The Hypothalamus Can Be Subdivided in Both Longitudinal and Medial-Lateral Directions

Parts of the hypothalamus are exposed at the base of the brain, surrounded by the circle of Willis. The mammillary bodies form the most posterior part of the hypothalamus and lie adjacent to the cerebral peduncles. Between the mammillary bodies and the optic chiasm and tract is a small swelling called the tuber cinereum. The median eminence arises from the tuber cinereum and narrows into the infundibulum, to which the pituitary gland is attached. These landmarks on the base of the brain are used to divide the hypothalamus longitudinally (Fig. 23-2) into an anterior region (above the optic chiasm, extending anteriorly to the lamina terminalis), a tuberal region (above and including the tuber cinereum), and a posterior region (above and including the mammillary bodies).

Figure 23-2 Longitudinal subdivisions of the hypothalamus, and the two lobes of the pituitary gland. III, Oculomotor nerve; LT, lamina terminalis (the membrane at the anterior end of the third ventricle); OC, optic chiasm.

The hypothalamus also gets divided up in a medial to lateral direction. The periaqueductal gray of the midbrain continues into the thin periventricular zone in the wall of the third ventricle. The fornix runs right through the longitudinal zones on its way to the mammillary body and is used as a landmark to divide the rest of the hypothalamus on each side into medial and lateral zones.

Hypothalamic Inputs Arise in Widespread Neural Sites

The hypothalamus receives lots of inputs (Fig. 23-3), but most of them are from two general categories: those from nuclei in the brainstem and spinal cord conveying information about the state of your body, and those from limbic structures like the amygdala, hippocampus, and septal nuclei. Inputs about the state of the body (“It’s getting warm in here,” or “Blood glucose is getting low”) arrive from places like the nucleus of the solitary tract by way of the dorsal longitudinal fasciculus, which travels through the periaqueductal gray into the periventricular zone; through the medial forebrain bundle, which travels through the reticular formation into the lateral hypothalamus; and as branches from tracts like the spinothalamic tract. Limbic inputs arrive from the amygdala, from the hippocampus (through the fornix), and from the septal nuclei and other sites (through the medial forebrain bundle); collectively they keep the hypothalamus updated on other aspects of the environment (“Not a good place to take off my shirt”).

Figure 23-3 Inputs to the hypothalamus.

Inputs also reach the hypothalamus from the retina and in the form of direct physical stimuli. Axons of some retinal ganglion cells terminate in the small suprachiasmatic nucleus on each side of the anterior hypothalamus. The suprachiasmatic nucleus is the “master clock” for most circadian rhythms, and information from the retina helps get these rhythms synchronized with the 24-hour day (THB6 Figure 23-5, p. 585). Finally, some hypothalamic neurons are sensory receptors themselves, directly responsive to temperature, blood osmolality, or the concentration of some chemicals in blood passing through the hypothalamus.

Hypothalamic Outputs Largely Reciprocate Inputs

Hypothalamic connections with visceral nuclei and limbic structures are largely reciprocal (Fig. 23-4). Projections through the dorsal longitudinal fasciculus and the medial forebrain bundle reach sites like the nucleus of the solitary tract, the dorsal motor nucleus of the vagus, and the intermediolateral cell column of the spinal cord (“Better start sweating”). Projections through the medial forebrain bundle and other routes reach the amygdala, septal nuclei, and other limbic structures (“Maybe I can find the thermostat”). (Hypothalamic output reaches the hippocampus through a more circuitous route utilizing the thalamus, as described in Chapter 24.) In addition, diffuse modulatory projections to the thalamus and cerebral cortex play a key role in sleep-wake cycles (see Fig. 22-5Fig. 22-6).

Figure 23-4 Outputs from the hypothalamus.

The Hypothalamus Controls Both Lobes of the Pituitary Gland

The final, and major, hypothalamic outputs control the pituitary gland (hypophysis) through two separate mechanisms (Fig. 23-5). (1) Hypothalamic neurons in the supraoptic and paraventricular nuclei are the source of antidiuretic hormone (vasopressin) and oxytocin. They transport these hormones down their axons to the posterior lobe of the pituitary (most of the neurohypophysis), where they are released into the circulation. (2) Hypothalamic neurons in and near the tuber cinereum produce small peptides that serve as releasing and inhibiting factors for the anterior lobe of the pituitary (most of the adenohypophysis). They transport these factors down their axons and release them into capillaries in the median eminence. These capillaries then converge into pituitary portal vessels that travel down the infundibular stalk to a second capillary bed in the anterior pituitary. The releasing and inhibiting factors leave the second capillary bed and control the production of anterior pituitary hormones.

Figure 23-5 Hypothalamic control of the pituitary gland.

Perforating Branches from the Circle of Willis Supply the Hypothalamus

The infundibular stalk sits right in the middle of the circle of Willis (THB6 Figure 6-3, p. 126), suggesting where the blood supply of the hypothalamus comes from—small perforating or ganglionic branches all around the circle.

The Hypothalamus Collaborates with a Network of Brainstem and Spinal Cord Neurons

The connections of the hypothalamus position it to control the whole gamut of homeostatic mechanisms and drive-related behaviors—temperature regulation, feeding and drinking, cardiovascular function, sexual behavior, hormonal regulation, aggression, and on and on. Although some of these activities mainly involve autonomic adjustments (e.g., cardiovascular reflexes), skeletal muscle plays a major role in others (e.g., breathing). Normal control of micturition (urination) provides a nice example of coordination among visceral afferents, conscious awareness, smooth muscle, skeletal muscle, and voluntary control.

Normal Micturition Involves a Central Pattern Generator in the Pons

The bladder is a container that spends most of its time storing urine; it can do this because the pressure inside the bladder is usually low—the smooth muscle in its wall (the detrusor) is relaxed, and the internal (smooth muscle) and external (skeletal muscle) sphincters in its neck are contracted. This storage mode is maintained by sympathetic inputs and by tonic firing of motor neurons to the external sphincter (Fig. 23-6A). Periodically the sphincters relax, the detrusor contracts, and urine is eliminated. Infants do this automatically, using spinal cord reflex circuitry that is suppressed later in life. For adults, entering this elimination mode is more complicated because decisions need to be made about appropriate times and places to urinate. The periaqueductal gray sums up inputs from the spinal cord (tension in the bladder wall), the hypothalamus, and cerebral cortex, then passes the decision along to the pontine micturition center, which executes it (Fig. 23-6B).

Figure 23-6 Control of micturition. A, In storage mode, sympathetic innervation relaxes the detrusor and contracts the internal sphincter. At the same time, lower motor neurons in S2 cause contraction of the external sphincter. B, In elimination mode, all of this is reversed by descending signals from the pontine micturition center.

Damage at various levels of the CNS causes an array of deficits in bladder control analogous to those affecting skeletal muscle. Lesions of the sacral spinal cord or cauda equina are much like lower motor neuron damage: The detrusor is unable to contract, so the bladder expands under relatively low pressure. Eventually enough pressure builds up for some urine to be expelled, but the bladder never empties. Damage between the midthoracic spinal cord and the pontine micturition center, in contrast, is like an upper motor neuron lesion. Spinal cord reflexes reemerge and become hyperactive, so the bladder tries to empty itself even when the volume is low. The external sphincter never gets a signal from the pontine micturition center telling it to relax, however, so the detrusor has to contract really hard to produce enough pressure to overcome it. Damage above the pons leaves all this circuitry intact but can remove some inputs to the pontine micturition center, causing a variety of problems like increased urgency or diminished social awareness.

The Hippocampus and Amygdala Are the Central Components of the Two Major Limbic Subsystems

The limbic system includes both cortical areas and noncortical structures and is divided into two subsystems, one centered on the hippocampus and the other on the amygdala. Each uses different areas of cerebral cortex as part of the bridge to multimodal association areas. The hippocampal subsystem is primarily involved in learning and memory, as described in Chapter 24.

The Amygdala Is Centrally Involved in Emotional Responses

The amygdala, a collection of nuclei located in the temporal lobe at the anterior end of the hippocampus, is a key link between experiences and emotional reactions to them. It receives a great deal of sensory information of all sorts, from the brainstem, thalamus, hypothalamus, olfactory bulb, and unimodal association areas (Fig. 23-7). It also receives inputs from anterior parts of the limbic lobe and nearby areas, including orbital and anterior temporal cortex and the insula. Connections with the hypothalamus travel through both the stria terminalis and a more diffuse pathway that passes underneath the lenticular nucleus. The stria terminalis, like the fornix, curves around with the lateral ventricle, but in this case travels just medial to the caudate nucleus; for much of its course it lies in the groove between the caudate nucleus and the thalamus.

Figure 23-7 Inputs to the amygdala.

Emotional experiences are accompanied by conscious awareness of the emotion, autonomic reactions, and heightened awareness of ongoing events, and outputs from the amygdala (Fig. 23-8) help mediate all of these. Outputs to anterior limbic cortex, both directly and through the dorsomedial nucleus of the thalamus, contribute to conscious experience. Those to the hypothalamus and to brainstem visceral nuclei mediate autonomic responses. Those to sensory cortical areas are even more extensive than inputs and help to enhance performance when the pressure is on. In addition, outputs to the ventral striatum get the basal ganglia into the act, and outputs to the hippocampus affect the probability that an event will be remembered (see Chapter 24).

Figure 23-8 Outputs from the amygdala.