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this tissue is purely ectodermal in origins.
first appears in members of coelenterate and are unipolar.
has least regeneration power.
structural and functional unit of nervous tissue is neuron or nerve cells.
the number of neurons in the human nervous system is 10^12.
- these are 3 types :-
unipolar or Pseudounipolar neurons
- in such neurons ,single process arises from the body of cell and bifuracetes ,to form the Axon and dendrites.
Examples are dorsal root ganglion of spinal cords.
they have two poles, one dendrite and one axon arising from the cell body.Example neurons arising from the retina and internal ear.
-such neurons have one or more than one dendrites.
- 99% of the dendrites are multipolar types.
- they are also called neuroglia.
- the number of glia cells is around 30 – 42 times the number of neurons.
- the glia cells around the neurons of the peripheral nervous system are called Schwann cells , while glia cells are the cental. nervous system which are of further 3 types
the structure of neurons
- it consists of three parts Dendrites,cyton, and axon.
- cytons contain the group of ribosomes and Nissl granules.
- such granules are present in the dendrites but absent in axon .
- the centrosome is absent in cytone and can't be the divide.
-the covering of-of Schwann cells around axon is called neurilemma.
- at nodes myelin sheats are absent while neurilemma is present:-
1 ganglia – they are the group of cytons or cell bodies.in CNS such group are called neurons.
2 nerves – the nerve is a group of axons in brain such group of axons is called tract.
Structures of Nerve
- the outer covering or the connective tissue covering the nerve is called epineurium.
- these are several bundles of neurons embedded in epineurium and such bundles are known to be fascicule.
- each nerve fasciculus has a connective tissue called perineurium.
-single neurons are also surrounded by connecting tissue covering called Endoneurium.
- so from out to in
Epineurium - periuneuium – Endoneurium
Types of nerves-
- on the basis of their function, the nerve is 3 types
1 sensory – these impulses is carried from receptors(skin, retina etc )to the CNS. Eg optic nerve.
2 moter – these are carried by an impulse from CNS to effector organs. Eg trochlear nerve.
3 mixed- in such nerves , some fibers carry impulses towards CNS and other away from the Brain or CNS.
Agrawal, sarita. principle of biology. 2nd edition . kathmandu: Asmita book Publication, 2068 ,2069
Mehta, Krishna Ram. Principle of biology. 2nd edition. kathmandu: Asmita, 2068,2069.
Jorden, S.L. principle of biology. 2nd edition . Kathmandu: Asmita book Publication, 2068.2069.