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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. The cell body was recognized as the location of genetic material within the cell.

More detailed information from electron microscopes and other technique 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 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 neurons failing to receive nerve growth factors die off, while those serving a useful function (presumably) continue to live and grow. The result is a selection process that optimizes development in the nervous system.

Neurons do not actually touch at chemical synapses; they are separated by a tiny cleft (the synaptic cleft). Molecules called transmitter substances (or neurotrans­mitters, or just transmitter) flow across the cleft when a synapse is activated.

Perhaps the best-known transmitters are the endogenous opiates called endorphins. They are not pleasure chemicals; they are produced in response to distressing pain.

Endorphins seem to be the mechanism behind placebo pain relief. Placebo pain relief is blocked by naloxone, which counteracts opiates.

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