Chapter 4: Excitable Tissue: Nerve

OBJECTIVES

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.

INTRODUCTION

The human central nervous system (CNS) contains about 10¹¹ (100 billion) neurons. It also contains 2–10 times this number of glial cells. The CNS is a complex organ; it has been calculated that 40% of the human genes participate, at least to a degree, in its formation. The neurons, the basic building blocks of the nervous system, have evolved from primitive neuroeffector cells that respond to various stimuli by contracting. In more complex animals, contraction has become the specialized function of muscle cells, whereas integration and transmission of nerve impulses have become the specialized functions of neurons. Neurons and glial cells along with brain capillaries form a functional unit that is required for normal brain function, including 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 (eg, cerebral ischemia, seizures, neurodegenerative diseases, and cerebral edema). 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 (eg, action potentials, receptor potentials, and synaptic potentials).

CELLULAR ELEMENTS IN THE CNS

GLIAL CELLS

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 ...