The pituitary is closely associated with the brain

The pituitary gland sits in a depression at the bottom of the skull just over the back of the roof of the mouth (Figure 42.5). It is attached by a stalk to the part of the brain called the hypothalamus, which is involved in many homeostatic regulatory systems (see Chapter 41).

The pituitary has two distinct parts that have different functions and separate origins during development. The anterior pituitary originates as an outpocketing of the embryonic mouth cavity, and the posterior pituitary originates as an outpocketing of the developing brain in the region that becomes the hypothalamus.

the posterior pituitary. The posterior pituitary releases two peptide hormones, antidiuretic hormone and oxytocin. Because these hormones are synthesized in neurons in the hypothalamus, they are called neurohormones. As antidi-uretic hormone and oxytocin are produced, they are packaged in vesicles. These vesicles are then transported down long extensions of the neurons, called axons, that run from the hypothalamus through the pituitary stalk and terminate in the posterior pituitary. The vesicles are stored in the

Posterior Pituitary And Hypothalamus

42.5 The Posterior Pituitary Releases Neurohormones The two hormones stored and released by the posterior pituitary are peptide neurohormones produced in the hypothalamus.

42.5 The Posterior Pituitary Releases Neurohormones The two hormones stored and released by the posterior pituitary are peptide neurohormones produced in the hypothalamus.

axon terminals until a nerve impulse stimulates their release (see Figure 42.5). How do the vesicles move down the axons? Proteins called kinesins grab onto the vesicles and, powered by ATP, "walk" step by step down microtubules in the axons.

The main action of antidiuretic hormone (ADH) in mammals and birds is to increase the amount of water conserved by the kidneys. When ADH secretion is high, the kidneys resorb water and produce only a small volume of highly concentrated urine. When ADH secretion is low, the kidneys produce a large volume of dilute urine. The posterior pituitary increases its release of ADH whenever blood pressure falls or the blood becomes too salty. We will discuss the mechanism of ADH action in Chapter 51. ADH is also known as vasopressin because it also causes the constriction of peripheral blood vessels as a means of elevating blood pressure.

When a woman is about to give birth, her posterior pituitary releases oxytocin, which stimulates the contractions of the uterus that deliver the baby. Oxytocin also brings about the flow of milk from the mother's breasts. The baby's suckling stimulates neurons in the mother, causing the secretion of oxytocin. Even the sight and sounds of her baby can cause a nursing mother to secrete oxytocin and release milk from her breasts.

the anterior pituitary. Four peptide and protein hormones released by the anterior pituitary (thyrotropin, adrenocorticotropin, luteinizing hormone, and follicle-stimulating hormone) are tropic hormones, which control the activities of other endocrine glands (see Figure 42.7). Each tropic hormone is produced by a different type of pituitary cell. We will say more about these tropic hormones when we describe their target glands (thyroid, adrenal cortex, testes, and ovaries) later in this chapter and in the next.

The other peptide-and protein hormones produced by the anterior pituitary influence tissues that are not endocrine glands. These hormones are growth hormone, pro-lactin, melanocyte-stimulating hormone, endorphins, and enkephalins.

Growth hormone (GH) acts on a wide variety of tissues to promote growth directly and indirectly. One of its important direct effects is to stimulate cells to take up amino acids. Growth hormone also promotes growth indirectly by stimulating the liver to produce chemical messages called somato-medins or insulin-like growth factors (IGFs), which stimulate the growth of bone and cartilage. Thus, in some of its actions, growth hormone can also be considered a tropic hormone in that it stimulates cells to produce and release other hormones.

Overproduction of growth hormone in children causes gi-gantism (individuals may grow to nearly 8 feet tall; Figure 42.6). Underproduction causes pituitary dwarfism, in which individuals fail to reach normal adult height.* Beginning in the late 1950s, children diagnosed as having a serious deficiency of growth hormone were treated with growth hormone extracted from human pituitaries from cadavers. The treatment was successful in stimulating substantial growth, but a year's supply the hormone for one individual required up to 50 pituitaries! In the mid-1980s, scientists using genetic

*Pituitary dwarfism, in which the individual is short but normally proportioned, is a distinct condition from the more common achondroplasia, in which the trunk and head are normal size but the bones of the arms and legs are foreshortened. Although the result of both conditions is short stature, the underlying causes are completely different.

Giant Actor Basketball Player

42.6 Effects of Excess Growth Hormone At 7'7", basketball player turned actor Gheorghe Muresan (shown here in the 1998 film My Giant) is the tallest man ever to play in the U.S. National Basketball Association. Mureson was born in Romania to parents who were both under 6 feet tall; his gigantism results from overproduction of pituitary growth hormone during childhood.

42.6 Effects of Excess Growth Hormone At 7'7", basketball player turned actor Gheorghe Muresan (shown here in the 1998 film My Giant) is the tallest man ever to play in the U.S. National Basketball Association. Mureson was born in Romania to parents who were both under 6 feet tall; his gigantism results from overproduction of pituitary growth hormone during childhood.

engineering technology isolated the gene for human growth hormone and introduced it into bacteria that could be grown in large quantities, making it possible to purify enough of the hormone to make it more widely available.

Prolactin stimulates breast development and the production and secretion of milk in female mammals. In some mammals, prolactin also functions as an important hormone during pregnancy. In human males, prolactin plays a role, along with other pituitary hormones, in controlling the endocrine function of the testes.

Endorphins and enkephalins are the body's natural opiates. In the brain, these molecules act as neurotransmitters in pathways that control pain. The significance of their release from the anterior pituitary is unknown. Interestingly, the production of endorphins and enkephalins in the pituitary is governed by the same gene that encodes at least two other pituitary hormones. This gene encodes a large parent molecule called pro-opiomelanocortin. This large protein molecule is cleaved to produce several peptides, some of which have hormonal functions. Adrenocorticotropin, melanocyte-stimulat-

ing hormone, endorphins, and enkephalins all result from the cleavage of pro-opiomelanocortin.

the anterior pituitary is controlled by hypothalamic neurohormones. The secretion of hormones by the anterior pituitary is largely under the control of neurohormones from the hypothalamus. The hypothalamus receives information about conditions in the body and in the external environment through both neuronal and hormonal signals. If the connection between the hypothalamus and the pituitary is experimentally cut, pituitary hormones are no longer released in response to changes in the internal or external environment. When pituitary cells were maintained in culture, extracts of hypothalamic tissue stimulated some of those cells to release their hormones into the culture medium. Therefore, scientists hypothesized that secretions of the hypothalamic cells control the activities of anterior pituitary cells.

Although hypothalamic neurons do not extend into the anterior pituitary as they do into the posterior pituitary, a special set of portal blood vessels connects the hypothalamus and the anterior pituitary (Figure 42.7). It was thus proposed that secretions from neurons in the hypothalamus enter the blood and are conducted down the portal vessels to

Hypothalamus

Hypothalamus

Inflowing blood

Neurohormonal Regulation

Nerve endings of hypothalamic neurons secrete neurohormones near capillaries that give rise to portal vessels.

Neurohormones from the portal vessels stimulate or inhibit the release of anterior pituitary hormones.

Anterior pituitary hormones leave the gland via the blood.

42.7 Hormones from the Hypothalamus Control the Anterior Pituitary Neurohormones produced in tiny quantities by cells in the hypothalamus are transported to the anterior pituitary through a system of portal blood vessels.These releasing and release-inhibiting hormones control the activities of endocrine cells in the anterior pituitary.

Nerve endings of hypothalamic neurons secrete neurohormones near capillaries that give rise to portal vessels.

Inflowing blood

Neurohormones from the portal vessels stimulate or inhibit the release of anterior pituitary hormones.

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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Responses

  • temshe
    What attaches pituitary to hypothalamus?
    7 years ago
  • Hannah
    What is neurohormones?
    6 years ago
  • Bisrat
    How does the pituitary gland relate to the plasma membrane?
    4 years ago
  • prospero smallburrow
    Which part of the brain is closely associated with the pituitary gland?
    4 years ago
  • JEAN
    Where neurohormones are stored in posterior pitutary?
    3 years ago
  • kifle
    How pitutary controls the activity of other endocrine glands?
    2 years ago

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