Figure 148. Neurons are broadly divided into four main types based on the number and placement of axons:
(1) unipolar, (2) bipolar, (3) multipolar, and (4) pseudounipolar.
Glia Although glia are frequently thought of as the supporting cast of the nervous system, there are
ten times more glial cells in the brain than there are neurons. Without the crucial functions performed
by these glial cells, neurons would not be able to function. In addition to providing myelin sheaths
around axons and buffering ions and chemicals that might otherwise harm neurons, glia also direct
developing neurons to their destinations. They also play a part in reacting to nerve activity and
controlling communication between nerve cells, as recently discovered by scientists. Glial
dysfunction can have disastrous effects; most brain tumors are brought on by glial cell mutations.
Types of Glia There are several different types of glia with different functions, two of which are shown in
Figure 149.
Astrocytes , shown in Figure 150a make contact with both capillaries and neurons in the
CNS. They give nutrients and other materials to neurons, control the levels of ions and chemicals in
the extracellular fluid, and support synapses structurally. The blood-brain barrier, a structure that
prevents toxic substances from entering the brain, is also formed by astrocytes. Through calcium
imaging studies, it has been demonstrated that astrocytes in particular activate in response to nerve
activity, transmit calcium waves between astrocytes, and regulate the activity of nearby synapses.
Satellite glia provide nutrients and structural support for neurons in the PNS.
Microglia scavenge and
degrade dead cells and protect the brain from invading microorganisms.
Oligodendrocytes , shown
in Figure 150b form myelin sheaths around axons in the CNS. One axon can be myelinated by several
oligodendrocytes, and one oligodendrocyte can provide myelin for multiple neurons. This is
distinctive from the PNS where a single
