Serotonin

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Figure 58-2

(acetylcholine)

Locus ceruleus^ (norepinephrine)

Nuclei of the raphe (serotonin)

relation to the basal ganglia. It lies anteriorly in the superior mesencephalon, and its neurons send nerve endings mainly to the caudate nucleus and putamen of the cerebrum, where they secrete dopamine. Other neurons located in adjacent regions also secrete dopamine, but they send their endings into more ventral areas of the brain, especially to the hypothalamus and the limbic system. The dopamine is believed to act as an inhibitory transmitter in the basal ganglia, but in some other areas of the brain it is possibly excitatory. Also, remember from Chapter 56 that destruction of the dopaminergic neurons in the substantia nigra is the basic cause of Parkinson's disease.

3. The raphe nuclei and the serotonin system. In the midline of the pons and medulla are several thin nuclei called the raphe nuclei. Many of the neurons in these nuclei secrete serotonin. They send fibers into the diencephalon and a few fibers to the cerebral cortex; still other fibers descend to the spinal cord. The serotonin secreted at the cord fiber endings has the ability to suppress pain, which was discussed in Chapter 48. The serotonin released in the diencephalon and cerebrum almost certainly plays an essential inhibitory role to help cause normal sleep, as we discuss in Chapter 59.

4. The gigantocellular neurons of the reticular excitatory area and the acetylcholine system. Earlier we discussed the gigantocellular neurons (the giant cells) in the reticular excitatory area of the pons and mesencephalon. The fibers from these large cells divide immediately into two branches, one passing upward to the higher levels of the brain and the other passing downward through the reticulospinal tracts into the spinal cord. The neurohormone secreted at their terminals is acetylcholine. In most places, the acetylcholine functions as an excitatory neurotransmitter. Activation of these acetylcholine neurons leads to an acutely awake and excited nervous system.

Other Neurotransmitters and Neurohormonal Substances Secreted in the Brain. Without describing their function, the following is a list of still other neurohormonal substances that function either at specific synapses or by release into the fluids of the brain: enkephalins, gamma-aminobutyric acid, glutamate, vasopressin, adrenocorticotropic hormone, epinephrine, histamine, endorphins, angiotensin II, and neurotensin. Thus, there are multiple neurohormonal systems in the brain, the activation of each of which plays its own role in controlling a different quality of brain function.

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