16.3 Histology and Function of Neuroglia

Neuroglia of the PNS

Neuroglia of the PNS completely surround axons and cell bodies. The two types of glial cells in the PNS are Schwann cells (neurolemmocytes) and satellite cells

Myelination in the PNS

Axons that are surrounded by a multilayered lipid and protein covering, called the myelin sheath, are said to be myelinated (MĪ′‐e‐li‐nāt′‐ed) (Figure 16.8a). The sheath electrically insulates the axon of a neuron and increases the speed of nerve impulse conduction. Axons without such a covering are said to be unmyelinated

ependymal cells

Ependymal cells are cuboidal to columnar cells arranged in a single layer that possess microvilli and cilia. These cells line the ventricles of the brain and central canal of the spinal cord (spaces filled with cerebrospinal fluid). Functionally, ependymal cells produce (possibly), monitor, and assist in the circulation of cerebrospinal fluid. They also form the blood-cerebrospinal fluid barrier

gray and white matter

In the spinal cord, the white matter surrounds an inner core of gray matter shaped like a butterfly or the letter H (in transverse section); in the brain, a thin shell of gray matter covers the surface of the largest portions of the brain, the cerebrum and cerebellum (Figure 16.9). When used to describe nervous tissue, a nucleus is a cluster of neuronal cell bodies within the CNS. (Recall that the term ganglion refers to a similar arrangement within the PNS.) Many nuclei of gray matter also lie deep within the brain. Much of the CNS white matter consists of tracts, bundles of axons in the CNS that extend for some distance up or down the spinal cord or connect parts of the brain with each other and with the spinal cord

microglia

Microglial cells or microglia are small cells with slender processes that give off numerous spinelike projections. Unlike other neuroglial cells, which develop from the neural tube, microglial cells originate in red bone marrow and migrate into the CNS as it develops. Microglial cells function as phagocytes. Like tissue macrophages, they remove cellular debris formed during normal development of the nervous system and phagocytize microbes and damaged nervous tissue.

Two types of neuroglia produce myelin sheaths:

Schwann cells (in the PNS) and oligodendrocytes (in the CNS). In the PNS, Schwann cells begin to form myelin sheaths around axons during fetal development. Each Schwann cell wraps about 1 millimeter (1 mm = 0.04 in.) of a single axon's length by spiraling many times around the axon (Figure 16.8a). Eventually, multiple layers of glial plasma membrane surround the axon, with the Schwann cell's cytoplasm and nucleus forming the outermost layer. The inner portion, consisting of up to 100 layers of Schwann cell membrane, is the myelin sheath. The outer nucleated cytoplasmic layer of the Schwann cell, which encloses myelinated or unmyelinated axons, is the neurolemma (sheath of Schwann). A neurolemma is found only around axons in the PNS. When an axon is injured, the neurolemma aids regeneration by forming a regeneration tube that guides and stimulates regrowth of the axon. Gaps in the myelin sheath, called nodes of Ranvier (RON‐vē‐ā) (myelin sheath gaps), appear at intervals along the axon (see Figures 16.3 and 16.7a). Each Schwann cell wraps one axon segment between two nodes. Nerve impulses are conducted more rapidly in myelinated axons because the impulses are formed more quickly at nodes of Ranvier and appear to "leap" from node to node as opposed to being conducted more slowly through every part of the membrane in unmyelinated axons. In the CNS, an oligodendrocyte myelinates parts of several axons. Each oligodendrocyte puts forth about 15 broad, flat processes that spiral around CNS axons, forming a myelin sheath (see Figure 16.6). A neurolemma is not present, however, because the oligodendrocyte cell body and nucleus do not envelop the axon. Nodes of Ranvier are present, but they are fewer in number. Axons in the CNS display little regrowth after injury. This is thought to be due, in part, to the absence of a neurolemma, and in part to an inhibitory influence on axon regrowth exerted by the oligodendrocytes. The amount of myelin increases from birth to maturity, and its presence greatly increases the speed of nerve impulse conduction. An infant's responses to stimuli are neither as rapid nor as coordinated as those of an older child or a young adult, in part because myelination is still in progress during infancy. Demyelination (dē‐mī‐e‐li‐NĀ‐shun) refers to the loss or destruction of myelin sheaths around axons. It may result from disorders such as multiple sclerosis or Tay‐Sachs disease, or from medical treatments such as radiation therapy and chemotherapy. Any single episode of demyelination may cause deterioration of affected nerves.

Schwann cells

also called neurolemmocytes (noo′‐rō‐LEM‐mō‐sīts), are flat cells that encircle PNS axons. Like the oligodendrocytes of the CNS, they form the myelin sheath around axons. A single oligodendrocyte myelinates several axons (Figure 16.6), but each Schwann cell myelinates a single axon (Figure 16.7a; see also Figure 16.8a, c). A single Schwann cell can also enclose as many as 20 or more unmyelinated axons (axons that lack a myelin sheath) (Figure 16.7b). Schwann cells participate in axon regeneration, which is more easily accomplished in the PNS than in the CNS.

Neuroglia of the CNS

an be distinguished on the basis of size, cytoplasmic processes, and intracellular organization into four types: astrocytes, oligodendrocytes, microglia, and ependymal cells

satellite cells

are flat cells that surround the cell bodies of neurons of PNS ganglia (Figure 16.7c). (Recall that ganglia are collections of neuronal cell bodies outside the CNS.) Besides providing structural support, satellite cells regulate the exchange of materials between neuronal cell bodies and interstitial fluid.

astrocytes

are the largest and most numerous of the neuroglia. They are star‐shaped cells that have many armlike processes. There are two types of astrocytes. Protoplasmic astrocytes have many short branching processes and are found in gray matter (described shortly). Fibrous astrocytes have many long unbranched processes and are located mainly in white matter (also described shortly). The processes of astrocytes make contact with blood capillaries, neurons, and the pia mater (a thin membrane around the brain and spinal cord). Astrocytes have the following functions: (1) They contain microfilaments that provide them with considerable strength, which enables them to support neurons. (2) Since neurons of the CNS must be isolated from various potentially harmful substances in blood, the endothelial cells of CNS blood capillaries have very selective permeability characteristics. Processes of astrocytes wrapped around blood capillaries secrete chemicals that maintain the unique permeability characteristics of the endothelial cells. In effect, the endothelial cells create a blood-brain barrier, which restricts the movement of substances between the blood and interstitial fluid of the CNS. Details of the blood-brain barrier are discussed in Chapter 18. (3) In the embryo, astrocytes secrete chemicals that appear to regulate the growth, migration, and interconnections among neurons in the brain. (4) Astrocytes help to maintain the appropriate chemical environment for the generation of nerve impulses. For example, they regulate the concentration of important ions such as K+; take up excess neurotransmitters; and serve as a conduit for the passage of nutrients and other substances between blood capillaries and neurons. (5) Astrocytes may also play a role in learning and memory by influencing the formation of neural synapses.

Neuroglia (glial cells) (glia)

constitute about half the volume of the CNS. Their name derives from the idea of early histologists that they were the "glue" that held nervous tissue together. We now know that neuroglia are not merely passive bystanders but rather active participants in nervous tissue function. Generally, neuroglia are smaller than neurons, and as stated previously they are much more numerous. In contrast to neurons, glia do not generate or propagate nerve impulses, and they have the ability to multiply and divide in the mature nervous system. In cases of injury or disease, neuroglia multiply to fill in the spaces formerly occupied by neurons. Brain tumors derived from glia, called gliomas (glē‐Ō‐mas), tend to be highly malignant and rapidly growing. Of the six types of neuroglia, four—astrocytes, oligodendrocytes, microglia, and ependymal cells—are found only in the CNS. The remaining two types—Schwann cells (neurolemmocytes) and satellite cells—are present in the PNS.

white matter

is aggregations of myelinated and unmyelinated axons of many neurons. The whitish color of myelin gives white matter its name

gray matter

of the nervous system contains neuronal cell bodies, dendrites, unmyelinated axons, axon terminals, and neuroglia. It appears grayish, rather than white, because the Nissl bodies impart a gray color and there is little or no myelin in these areas. Blood vessels are present in both white and gray matter.

oligodendrocytes

resemble astrocytes, but are smaller and contain fewer processes. Oligodendrocyte processes are responsible for forming and maintaining the protective covering around CNS axons. As you will see shortly, the myelin sheath is a lipid and protein covering around some axons that insulates the axon and increases the speed of nerve impulse conduction.