Describe the physiologic and anatomic relationships between the hypothalamus and the anterior and the posterior pituitary.
Understand the integration of hypothalamic and pituitary function and identify the 2 different pathways used for hypothalamic–pituitary interactions.
Identify the appropriate hypothalamic releasing and inhibitory factors controlling the secretion of each of the anterior pituitary hormones.
Differentiate between the routes of transport of hypothalamic neuropeptides to the posterior and anterior pituitary.
Identify the mechanisms that control the release of oxytocin and arginine vasopressin (AVP).
Understand the physiologic target organ responses and the cellular mechanisms of oxytocin and AVP action.
The hypothalamus is the region of the brain involved in coordinating the physiologic responses of different organs that together maintain homeostasis. It does this by integrating signals from the environment, from other brain regions, and from visceral afferents and then stimulating the appropriate neuroendocrine responses. In doing so, the hypothalamus influences many aspects of daily function, including food intake, energy expenditure, body weight, fluid intake and balance, blood pressure, thirst, body temperature, and the sleep cycle. Most of these hypothalamic responses are mediated through hypothalamic control of pituitary function (Figure 2–1). This control is achieved by 2 mechanisms: (1) release of hypothalamic neuropeptides synthesized in hypothalamic neurons and transported through the hypothalamo-hypophyseal tract to the posterior pituitary, and (2) neuroendocrine control of the anterior pituitary through the release of peptides that mediate anterior pituitary hormone release (hypophysiotropic hormones) (Figure 2–2). Because of this close interaction between the hypothalamus and the pituitary in the control of basic endocrine physiologic function, they are presented as an integrated topic.
Anatomical and functional relationship between the hypothalamus and the pituitary. The hypothalamus is anatomically and functionally linked with the anterior and posterior pituitary. They are closely related because of the portal system of blood supply. The superior, medial, and inferior hypophyseal arteries provide arterial blood supply to the median eminence and the pituitary. Magnocellular neurons of the supraoptic and paraventricular nuclei have long axons that terminate in the posterior pituitary. The axons of parvicellular neurons terminate in the median eminence where they release their neuropeptides. The long portal veins drain the median eminence, transporting the peptides from the primary capillary plexus to the secondary plexus that provides blood supply to the anterior pituitary. The release of hypothalamic neuropeptides is regulated by afferent signals from other brain regions, from visceral afferents by circulating levels of substrates and hormones. The sleep/awake state of the individual, the light variations, noise, fear, anxiety, and visual images all are examples of signals that are integrated by the hypothalamus and that are involved in the regulation of hypothalamic neuropeptide release and control of pituitary function. The hormones released from the anterior and posterior pituitary regulate vital body functions maintaining homeostasis.
(Adapted with permission from Melmed S: Medical progress: Acromegaly, N Engl J Med