The Limbic System and the Hypothalamus—Behavioral and Motivational Mechanisms of the Brain

Control of behavior is a function of the entire nervous system. Even the wakefulness and sleep cycle discussed in Chapter 60 is one of our most important behavioral patterns.

In this chapter, we deal first with the mechanisms that control activity levels in different parts of the brain. Then we discuss the causes of motivational drives, especially motivational control of the learning process and feelings of pleasure and punishment. These functions of the nervous system are performed mainly by the basal regions of the brain, which together are loosely called the limbic system, meaning the “border” system.

Activating—Driving Systems Of The Brain

Without continuous transmission of nerve signals from the lower brain into the cerebrum, the cerebrum becomes useless. In fact, severe compression of the brain stem at the juncture between the mesencephalon and cerebrum, as sometimes results from a pineal tumor, often causes the person to enter into unremitting coma lasting for the remainder of his or her life.

Nerve signals in the brain stem activate the cerebrum in two ways: (1) by directly stimulating a background level of neuronal activity in wide areas of the brain and (2) by activating neurohormonal systems that release specific facilitory or inhibitory hormone-like neurotransmitters into selected areas of the brain.

Control Of Cerebral Activity By Continuous Excitatory Signals From The Brain Stem

Reticular Excitatory Area of the Brain Stem—a Driver of Brain Activity

Figure 59-1 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 called the bulboreticular facilitory area . We also discuss this area in Chapter 56 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 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.

Figure 59-1, The excitatory-activating system of the brain. Also shown is an inhibitory area in the medulla that can inhibit or depress the activating system.

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 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 Reticular Excitatory Area by Peripheral Sensory Signals

The level of activity of the reticular 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 Causedby Feedback Signals Returning From the Cerebral Cortex

Not only do excitatory signals pass to the cerebral 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 brain thought processes or by 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 is a positive feedback mechanism 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 via return fibers. Activation of these back-and-forth reverberation signals has been suggested to establish long-term memories.

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.

A Reticular Inhibitory Area Is Located in the Lower Brain Stem

Figure 59-1 shows another area that is important in controlling brain activity—the reticular inhibitory area, located medially and ventrally in the medulla. In Chapter 56 , we learned that this area can inhibit the reticular facilitory area of the upper brain stem and thereby decrease activity in the superior portions of the brain. One of the mechanisms for this activity is to excite serotonergic neurons, which in turn secrete the inhibitory neurohormone serotonin at crucial points in the brain; we discuss this concept in more detail later.

Neurohormonal Control of Brain Activity

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 physiological mechanism is often used to control brain activity. This mechanism 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 59-2 shows three neurohormonal systems that have been studied in detail in the rat brain: (1) a norepinephrine system, (2) a dopamine system, and (3) a serotonin system. Norepinephrine usually functions as an excitatory hormone, whereas serotonin is usually 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 norepinephrine 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.

Figure 59-2, Three neurohormonal systems that have been mapped in the rat brain—a norepinephrine system, a dopamine system, and a serotonin system.

Neurohormonal Systems in the Human Brain

Figure 59-3 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 systems 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 59-2 , 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. Chapter 60 describes how this system probably plays an important role in causing dreaming, thus leading to a type of sleep called rapid eye movement (REM) sleep.
2.
The substantia nigra and the dopamine system. The substantia nigra is discussed in Chapter 57 in 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 57 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 49 . 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 60 .
4.
The gigantocellular neurons of the reticular excitatory area and the acetylcholine system. We previously discussed the gigantocellular neurons (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.

Figure 59-3, Multiple centers in the brain stem. These neurons secrete different transmitter substances (specified in parentheses). The neurons send control signals upward into the diencephalon and cerebrum and downward into the spinal cord.

Other Neurotransmitters and Neurohormonal Substances Secreted in the Brain

Without describing their function, the following is a partial 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, α-melanocyte stimulating hormone (α-MSH), neuropeptide-Y (NPY), 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.

Limbic System

The word “limbic” means “border.” Originally, the term “limbic” was used to describe the border structures around the basal regions of the cerebrum, but as we have learned more about the functions of the limbic system, the term limbic system has been expanded to mean the entire neuronal circuitry that controls emotional behavior and motivational drives.

A major part of the limbic system is the hypothalamus, with its related structures. In addition to their roles in behavioral control, these areas control many internal conditions of the body, such as body temperature, osmolality of the body fluids, and the drives to eat and drink and to control body weight. These internal functions are collectively called vegetative functions of the brain, and their control is closely related to behavior.

Functional Anatomy of the Limbic System—Key Position of the Hypothalamus

Figure 59-4 shows the anatomical structures of the limbic system, demonstrating that they are an interconnected complex of basal brain elements. Located in the middle of all these structures is the extremely small hypothalamus, which from a physiological point of view is one of the central elements of the limbic system. Figure 59-5 illustrates schematically this key position of the hypothalamus in the limbic system and shows other subcortical structures of the limbic system surrounding it, including the septum, paraolfactory area, anterior nucleus of the thalamus, portions of the basal ganglia, hippocampus, and amygdala.

Figure 59-4, Anatomy of the limbic system, shown in the dark pink area.

Figure 59-5, The limbic system, showing the key position of the hypothalamus.

Surrounding the subcortical limbic areas is the limbic cortex, composed of a ring of cerebral cortex on each side of the brain—(1) beginning in the orbitofrontal area on the ventral surface of the frontal lobes, (2) extending upward into the subcallosal gyrus, (3) then over the top of the corpus callosum onto the medial aspect of the cerebral hemisphere in the cingulate gyrus, and finally (4) passing behind the corpus callosum and downward onto the ventromedial surface of the temporal lobe to the parahippocampal gyrus and uncus.

Thus, on the medial and ventral surfaces of each cerebral hemisphere is a ring of mostly paleocortex that surrounds a group of deep structures intimately associated with overall behavior and emotions. In turn, this ring of limbic cortex functions as a two-way communication and association linkage between the neocortex and the lower limbic structures.

Many of the behavioral functions elicited from the hypothalamus and other limbic structures are also mediated through the reticular nuclei in the brain stem and their associated nuclei. We pointed out in Chapter 56 , as well as earlier in this chapter, that stimulation of the excitatory portion of this reticular formation can cause high degrees of cerebral excitability while also increasing the excitability of much of the spinal cord synapses. In Chapter 61 , we see that most of the hypothalamic signals for controlling the autonomic nervous system are also transmitted through synaptic nuclei located in the brain stem.

An important route of communication between the limbic system and the brain stem is the medial forebrain bundle, which extends from the septal and orbitofrontal regions of the cerebral cortex downward through the middle of the hypothalamus to the brain stem reticular formation. This bundle carries fibers in both directions, forming a trunk line communication system. A second route of communication is through short pathways among the reticular formation of the brain stem, thalamus, hypothalamus, and most other contiguous areas of the basal brain.

The Hypothalamus, a Major Control Headquarters for the Limbic System

The hypothalamus, despite its small size of only a few cubic centimeters (weighing only about 4 grams), has two-way communicating pathways with all levels of the limbic system. In turn, the hypothalamus and its closely allied structures send output signals in three directions: (1) backward and downward to the brain stem, mainly into the reticular areas of the mesencephalon, pons, and medulla, and from these areas into the peripheral nerves of the autonomic nervous system; (2) upward toward many higher areas of the diencephalon and cerebrum, especially to the anterior thalamus and limbic portions of the cerebral cortex; and (3) into the hypothalamic infundibulum to control or partially control most of the secretory functions of both the posterior and the anterior pituitary glands.

Thus, the hypothalamus, which represents less than 1% of the brain mass, is one of the most important of the control pathways of the limbic system. It controls most of the vegetative and endocrine functions of the body and many aspects of emotional behavior.

Vegetative and Endocrine Control Functions of the Hypothalamus

The different hypothalamic mechanisms for controlling multiple functions of the body are so important that they are discussed in multiple chapters throughout this text. For example, the role of the hypothalamus to help regulate arterial pressure is discussed in Chapter 18 , thirst and water conservation in Chapter 30 , appetite and energy expenditure in Chapter 72 , temperature regulation in Chapter 74 , and endocrine control in Chapter 76 . To illustrate the organization of the hypothalamus as a functional unit, we summarize the a few of its vegetative and endocrine functions here as well.

Figures 59-6 and 59-7 show enlarged sagittal and coronal views of the hypothalamus, which represents only a small area in Figure 59-4 . Especially note in Figure 59-6 the multiple activities that are excited or inhibited when respective hypothalamic nuclei are stimulated. In addition to these centers, a large lateral hypothalamic area (shown in Figure 59-7 ) is present on each side of the hypothalamus. The lateral areas are especially important in controlling thirst, hunger, and many of the emotional drives.

Figure 59-6, Control centers of the hypothalamus (sagittal view). GI, Gastrointestinal.

Figure 59-7, Coronal view of the hypothalamus, showing the mediolateral positions of the respective hypothalamic nuclei.

A word of caution must be issued when studying these diagrams because the areas that cause specific activities are not nearly as accurately localized as suggested in the figures. Also, it is not known whether the effects noted in the figures result from stimulation of specific control nuclei or merely from activation of fiber tracts leading from or to control nuclei located elsewhere. With this caution in mind, we can give the following general description of the vegetative and control functions of the hypothalamus.

Cardiovascular Regulation

Stimulation of different areas throughout the hypothalamus can cause many neurogenic effects on the cardiovascular system, including changes in arterial pressure and heart rate. In general, stimulation in the posterior and lateral hypothalamus increases the arterial pressure and heart rate, whereas stimulation in the preoptic area often has opposite effects, causing a decrease in both heart rate and arterial pressure. These effects are transmitted mainly through specific cardiovascular control centers in the reticular regions of the pons and medulla.

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