Nervous tissue comprises two types of cells: nerve cells or neurons and glars cells. Neurons have numerous long processes and transmit nerve impulse. The glars cells have short processes and they support and protect neurons. A neuron has a large cell body with two or more thin protoplasmic processes extending from it. One of the processes, called the axon, is long and conducts nerve impulse away from the cell body. It ends in a number of small branches on muscles fibers, gland cells or other neurons. The remaining processes (one or more) conduct nerve impulses towards the cell body and are called dendrites or dendraons.
The axon terminals may also form between the axon terminal and cell bodies and, even, between axons of other neurons. Nerve impulses pass between neurons through the synapse with the help of chemicals such as acetylcholine, which are termed neuro-transmitters. The neurons are of three types based on the number of nerve processes:
- Unipoplar neurons, which have only axon and another but no dendron and are found only in early embryos.
- Bipolar neurons which have two processes, one axon and another dendron, and are found in olfactory epithelium and retina of eye.
- Multi-polar neurons, which have many processes arising from cell body, out of them one (larger) act as axon and the remaining as dendron. Multipolar neurons are found in brain and spinal cord.
The cell body called cyton, is the part of as neuron that contains the nucleus and the cytoplasm. The nucleus is spherical and large with a prominent nucleus. The cytoplasm contains rough endoplasmic reticulum and free ribosomes, which in a properly stained cell appear as granular areas called Nissle bodies. The extended axon or dendrite of a neuron is called a nerve fiber. Nerve fibers are collected into bundles. Each nerve fibers is covered with a continuous sheath, called the neurolemma. The neurolemma sheath is made up of a single layer of flat expanded schwann cells.