Serotonergic system of the brain

The serotonin (5-HT) system consists of a diverse group of neurons (the cell bodies of which are located in the brainstem raphe nuclei, the reticular formation, and complex axonal systems) that innervate virtually the whole central nervous system. Some of the serotonin neurons do not make synaptic contacts but, rather, terminate in a field of neurons that are influenced by the serotonin released at the terminals. Serotonin neurons are found abundantly in the limbic structures that contribute to emotional behavior and learning. The hippocampus, a component of the limbic system, is richly innervated by serotonergic, noradrenergic, and other neurotransmitter systems. There are several axonal pathways from the dorsal and median raphe 5-HT neurons to the hippocampus. Some fine axons in this pathway are distributed to the CA1 and CA3 layers of the hippocampus and are susceptible to damage by serotonin neurotoxins. In contrast to the hippocampus, serotonin innervation of the cerebellum is rather sparse. However, there is a complex and well-organized modulation of glutaminergic transmission via 5-HT receptors in the cerebellum. This interaction may play an important role in the cerebral control of movement and ataxia as shown by the efficacy of 5-HT1A antagonists in clinical studies of cerebellar ataxia. (Contributed by Dr. K K Jain.)