Review of Medical Physiology - William F. Ganong 2002

Functions of the Nervous System
Central Regulation of Visceral Functions
Hypothalamus — Anatomical Details

The Hypothalamus (Fig. 14-2) is part of the anterior region of the Diencephalon, located rostral to the interpeduncular nuclei and ventral to the hypothalamic sulcus. It contains A number of nuclei and nuclear areas.

Connection with the Pituitary Gland

There are neural connections between the hypothalamus and the posterior pituitary, as well as both neural and vascular connections with the anterior pituitary. Embryologically, the posterior pituitary develops from an evagination of the floor of the Third ventricle. It is formed primarily by the terminal segments of axons extending from Cell bodies in the supraoptic and paraventricular nuclei (Fig. 14-3) via the hypothalamohypophysial tract. The vast majority of supraoptic fibers terminate in the posterior pituitary, whereas some paraventricular fibers end in the median eminence. Both the anterior pituitary and the pars intermedia develop embryonically from Rathke's pouch, an evagination of the roof of the Pharynx (see Fig. 22-1). Sympathetic nerve fibers enter the anterior lobe through its capsule, and parasympathetic fibers branch off from the petrosal nerves, though some nerve fibers also reach it via the hypothalamus. The hypophysial portal vessels provide a direct vascular link between the hypothalamus and the anterior pituitary. Arterial branches arising from the Internal Carotid Artery and the circle of Willis form a fenestrated capillary network—the primary plexus—on the ventral surface of the hypothalamus (see Fig. 14-2). Loops of these capillaries also penetrate the median eminence. The capillaries drain into the Vessels of the hypophysial portal system, through which Blood flows via the pituitary stalk to the capillaries of the anterior lobe. This system begins and ends in capillaries; its blood does not pass through The Heart and, therefore, constitutes a true portal system. In birds and certain mammals, including humans, there is no other arterial Blood supply to the anterior lobe other than the portal vessels and anastomoses with the capillaries of the posterior lobe.

In other mammals, a portion of the blood reaches the anterior lobe via a separate network of anterior hypophysial Arteries, although the majority still flows through the portal vessels. The median eminence is generally considered part of the ventral hypothalamus, from which the portal vessels originate. This area lies outside the blood-Brain barrier (see Chapter 32).

Afferent and Efferent Connections of the Hypothalamus

The major afferent and efferent Neural Pathways to and from the hypothalamus consist primarily of unmyelinated fibers. There are extensive connections between the hypothalamus and the limbic system, as well as crucial pathways linking the hypothalamus with the tegmental nuclei of the Midbrain and Hindbrain.

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Fig. 14-2. The human hypothalamus with a schematic representation of the hypophysial portal vessels.

Norepinephrine-secreting Neurons with cell bodies located in the hindbrain terminate in numerous Regions of the hypothalamus (see Fig. 15-6); paraventricular Nucleus neurons, which presumably secrete oxytocin and vasopressin respectively, project to the hindbrain and Spinal Cord; and epinephrine-secreting neurons with cell bodies in the hindbrain terminate in the ventral hypothalamus.

Additionally, there is an intrahypothalamic dopaminergic neuronal system with cell bodies in the arcuate nucleus and terminals ending on (or near) the capillaries that form the portal vessels of the median eminence. Serotonergic neuronal projections extend to the hypothalamus from the reticular nuclei of the pontine raphe.