THE CELLS OF THE NERVOUS SYSTEM— neurons and glia—share many characteristics with cells in general. However, neurons are specially endowed with the ability to communicate precisely and rapidly with other cells at distant sites in the body. Two features give neurons this ability.
First, they have a high degree of morphological and functional asymmetry: Neurons have receptive dendrites at one end and a transmitting axon at the other. This arrangement is the structural basis for unidirectional neuronal signaling.
Second, neurons are both electrically and chemically excitable. The cell membrane of neurons contains specialized proteins—ion channels and receptors—that facilitate the flow of specific inorganic ions, thereby redistributing charge and creating electrical currents that alter the voltage across the membrane. These changes in charge can produce a wave of depolarization in the form of action potentials along the axon, the usual way a signal travels within the neuron. Glia are less excitable, but their membranes contain transporter proteins that facilitate the uptake of ions as well as proteins that remove neurotransmitter molecules from the extracellular space, thus regulating neuronal function.
There are hundreds of distinct types of neurons depending on their dendritic morphology, pattern of axonal projections, and electrophysiological properties. This structural and functional diversity is largely specified by the genes expressed by each neuronal cell type. Although neurons all inherit the same complement of genes, each expresses a restricted set and thus produces only certain molecules—enzymes, structural proteins, membrane constituents, and secretory products—and not others. In large part, this expression depends on the cell’s developmental history. In essence, each cell is the set of molecules it expresses.
There are also many kinds of glial cells that can be identified based on their unique morphological, physiological, and biochemical features. The diverse morphologies of glial cells suggest that glia are probably as heterogeneous as neurons. Nonetheless, glia in the vertebrate nervous system can be divided into two major classes: macroglia and microglia. There are three main types of macroglia: oligodendrocytes, Schwann cells, and astrocytes. In the human brain, about 90% of all glial cells are macroglia. Of these, approximately half are myelin-producing cells (oligodendrocytes and Schwann cells) and half are astrocytes. Oligodendrocytes provide the insulating myelin sheaths of the axons of some neurons in the central nervous system (CNS) (Figure 7–1A). Schwann cells myelinate the axon of neurons in the peripheral nervous system (Figure 7–1B); nonmyelinating Schwann cells have other functions, including promoting development, maintenance, and repair at the neuromuscular synapse. Astrocytes owe their name to their irregular, roughly star-shaped cell bodies and large numbers of processes; they support neurons and modulate neuronal signaling in several ways (Figure 7–1C). Microglia are the brain’s resident immune cells and phagocytes, but also have homeostatic functions in the healthy brain.
The principal types of glial cells are oligodendrocytes and astrocytes in the central nervous ...