The nervous system is
made up of nervous tissues. It is formed of three distinct components namely
the neurons, neuroglia and nerve fibres.
(i) Neuron or nerve
neuron or nerve cell is the structural and functional unit of the
nervous system. It is the longest cell of the human body with a length of over
100mm. These cells are
highly specialised to detect, receive and transmit different kinds of stimuli.
Information is conducted through neurons in the form of electrical impulses
from one part of the body to another.
(ii) Neuroglia: Neuroglia are also
called as glial cells. They are non-exciting, supporting cell of
the nervous system. They do not initiate or conduct nerve impulses.
(iii) Nerve fibres: The nerve fibres are the
long slender processes of neurons. A number of nerve fibres are bundled
up together to form nerves.
A neuron typically
consists of three basic parts: Cyton, Dendrites and Axon.
(i) Cyton: Cyton is also called
cell body or perikaryon. It has a central nucleus with abundant
cytoplasm called neuroplasm. The cytoplasm has large granular body
called Nissl’s granules and the other cell organelles like mitochondria,
ribosomes, lysosomes, and endoplasmic recticulum. Neurons do not have the
ability to divide. Several neurofibrils are present in the cytoplasm that help
in transmission of nerve impulses to and from the cell body.
(ii) Dendrites: These are the numerous
branched cytoplasmic processes that project from the surface of the cell
body. They conduct nerve impulses towards the cyton. The branched projections
increase the surface area for receiving the signals from other nerve cells.
(iii) Axon: The axon is a single,
elongated, slender projection. The end of axon terminates as fine
branches which terminate into knob like swellings called synaptic knob.
The plasma membrane of axon is called axolemma, while the
cytoplasm is called axoplasm. It carries impulses away from the cyton.
The axons may be covered by a protective sheath called myelin sheath
which is further covered by a layer of Schwann cells called neurilemma.
Myelin sheath breaks at intervals by depressions called Nodes of Ranvier.
The region between the nodes is called as internode. Myelin
sheath acts as insulator and ensures rapid transmission of nerve impulses.
Synapse: A junction between
synaptic knob of axon of one neuron and dendron of next neuron is called
synaptic junction. Information from one neuron can pass to
another neuron through these junctions with the release of chemicals known as
neurotransmitters from the synaptic knob.
The neurons may be of
different types based on their structure and functions.
Structurally the neurons
may be of the following types:
(i) Unipolar neurons: Only one nerve process
arises from the cyton which acts as both axon and dendron.
(ii) Bipolar neurons: The cyton gives rise
to two nerve processes of which one acts as an axon while another as a
(iii) Multipolar neurons: The cyton gives rise
to many dendrons and an axon
On the basis of
functions neurons are categorised as:-
(i) Sensory or afferent
carry impulses from the sense organ to the central nervous system.
(ii) Motor or efferent
carry impulses from the central nervous system to effector organ such as
the muscle fibre or the gland.
impulses betweeen sensory and motor neurons.
Nerve fibres are of two
types based on the presence or absence of myelin sheath.
(i) Myelinated nerve fibre: The axon is covered with myelin sheath
(ii) Non-myelinated nerve fibre: The axon is not covered by myelin
non-myelinated nerve fibres form the white matter and grey matter of the brain.