Definition

Nerve cells, also known as neurons. Neurons are the primary structural and functional unit of the nervous system. They are responsible for conducting nerve impulses from one part of the body to another. They are the basic information-processing units of the nervous system. The human nervous system contains about 10¹² (12 trillion) neurons.

The neurons are highly differentiated and specialized excitable cell, their specific activity is:

  1. The reception of stimuli
  2. The generation of nerve impulses
  3. The transmission of impulses to other nerve cells or the effectors (muscle fibers or glands)

Structure of Neurons

Most of the neurons consist of the following distinct parts:
  • Cell body, Soma or perikaryon
  • Axon
  • Dendrite

1. Cell body, Soma or perikaryon

This contains a well-defined nucleus and nucleolus surrounded by a granular cytoplasm; the latter is known as perikaryon (around the nucleus). Nerve cells possess a single nucleus except those of the sympathetic and sensory ganglia, which are binucleated.
The cytoplasm possesses mitochondria, ribosomes, endoplasmic reticulum, Golgi apparatus, and lysosomes. Some neurons are rich in melanin, others have copper, iron, or zinc.

The nerve cell body is the most crucial part of neurons; if it is destroyed (e.g. by toxins, by anoxia, by viral infections like poliomyelitis) the complete neuron dies.

2. Axons

This is the name given to the longest process of the neurons and is also known as a nerve fiber. Axon moves impulses away from the nerve cell body to other neurons or muscular or glandular tissues.

The axon emerges from the cell body at the region termed the axon hillock. The axon consists of a central core of a jelly-like semifluid substance known as the axoplasm surrounded by a membrane called axolemma.

In the body a nerve fiber carries impulses in one direction only, if a fiber carries impulses from the central nervous system (CNS) to the periphery, it is known as efferent. While a nerve fiber carrying impulses from the periphery towards the CNS, it is known as an afferent nerve fiber.

Size

Axons are variable in length from a few millimeters (in the brain) to a meter or more between the spinal cord and toes.

a. axon collateral

The axons do not branch as much as dendrites but many axons along their course show side branching known as collaterals.

b. Axon terminals

The axons and axon collaterals terminate by branching into many fine filaments known as terminals.

Dendrites

These are highly branched, the cytoplasmic processes of the nerve cell body. They are responsible for receiving information and conducting it towards the cell body. The dendrites have all the components present in the cytoplasm of the nerve cell body.

Classification of neurons

The various neurons in the body are categorized on the basis of their structure and function. In other words, neurons are classified histologically as well as physiologically.

Histological classification of neurons

a. Unipolar neurons (mono-polar)

This type of neuron has only one process which functionally an axon. In man, these neurons are just in one place. 
Example: Mesencephalic nucleus of the trigeminal nerve.

b. Pseudo unipolar neurons

In this type of neuron, the axon and dendrites arise from a common stem that divides into two processes, one acting as the dendrite and the other as the axon. 
Example: Posterior spinal root ganglia.

c. Bipolar neurons

In this type of neuron, there is one axon and one dendrite, each arising at different sites of the cell body opposite to each other and having the same size.
Example: Retina of the eye, olfactory mucous membrane, and inner ear.

d. Multipolar neurons

These neurons have a large number of dendrites arising from the cell body. The dendrites may arise from one pole of the cell body or may arise from all portions of the cell body. There is only one long procedure that represents the axon. Most neurons of the CNS are of this type.
Example: Brain and spinal cord.

e. Neurons without axons

A few neurons in the central nervous system possess dendrites but no axons. In these cells, nervous connections occur in any direction along with their dendrite-like processes.
Example: Amacrine cell in the retina.


Physiological classification of neurons

The physiological classification of neurons is based on the directions in which they transfer impulses.

Sensory or receptor neurons

These types of neurons receive afferent fibers, responsible for numerous sensations.
Example: Pain, temperature, touch, pressure, light, sound, smell, and taste neurons.

Motor or effector neurons

These neurons send impulses from the brain and spinal cord to the effectors, which may be muscles or glands.