A Concept of Hypothalamic Functioning

A CONCEPT OF HYPOTHALAMIC FUNCTIONING EVERETT W. BOVARD* The hypothalamus can be considered the final common pathway for environmental (i), and therefore neural, influence on the anterior pituitary . Hypothalamic areas found experimentally to control endocrine release from the anterior pituitary are shown in Figure I. On the basis of stimulation and lesion studies, the hypothalamus may be divided roughly into two functional zones. The tear-shaped dark area to the right in Figure ? is the posterior hypothalamic zone, extending from the mammillary bodies in an anterior and medial direction to the median eminence of the tuber cinereum, just above the pituitary. Stimulation of this posterior zone has been found by Hess (2) and others to elicit sympathetic autonomic reactions, such as a rise in blood pressure and dilatation ofthe pupils, andto increase general excitability ofthe motor system (2, 3). Stimulation and lesion studies plus recording of electrical activity under stress have suggested (1, 4, 5) that this posterior zone ofthe hypothalamus mediates both the pituitary adrenal-cortical and the sympathetic adrenalmedullary components ofthe response to stress (Fig. 1). More specifically, the pituitary-adrenal response to emotional or neurogenic stress (that is, stress that must necessarily be mediated by the central nervous system) would appear (1) to be mediated by release ofa humoral substance from nerve fiber endings in the median eminence into the primary capillary plexus (see Fig. 1) ofthe hypothalamic-hypophysial portal vein system. This substance is then presumably carried by the portal veins to the anterior pituitary, where it precipitates release of adrenocortico- * Department of Anatomy, Albert Einstein College of Medicine, New York 61, New York. This work, done at Montreal Neurological Institute, was made possible by a special research fellowship of the United States Public Health Service, sponsored by the National Institute of Mental Health. The writer would like to acknowledge with thanks the encouragement and special help of Dr. Peter Gloor, together with that of Dr. H. Koikegami on the neuroanatomy of the hypothalamus . 52 Everett W. Bovard · Hypothalamic Functioning Perspectives in Biology and Medicine · Autumn 1961 tropic hormone (ACTH) into the blood stream at a rate above the normal resting level ofACTH secretion. Final coordination and integration of neural impulses from different sources that influence the anterior pituitary in emotional stress could be provided by a dense matrix offine, unmyelinated nerve fibers located in the catjust above the median eminence in the arcuate nucleus, according to Metuzals (6). As he suggests, interaction between single nerve fibers Fig. i.—The hypothalamus and pituitary in median sagittal section. Shown are hypothalamic sites found through stimulation and lesion experiments to be associated with hormonal release into the blood stream. In the response to stress, ACTH is released from the anterior pituitary, probably under stimulus of a chemical transmitter carried from the median eminence by the portal veins (see diagram ), and then itselfstimulates release ofadrenal cortical hormones, such as cortisone and hydrocortisone , from the adrenal cortex. The posterior hypothalamic areas whose activity has been found to be associated with ACTH release are shown above. An equally important part ofthe response to stressisthe activity ofsympathetic areasin the posterior hypothalamus which, through thesplanchnic nerve, trigger therelease ofepinephrine and nor-epinephrine from the adrenal medulla. The posterior hypothalamic zone (to the right, tear-shaped) is shadowed inthe diagram; the anterior hypothalamic zone (to the left, oval) is light. Note the zone of overlap between the two areas near the median eminence,just abovethe pituitary. Note also that the presumed hypothalamic areafor stimulation of growth hormone (somatotropin, STH) release from the anterior pituitary is located in the anterior hypothalamus. Release ofoxytocichormone (OXT) and anti-diuretichormone (ADH) from the posterior lobe of the pituitary is believed to be triggered through neural connections of anterior hypothalamic nuclei with this lobe of the pituitary. Shown also are hypothalamic sites whose activity has been found to be related to release of gonadotropic hormone (GTH) and thyrotropic hormone (TSH) from the anterior pituitary. This diagram was prepared by Dr. and Mrs. Peter Gloor ofthe Montreal Neurological Institute and is reproduced with their kind permission. 53 crossing each other in this matrix or neuropil could provide for rapid and wide dispersion ofneural impulses in all directions. But the response...