T

Alpha Cells

Glucagon

Glucose

Liver

Liver

FIGURE 5 Negative feedback of glucose production by glucagon. (—) = inhibits, ( + ) = stimulates.

consequence of hormone secretion acts directly or indirectly on the secretory cell in a negative way to inhibit further secretion. A simple example from everyday experience is the thermostat. When the temperature in a room falls below some preset level, the thermostat signals the furnace to produce heat. When room temperature rises to the preset level, the signal from the thermostat to the furnace is shut off, and heat production ceases until the temperature again falls. This is a simple closed-loop feedback system and is analogous to the regulation of glucagon secretion. A fall in blood glucose detected by the alpha cells of the islets of Langerhans causes them to release glucagon, which stimulates the liver to release glucose and thereby increase blood glucose concentrations (Fig. 5). With restoration of blood glucose to the set-point level, further secretion of glucagon is inhibited. This simple example involves only secreting cells and responding cells. Other systems may be considerably more complex and involve one or more intermediary events, but the essence of negative feedback regulation remains the same: Hormones produce biologic effects that directly or indirectly inhibit their further secretion.

A problem that emerges with this system of control is that the thermostat maintains room temperature constant only if the natural tendency of the temperature is to fall. If the temperature were to rise, it could not be controlled by simply turning off the furnace. This problem is at least partially resolved in hormonal systems, because at physiologic set points the basal rate of secretion usually is not zero. In the preceding example, when blood glucose concentration rises, gluca-gon secretion can be decreased and therefore diminish the impetus on the liver to release glucose. Some regulation above and below the set point can therefore be accomplished with just one feedback loop; this mechanism is seen in some endocrine control systems. Regulation is more efficient, however, with a second, opposing loop, which is activated when the controlled variable deviates in the opposite direction. With regulation of blood glucose, for example, that second loop is provided by insulin. Insulin inhibits glucose production by the liver and is secreted in response to an elevated blood glucose level (Fig. 6). Protection against deviation in either direction is often achieved in biologic systems by the opposing actions of antagonistic control systems.

Closed-loop negative feedback control as described earlier can maintain conditions only at a state of

Glucagon

Normal Range

Hormone Secretion Rate

Glucagon

Normal Range

Hormone Secretion Rate

100 200 Blood Glucose Concentration

FIGURE 6 Negative feedback regulation of blood glucose concentration by insulin and glucagon. (") = inhibits, ( + ) = stimulates.

100 200 Blood Glucose Concentration

Alpha Cells (+)

FIGURE 6 Negative feedback regulation of blood glucose concentration by insulin and glucagon. (") = inhibits, ( + ) = stimulates.

constancy. Such systems are effective in guarding against upward or downward deviations from some predetermined set point, but changing environmental demands often require temporary deviation from constancy. This is accomplished in some cases by adjusting the set point, and in other cases, by a signal that overrides the set point. For example, epinephrine secreted by the adrenal medulla in response to some emergency inhibits insulin secretion and increases glucagon secretion, even though the concentration of glucose in the blood may already be high. Whether the set point is changed or overridden, deviation from constancy is achieved by the intervention of some additional signal from outside the negative feedback system. In most cases that additional signal originates with the nervous system.

Hormones also initiate or regulate processes that are not limited to steady or constant conditions. Virtually all of these processes are self-limiting, and their control resembles negative feedback, but of the open-loop type. For example, oxytocin is a hormone secreted by hypo-thalamic nerve cells whose axons terminate in the posterior pituitary gland. Its secretion is necessary for extrusion of milk from the lumen of the mammary alveolus into secretory ducts so that the infant suckling at the nipple can receive milk. In this case, sensory nerve endings in the nipple detect the signal and convey afferent information to the central nervous system, which in turn signals release of oxytocin from axon terminals in the pituitary gland. Oxytocin causes myoepithelial cells to contract, resulting in delivery of milk to the infant. When the infant is satisfied, the suckling stimulus at the nipple ceases.

Positive Feedback

Positive feedback means that some consequence of hormonal secretion acts on the secretory cells to stimulate further secretion. Rather than being self-limiting, as with negative feedback, the drive for secretion becomes progressively more intense. Positive feedback systems are unusual in biology, because they terminate with some cataclysmic, explosive event. A good example of a positive feedback system involves oxytocin and its other effect: causing contraction of uterine muscle during childbirth (Fig. 7). In this case the stimulus for oxytocin secretion is dilation of the uterine cervix. Upon receipt of this information through sensory nerves, the brain signals the release of oxytocin from nerve endings in the posterior pituitary gland. Enhanced uterine contraction in response to oxytocin results in greater dilation of the cervix, which strengthens the signal for oxytocin release and so on until the infant is expelled from the uterine cavity.

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Get Rid of Gallstones Naturally

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