The units which make up the nervous system are called nerve cells or neurons. So, neuron is the structural and functional unit of the nervous system. We can now say that- A neuron (also called neurone or nerve cell) is a cell that carries electrical impulses. Neurons are the basic units of the nervous system.
Every neuron is made of a cell body (also called a soma), dendrites and an axon. Dendrites and axons are nerve fibres. There are about 86 billion neurons in the human brain, which comprises roughly 10% of all brain cells. The neurons are supported by glial cells and astrocytes.
Sensory neurons carry signals from sense organs to the spinal cord and brain.
Relay neurons carry messages between sensory or motor neurons and the central nervous system (CNS).
Motor neurons carry signals from the CNS to muscles, motor neurons are connected to the relay neurons. The signal passes between the neurons via synapses. Synapses are microscopic voids between cells where chemicals are released from the axon terminal of one cell to specialized chemical receptors on the dendrite of the receiving cell.