Overview of Hypothalamic Nuclei

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).

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.

Only gold members can continue reading. Log In or Register to continue