This is the 2007 version. Click here for the 2017 chapter 02 table of contents.

Summary: Neurons

Neurons are the basic units of the nervous system. The classic doctrine, formulated in the early decades of the 1900s, identified basic parts of the neuron. Dendrites were said to be input areas, axons output areas, and the cell body was recognized as the location of genetic material within the cell. Later findings complicated this picture. For example, both dendrites and axons act as input and output structures.

Neurons are connected in a vast network. Each neuron may send messages to thousands of other neurons while receiving inputs from an equally large number. Neurons do not actually touch at chemical synapses; they are separated by a tiny cleft (the synaptic cleft) across which flows molecules called transmitter substances. Perhaps the best-known transmitters are the endogenous opiates called endorphins. They are not pleasure chemicals, and they are not necessarily produced in response to pain, only to stress.

Neurons die in great numbers throughout a person's life. This is one way the nervous system organizes itself. Each person has far more neurons than needed. The ones that fail to receive nerve growth factor die off, while the ones that serve a useful function continue to live and grow. The result is a selection process that optimizes development in the nervous system.


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