13. The Hypothalamus
Revised August 4, 2008
The objectives of this chapter are to:
- Describe and identify the anatomic features of the hypothalamus
- Describe the efferent connections of the hypothalamus with the autonomic nervous system.
- Compare neuroendocrine secretion and control in the anterior and posterior pituitary.
The hypothalamus, occupying just 4 grams of the adult human brain, regulates many functional systems that support life, including energy metabolism, food intake and body weight; fluid and electrolyte balance, thirst, and water intake; body temperature; immune response; circadian (24-hour) rhythms and sleep-wake cycles; and reproduction. Most of this regulation is achieved via hypothalamic control of the autonomic and endocrine systems, to be discussed below.
As described earlier, in chapters 1 and 2, the only parts of the hypothalamus that appear on the surface of the brain are the tuber cinereum and the paired mammillary bodies (#4737). This ventral area is outlined by the optic chiasm, optic tract, and interpeduncular fossa).
As seen in midsagittal sections, (fig 13a) the hypothalamic sulcus separates the hypothalamus from the thalamus. The rostral boundary of the hypothalamus is indicated by a line drawn between the anterior commissure (#4835) and the optic chiasm. Actually, this line corresponds to a thin wall of neural tissue called the lamina terminalis (#4525), which is the anterior wall of the third ventricle. The caudal boundary of the hypothalamus is approximated by a line extending from the posterior margin of the mammillary bodies to the posterior commissure.
On coronal sections (fig 13b), the fornix divides the hypothalamus into medial and lateral zones. The medial forebrain bundle (#11857) runs longitudinally through the lateral zone. This complex fiber group is a bidirectional pathway that connects the forebrain, hypothalamus, and brain stem. Transverse sections through the caudal hypothalamus show the mammillary bodies (#11860).
MOVIE Hypothalamus in relation to the ventricles (the hypothalamus is colored yellow)
Most of the functions of the hypothalamus are achieved via autonomic regulation and endocrine regulation.
I. Autonomic Regulation
The hypothalamus receives information from visceral sensory systems, especially via the vagus nerve and solitary nucleus.
Attention will be directed to how the hypothalamus, through its connections with preganglionic neurons, affects the autonomic nervous system. The paraventricular nucleus and adjacent lateral hypothalamus project to autonomic preganglionic neurons in the brain stem and spinal cord. This pathway begins in the medial forebrain bundle and continues into the lateral brain stem tegmentum (#6609). The axons to spinal preganglionic neurons form the hypothalamospinal tract, a central autonomic pathway. It descends through the cord in the dorsal part of the lateral funiculus.
Recall that the preganglionic cell bodies are located in specific cell groups in the CNS and that postganglionic cell bodies are located in autonomic ganglia in the PNS. The preganglionic cell bodies of the parasympathetic (craniosacral) division are in the Edinger-Westphal nucleus, superior salivatory nucleus, inferior salivatory nucleus, the dorsal motor nucleus of X, and in the intermediolateral cell column of sacral segments S-2, 3, and 4. Where are each of these nuclei located? Engrave on your heart that cranial nerves III, VII, IX, and X (#4184) contain preganglionic parasympathetic axons. Where are the preganglionic neurons of the sympathetic division located (fig 13d)?
Lesions affecting the hypothalamospinal tract in the brain stem or cervical cord cause Horner's syndrome. What is this syndrome and why is it caused by interruption of these fibers? One classical location for such a lesion is the dorsolateral aspect of the rostral medulla. Name the resultant syndrome. What other tracts and nuclei are affected in this syndrome, and what signs and symptoms occur?
II. Endocrine Regulation
A. The Hypothalamus and Anterior Pituitary
The hypothalamus controls the release of anterior pituitary hormones. Small neurons in the arcuate nucleus (in the tuber cinereum), periventricular zone and paraventricular nucleus produce releasing hormones and release-inhibiting hormones, e.g., growth hormone-releasing hormone and somatostatin, which inhibits release of growth hormone. These hormones are transported from the cell bodies down the axons in the short tuberoinfundibular tract into the median eminence (#11861) in the infundibulum ("funnel") at the junction of the tuber cinereum with the pituitary stalk.
The tuberoinfundibular tract axons end on the fenestrated capillaries in the median eminence (fig 13e). The hypophysial portal system connects the fenestrated capillaries of the median eminence with the fenestrated capillaries of the anterior pituitary (#9904 a, #6618). Therefore, releasing and release-inhibiting hormones discharged from axon terminals in the median eminence flow through the hypophysial portal system into the anterior pituitary. They stimulate or inhibit the release of the corresponding anterior pituitary hormones. The blood supply to this region can be seen in fig 13f.
B. The Hypothalamus and Posterior Pituitary.
Fig 13g shows the supraoptic nucleus (#11863) and the paraventricular nucleus (#11918) in the anterior hypothalamus. They stand out because their cell bodies are relatively large and deeply stained (#41909, as seen in the rat). The supraoptic nuclei are named for their location above the optic tract. The paraventricular nuclei are also named for their location, which is near the wall of the third ventricle. These neurons manufacture vasopressin (antidiuretic hormone, ADH) and oxytocin. The hormones are transported down the axons of the hypothalamo-neurohypophysial tract to the posterior pituitary (#9904 b, #5770) where the axons end on fenestrated capillaries (Fig 13h). Vasopressin and oxytocin are stored in the axon terminals and, in response to nerve impulses are released and enter the capillaries and, thus, the systemic circulation.
The supraoptic and paraventricular nuclei receive afferents directly and indirectly from several areas of the CNS that influence the neurosecretion of vasopressin and oxytocin. For example, near the supraoptic nuclei are neurons that act as osmoreceptors and detect change in the osmolality of the blood. Hyperosmolality leads to release of vasopressin, resulting in increased water retention in the kidney. Low vasopressin secretion results in increased output of dilute urine (polyuria), severe thirst, and increased drinking (polydipsia). This condition is called diabetes insipidus.
Stimulation of mechanoreceptors in the breast of a lactating mother reflexly causes the release of oxytocin, which causes contraction of the myoepithelial cells in the breast resulting in milk ejection. How does information from the mechanoreceptors in the nipple reach the supraoptic and paraventricular nuclei (#11913)?
- Hypothalamus in relation to the ventricles (same movie as above, the hypothalamus is colored yellow)
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