Several substances called neuropeptides occur in the brain and spinal cord.
They appear to be synthesized in neuron cell bodies and are composed of chains of two to thirty-nine amino acids. Some of them seem to serve as neurotransmitters, while others may act as neuromodulators substances that alter a neuron's response to a neurotransmitter.
The neuropeptides include two groups of neurotransmitters, called enkephalins and endorphins, whose molecules contain five amino acids each.
They influence the nervous system in much the same way as morphine, in that they relieve pain sensations.
Another neuropeptide, which consists of eleven amino acids and is widely distributed throughout the nervous system, is called substance P.
It seems to function as a neurotransmitter (or perhaps as a neuromodulator) in the neurons that transmit pain impulses into the spinal cord and on to the brain.
Some investigators think the enkephalins and endorphins may relieve pain by inhibiting the release of substance P from pain-transmitting neurons.
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