The Hypothalamus Is Composed of Anatomically Distinct Nuclei

The diencephalon includes the hypothalamus, thalamus, and subthalamus (Fig. 7.1). The rostral border of the hypothalamus is at the optic chiasm, and its caudal border is at the mammillary body.

On the basal surface of the hypothalamus, exiting the median eminence, the pituitary stalk contains the hypo-thalamo-hypophyseal portal blood vessels (see Fig. 32.3). Neurons within specific nuclei of the hypothalamus secrete releasing factors into these portal vessels. The releasing factors are then transported to the anterior pituitary, where they stimulate secretion of hormones that are trophic to other glands of the endocrine system (see Chapter 32).

The pituitary stalk also contains the axons of magnocel-lular neurons whose cell bodies are located in the supraop-tic and paraventricular hypothalamic nuclei. These axons form the hypothalamo-hypophyseal tract within the pituitary stalk and represent the efferent limbs of neuroendocrine reflexes that lead to the secretion of the hormones vasopressin and oxytocin into the blood. These hormones are made in the magnocellular neurons and released by their axon terminals next to the blood vessels within the posterior pituitary.

The nuclei of the hypothalamus have ill-defined boundaries, despite their customary depiction (Fig. 7.2). Many are named according to their anatomic location (e.g., anterior hypothalamic nuclei, ventromedial nucleus) or for the structures they lie next to (e.g., the periventricular nucleus surrounds the third ventricle, the suprachiasmatic nucleus lies above the optic chiasm).

Hypothalamus Optic Chiasm

^HIGUREHI^fc A midsagittal section through the human ^Btmmil^B^ brain, showing the most prominent structures of the brainstem (gray), diencephalon (red), and fore-brain (white). The cerebellum is also shown. (Modified from Kandel ER, Schwartz JH, Jessel TM. Principles of Neural Science. 3rd Ed. New York: Elsevier, 1991.)

^HIGUREHI^fc A midsagittal section through the human ^Btmmil^B^ brain, showing the most prominent structures of the brainstem (gray), diencephalon (red), and fore-brain (white). The cerebellum is also shown. (Modified from Kandel ER, Schwartz JH, Jessel TM. Principles of Neural Science. 3rd Ed. New York: Elsevier, 1991.)

The hypothalamus receives afferent inputs from all levels of the CNS. It makes reciprocal connections with the limbic system via fiber tracts in the fornix. The hypothalamus also makes extensive reciprocal connections with the brainstem, including the reticular formation and the medullary centers of cardiovascular, respiratory, and gastrointestinal regulation. Many of these connections travel within the medial forebrain bundle, which also connects the brainstem with the cerebral cortex.

Several major connections of the hypothalamus are oneway rather than reciprocal. One of these, the mammil-lothalamic tract, carries information from the mammillary bodies of the hypothalamus to the anterior nucleus of the thalamus, from where information is relayed to limbic regions of the cerebral cortex. A second one-way pathway carries visual information from the retina to the suprachi-asmatic nucleus of the hypothalamus via the optic nerve. Through this retinal input, the light cues of the day/night cycle entrain or synchronize the "biological clock" of the brain to the external clock. A third one-way connection is the hypothalamo-hypophyseal tract from the supraoptic and paraventricular nuclei to the posterior pituitary gland. The hypothalamus also projects directly to the spinal cord to activate sympathetic and parasympathetic preganglionic neurons (see Chapter 6).

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Essentials of Human Physiology

Essentials of Human Physiology

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Responses

  • yrj
    Where is the optic chiasm located?
    8 years ago
  • baldovino
    What structure surrounds the pituitary gland?
    8 years ago

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