The nervous tissue is composed of two distinct types of cells: the excitable and the non excitable. The excitable cells of the nervous system are known as neurons. They form the structural and functional unit of the nervous system and are responsible for all its activities.
Structure of a neuron:
Each nerve cell or neuron has:
- A cell body or perikaryon, having central nucleus and Nissl’s granules in its cytoplasm.
- Cell processes called neurites, which are of two types. Many short afferent processes, which are freely branching and varicose, are called dendrites.
A single long efferent process called axon, which may give off occasional branches (collaterals) and is of uniform diameter. The terminal branches of the axon are called axon terminals or telodendria.
The cell bodies (somata) of the neurons form grey matter and nuclei in the CNS (Central Nervous System), and ganglia in the PNS (Peripheral Nervous System). The cell processes (axons) form tracts in the CNS, and nerves in the PNS.
Types of neurons:
Neurons can be classified in several ways.
Classification according to the number of processes:
According to the number of their processes (neurites) the neurons can be divided into the following types:
- Unipolar neurons; e.g. mesencephalic nucleus;
- Pseudo-unipolar neurons, e.g. sensory ganglia or spinal ganglia
- Bipolar neurons, e.g. spiral and vestibular ganglia and bipolar neurons of retina.
- Multipolar neurons, e.g. neurons in cerebrum and cerebellum.
Classification according to the length of axon:
According to the length of axon, the neurons are classified as
- Golgi type I neurons, with a long axon
- Golgi type II neurons (microneurons), with a short or no axon.
Dynamic polarity of neurons:
The neurons show dynamic polarity in their processes. The impulse flows towards the soma in the dendrites, and away from the soma in the axon. However, in certain microneurons, where the axon is absent, the impulse can flow in either direction through their dendrites.