Figure 1 shows a general system for controlling the activity level 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 56 because it is the same brain stem reticular area that transmits facilitory signals down ward 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 signals 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 signals originate from large neuronal cell bodies that lie throughout the brain stem reticular area. Their nerve endings release the neurotransmitter acetylcholine, which serves as an excitatory agent that lasts 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 signals 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 throughout 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 the longer-term background excitability level of the brain.
Excitation of the Excitatory Area by Peripheral Sensory Signals. The level of activity of the excitatory area in the brain stem, and therefore the level of activity of the entire brain, is determined to a great extent by the number and type of sensory signals that enter the brain from the periphery. Pain signals in particular increase activity in this excitatory area and therefore strongly excite the brain to attention.
The importance of sensory signals in activating the excitatory area is demonstrated by the effect of cutting the brain stem above the point where the fifth cerebral nerves enter the pons. These nerves are the highest nerves entering the brain that transmit significant numbers of somatosensory signals into the brain. When all these input sensory signals are gone, the level of activity in the brain excitatory area diminishes abruptly, and the brain proceeds instantly to a state of greatly reduced activity, approaching a permanent state of coma. However, when the brain stem is transected below the fifth nerves, which leaves much input of sensory signals from the facial and oral regions, the coma is averted.
Increased Activity of the Excitatory Area Caused by Feedback Signals Returning From the Cerebral Cortex. Not only do excitatory signals pass to the cere bral cortex from the bulboreticular excitatory area of the brain stem, but feedback signals also return from the cerebral cortex back to this same area. Therefore, any time the cerebral cortex becomes activated by either brain thought processes or motor processes, signals are sent from the cortex to the brain stem excitatory area, which in turn sends still more excitatory signals to the cortex. This process helps to maintain the level of excitation of the cerebral cortex or even to enhance it. This mechanism is a general mechanism of positive feedback that allows any beginning activity in the cerebral cortex to support still more activity, thus leading to an “awake” mind.
The Thalamus Is a Distribution Center That Controls Activity in Specific Regions of the Cortex. As pointed out in Chapter 58, almost every area of the cerebral cortex connects with its own highly specific area in the thalamus. Therefore, electrical stimulation of a specific point in the thalamus generally activates its own specific small region of the cortex. Furthermore, signals regularly reverberate back and forth between the thalamus and the cerebral cortex, with the thalamus exciting the cortex and the cortex then re-exciting the thalamus by way of return fibers. It has been suggested that the thinking process establishes long-term memories by activating such back and-forth reverberation of signals.
Whether the thalamus also functions to call forth specific memories from the cortex or to activate specific thought processes is still unclear, but the thalamus does have appropriate neuronal circuitry for these purposes.
