There are two kinds of cells in the nervous system: glial cells and neurons. Glial cells, which make up the support structure of the nervous
system, perform four functions:
- Provide structural support to the neurons
- Insulate neurons
- Nourish neurons
- Remove waste products
The other cells, neurons, act as the communicators of the nervous
system. Neurons receive information, integrate it, and pass it along. They
communicate with one another, with cells in the sensory organs, and with muscles and
Each neuron has the same structure:
- Each neuron has a soma, or cell body, which is the central
area of the neuron. It contains the nucleus and other structures common to all
cells in the body, such as mitochondria.
- The highly branched fibers that reach out from the neuron are called dendritic trees. Each branch is called a dendrite.
Dendrites receive information from other neurons or from sense organs.
- The single long fiber that extends from the neuron is called an axon. Axons send information to other neurons, to muscle cells, or to
gland cells. What we call nerves are bundles of axons coming from
- Some of these axons have a coating called the myelin sheath.
Glial cells produce myelin, which is a fatty substance that protects the nerves.
When an axon has a myelin sheath, nerve impulses travel faster down the axon.
Nerve transmission can be impaired when myelin sheaths disintegrate.
- At the end of each axon lie bumps called terminal buttons. Terminal
buttons release neurotransmitters, which are chemicals
that can cross over to neighboring neurons and activate them. The junction
between an axon of one neuron and the cell body or dendrite of a neighboring
neuron is called a synapse.
Communication Between Neurons
In 1952, physiologists Alan Hodgkin and Andrew
Huxley made some important discoveries about how neurons transmit
information. They studied giant squid, whose neurons have giant axons. By
putting tiny electrodes inside these axons, Hodgkin and Huxley found that
nerve impulses are really electrochemical reactions.