Overview of Hypothalamic Cell Groups
The hypothalamus consists of a complex assemblage of cell groups. The borders of these cell groups often are not quite as distinct as those shown in the drawings, but the different cell groups are also distinguished based upon their neurotransmitters, functions, and connections.
|GENERAL TOPOGRAPHY OF THE HYPOTHALAMUS|
In general, the hypothalamus can be divided into three tiers of nuclei. Most medially, along the wall of the third ventricle, is the periventricular nucleus, shown here in green. Along the base of the periventricular nucleus is an expansion laterally along the edge of the median eminence, known as the arcuate or infundibular nucleus. The periventricular stratum contains many neurons that make releasing or release-inhibiting hormones (see Plate 5-6) and whose axons end on the capillary loops of the hypophysial portal vessels in the median eminence. Many axons from the brainstem run through the periventricular gray matter, in the posterior longitudinal fasciculus, and into the periventricular region of the hypothalamus.
The next tier of nuclei is sometimes called the medial tier. These nuclei are generally involved in intrinsic connections within the hypothalamus that allow integration of various functions. The most rostral of the medial nuclei is the medial preoptic region (orange), which sits along the wall of the third ventricle as it opens. Along the anterior wall of the third ventricle is the median preoptic nucleus (not shown here). These two cell groups are involved in integrating control of body temperature with fluid and electrolyte balance, wake-sleep cycles, and reproductive function.
The next most caudal region is called the anterior hypothalamic area (purple). At the base of the anterior hypothalamic area, just above the optic chiasm, is the suprachiasmatic nucleus (see Plate 5-5). These structures are involved in regulating circadian rhythms. The suprachiasmatic nucleus is the body’s main biologic clock, and it sets the timing of rhythms of sleep, feeding, body temperature, and reproduction. These functions are controlled by means of outputs to the portion of the anterior hypothalamic area between the suprachiasmatic nucleus and the paraventricular nucleus (blue), called the subparaventricular zone.
The supraoptic and paraventricular nuclei are also at this anterior level in the medial tier. Both nuclei contain large numbers of oxytocin and vasopressin neurons, whose axons travel through the pituitary stalk in the tuberohypophysial tract, to the posterior pituitary gland, where they release their hormones into the circulation. The paraventricular nucleus also contains neurons that make releasing hormones (especially corticotrophic-releasing hormone) and project to the median eminence. A third population of neurons in the paraventricular nucleus sends axons through the medial forebrain bundle in the lateral hypothalamus to the brainstem and spinal cord, to control both the sympathetic and parasympathetic nervous systems. Many of these neurons use either oxytocin or vasopressin as a central neurotransmitter in this autonomic pathway, but they are an entirely separate set of neurons from those that send axons to the posterior pituitary gland.
Just caudal to the anterior hypothalamic area, in the tuberal level of the hypothalamus, the medial tier contains three cell groups. The ventromedial nucleus (tan) sits just above the median eminence and is mainly involved in feeding, aggression, and sexual behavior. The dorsomedial nucleus (yellow), which is just dorsal to it, has extensive outputs to much of the rest of the hypothalamus. The subparaventricular zone sends cir-cadian outputs to both the dorsomedial and ventromedial nuclei, and the dorsomedial nucleus uses this input to organize circadian cycles of wake-sleep, corticosteroid secretion, feeding, and other behaviors. The dorsal hypothalamic area, just above the dorsomedial nucleus, contains neurons that are involved in regulating body temperature.
At the most posterior end of the hypothalamus, the mammillary bodies form a prominent pair of protuberances along the base of the brain. Despite having very clear-cut, heavily myelinated connections, the function of the mammillary nuclei remains mysterious. They receive a major brainstem input from the mammillary peduncle and a large bundle of efferents from the hippocampal formation through the fornix. The large fiber bundle that emerges from the mammillary body splits into a mammillotegmental tract to the brainstem and a mammillothalamic tract to the anterior thalamic nucleus. Neurons in the mammillary body appear to be concerned with head position in space, and may be related to hippocampal circuits that remember the positions of objects in space (so-called place cells). However, lesions of the mammillary bodies in primates have relatively subtle effects on memory.
The lateral tier of the hypothalamus includes the lateral preoptic and lateral hypothalamic areas. These regions are traversed by the medial forebrain bundle, which connects the brainstem below with the hypothalamus and the forebrain above. Many neurons in the lateral hypothalamic area project through the medial forebrain bundle, either to the basal forebrain or cerebral cortex, or to the brainstem or spinal cord. Among these are the neurons that contain the peptides orexins (also known as hypocretins) or melanin-concentrating hormone (MCH). These neurons are involved in regulating wake-sleep cycles as well as metabolism, feeding, and other types of motivated behaviors. Loss of the orexin neurons causes the disorder known as narcolepsy (see Plate 5-22).
|OVERVIEW OF HYPOTHALAMIC NUCLEI|
At the posterior hypothalamic level, there is also a cluster of histaminergic neurons, called the tuberomammillary nucleus, in the lateral hypothalamus adjacent to the mammillary body. These neurons play a role in regulation of wakefulness and body temperature and have projections from the cerebral cortex to the spinal cord. The posterior hypothalamic area sits just above the mammillary body. In humans, many of the orexin, MCH, and histaminergic neurons are found in this region.