The medial zone is divided into three regions: supraoptic, tuberal, and mammillary. There are four nuclei in the supraoptic region, which play a role in thermoregulation, osmoregulation, and the sleep–wake cycle. The supraoptic and paraventricular nuclei in this region synthesize antidiuretic hormone or vasopressin, which stimulates water uptake and oxytocin for stimulation of uterine contractions and lactation in the mammary glands. Axons from these nuclei are conveyed by the hypothalamohypophyseal tract to the posterior pituitary for release into the circulatory system (eFig. 9.89). A third nuclear group in this region is the suprachiasmatic nucleus. This nucleus receives input directly from the retina to influence circadian rhythms, which contribute to the light–dark cycle. The anterior nucleus is the final group and functions predominantly in regulating body temperature.
The tuberal region in the medial zone contains three nuclei: ventromedial, dorsomedial, and arcuate. The largest and best defined is the ventromedial nucleus, which functions as a satiety center to decrease feeding behavior. Posterior to the ventromedial nucleus is the dorsomedial nucleus, which functions in the behavioral expression of rage or aggressive behavior. Finally, the arcuate nucleus serves as a center for releasing hormones, which are transmitted by the tuberoinfundibular tract and hypophysial portal system to the anterior pituitary (eFig. 9.89).
The mammillary region or mammillary body is the final group of nuclei in the medial zone. Four nuclei comprise this region: medial, intermediate, lateral mammillary, and posterior hypothalamic. The best defined is the medial mammillary nucleus, as it is the primary site for the termination of axons from the postcommissural fornix. This pathway originates from the subiculum of the hippocampal complex and plays a key role in memory. The medial mammillary nuclei also connects to structures of the limbic system.
The periventricular zone resides medial to the medial zone and adjacent to the ependymal cells of the third ventricle. Neurons from this zone predominantly synthesize releasing hormones. Axons from these neuronal cells project through the tuberoinfundibular tract to the hypophysial portal system to influence release of hormones from the anterior pituitary.
Through the review of the hypothalamus thus far, it is apparent that this small, 4-gram structure has a significant role in regulating visceral, endocrine, and behavioral system functions through multiple pathways. It is important to remember the majority of the pathways mentioned represent input–output relationships between the hypothalamus and other structures. For a review of neural and non-neural inputs and outputs, refer to the summary figures (eFig. 9.92A and B).
Part XI: Olfactory and
The sense of olfaction has a role in both pleasurable experiences and survival. The same receptors that allow us to enjoy the food we consume or experience odorants in the environment also help us avoid spoiled food or potentially hazardous situations like a fire. Unlike the other special sensory system pathways, the olfactory sensory pathway is unique in that it does not have a thalamic relay before reaching the primary olfactory cortex. In this section we will review the course of the neurons in the olfactory system and their connection to the limbic system.
Three types of olfactory receptors make up the olfactory epithelium along the lateral and septal walls of the nasal cavity. These cells allow for regeneration (basal stem cells), support (sustentacular cells), and transmission of information (olfactory receptor neurons). Each olfactory receptor neuron has an olfactory vesicle with cilia that contain receptors for odorant molecules and an unmyelinated axon that passes through the cribriform plate to terminate in the olfactory bulb (eFig. 9.93). As the olfactory receptor neurons originate embryologically from the CNS, they are considered part of the CNS and not the PNS.