Morphology of Nerve Cell
In a human body there are different types of nerve cells and the number of nerve cells in Central nervous System only is estimated to be about 100 billion.Though the nerve cells are different in shape and size, most of them has the same parts as a typical spinal motor neuron.Along with the neurons, supporting cells are also present.Now let us see the structure of neurons in detail.
Like any typical cell, neurons has got a cell body.From the cell body five to seven processes called dendrites extend outward.In the cerebral and cerebellar cortex, the dendrites have small knobby projection called dendritic spines.The next important structure seen in a typical neuron is called axon.The long fibrous axon originates from a somewhat thickened area of the cell body called the axon hillock.The first portion of the axon is called initial segment.The axon also divides into terminal branches and each of the branches end in a number of synaptic knobs.The knobs are called terminal button or axon telodendria.They contain granules in which neurotransmitter secreted by the nerves are stored.They are released when the impulse reach the knob and is strong enough to pass into the next neuron.
The next part of neuron we consider is myelin sheath.Myelin is a protein-lipid complex that is wrapped around an axon.Outside the central nervous system, the myelin is produced by schwann cells or glia-like cells found along the axon.The schwann cells will wrap its membrane around the an axon upto 100 times.After wrapping, the whole sheath of membrane is compacted with the help of proteins called Protein zero.Thus an insulating layer is formed along the axon.But myelin sheath does not cover the entire axon.The envelop is absent in the ending and also in the constrictions called Nodes of Ranvier.These are 1mm apart periodic constrictions, and in a mode of conduction called saltatory conduction, the impulse jump from one node to another.And so a neuron with myelin sheath is called myelinated neuron.But all neurons are not myelinated and in some other types, the covering are produced by a different type of cells.
The mammalian cells which are not myelinated are called unmyelinated neurons.They are simply surrounded by Schwann cells and the membrane is not wrapped to produce myelin.In another set of neurons in CNS of mammals, the neurons are myelinated but Schwann cells are replaced by oligodendrogliocytes.The main difference is that when schwann cells is confined to only a single neuron, oligodendrogliocytes are cells that send of multiple processes which forms myelin on more than one axons.Thus myelin coat present in an axon provides an insulating coat and altogether it helps in the conduction of impulses.In an auto immune disease called multiple sclerosis, destruction of myelin occurs in the CNS.
Though i have explained the basic parts of neurons with reference to a typical spinal motor neurons, it does not explain the morphology of all types of neurons.There are many variations and types.So to conclude the morphology of nerve cells we can classify the neurons as a whole into four important zone based on functional aspect.(1).A receptor or dendritic zone where potential changes are integrated.(2).A zone where propagating action potential are generated.(3).An axonal process that transmits impulse to nerve endings.(4).The nerve ending where action potential causes release of synaptic neuro transmitters.
Based on this four zone the morphology of most type of neurons can be explained.
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