Neurons

What is the function of neurons?

The messages carried by the nervous system are electrical signals called impulses. Nervous system impulses are transmitted by cells called neurons.

Types of Neurons Neurons can be classified into three types according to the direction in which an impulse travels. Sensory neurons carry impulses from the sense organs, such as the eyes and ears, to the spinal cord and brain. Motor neurons carry impulses from the brain and the spinal cord to muscles and glands. Interneurons do the high-level work. They process information from sensory neurons and then send commands to other interneurons or motor neurons.

Structure of Neurons Although neurons come in many shapes and sizes, they all have certain features in common. As shown in Figure 31–2, the largest part of a typical neuron is its cell body, which contains the nucleus and much of the cytoplasm.

Spreading out from the cell body are short, branched extensions called dendrites. Dendrites receive impulses from other neurons and carry impulses to the cell body. The long fiber that carries impulses away from the cell body is the axon. An axon ends in a series of small swellings called axon terminals. Neurons may have dozens of dendrites, but usually they have only one axon. In most animals, axons and dendrites of different neurons are clustered into bundles of fibers called nerves. Some nerves contain fibers from only a few neurons, but others contain hundreds or even thousands of neurons.

In some neurons, the axon is surrounded by an insulating membrane known as the myelin (MY uh lin) sheath. The myelin sheath that surrounds a single, long axon has many gaps, called nodes, where the axon membrane is exposed. As an impulse moves along the axon, it jumps from one node to the next. This arrangement causes an impulse to travel faster than it would through an axon without a myelin sheath.

In Your Notebook Make a two-column table that lists the structures of a neuron in one column and their functions in the next column.