Increasing or decreasing secretion of a hormone results in increased or decreased blood levels of the hormone.
Control
of Hormones
Increasing or decreasing secretion of a hormone results in
increased or decreased blood levels of the hormone. This is because hormones
are continually excreted in the urine and broken down mostly by liver enzymes. Negative feedback is the mechanism that controls
hormone secretion. It is triggered by an internal or external stimulus. FIGURE 16-5 shows the steps of hormone
secretion:
1.The nervous system directly stimulates certain glands
2. The hypothalamus regulates the release of hormones from
the anterior pituitary gland, with its proximity allowing constant
communication about the internal environment.
3. Other
glands respond directly to internal changes.
As hormone levels rise, negative feedback inhibits the
system and secretion decreases. As blood levels of hormones decrease, the
system starts up again. Hormones in the bloodstream fluctuate resultantly, but
remain relatively stable. Three types of stimuli trigger endocrine gland
actions:
■■ Humoral stimuli: Changing blood levels of certain vital
ions and nutrients. They are the simplest type of endocrine controls. For
example, when para-thyroid gland cells or chief cells monitor bloodcalcium andsecreteparathyroid
hormone(PTH) when they detect lower than normal levels Humoral stimuli are also
linked to release of insulin from the pancreas and aldosterone from theadrenal
cortex.
■■Neural stimuli: Nerve fiber stimulation. Thebest example
is how the sympathetic nervoussystem responds to stress by stimulating
theadrenal medulla to release norepinephrine andepinephrine.
■■Hormonal stimuli: Hormone release due to theproduction of
other hormones. For example,when the hypothalamus regulates secretion ofmost
anterior pituitary hormones via the actionsof its own releasing hormones and
inhibitinghormones. Then, many anterior pituitary hormones act to stimulate
other endocrine organsto cause them to release their own hormones.Increasing
blood levels of the hormones fromthe final target glands then inhibit the
releaseof anterior pituitary hormones and, therefore,their own release.
Remember that multiple hormones may act on the same target
cells at the same time, often with unpre-dictable reactions. There are three
basic types of hor-mone interaction:
■■Permissiveness: One hormone is not able to exertits full
effects without the presence of anotherhormone. For example, when reproductive
system hormones require the presence of thyroidhormone to develop reproductive
structures at 0 their normal times in the life span. If the thyroidhormone is
lacking, reproductive development isdelayed.
■■Synergism: One hormone produces the sameeffects as another
hormone at the target cell,amplifying their combined effects. For example,when
glucagon from the pancreas, along with epinephrine, signals the liver to
release glucose intothe blood.
■■ Antagonism:
One hormone opposes anotherhormone’s actions. For example, when insulin is
antagonized by glucagon. Insulin lowers blood glucose levels while glucagon raises
them, an opposite effect.
Hormones may also produce complimentary yet different
effects in certain organs and tissues described as integrative effects.
1. Explain the three types of stimuli that control hormone
release.
2. Define the three basic types of hormone interaction.
3. Explain cyclic adenosine monophosphate
TH 2019 - 2024 pharmacy180.com; Developed by Therithal info.