Serotonin

The raphe nuclei, running rostro-caudally along the midline of the pons and medulla, contain the majority of the serotonergic neurons of the brain. The posterior groups send descending projections to the cerebellum and to sensory and motor neurons and also modulate neuronal excitability. The ascending forebrain projections arise from the anterior groups. The dorsal raphe projects to frontal cortex and striatum and the median raphe projects to the hippocampus and the septum.

Neurons of the raphe nuclei fire tonically during active alert wakefulness when there is EEG desyn-chrony, but they fire at very low rates during sleep and are silent during rapid eye movement sleep when the EEG is also desynchronized. However, lesions of the raphe nuclei result in reduction of forebrain serotonin and a correlated reduction in time spent sleeping. Furthermore, drugs that acutely reduce forebrain serotonin also lead to a reduction in sleep. With chronic inhibition of serotonin synthesis (maintaining levels of serotonin below 10% of normal), there is nevertheless a partial recovery of sleep. This evidence suggests that serotonin is necessary for the expression of normal sleep-wake cycles, although serotonin does not appear to be necessary for initiation of sleep.

A role for serotonin in cognitive processes is suggested by the specificity of the neuronal responses during alert wakefulness. There is evidence that serotonin might be involved in processes of behavioral inhibition, particularly the ability to withhold punished responses. Such deficits would be manifest as impulsivity.

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