Chapter  21:  Nerve Cells

Neonatal rat cortical brain cells, cultured for 25 days in vitro, stained with a fluorescent antibody to the cytoskeletal intermediate filament protein GFAP (Glial Fibrillary Acidic Protein, green) and with the dye DAPI that causes DNA to fluoresce blue. Two distinct types of astrocytes (green cells) are present in this culture, along with other types of cells (non-green) that appear as isolated blue nuclei. [Photograph courtesy of Nancy Kedersha.]

The nervous system regulates all aspects of bodily function and is staggering in its complexity. The human brain — the control center that stores, computes, integrates, and transmits information — contains about 10¹² neurons (nerve cells), each forming as many as a thousand connections with other neurons. Millions of specialized neurons sense features of both the external and internal environments and transmit this information to the brain for processing and storage. Millions of other neurons regulate the contraction of muscles and the secretion of hormones. The nervous system also contains glial (neuroglial) cells that occupy the spaces between neurons and modulate their functions (see chapter opening figure).

The structure and function of individual nerve cells is understood in great detail, perhaps in more detail than for any other type of cell. The function of a neuron is to communicate information, which it does by two methods. Electric signals process and conduct information within a cell, while chemical signals transmit information between cells, utilizing processes similar to those employed by other types of cells to signal each other (Chapter 20). Sensory neurons have specialized receptors that convert diverse types of stimuli from the environment (e.g., light, touch, sound, odorants) into electric signals. These electric signals are then converted into chemical signals that are passed on to other cells called interneurons, which convert the information back into electric signals. Ultimately the information is transmitted to muscle-stimulating motor neurons or to other neurons that stimulate other types of cells, such as glands.

The output of a nervous system is the result of its circuit properties, that is, the wiring, or interconnections, between neurons, and the strength of these interconnections. Complex aspects of the nervous system, such as vision and consciousness, cannot be understood at the single-cell level, but only at the level of networks of nerve cells that can be studied by techniques of systems analysis. The nervous system is constantly changing; alterations in the number and nature of the interconnections between individual neurons occur, for example, in the development of new memories.

In this chapter we focus on how individual neurons function and how small groups of cells function together. A great deal of information has been gleaned from simple nervous systems. Squids and sea slugs have large neurons that are relatively easy to identify and manipulate experimentally. Moreover, in these species, only a few identifiable neurons may be involved in a specific task; thus their function can be studied in some detail. Analyses of humans, mice, nematodes, and flies with mutations that affect specific functions of the nervous system have provided important insights, as have molecular cloning of key neuron proteins, such as ion channels and receptors. Genetic and molecular studies on the development of the nervous system, detailed in Chapter 23, have elucidated how neurons form and maintain specific connections with other neurons and other types of cells. Because the principles studied are basic, all of these findings are applicable to complex nervous systems, including that of humans.