Introduction
Neural tissue exerts the greatest control over the body’s responsiveness to changing condition. Nervous tissue is specialized to transmit messages in our body. Brain, spinal cord and nerves are all composed of nervous tissue. Nervous tissue contains highly specialized cells called nerve cells or neurons. Neuroglia make up more than one-half the volume of neural tissue in our body.
Neurons
Neurons are the structure and functional units of nervous tissue and are excitable cells. Each neuron has following parts:-
1. Cyton or Cell body:- It consists of a central nucleus tissue and are cytoplasm (neuroplasm) with characteristic deeply stained particles, called Nissl’s granules. Nissl’s granules are large and irregular masses of ribosomes and RER
Dendrites:- These are short and branded processes arising from the cyton. They carry impulses towards the cell body.
Axon:- It is a single long cylindrical projection emerging from cyton. The axon ends in a group of branches (axon endings), termed as terminal arborizations. Nissl’s granules are absent in axon. The axon carries messages away from the cyton.
Axon is surronnded by a sheath (Neurilemma) formed by neuroglial cells called Schwann cells . This sheath is called myelin sheath which is a discontinuous covering and is absent at Nodes of Ranvier.
4. Synapse:- When a neuron is suitably stimulated, an electrical disturbance is generated which swiftly travels along its plasma membrane. Arrival of the disturbance at the neuron’s endings, or output zone, triggers events that may cause stimulation or inhibition of adjacent neurons and other cells. .
The terminal arborization of axon of one neuron is very closely placed to the dendrites of another neuron to carry impulses from one neuron to another neuron. This close proximity is called synapse
Nerves impulses pass between neurons through the synapse with the help of chemicals called Neurotransmitters (e.g. Acetylcholine).
Nerve fibres (ensheathed axons) are of two types:- Whitish medullated.and greyish non-medullated.
Types of Neurons:-
The neurons are of four types based on the number of nerve processes:-
(a) Unipolar neurons:- Which have only one axon or process. They are found in early embryos.
(b) Bipolar neurons:– Which have two processes, one axon and another Dendron. They are found in olfactory epithelium and retina of eye.
(c) Multipolar neurons:- Which have many processes arising from cell body; out of them one is longer and acts as an axon and the remaining as dendrons. Multipolar neurons are most common and are found in brain and spinal cord.
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(d) Pseudo- unipolar neurons:- They are actually bipolar but appear initially like unipolar. A single process arises which divides to form dendrites and an axon. This is found in dorsal root ganglion of the spinal nerve.
Apolar/Non polar neurons:- Primitive neurons that first appeared in cnidarians. They have multiple neuritis with undefined polarity.
Neuroglial cells/ Neuroglia
These are specialized cells found in the brain and spinal cord supporting the neurons. About more than 50% of brain cells are neuroglial cells. These cells have different shapes and are thought to separate and insulate adjacent neurons, so that impulses pass from one neuron to the next only over the synapse, where the packing cells are missing
Types of Glial Cells:- these cells lack Nissl’s granules and are mostly ectodermal in origin.
(i) Astrocytes /Macrocytes:- They are large in size with a number of protoplasmic processes. They form maximum number of glial cells. They help in repair of nervous tissue and form blood brain barrier.
(ii) Oligodendrocytes:– They are with few protoplasmic processes and form myelin sheath in CNS.
There is no neurolemma inside the central nervous system. In the absence of Schwann cells, myelin is formed by the spiral wrapping of the nerve fibres by processes of Oligodendrocytes.
(iii) Microglial cells:- They are mesodermal in origin. They are amallest in size with few feathery processes and help in phagocytosis.
