In multicellular organisms, the various activities of the body are controlled or regulated by two different systems. One is chemical system which includes specific substances called hormones secreted by endocrine cells.
The other system is the nervous system in which electrical signals are generated in the form of nerve impulses which are usually associated with the fast responses to environmental stimuli. The nervous system of higher animals includes the brain, spinal cord and nerves.
The brain and spinal cord constitute the central nervous system. The nerves and their branches constitute the peripheral nervous system. The fundamental unit of the nervous system is the nerve cell or neuron.
Neuron is an irregularly shaped structure in the centre of which there lies a spherical nucleus with prominent nucleolus and fine chromatin granules but no centriole. That is why mature nerve cells or neurons cannot divide. It contains a number of processes such as dendrites, axon and collaterals.
On the basis of functional properties, neurons may be (i) afferent which carry sensory information in the form of nerve impulses from the periphery of the body, (ii) efferent which carry impulses from central nervous system to effector organs such as muscles and glands (iii) internuncial are found between afferent (sensory) and efferent (motor) neurons and (iv) neurosecretory which produce neurohormones.
The functional junction between the dendritic region of one neuron or nerve and the axon ending of another neuron is commonly called synaptic region or synapse. The characteristic feature of synapse is that
(i) It transmits impulses from one neuron to another neuron (ii) it can transmit impulses only in one direction. When a wave or nerve impulse reaches the synapse, it brings about the release of a chemical compound (neurohumor) which is known as transmitter; this substance is responsible for the conduction of nerve impulse through a synapse.
This transmitter is called acetylcholine. Acetylcholine is secreted only by axon terminations as a result of which the axon is not excited anywhere and the nerve impulses always travel in a single direction.