Our Marvelous Bodies: An Introduction to the Physiology of Human Health

By Gary F. Merrill | Go to book overview

2
Understanding the Mammalian
Nervous ystem

What Are Neurons?

Neurons, or brain cells, come in multiple shapes and sizes. Their common purpose is to communicate. Neurons consist of three basic components that enable them to communicate. These include the cell body or soma, an axon or cable that extends to an adjacent neuron or other effector, and dendrites. Dendrites are shaft-like projections that arise from the cell body and make contact with multiple other neurons in the vicinity of the original cell. Physiologically, dendrites receive incoming information. That information is integrated and analyzed in the cell body. A response is then conducted down the axon to another neuron or some other effector such as a skeletal muscle cell. Neurons are the functional units of all the body's nervous systems.

No one knows how many neurons are present in the human body. In late 2006, I heard a mammalian neuroscientist say that in one cubic millimeter of tissue from the rat cerebral cortex, there are more neurons than there are stars in the universe. I don't know how he arrived at this figure. However, as the universe is composed mainly of cold, dark, matter—as much as 70 percent as estimated by some astrophysicists—so is the human nervous system composed of cells and tissues other than neurons. One of the most abundant of these is the glial cell. Glial cells are thought to be supportive to neurons. They influence blood flow, provide structural support, influence remodeling after disease and damage, and are thought to contribute to the neuron's communicative/metabolic functions.

Neurons communicate via two processes. The first is called conduction and the second transmission. I will say more about each later, but for now conduction refers to the physiological processes involved in getting a message from the cell body, down the axon, and to the effector.

Nerve axons can be myelinated or nonmyelinated. Myelin is an insulationlike fatty sheath that encases the axon but is disrupted at regular intervals along

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