Relationship between hypothalamus and pituitary gland

The hypothalamus plays a very important role in the maintenance of homeo-stasis. It carries out this function, in large part, by regulating the activities of the neurohypophysis and the adenohypophysis. For example, the hypothalamus processes signals from other regions of the nervous system including information regarding pain and emotional states such as depression, anger, and excitement. In addition, because it is not protected by the blood-brain barrier, it monitors the composition of the blood and helps to regulate the concentration of nutrients, electrolytes, water, and hormones. In other words, it is an important processing center for information concerning the internal environment. This information is then used to control the release of hormones from the pituitary. Due to their embryonic origins, the neurohypophysis and the adenohypophysis are regulated by the hypothalamus, using two very different mechanisms:

• Neuronal signals

• Hormonal signals

As discussed previously, the neurohypophysis has a direct anatomical connection to the hypothalamus. Therefore, the hypothalamus regulates the release of hormones from the neurohypophysis by way of neuronal signals. Action potentials generated by the neurosecretory cells originating in the hypothalamus are transmitted down the neuronal axons to the nerve terminals in the neurohypophysis and stimulate the release of the hormones into the blood. The tracts formed by these axons are referred to as hypothalamic-hypophyseal tracts (see Figure 10.2). The action potentials are initiated by various forms of sensory input to the hypothalamus. Specific forms of sensory input that regulate the release of ADH and oxytocin are described in subsequent sections in this chapter.

The adenohypophysis does not have a direct anatomical connection with the hypothalamus; therefore, regulation of hormone secretion by way of neuronal signals is not possible. Instead, these two structures are associated by a specialized circulatory system and the secretion of hormones from the adenohypophysis is regulated by hormonal signals from the hypothalamus (see Figure 10.2). Systemic arterial blood is directed first to the hypothalamus. The exchange of materials between the blood and the interstitial fluid of the hypothalamus takes place at the primary capillary plexus. The blood then flows to the adenohypophysis through the hypothalamic-hypophyseal portal veins. Portal veins are blood vessels that connect two capillary beds. The second capillary bed in this system is the secondary capillary plexus located in the adenohypophysis.

Located in close proximity to the primary capillary plexus in the hypothalamus are specialized neurosecretory cells. In fact, the axons of these cells terminate on the capillaries. The neurosecretory cells synthesize two types of hormones: releasing hormones and inhibiting hormones (see Table 10.2). Each of these hormones helps to regulate the release of a particular hormone from the adenohypophysis. For example, thyrotropin-releasing hormone produced by the neurosecretory cells of the hypothalamus stimulates secretion of thyrotropin from the thyrotrope cells of the adenohypophysis. The hypo-thalamic-releasing hormone is picked up by the primary capillary plexus; travels through the hypothalamic-hypophyseal portal veins to the anterior pituitary; leaves the blood by way of the secondary capillary plexus; and exerts its effect on the appropriate cells of the adenohypophysis. The hypo-physeal hormone, in this case, thyrotropin, is then picked up by the secondary capillary plexus, removed from the pituitary by the venous blood, and delivered to its target tissue.

A noteworthy feature of this specialized circulation is that the regulatory hypothalamic hormones are delivered directly to the adenohypophysis by the portal system. Therefore, the concentration of these hormones remains very high because they are not diluted in the blood of the entire systemic circulation.

Table 10.2 Summary of Major Hormones

Location

Hormone

Target tissues

Hypothalamus

Adenohypophysis

Releasing and inhibiting hormones (GnRH, TRH, CRH, PRF, PIH, GHRH, GHIH)

Antidiuretic hormone ( ADH) Oxytocin

Follicle-stimulating hormone (FSH)

Luteinizing hormone (LH)

Adenohypophysis

Kidney

Arterioles

Uterus

Mammary glands Females: ovaries

Males: testes Females: ovaries

Thyroid-stimulating hormone (TSH) Adrenocorticotropic hormone (ACTH) Prolactin (PRL) Growth hormone (GH)

Males: testes Thyroid gland

Adrenal cortex

Mammary glands Bone, visceral tissues

Thyroid gland

Triiodothyronine (T3) Tetraiodothyronine (T 4)

Most tissues

Major functions of hormone

Control of release of hormones from the adenohypophysis

Promotes reabsorption of water Vasoconstriction Contraction of smooth muscle Ejection of milk

Development of follicles; secretion of estrogen Spermatogenesis

Rupture of follicle and ovulation; secretion of estrogen and progesterone from corpus luteum Secretion of testosterone Secretion of thyroid hormones (T 3,T4)

Secretion of Cortisol

Breast development; lactation Growth of skeleton and visceral tissues; increase blood glucose; protein synthesis; increase blood fatty acids Growth and maturation, normal neurological development and function; increase in metabolic rate

Parathyroid glands Adrenal medulla Adrenal cortex

Pancreas

Calcitonin

Parathyroid hormone (PTH)

Epinephrine and norepinephrine Mineralocorticoids

(aldosterone) Glucocorticoids (Cortisol)

Androgens Insulin

Glucagons

Bone

Bone, kidneys, intestine

Adrenergic receptors throughout the body Kidney

Most tissues

Various tissues Most tissues

Most tissues

Decrease blood calcium Increase blood calcium; decrease blood phosphate; activation of vitamin D 3 "Fight-or-flight" response; reinforces effects of the sympathetic nervous system Reabsorption of sodium; excretion of potassium

Increase blood glucose and fatty acids; adaptation to stress Secondary sex characteristics in females Cellular uptake, utilization and storage of glucose, fatty acids and amino acids Increase blood glucose and fatty acids

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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