The nervous system is composed of two divisions, the central nervous system (CNS) and peripheral nervous system (PNS). The CNS contains the brain and the spinal cord and the PNS consists of thousands of nerves that connect the spinal cord to muscles and sensory receptors.

A peripheral nerve is composed of nerve bundles (fascicles) that contain hundreds of individual nerve fibers (neurons). Neurons consist of dendrites, axon, and cell body. The dendrites are the tree-like structures that receive signals from other neurons and from special sensory cells that sense the body’s surrounding environment. The cell body is the headquarters of the neuron and contains its genetic information in the form of DNA. The axon transmits signals away from the cell body to other neurons.

Many neurons are insulated like pieces of electrical wire. This insulation protects them and also allows their signals to move faster along the axon. Without this insulation, signals from the brain might never reach the outlying muscle groups in the limbs.

The operation of the nervous system depends on the flow of communication between neurons. For an electrical signal to travel between two neurons, it must first be converted to a chemical signal, which then crosses a space of about a millionth of an inch wide. The space is called a synapse, and the chemical signal is called a neurotransmitter.

Neurotransmitters allow the billions of neurons in the nervous system to communicate with one another, making the nervous system the master communication system of the body.