This video describes the hormones produced by the hypothalamus and how it controls the pituitary gland and plays a role in homeostasis.
HYPOTHALAMUS AND PITUITARY
The hypothalamus is the control centre of the brain and is involved in many aspects of homeostasis by being part of both the nervous system (composed of neurons in the brain) and the endocrine system (secretes hormones into the bloodstream). The hypothalamus is directly connected to the posterior pituitary gland via neurons and to the anterior pituitary via the hypophyseal portal blood system (Figure 17.5). The hypothalamus produces releasing hormones or inhibiting hormones that travel through the hypophyseal portal system to the anterior pituitary gland, and these hormones then signal the pituitary to produce specific hormones (Table 17.1). The hypothalamus also produces two hormones, oxytocin and antidiuretic hormone (ADH), also called vasopressin, which it transports to the posterior pituitary gland. The posterior pituitary does not produce any hormones itself. The neuronal cell bodies are located in the hypothalamus where the hormones are produced. These hormones are then transported along the axons and released and stored in the axon terminals in the posterior pituitary until they need to be released into the bloodstream. Behind the hypothalamus is the pineal gland that produces the hormone melatonin, which is involved in inducing sleep.
Posterior Pituitary Hormones
Oxytocin Oxytocin is produced in the hypothalamus, stored in the posterior pituitary gland, and released during childbirth. The main functions of oxytocin are to cause uterine contractions and the ejection of breast milk from the mammary glands during breast-feeding. Uterine contractions and oxytocin release is an example of a positive feedback mechanism since the contracting uterus further stimulates the release of more oxytocin, which causes the uterus to contract, and so on. Oxytocin also acts as neuromodulator in the brain. It plays a role in maternal bonding with the infant, bonding and feelings of love for another person, and feelings for another person such as empathy. It also plays a role in orgasm, the regulation of the immune response, and decreases anxiety. Oxytocin’s role in decreasing inflammation and promoting wound healing supports the idea that positive social interactions are good for our health.
Antidiuretic Hormone (ADH) Antidiuretic hormone (ADH), also called vasopressin, is produced in the hypothalamus and stored in the posterior pituitary gland. ADH is released from the posterior pituitary gland when blood volume/blood pressure is low. An antidiuretic has the opposite effect of a diuretic. Diuretics are substances that cause water to be excreted by the kidneys. ADH acts on the distal tubule and collecting ducts of the kidneys to reabsorb water, which restores blood volume and therefore blood pressure. ADH causes blood vessels to constrict to maintain blood pressure during periods of lower blood volume.
Did You Know?
The drug ecstasy is referred to as the “love drug” because it causes the hypothalamus to release large amounts of oxytocin into the blood.
Anterior Pituitary Hormones
Growth Hormone Human growth hormone (GH) (also called somatotropin) is a protein hormone that is released from the anterior pituitary gland in response to GHRH produced by the hypothalamus. The main stimuli for the production of GHRH is deep sleep, high-intensity exercise, periods of increased androgen production (growth periods during childhood and puberty), and fasting (period between meals). Adolescents produce twice as much growth hormone as an adult. Even during adulthood, GH is very important for maintaining healthy cell division to replace old and damaged cells; a lack of GH results in increased signs of aging. As people age, less GH is secreted, and so it is very important to include exercise in a daily routine.
Growth hormone performs the following functions in the body:
- Promoting bone growth (height) in children and adolescents. GH causes calcium to be incorporated into bones, and decreases osteoporosis in adults.
- Stimulating protein synthesis. This occurs particularly in skeletal muscle where protein is required to increase muscle strength.
- Aiding metabolism. GH causes the liver to break down glycogen and fat and thereby increases the level of blood sugar so fuel is available for cells.
- Stimulating the immune system. Stress hormones inhibit growth hormone and inhibit the immune system.
Excessive GH production during childhood and puberty can cause gigantism, too much GH produced after puberty causes acromegaly, and too little GH production causes dwarfism. Where early diagnosis is possible, these conditions are much less common. Dwarfism can now be prevented in children with exogenous growth hormone produced through genetic engineering.
Did You Know?
If you have a mild cold without a fever, moderate- to high-intensity exercise can help you get rid of the cold faster because you will stimulate the production of growth hormone and give your immune system a boost. Exercising while you have an infection that causes a fever will divert too much energy from your immune system and can make you feel worse.
Thyroid-Stimulating Hormone The anterior pituitary gland produces thyroid-stimulating hormone (TSH) in response to the secretion of thyrotropin-releasing hormone (TRH) from the hypothalamus. The hypothalamus releases TRH when the blood levels of thyroid hormones T3 and T4 are low. TSH acts on the thyroid gland and is regulated, through negative feedback, by the concentration of thyroid hormones in the blood; a high level of circulating thyroid hormones inhibits the release of TSH from the anterior pituitary gland.
Did You Know?
Having low blood levels of thyroid hormones can contribute to depression because T3 and T4 play a role in the release of serotonin, also referred to as “the good mood molecule.”
Adrenocorticotropic Hormone The anterior pituitary gland releases adrenocorticotropic hormone (ACTH) in response to the secretion of corticotrophin-releasing hormone (CRH) from the hypothalamus. ACTH acts on the adrenal cortex and regulates the production of glucocorticoids, mainly cortisol, in response to stress. ACTH causes cortisol to be produced by the zona fasciculata region of the adrenal cortex from cholesterol, and it stimulates adrenal cells to take up cholesterol from low-density lipoproteins (LDLs) in the blood.
Follicle-Stimulating Hormone (FSH) and Luteinizing Hormone (LH) FSH and LH have important roles in the stimulation of the gonads during puberty and reproduction. FSH is the primary hormone that stimulates development of the sperm in the testes and oocytes in the ovaries. LH stimulates the gonads to produce the hormones testosterone, estrogen, and progesterone. Males and females secrete all three hormones; however, females have much more of the aromatase enzyme that converts testosterone into estrogen.
Prolactin Prolactin (PRL) is produced in the anterior pituitary gland and released in response to the secretion of prolactin-releasing hormone (PRH), which is produced in the hypothalamus. Prolactin is blocked by prolactin-inhibiting hormone (PIH), also produced in the hypothalamus. The primary role of prolactin is in lactation. During pregnancy it stimulates the growth of the female mammary glands and their production of milk after delivery of the baby. Recall that the posterior pituitary hormone oxytocin causes the ejection of the milk. As progesterone levels decrease at the end of pregnancy, the hypothalamus begins to produce PRH. Prolactin also plays a role in surfactant production in the fetus at the end of the pregnancy, and this helps the mother’s immune system not reject the infant.
Prolactin is produced in both men and women after orgasm, causing a satisfied, relaxed feeling. Inappropriately high prolactin levels can cause impotence in males. In females, prolactin inhibits the production of FSH and LH during lactation after a baby is born. This biological mechanism is meant to prevent another pregnancy before the newborn has developed enough to no longer need the mother’s breast milk for nutrition. However, this inhibition is weak, so some FSH and LH is still produced, and many pregnancies occur while mothers are still nursing their newborns.
