BIOA02H3 Chapter Notes -Negative Feedback, Blood Sugar, Somatostatin

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25 Feb 2011
41.1 What Are Hormones and How Do They Work?
-Control and regulation require information.
-In multi-cellular animals, most of this information is transmitted as electric signals and as
chemical signals.
The electrical signals are impulses generated by the nervous system, conducted along
cell processes of nerve cells to their targets on specific cells.
The chemical signals are hormones, secreted by cells of the endocrine system into
the extracellular fluid.
-The two informational systems of the body are the nervous and endocrine systems.
Hormones can act locally or at a distance
-Endocrine Cells: The cells that secrete hormones.
-Target Cells: The cells that have receptors for hormones.
-Circulating Hormones: Hormones secreted into the extracellular fluid can diffuse into the
blood, which distributes them throughout the body so they can activate target cells far from
the site of release.
Testosterone is a circulating hormone.
-Some hormones are released in such tiny quantities, or are so rapidly inactivated by enzymes,
or taken up so efficiently by local cells that they never diffuse into the blood in sufficient
amounts to act on distant cells.
-Paracrine Hormones: Hormones that affect only target cells near their release site.
e.g., histamine, one of the mediators of inflammation.
-The most local action of a hormone can have is when there are receptors on the same cell that
released it.
-When a hormone influences the cell that released it, it is said to have autocrine function.
Such functions can provide negative feedback to control rates of secretion.
-Some endocrine cells exist as single cells within a tissue.
-Hormones of the digestive tract are secreted by isolated endocrine cells in the wall of the
stomach and small intestines.
-Endocrine Glands: Hormones that are secreted by aggregations of endocrine cells forming
secretory organs.
“endocrine”: the glands do not have ducts that lead to the outside of the body; they
secrete their products directly into the extracellular fluid.
A single endocrine gland may secrete several different hormones.
Exocrine Glands are contrasting; they have ducts that carry their products to the
surface of the skin (e.g., sweat glands) or to the surface of the body passageway that
leads to the outside of the body (salivary glands).
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Hormonal communication arose in early evolution
-Plants do not have nervous systems, but they do have hormones.
-The most primitive of the multi-cellular animals, the sponges, also do not have a nervous
system but they do have chemical communication.
-Even a protest, the social amoeba, which produces multi-cellular fruiting bodies by the
aggregation of individuals, coordinates the aggregation with a chemical signal, cAMP.
Hormones from the head control molting in insects
-The largest groups of arthropods are the insects, and like all arthropods they have rigid
Their growth is episodic, punctuated with molts (shedding of the exoskeleton).
Each growth stage between two molts is called an instar.
-Sir Vincent Wigglesworth
He conducted experiments on the blood-sucking bug Rhodnius prolixus.
Rhodnius prolixus
oA hardy experimental animal.
oIt can live a long time even after it is decapitated.
He formed a hypothesis that something diffusing slowly form the head of the bug
controls molting.
He decapitated two Rhodnius: one shortly after its blood meal and another that had
its blood meal a week earlier.
oThe two decapitated bodies were connected with a short piece of glass tubing
that allowed body fluid transfer between them. They both molted.
-Two hormones working in sequence regulate molting:
1.Prothoracicotropic hormone (PTTH)
Cells in the brain produce PTTH and because of this it is called the brain
It is transported to and stored in a pair of structures called the corpora cardiac
attached to the brain.
After appropriate stimulation (which for Rhodnius is a blood meal), it is
released from these structures and it diffuses in the extracellular fluid to an
endocrine gland, the prothoracic gland.
It stimulates the prothoracic gland to release the hormone ecdysone.
It is a lipid-soluble steroid molecule that readily enters its target cells (mostly
cells of the epidermis).
In target cells, it binds to a receptor that is probably ancestral to the vertebrate
testosterone receptor.
The hormone–receptor complex acts as a transcription factor and induces
expression of the genes for enzymes involved in digesting the old cuticle and
secreting a new one.
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-The control of molting by PTTH and ecdysone is a general arthropod hormonal control
mechanism and is an example of how a hormonal system works with the nervous system to
integrate diverse information and induce a long-term effect.
-The nervous system receives various types of information (such as day length, temperature,
crowding, and nutrition) that help determine the optimal timing for growth and development.
-The nervous system (the brain) then controls the endocrine gland (the prothoracic gland)
producing the hormone (ecdysone) that orchestrates the physiological processes involved in
development and molting.
Juvenile hormone controls development in insects
Juvenile Hormone: in insects, a hormone maintaining larval growth and preventing maturation
or pupation.
Hormones can be divided into three chemical groups
-There is enormous diversity in the chemical structure of hormones, but most of them can be
divided into three groups:
1.Peptides and polypeptides are the majority of hormones.
e.g., insulin.
These hormones are water-soluble and are this easily transported in the blood.
They, however, cannot pass readily through lipid-rich cell membranes.
They are packaged in vesicles in the cells that make them and are released by
2.Steroid hormones
e.g., testosterone and estrogen (they are derivatives of the steroid cholesterol).
They are lipid-soluble and easily dissolve in and pass through cell membranes.
They diffuse out of the cells that make them as they are synthesized.
They are not soluble in blood and therefore they must be bound to carrier
proteins in order to be transported to their target cells.
3.Amine hormones
e.g., thyroxine.
They are mostly derivatives of the amino acid tyrosine.
Some are water-soluble and others are lipid-soluble.
Their modes of release differ accordingly.
Hormone receptors are found on the cell surface or in the cell interior
-The chemical structure of hormones is related to the location of their receptors.
-Lipid-soluble hormones can diffuse through plasma membranes, and therefore their receptors
are inside the cell, in either the cytoplasm or the nucleus.
-In most cases, the complex formed by the lipid-soluble hormone and its receptor acts by
altering gene expression in the cell.
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