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Control of the Pituitary Gland: Neurohypophysis

Control of the Pituitary Gland: Neurohypophysis

Author: Aaron Mullally
Description:

- review the terms adenohypophysis and neurohypophysis

- know that the neurohypophysis is directly connected to the hypothalamus

- know which centers project their axons into the neurohypophysis

- know which hormones are stored and secreted from the neurohypophysis

This packet covers how the posterior lobe of the pituitary gland is controlled by the hypothalamus. Before going through this packet make sure you have gone through the packet covering the embryologic development of the pituitary gland first:

http://www.sophia.org/packets/embryologic-development-of-the-pituitary-gland

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Tutorial

Overview

The control if the posterior lobe (neurohypophysis) is much more simple, in my opinion. A big part of this is because of its embryonic origins, from the diencephalon (hypothalamus) itself. Since the neurohypophysis is a down growth from the hypothalamus itself, the two are directly connected. Neurons from the supraoptic nuclei and paraventricular centers of the hypothalamus project directly into the neurohypophysis and stimulate and inhibit this part of the gland to secrete its hormones, when necessary. The two hormones that are secreted from the neurohypophysis are vasopressin (otherwise known as anti-diuretic hormone {ADH}) and oxytocin. These hormones are synthesized by the neurons from these nuclei and are transported and stored in the neurohypophysis until needed.

The supraoptic nuclei are located just superior to the optic chiasm, hence the name. These neurons are the osmoreceptors of the brain (osmo- water; receptor = transducer). This is an important area to remember because you will return to it when discussing water balance of the body. The osmoreceptors found in this center are stimulated by an increase in extracellular sodium (↑ osmolarity). This increase in extracellular sodium is caused by the loss of water. For example when we sweat we do lose sodium from our plasma but not in proportion to the amount of water that's lost. So as we sweat profusely we lose more water than sodium, which creates an imbalance that favors a higher concentration of sodium in proportion to water in the body. This is why hydration before, during (if possible) and after exercise are critical. When our osmolarity rises, these receptors are activated and they generate action potentials to the nuerohypophysis and stimulate the secretion of ADH. ADH's target is the kidneys, where it stimulates renal retention of water to counterbalance the water loss.

The paraventricular nucleus is found surrounding the third ventricle, hence the name. The neurons in this nucleus synthesize the hormone oxytocin and store it in the neurohypophysis until needed. Oxytocin is a hormone that is more utilized by females than males. This hormone is primarily used for contracting the uterus and the ejection of breast milk. When the uterus is stretched to a certain point during gestation the stretch receptors associated with it will generate action potentials to the paraventricular nucleus, stimulating the release of oxytocin. As oxytocin levels rise and the uterus contracts with more force, more and more oxytocin is relased thus increaing the force of contractions and shortening the time intervals in between. This is one of the few examples of a positive feedback mechanism in the body. When the child suckles on the nipple at the beginning of feeding, this stimulates the secretion of oxytocing and the ejection of breast milk.

Source: Mind of Aaron

Neurohypophysis Control

This video covers how the hypothalamus controls the activity of the posterior pituitary (neurohypophysis)

Source: Self made

Neurohypophysis Control Image

Source: Self made