The structural and functional unit of the nervous system is a highly specialized cell called the nerve cell or neuron. Each nerve cell consists of three principal parts.
i. Cell body or cyton
ii. Axon
ADVERTISEMENTS:
iii. Dendrites
(i) Cell body:
The cell body or cyton (or Perikaryon) has a large, central nucleus surrounded by the cytoplasm. In the cytoplasm (also called neuroplasm), Nissl granules and neurofibrils are present. Cell organelles like mitochondria, golgi apparatus, endoplasmic reticulum, microfilaments and microtubules, are also present in the cytoplasm. There is no centrosome in cyton because the nerve cells have lost the ability to divide.
Image Source: intropsych.com
(ii) Dendrites:
Several short, thread-like branches called dendrites (Greek: dendron means tree) arise from the cell body. The dendrites conduct nerve impulse to cyton.
(iii) Axon:
ADVERTISEMENTS:
One of the branches grows very large in comparison to others. This branch is called the axon. The axon is covered by three layers.
i. Axolemma (the innermost layer)
ii. Myelin sheath or medullary sheath (the middle layer)
iii. Neurolemma (the outermost white insulating sheath)
ADVERTISEMENTS:
The axolemma and neurolemma are continuous sheaths, whereas the myelin sheath is not a continuous one. It is constricted at intervals. These constrictions are known as nodes of Ranvier. The axon ends have swollen bulb like ends which store acetylcholine (a neurotransmitter). These are called axon endings. Axon endings are closely placed near the dendrites of another neuron but are not connected. Such gaps in between are called synaptic clefts or synapse.