In most animals, the axon is the long slender part of the nerve cell, or neuron, that conducts electrical impulses from the cell body to the dendrites at the other end of the cell. The axon can be short or very long, up to 3.25 feet (1 m) in length in humans and other species of animals, and it can be a single strand of nerve tissue or it can branch. Each time an axon branches off from the main axon, the extra segment is called an axon branch.
Most but not all nerve cells contain an axon. When an axon is present within the neuron, there is never more than one. Branching is not uncommon, with most axons having at least one axon branch and others having many, even hundreds of branches from the main axon. At the terminal end of an axon or an axon branch, the electrical impulse is then passed to another nerve cell via the dendrites or to or from another type of cell.
The electrical transmission between cells always occurs at the synapse, where the end of the axon branch comes in very close contact with the other cell. To pass the nerve impulse to another cell, either electrical or electrochemical methods are used. If the impulse is passed chemically, then neurotransmitter molecules are used. These are released from the terminal ends of the axon branch and fill the synapse. Once enough neurotransmitter has diffused across to the other cell, it is stimulated to action.
There are basically three different types of neurons, which can be classified based on the direction of the electrical, or nerve, impulse. This means that the impulse can travel in different directions along an axon branch depending on the type of neuron. The three groups of nerve cells are motor neurons, sensory neurons and interneurons, which are found solely within the central nervous system.
Motor neurons transmit impulses from the central nervous system to the rest of the body, particularly to the muscles. The messages carried along motor neurons are usually in response to stimuli that have been sent to the brain via the sensory neurons. Sensory neurons receive input from the cells of the body and carry it to the central nervous system. The receptors at the ends of the axon branch are stimulated by external means, including touch, pressure, heat, sound, light and pain. Different receptors occur in different areas depending on the stimuli, but all convert it to electrical impulses.