Aside from direct control of brain activity by specific transmission of nerve signals from the lower brain areas to the cortical regions of the brain, still another physiologic mechanism is very often used to control brain activity. This is to secrete excitatory or inhibitory neurotransmitter hormonal agents into the substance of the brain. These neurohormones often persist for minutes or hours and thereby provide long periods of control, rather than just instantaneous activation or inhibition.
Figure 58-2 shows three neurohormonal systems that have been mapped in detail in the rat brain: (1) a norepinephrine system, (2) a dopamine system, and (3) a serotonin system. Norepinephrine usually functions
Three neurohormonal systems that have been mapped in the rat brain: a norepinephrine system, a dopamine system, and a serotonin system. (Adapted from Kelly, after Cooper, Bloom, and Roth, in Kandel ER, Schwartz JH: Principles of Neural Science, 2nd ed. New York: Elsevier, 1985.)
as an excitatory hormone, whereas serotonin usually is inhibitory, and dopamine is excitatory in some areas but inhibitory in others. As would be expected, these three systems have different effects on levels of excitability in different parts of the brain. The norepi-nephrine system spreads to virtually every area of the brain, whereas the serotonin and dopamine systems are directed much more to specific brain regions— the dopamine system mainly into the basal ganglial regions and the serotonin system more into the midline structures.
Neurohormonal Systems in the Human Brain. Figure 58-3 shows the brain stem areas in the human brain for activating four neurohormonal systems, the same three discussed for the rat and one other, the acetylcholine system. Some of the specific functions of these are as follows:
1. The locus ceruleus and the norepinephrine system. The locus ceruleus is a small area located bilaterally and posteriorly at the juncture between the pons and mesencephalon. Nerve fibers from this area spread throughout the brain, the same as shown for the rat in the top frame of Figure 58-2, and they secrete norepinephrine. The norepinephrine generally excites the brain to increased activity. However, it has inhibitory effects in a few brain areas because of inhibitory receptors at certain neuronal synapses. In Chapter 59, we will see that this system probably plays an important role in causing dreaming, thus leading to a type of sleep called rapid eye movement sleep (REM sleep).
2. The substantia nigra and the dopamine system. The substantia nigra is discussed in Chapter 56 in
To diencephalon and cerebrum
To diencephalon and cerebrum
Locus ceruleus^ (norepinephrine)
Nuclei of the raphe (serotonin)
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.