Among mammal, the human nervous system is the most developed and complex. The brain acts as the main coordinator. There is a network of nerve cells in the entire body. The structural and functional unit of the nervous system is the nerve cell. The brain and network of nerves are called nervous system. The nerve cell is the longest cell of the body. The nerve cell has three parts- soma, dendrites and axon. The soma is like a simple cell. It has a nucleus and various cell organelles. The soma has nissel granules. It lacks vacuoles The dendrites are small and branched. They receive impulses and conduct them towards soma. Axon is a slender and fibre like an extension. It conducts the nerve impulses away from the soma. The terminal part of an axon of the nerve cell is found in contact with the dendrites of another nerve cell. This functional junction of two nerve cells is called synapsis.
Conduction of impulse: The sensory nerves carry the impulses to the controlling centre(brain or spinal cord). In the controlling centre, the impulses are analysed and its responses are decided. The motor nerve carry the responses, in the form of impulses to the organ, where its reaction is to take place. This process is called response to a particular stimulus. The membrane covering the neuron is electrically charged. The positive charge (+) is on the outer surface, while the negative charge (-) is on the inner surface.
These two charges are unequal, hence there is a potential difference in the two surfaces of the membrane. As soon as the stimulus is received by the dendrites, the positively charged ions begin to move inside and the negatively charged ions begin to move outside. This causes an impulse and depolarisation stage is created. After passing the impulse, polarisation stage is restored. As the impulses move forward again, depolarisation stage is reached. In this way, the process of polarisation and depolarisation continues and the impulse moves forward. When the impulse reaches the end of the axon, three hormones- adrenaline, acetylcholine, serotonin are secreted. These help in passing of the impulse from one neuron to another, at the synapse.
