Serotonin: Neurotransmitter Extraordinare
Serotonin was first isolated from the blood in 1948. Later it was identified in the central nervous system. Like most neurotransmitters it has a fairly simple chemical structure, but has many complex functions. It is synthesized from tryptophan and has the chemical name 5-hydroxytryptamine, or 5-HT.
Serotonin has a variety of effects in both the central and peripheral nervous system. It is found in three main areas in the body, including the intestinal wall, blood vessels, and the central nervous system. It is here that it has been studied the most. Serotonin is involved in controlling appetite, memory and learning, temperature regulation, mood, behavior (including sexual and hallucinogenic behavior), cardiovascular function, muscle contraction, endocrine regulation, depression, and sleep. Interestingly, with all of these functions. the average human only has 10 mg of serotonin in his or her body at any one time.
The seretoninergic system develops from the midline of the floor of the fourth ventricle. Therefore, the highest concentrations of 5-HT are found in the dorsal raphe nucleus of the brain. It is here that it is thought to control sleep to a certain extent. It acts as an inhibitor. In other words, when the 5-HT receptors are stimulated, sleep is inhibited. Inversely, a bout of REM sleep is triggered by decreased activity in the raphe nucleus. In anticipation of arousal, the neuronal activity returns to basal level several seconds before the end of the REM stage. This relationship can be seen in the following graph. (Reference p288 from text)
The PGO waves are the first indication of REM sleep and the unit which activity is shown is one from the dorsal raphe nucleus.
There is also evidence to suggest that the neurotransmitter norepinephrine is involved in the inhibition of sleep. There is no evidence yet of a transmitter that gives excitatory input. The combination of at least these two inhibitory neurotransmitters results in the activation of acetylcholinergic neurons in the dorsolateral pons. This activation results in PGO waves, REM, cortical desynchrony, and muscular paralysis. There are still many unanswered questions about the role of neurotransmitters in regulating sleep. Future research is concentrating on what causes the change in concentrations of serotonin to occur and what transmitter increases at the beginning of REM sleep.
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