Physiology 5th Ed.

HYPOTHALAMIC-PITUITARY RELATIONSHIPS

The hypothalamus and pituitary gland function in a coordinated fashion to orchestrate many of the endocrine systems. The hypothalamic-pituitary unit regulates the functions of the thyroid, adrenal, and reproductive glands and also controls growth, milk production and ejection, and osmoregulation. It is important to visualize the anatomic relationships between the hypothalamus and the pituitary because these relationships underlie the functional connections between the glands.

The pituitary gland, which also is called the hypophysis, consists of a posterior lobe and an anterior lobe. The posterior lobe (or posterior pituitary) is also called the neurohypophysis. The anterior lobe (or anterior pituitary) is also called the adenohypophysis. The hypothalamus is connected to the pituitary gland by a thin stalk called the infundibulum. Functionally, the hypothalamus controls the pituitary gland by both neural and hormonal mechanisms (Fig. 9-9).

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Figure 9–9 Schematic figure showing the relationship between the hypothalamus and the posterior and anterior lobes of the pituitary gland. Pink circles are posterior pituitary hormones; yellow circles are hypothalamic hormones; triangles are anterior pituitary hormones. ADH, Antidiuretic hormone; TRH, thyrotropin-releasing hormone; TSH, thyroid-stimulating hormone.

Relationship of the Hypothalamus to the Posterior Pituitary

The posterior lobe of the pituitary gland is derived from neural tissue. It secretes two peptide hormones, antidiuretic hormone (ADH) and oxytocin, which act on their respective target tissues—the kidney, the breast, and the uterus.

The connections between the hypothalamus and the posterior lobe of the pituitary are neural. In fact, the posterior pituitary is a collection of nerve axons whose cell bodies are located in the hypothalamus. Thus, the hormones secreted by the posterior lobe (ADH and oxytocin) are actually neuropeptides; in other words, they are peptides released from neurons.

The cell bodies of ADH- and oxytocin-secreting neurons are located in supraoptic and paraventricular nuclei within the hypothalamus. Although both hormones are synthesized in both nuclei, ADH is primarily associated with supraoptic nuclei and oxytocin is primarily associated with paraventricular nuclei.

Once synthesized in the cell bodies, the hormones (i.e., neuropeptides) are transported down the axons in neurosecretory vesicles and stored in bulbous nerve terminals in the posterior pituitary. When the cell body is stimulated, the neurosecretory vesicles are released from the nerve terminals by exocytosis and the secreted hormone enters nearby fenestrated capillaries. Venous blood from the posterior pituitary enters the systemic circulation, which delivers the hormones to their target tissues.

In summary, the relationship between the hypothalamus and the posterior pituitary is straightforward—a hormone-secreting neuron has its cell body in the hypothalamus and its axons in the posterior lobe of the pituitary.

Relationship of the Hypothalamus to the Anterior Pituitary

The anterior lobe of the pituitary gland is derived from primitive foregut. Unlike the posterior lobe, which is neural tissue, the anterior lobe is primarily a collection of endocrine cells. The anterior pituitary secretes six peptide hormones: thyroid-stimulating hormone (TSH), follicle-stimulating hormone (FSH), luteinizing hormone (LH), growth hormone, prolactin, and adrenocorticotropic hormone (ACTH).

The nature of the relationship between the hypothalamus and the anterior pituitary is both neural and endocrine (in contrast to the posterior lobe, which is only neural). The hypothalamus and anterior pituitary are linked directly by the hypothalamic-hypophysial portal blood vessels, which provide most of the blood supply to the anterior lobe.

There are both long and short hypophysial portal vessels, which are distinguished as follows: Arterial blood is delivered to the hypothalamus via the superior hypophysial arteries, which distribute the blood in a capillary network in the median eminence, called the primary capillary plexuses. These primary capillary plexuses converge to form the long hypophysial portal vessels, which travel down the infundibulum to deliver hypothalamic venous blood to the anterior lobe of the pituitary. A parallel capillary plexus forms from the inferior hypophysial arteries in the lower portion of the infundibular stem. These capillaries converge to form the short hypophysial portal vessels, which deliver blood to the anterior lobe of the pituitary. In summary, the blood supply of the anterior pituitary differs from that of other organs: Most of its blood supply is venous blood from the hypothalamus, supplied by the long and short hypophysial portal vessels.

There are two important implications of the portal blood supply to the anterior lobe of the pituitary: (1) The hypothalamic hormones can be delivered to the anterior pituitary directly and in high concentration, and (2) the hypothalamic hormones do not appear in the systemic circulation in high concentrations. The cells of the anterior pituitary, therefore, are the only cells in the body to receive high concentrations of the hypothalamic hormones.

The functional connections between the hypothalamus and the anterior lobe of the pituitary now can be understood in the context of the anatomic connections. Hypothalamic-releasing hormones and release-inhibiting hormones are synthesized in the cell bodies of hypothalamic neurons and travel down the axons of these neurons to the median eminence of the hypothalamus. Upon stimulation of these neurons, the hormones are secreted into the surrounding hypothalamic tissue and enter the nearby capillary plexus. The blood from these capillaries (now venous blood) drains into the hypophysial portal vessels and is delivered directly to the anterior lobe of the pituitary. There, the hypothalamic hormones act on the cells of the anterior lobe, where they stimulate or inhibit the release of the anterior pituitary hormones. The anterior pituitary hormones then enter the systemic circulation, which delivers them to their target tissues.

The hypothalamic-anterior pituitary relationship can be illustrated by considering the TRH–TSH–thyroid hormone system. TRH is synthesized in hypothalamic neurons and secreted in the median eminence of the hypothalamus, where it enters capillaries and then hypophysial portal vessels. It is delivered in this portal blood to the anterior lobe of the pituitary, where it stimulates TSH secretion. TSH enters the systemic circulation and is delivered to its target tissue, the thyroid gland, where it stimulates secretion of thyroid hormones.