Nerve Cell - The Basic Unit
The basic unit of the nervous system is the nerve cell or neuron. A neuron is to the nervous system what a brick is to the building. It has a cell body termed as perikaryon and one or more processes termed as neurites. You will see that, functionally, there are three types of neurons. Specific neurons are specialised for perceiving changes in the external or internal environment and transforming or transducing these stimuli into nerve impulses. We call them sensory neurons. A number of neurones are specialised for receiving nerve impulses from other neurones and transmitting these impulses to effector organs such as muscles or glands. These neurones are called motor neurones. Generally, intervening between sensory neurons and motor neurons are one or more association neurons. Association neurones receive impulses from sensory neurones and other association neurons. They as well transmit impulses to other association neurons and motor neurons. So, one or more association neurons may intervene between sensory neuron and motor neuron. Though, sensory neuron may as well transmit impulses direct to motor neuron.