Transmitters of the CNS

In contrast to the peripheral nervous system, in which only three compounds—acetylcholine, norepinephrine, and epinephrine—serve as neurotransmitters, the CNS contains at least 21 compounds that serve as neurotransmitters (Table 20–1). Furthermore, since there are numerous sites within the CNS for which no transmitter has been identified, it is clear that additional compounds, yet to be discovered, also mediate central neurotransmission.

It is important to note that none of the compounds that are thought to be CNS neurotransmitters has actually been proved to serve this function. The reason for uncertainty lies with the technical difficulties involved in CNS research. However, although absolute proof may be lacking, the evidence supporting a neurotransmitter role for several compounds (eg, dopamine, norepinephrine, serotonin, enkephalins) is completely convincing.

Although much is known about the actions of CNS transmitters at various sites in the brain and spinal cord, it is not usually possible to relate these known actions in a precise way to behavioral or psychologic processes. For example, although we know the locations of specific CNS sites at which norepinephrine appears to act as a transmitter, and although we know the effect of norepinephrine at most of these sites (suppression of neuronal excitability), we do not know the precise relationship between suppression of neuronal excitability at each of these sites and the impact of that suppression on the overt function of the organism. This example illustrates the general state of our knowledge of CNS transmitter function: We have a great deal of detailed information about the biochemistry and electrophysiology of CNS transmitters, but we are as yet unable to assemble those details into a completely meaningful picture.