After studying this chapter, you should be able to:
Name the various types of glia and their functions.
Name the parts of a neuron and their functions.
Describe the chemical nature of myelin, and summarize the differences in the ways in which unmyelinated and myelinated neurons conduct impulses.
Describe orthograde and retrograde axonal transport.
Describe the changes in ionic channels that underlie the action potential.
List the various nerve fiber types found in the mammalian nervous system.
Describe the function of neurotrophins.
The human central nervous system (CNS) contains about 1011 (100 billion) neurons and 2–10 times this number of glial cells. It has been calculated that 40% of the human genes participate, at least to a degree, in the formation of the CNS. Neurons and glial cells along with brain capillaries form a functional unit that is required for normal brain function; their physiologic duties include synaptic activity, extracellular fluid homeostasis, energy metabolism, and neural protection. Disturbances in the interaction of these elements are the pathophysiological basis for many neurologic disorders. This chapter describes the cellular components of the CNS and the excitability of neurons, which involves the genesis of electrical signals that enable neurons to integrate and transmit impulses.
CELLULAR ELEMENTS IN THE CNS
For many years following their discovery, glial cells (or glia) were viewed as CNS connective tissue. In fact, the word glia is Greek for glue. However, today these cells are recognized for their role in communication within the CNS in partnership with neurons. Unlike neurons, glial cells continue to undergo cell division in adulthood and their ability to proliferate is particularly noticeable after brain injury (eg, stroke).
There are two major types of glial cells in the vertebrate nervous system: microglia and macroglia. Microglia are immune system cells; they are scavenger cells that resemble tissue macrophages and remove debris resulting from injury, infection, and disease (eg, multiple sclerosis [MS], AIDS-related dementia, Parkinson disease, and Alzheimer disease). Microglia arise from macrophages outside of the nervous system and are physiologically and embryologically unrelated to other neural cell types.
There are three types of macroglia: oligodendrocytes, Schwann cells, and astrocytes (Figure 4–1). Oligodendrocytes and Schwann cells are involved in myelin formation around axons in the CNS and peripheral nervous system, respectively. Astrocytes, which are found throughout the brain, are further subdivided into two groups. Fibrous astrocytes, which contain many intermediate filaments, are found primarily in white matter. Protoplasmic astrocytes are found in gray matter and have a granular cytoplasm. Both types of astrocytes send processes to blood vessels, where they induce capillaries to form the tight junctions making up the blood–brain barrier. They also send processes that envelop synapses and the surface of nerve cells. Protoplasmic astrocytes have a membrane potential that varies with the ...