Nervous tissue originates from ectoderm and is specialized for receiving stimuli (Excitability or Irritability) and transmit message (conductivity).
Fig: Nervous system
Nerve cell is made up of cell body & cell process – (Dendron and Axon = Neurites)
(i) Cell body or Cyton or soma or perikaryon:-
(i) Dendron:– It is small cell process. It's fine branches are called dendrites. Some receptors are found on the dendrites, so dendron receive the stimuli & produce centripetal (towards the cell body) conduction.
(ii) Axon (Long process = Axon = Nerve fibre) – It is longest cell process of cyton, its diameter is uniform. It contain axoplasm.
Differences between Axon & Dendron:
|It is always single.||One or more.|
|It has Neurofibrils but no Nissl's granules.||It has both.|
|It is long sized process.||Small sized|
|Nerve impulse travels away from the cell body. (Centrifugal)||Nerve impulse travels towards the cell body.(Centripetal)|
Axon is covered by a layer of phospholipids (sphingomyelin) which is called as medulla or myelin sheath.
Myelinogenesis in the Peripheral nervous system (PNS):– In the peripheral nerves, myelinogenesis begins with the deposition of myelin sheath in concentric layer around the axon by schwann cells.
Myelinogenesis in the Central nervous system (CNS) :– Neurilemma or schwann cells are not present therefore myelinogenesis process occurs with the help of oligodendrocytes (Neuroglia).
Collaterals of Axon – These are small process or branches of axon. It's structure is similar to axon. It help in the conduction of nerve impulse in more area.
Gray matter:– It is composed of nerve cells. It consist of cytons & non-medullated nerve fibres (Grey fibers).
White matter – It contain myelinated nerve fibres (White fibres).
TYPES OF NEURONS:
Apolar/Nonpolar Neuron:– No definite dendron/axon. Cell process are either absent or if present are not differentiated in axon and dendrons. Nerve impulse radiates in all directions. e.g. Hydra, Amacrine cell of Retina.
Pseudounipolar:– In this type, nerve cell has only axon but a small process develop from axon which act as dendron. e.g. Dorsal root ganglia of spinal cord.