Keywords
autonomic regulation, akinetic mutism, amygdala, hippocampus, cingulate cortex, pituitary
Chapter Outline
The Hypothalamus Coordinates Drive-Related Behaviors, 150
The Hypothalamus Can Be Subdivided in Both Longitudinal and Medial-Lateral Directions, 150
Hypothalamic Inputs Arise in Widespread Neural Sites, 150
Hypothalamic Outputs Largely Reciprocate Inputs, 151
Perforating Branches From the Circle of Willis Supply the Hypothalamus, 152
The Hypothalamus Collaborates With a Network of Brainstem and Spinal Cord Neurons, 152
Limbic Structures Are Interposed Between the Hypothalamus and Neocortex, 152
The Cingulate Gyrus, Hippocampus, and Amygdala Are Central Components of the Limbic Subsystem, 153
The Cingulate Cortex Acts as the Gateway Between the Limbic System and Neocortex, 153
The Amygdala Is Centrally Involved in Emotional Responses, 153
There is a whole sphere of mental activity that goes beyond simple perception of stimuli and logical formulation of responses. We have drives and urges, and most of our experiences are emotionally colored. This emotional coloring and its relationship with basic drives is the province of the limbic system . The hypothalamus regulates autonomic function and drive-related behavior, and limbic structures serve as bridges between the hypothalamus and neocortex.
The Hypothalamus Coordinates Drive-Related Behaviors
The hypothalamus is a nodal point in the neural circuits underlying drive-related behaviors ( Fig. 23.1 ). It’s got interconnections with visceral parts of the nervous system, through which it is informed of and controls things like blood glucose, blood pressure, and body temperature. The hypothalamus has interconnections with limbic structures, through which you become aware of homeostatic needs (“I’m hungry”). Finally, the hypothalamus has not just neural outputs but also ways to control the pituitary gland .
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).
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
Most inputs from the forebrain arise in limbic structures.
Inputs from the brainstem and spinal cord traverse the medial forebrain bundle and dorsal longitudinal fasciculus.
The hypothalamus contains intrinsic sensory neurons.
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. Finally, some hypothalamic neurons are sensory receptors themselves, directly responsive to temperature, blood osmolality, or the concentration of some chemicals or hormones 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 for autonomic responses (“Better start sweating”). Projections through the medial forebrain bundle and other routes reach the amygdala, septal nuclei, and other limbic structures for cognitive responses (“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 Figs. 22.5 and 22.6 ).
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 (i.e., thyrotropin-releasing hormone, corticotropin-releasing hormone, gonadotropin-releasing hormone, growth hormone-releasing hormone, somatostatin, and dopamine) 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 (i.e., thyroid-stimulating hormone, adrenocorticotropic hormone, follicle-stimulating hormone, luteinizing hormone, growth hormone, and prolactin) that eventually are released into circulation.
