Control of Cerebral Activity by Continuous Excitatory Signals from the Brain Stem

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Reticular Excitatory Area of the Brain Stem

Figure 58-1 shows a general system for controlling the level of activity of the brain. The central driving component of this system is an excitatory area located in the reticular substance of the pons and mesencephalon. This area is also known by the name bulboreticular facilitory area. We also discuss this area in Chapter 55 because it is the same brain stem reticular area that transmits facilitory signals downward to the spinal cord to maintain tone in the antigravity muscles and to control levels of activity of the spinal cord reflexes. In addition to these downward signals, this area also sends a profusion of signals in the upward direction. Most of these go first to the thalamus, where they excite a different set of neurons that transmit nerve signals to all regions of the cerebral cortex as well as to multiple subcortical areas.

The signals passing through the thalamus are of two types. One type is rapidly transmitted action potentials that excite the cerebrum for only a few milliseconds. These originate from large neuronal cell bodies that lie throughout the

Thalamus

Thalamus

Excitatory area

5th Cranial nerve

Excitatory area

5th Cranial nerve

Inhibitory area

Figure 58-1

Figure 58-1

Excitatory-activating system of the brain. Also shown is an inhibitory area in the medulla that can inhibit or depress the activating system.

brain stem reticular area. Their nerve endings release the neurotransmitter substance acetylcholine, which serves as an excitatory agent, lasting for only a few milliseconds before it is destroyed.

The second type of excitatory signal originates from large numbers of small neurons spread throughout the brain stem reticular excitatory area. Again, most of these pass to the thalamus, but this time through small, slowly conducting fibers that synapse mainly in the intralaminar nuclei of the thalamus and in the reticular nuclei over the surface of the thalamus. From here, additional small fibers are distributed everywhere in the cerebral cortex. The excitatory effect caused by this system of fibers can build up progressively for many seconds to a minute or more, which suggests that its signals are especially important for controlling longer-term background excitability level of the brain.

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