron is not continuous with that of another.
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