Supporting Cells

Unlike other organs that are "packaged" in connective tissue derived from mesoderm (the middle layer of embryonic tissue), the supporting cells of the nervous system are derived from the same embryonic tissue layer (ectoderm) that produces neurons.

■ Figure 7.3 The relationship between the CNS and PNS. Sensory and motor neurons of the peripheral nervous system carry information into and out of, respectively, the central nervous system (brain and spinal cord).

The Nervous System: Neurons and Synapses 155

There are two types of supporting cells in the peripheral nervous system:

There are four types of supporting cells, called neuroglial (or glial) cells, in the central nervous system (fig. 7.5):

oligodendrocytes, which form myelin sheaths around axons of the CNS;

microglia, which migrate through the CNS and phagocytose foreign and degenerated material;

Schwann cells, which form myelin sheaths around 1.

peripheral axons; and satellite cells, or ganglionic gliocytes, which support 2.

neuron cells bodies within the ganglia of the PNS.

vllv f

■ Figure 7.4 Three different types of neurons. Pseudounipolar neurons, which are sensory, have one process that splits. Bipolar neurons, found in the retina and cochlea, have two processes. Multipolar neurons, which are motor and association neurons, have many dendrites and one axon.

■ Figure 7.5 The different types of neuroglial cells. Myelin sheaths around axons are formed in the CNS by oligodendrocytes. Astrocytes have extensions that surround both blood capillaries and neurons. Microglia are phagocytic, and ependymal cells line the brain ventricles and central canal of the spinal cord.

Table 7 .3 Supporting Cells and Their Functions*

Cell Type

Location

Functions

Schwann cells

PNS

Surround axons of all peripheral nerve fibers, forming a neurilemmal sheath, or sheath of Schwann; wrap around many peripheral fibers to form myelin sheaths; also called neurolemmocytes

Satellite cells

PNS

Support functions of neurons within sensory and autonomic ganglia; also called ganglionic gliocytes

Oligodendrocytes

CNS

Form myelin sheaths around central axons, producing "white matter" of the CNS

Microglia

CNS

Phagocytose pathogens and cellular debris in the CNS

Astrocytes

CNS

Cover capillaries of the CNS and induce the blood-brain barrier; interact metabolically with neurons and modify the extracellular environment of neurons

Ependymal cells

CNS

Form the epithelial lining of brain cavities (ventricles) and the central canal of the spinal cord; cover tufts of capillaries

to form choroid plexuses—structures that produce cerebrospinal fluid

*Supporting cells in the CNS are known as neuroglia.

to form choroid plexuses—structures that produce cerebrospinal fluid

*Supporting cells in the CNS are known as neuroglia.

- Schwann cell

Axon

Axon

Sheath of Schwann (Neurilemma)

Myelin sheath

■ Figure 7.6 The formation of a myelin sheath around a peripheral axon. The myelin sheath is formed by successive wrappings of the Schwann cell membranes, leaving most of the Schwann cell cytoplasm outside the myelin. The sheath of Schwann is thus external to the myelin sheath.

3. astrocytes, which help to regulate the external environment of neurons in the CNS; and

4. ependymal cells, which line the ventricles (cavities) of the brain and the central canal of the spinal cord.

A summary of the supporting cells is presented in table 7.3. Recent evidence suggests a more exciting role for the ependymal cells that line the ventricles of the brain, and also for the astrocytes immediately adjacent to this region—they can function as neural stem cells. That is, they can divide and their progeny can differentiate (specialize) along different lines, to become new neurons and neuroglial cells. Reptile and bird brains have been known to generate new neurons throughout life, but only recently has this ability been demonstrated in mammalian (including human) brains.

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