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Drives and Emotions: The Hypothalamus and Limbic System
We seldom perceive things in a completely neutral fashion. Various sights and sounds make us happy, sad, or angry; some tastes and smells are extremely gratifying, others disgusting. Such feelings engendered by sensory inputs are ultimately the result of brains wired to promote survival and reproduction, and they are variable depending on current physiological needs and the social situation. Food, for example, becomes more attractive when one is hungry and less attractive when one is satiated or during tense social situations. Therefore the anatomical substrate for emotions, feelings, and the motivation for actions involve conscious awareness or context-dependent modulation that includes neocortical areas. These cortical areas must be heavily interconnected with the hypothalamus, because sensory inputs that elicit an emotion also initiate autonomic responses such as salivating and gearing up the alimentary tract, or pumping adrenaline and diverting blood to skeletal muscles.
The limbic system is the name given to the portions of the brain primarily concerned with turning motivations (e.g., avoid pain, approach rewards) and physiological drives (hunger, thirst) into behaviors supported by autonomic states. The limbic system includes cortical and subcortical components: the subcortical components are the hippocampus, the amygdala, and the hypothalamus. The cortical components are the cingulate cortex, the parahippocampal gyri (see Fig. 3.14 ), and some phylogenetically older areas of the insula and the prefrontal cortex. In view of the preceding discussion, it is not surprising that the subcortical components of the limbic system appeared early in vertebrate phylogeny and that, in terms of its connections, the limbic system is interposed between the hypothalamus and the neocortex. That is, limbic structures serve as bridges between autonomic and voluntary responses to changes in the environment. Our response to a chilly room provides a simple example. There are autonomic responses to the chill, coordinated by the hypothalamus, such as cutaneous vasoconstriction and shivering. We also become consciously aware of the chill and may choose to put on more clothes or throw another log on the fire.
The Hypothalamus Coordinates Drive-Related Behaviors
The hypothalamus ( Fig. 23.1 ) is a small portion of the diencephalon (weighing only about 4 g), but it is important as a nodal point in pathways concerned with autonomic, endocrine, emotional, and somatic functions ( Fig. 23.2 ) a
a Although it often appears in diagrams such as Fig. 23.2 that the hypothalamus-autonomic-pituitary system works in a realm separate and distinct from that controlling skeletal muscle, this is not the case. We have only one nervous system, and its parts work in a coordinated way to achieve desired outcomes. A simple set of examples is provided by the great variety of afferent and efferent limbs in reflexes that include autonomic fibers: light causes parasympathetic pupillary responses, a sudden loud noise is likely to cause sympathetic responses, cold causes shivering, and so on.
that are generally designed to maintain our internal environment in a physiological range (i.e., to promote homeostasis). For example, stimulation of appropriate hypothalamic areas in experimental animals can cause vasodilation, feeding behavior, or alterations of pituitary function. Its functions extend beyond this, however, into more complex interactions involved in motivational drives and emotional behaviors; stimulation of overlapping hypothalamic areas can elicit responses such as rage, sleep, or sexual behavior. Participation in these multiple functions is based on a widespread set of connections that fall into three principal categories: (1) interconnections with various cortical and subcortical components of the limbic system that coordinate motivated behaviors, (2) outputs that influence the pituitary gland coordinating hormonal changes, and (3) interconnections with various visceral and somatic nuclei, motor and sensory, of the brainstem and spinal cord. The hypothalamus is divided into a number of nuclei and areas, as described shortly. Each of these different nuclei and areas has more or less distinctive connections, but for the sake of simplicity, most of these connections are discussed here as though the hypothalamus were primarily a uniform structure.
The Hypothalamus Can Be Subdivided in Both Longitudinal and Medial-Lateral Directions
The inferior surface of the hypothalamus (see Figs. 3.16 and 3.17 ), exposed directly to subarachnoid space, is bounded by the optic chiasm, the optic tracts, and the posterior edge of the mammillary bodies. This area, exclusive of the mammillary bodies, is called the tuber cinereum (Latin for “gray swelling”; Fig. 23.3A ). The median eminence protrudes ventrally from the surface of the tuber cinereum and is continuous with the infundibular stalk, which in turn is continuous with the posterior lobe of the pituitary. The infundibular stalk and posterior lobe together constitute the neurohypophysis.
The medial surface of the hypothalamus (see Fig. 23.3B ) extends anteriorly to the lamina terminalis, superiorly to the hypothalamic sulcus, and posteriorly to the caudal edge of the diencephalon. As in the case of the brainstem, transverse planes are sometimes used to define the anterior and posterior boundaries—in this case, a plane passing through the posterior commissure and the posterior edge of the mammillary bodies, and another passing through the anterior edge of the optic chiasm and the posterior edge of the anterior commissure. Also as in the case of the brainstem, however, functionally related neural tissue continues through both these planes. For example, the neural tissue immediately in front of the anterior plane is structurally and functionally continuous with the hypothalamus (see Fig. 23.3B and C ). Therefore this region, the preoptic area, is treated instead as part of the anterior hypothalamus.
The hypothalamus can be subdivided from front to back into anterior, tuberal, and posterior regions. The anterior region is the part above the optic chiasm, the tuberal region is the part above and including the tuber cinereum, and the posterior region is the part above and including the mammillary bodies. In addition, the entire hypothalamus of each side is divided into three longitudinal zones (see Fig. 23.3D ). The thin periventricular zone, making up the wall of the third ventricle, is a rostral continuation of the periaqueductal gray. The lateral zone, lateral to the fornix as this fiber bundle traverses the hypothalamus, is a rostral continuation of the reticular formation. The periventricular and lateral zones include collections of neurons and are avenues through which ascending and descending axons enter, leave, or traverse the hypothalamus; the medial zone between them is populated by a series of hypothalamic nuclei ( Fig. 23.4 and Table 23.1 ).
TABLE 23.1
Hypothalamic Nuclei
| Region | Periventricular Zone | Medial Zone | Lateral Zone |
|---|---|---|---|
| Anterior | Suprachiasmatic nucleus | Medial preoptic nucleus Anterior nucleus Paraventricular nucleus Supraoptic nucleus |
Lateral preoptic nucleus Lateral nucleus |
| Tuberal | Arcuate nucleus | Dorsomedial nucleus Ventromedial nucleus |
Lateral tuberal nuclei Tuberomammillary nucleus Lateral nucleus |
| Posterior | Mammillary body Posterior nucleus |
Lateral nucleus |
The periventricular zone is traversed by the dorsal longitudinal fasciculus, a bundle of hypothalamic afferents and efferents (see Fig. 23.6B and C ). The periventricular zone also contains a small suprachiasmatic nucleus and a larger arcuate nucleus. The suprachiasmatic nucleus is tiny—less than 1 mm 3 and fewer than 10,000 neurons on each side—but it is the “master clock” for our circadian rhythms ( Fig. 23.5A ). The free-running period of cells in the suprachiasmatic nucleus is typically about 25 hours (see Fig. 23.5B ), but it receives direct projections from the retina that entrain it to the actual day length. Its neurons also contain numerous melatonin receptors, and the nighttime rise in pineal melatonin secretion is thought to provide an additional signal that helps “set” the circadian clock. The arcuate nucleus, as described later in this chapter, is critically involved in feeding behavior.
The lateral zone consists mainly of scattered cells interspersed among the longitudinally running fibers of the medial forebrain bundle. Anteriorly, it is continuous with the lateral preoptic nucleus, an important sleep-promoting area (see Fig. 22.34 ); caudally, it is continuous with the midbrain reticular formation. Part of the supraoptic nucleus intrudes into it, as do clumps of cells called lateral tuberal nuclei. It also contains the small tuberomammillary nucleus, the source of histaminergic fibers that project widely to the cerebral cortex and thalamus and participate in the sleep-wake cycle (see Fig. 22.31 ). Otherwise it is undivided.
The medial zone contains a number of nuclei (see Table 23.1 ). Anteriorly, these include two distinctive nuclei containing large neurosecretory cells: the supraoptic and paraventricular nuclei. The supraoptic nucleus sits astride the optic tract, extending into the lateral hypothalamic zone; the paraventricular nucleus is higher up in the wall of the third ventricle, near the anterior commissure. Most cells of the supraoptic nucleus and many cells of the paraventricular nucleus secrete hormones that travel down the axons of these cells and are released in the neurohypophysis. The hormones involved and the pathway traversed by them are discussed later in this chapter (see Fig. 23.10 ). The medial tuberal region is subdivided into dorsal and ventral portions called the dorsomedial and ventromedial nuclei, respectively. In addition, clusters of orexin-containing neurons near the fornix, extending into the lateral and posterior hypothalamus, are the source of a second set of widespread wakefulness-promoting projections (see Fig. 22.31 ). The medial mammillary region contains the mammillary body (actually a complex of several nuclei) and the posterior hypothalamic nucleus, part of which is continuous with the periaqueductal gray matter of the midbrain.
Hypothalamic Inputs Arise in Widespread Neural Sites
Neural inputs to the hypothalamus arise in two general areas ( Fig. 23.6 ): (1) various parts of the forebrain, particularly components of the limbic system and (2) the brainstem and spinal cord. Afferents from the brainstem and spinal cord convey visceral and somatic sensory information, whereas those from limbic structures convey information relevant to the role of the hypothalamus in mediating many of the autonomic and somatic aspects of affective states. The connections of limbic components with one another and with the hypothalamus are discussed later in this chapter and are mentioned only briefly here.
Most Inputs From the Forebrain Arise in Limbic Structures
Major forebrain afferents to the hypothalamus arise in the (1) septal nuclei and nearby parts of the basal forebrain, including the ventral striatum; (2) hippocampus; (3) amygdala; (4) insula, orbitofrontal cortex along with a few other related cortical areas; and (5) retina.
The septal nuclei (see Fig. 25.2 ), prominent components of the limbic system located adjacent to the septum pellucidum, project fibers to the hypothalamus through the medial forebrain bundle. The medial forebrain bundle is built like a frayed rope, with fibers entering and leaving it at many levels as it traverses the lateral hypothalamic zone and extends into the brainstem tegmentum. This is a bidirectional bundle that also contains afferents from the brainstem to the hypothalamus, hypothalamic efferents passing rostrally and caudally, fibers interconnecting different hypothalamic levels, and fibers passing through the hypothalamus on their way to someplace else.
The major output from the hippocampus is contained in the fornix. This fiber bundle arches around under the corpus callosum and through the hypothalamus, where many of its fibers reach the mammillary body (see Fig. 24.19 ).
The amygdala projects fibers to the hypothalamus by two different routes. Some travel through the stria terminalis, a long, curved fiber bundle that accompanies the caudate nucleus. Others take a shorter course and pass under the lenticular nucleus directly to the hypothalamus (see Fig. 23.20 ).
There are direct projections from the cerebral cortex to the hypothalamus. These arise mainly in the orbital and medial prefrontal cortex of the frontal lobe and in the insula and join the medial forebrain bundle. There are also contributions from the cingulate gyrus and some other cortical areas.
Finally, the projections from photosensitive retinal ganglion cells to the suprachiasmatic nucleus play a key role in entraining circadian rhythms.
Inputs From the Brainstem and Spinal Cord Traverse the Medial Forebrain Bundle and Dorsal Longitudinal Fasciculus
An assortment of sensory inputs (in addition to those from the retina) reaches the hypothalamus by several routes. Some involve synapses in various portions of the reticular formation and periaqueductal gray; others arrive directly from sites such as the solitary and parabrachial nuclei. Some of these afferents travel in the medial forebrain bundle; others are contained in the dorsal longitudinal fasciculus (see Fig. 23.6B and C ), a collection of thinly myelinated fibers that pass through the periventricular and periaqueductal gray of the brainstem and then fan out in the hypothalamic wall of the third ventricle. Still other afferents enter the hypothalamus as collaterals of fibers in other pathways such as the spinothalamic tract.
Ascending axons from brainstem monoamine-containing neuronal groups—locus ceruleus, raphe nuclei, and the ventral tegmental area—also traverse the medial forebrain bundle on their way to innervate the cerebral cortex (see Fig. 11.24, Fig. 11.26, Fig. 11.27 ). Along the way, some terminate in the hypothalamus.
The Hypothalamus Contains Intrinsic Sensory Neurons
In addition to receiving various types of visceral and somatic information through the brainstem pathways just mentioned, the hypothalamus contains neurons that are directly responsive to physical stimuli. Some of these cells are sensitive to the temperature of the hypothalamus, whereas the activity of others is sensitive to such things as blood osmolality ( Fig. 23.7 ) or the concentration of glucose or certain hormones in blood passing through the hypothalamus.
Hypothalamic Outputs Largely Reciprocate Inputs
Some efferent pathways from the hypothalamus reciprocate the afferent pathways ( Fig. 23.8 ). Thus the hypothalamus projects to the septal nuclei, hippocampus, amygdala, brainstem, and spinal cord by way of the same fiber bundles that carry afferents to the hypothalamus. Efferents to the cerebral cortex, rather than focusing on limbic areas, blanket the cortex with widespread projections in a manner similar to that of the monoamine-containing fibers from the brainstem; the two most prominent examples are histaminergic fibers from the tuberomammillary nucleus and orexinergic fibers from the tuberal and posterior hypothalamus. A few pathways are totally or predominantly efferent in nature. The prominent mammillothalamic tract, for example, passes from the mammillary body to the anterior nucleus of the thalamus. Shortly after leaving the mammillary body, many of these same axons send branches to the midbrain reticular formation through the mammillotegmental tract.
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