(a) The unit which makes up the nervous system is called a neuron.
(b) A microscopic gap between a pair of adjacent neurons over which nerve impulses pass when going from one neuron to the next is called a synapse. Synapse between two neurons acts as a one way valve which allows electrical impulses to pass in one direction only. This happens as follows: When an electrical impulse coming from the receptor reaches the end of the axon of sensory neuron, then the electrical impulse releases tiny amount of a chemical substance called neuro transmitter substance into the synapse between two adjacent neurons. This substance crosses the synapse and starts a similar electrical impulse in the dendrite of the next neuron. In this way, the electrical impulses passes from one neuron to the next across the synapse.