The Neuron

The basic unit of the nervous system is the neuron or nerve cell. The first thing we will see is that there are a lot of them. Nobody has counted all of the neurons in the human body, but estimates run from 10-12 billion to much higher. For the sake of comparison, a cockroach has about 250,000 neurons, while a chimpanzee has 3.5 billion (with the same caveats about numbers as above). The point to remember is that there are a great many, and that the large number contributes to the complexity of the nervous system. When one starts to consider the connections between nerve cells, then the numbers truly become astronomical because a neuron may connect to as many as 75 other neurons. Most of the neurons in the human body, about 70%, are in the central nervous system.

Neurons can be divided into three types, depending on their function. Afferent or sensory neurons run from the sense organs to the central nervous system. Efferent or motor neurons run from the central nervous system to the muscles, and interneurons or multipolar neurons are found chiefly within the brain and they are multiply connected to other neurons.

Axons are a special part of a neuron. They are the part that carries the nerve impulse down the cell. Nothing in the axon predisposes it to carry the nerve impulse from the cell body to the terminal buttons. If you electrically stimulate an isolated axon in the middle, the nerve impulse will run in both directions. It is the organization of the nerves that ensures that the impulses travel in the proper direction.

URLs

Organization of the Nervous System--tutorial, adv., long, graphics

Comprehensive tutorial on the basics of the nervous system with line drawings; includes sections on neurons and glia, terminology, subdivisions of the nervous system, peripheral nervous system, spinal nerves, autonomic nervous system, and a self-quiz. http://www.cc.emory.edu/ANATOMY/AnatomyManual/nervous_system.html

The Neurons and the Nervous System--tutorial, interm., medium, links, graphics

Another tutorial on the nervous system, sections include the basic neuron, electrical activity in neurons, chemical activity in neurons, the nervous system, and others. http://eleceng.ukc.ac.uk/~sd5/research/nn_index.html

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