Download ANSWERS TO REVIEW QUESTIONS – CHAPTER 27

ANSWERS TO REVIEW QUESTIONS – CHAPTER 27
1.
Describe two basic differences in the modes of action of water-soluble and lipid-soluble
hormones. (pp. 638–639)
Water-soluble hormones usually interact with receptors on the cell surface. These receptors often span
the cell membrane and frequently link up with a second messenger inside the cell. Lipid-soluble
hormones are able to penetrate the cell membrane and usually interact with intracellular receptors.
2.
With reference to hormone actions, what is meant by the terms autocrine, paracrine and
endocrine? (pp. 637–638)
The three terms generally refer to the distance the target is away from the secreting cell.
Autocrine hormones interact with receptors on the surface of the same cell that was responsible for
their secretion, and as such can be seen as the ultimate in locally acting hormones.
Paracrine hormones usually act over only very short distances, travelling to their site of action by
diffusion through the extracellular fluid. However, they can sometimes diffuse into the blood and affect
more distant organs.
Endocrine hormones are usually secreted into the circulating blood (or circulating haemolymph, or
even extracellular fluid in some invertebrates) and are usually targeted at organs some distance away
from the secreting cell.
3.
What is meant by neuroendocrine control? Describe (1) a neurosecretory cell and (2) a
neurohaemal organ. (pp. 639–641)
Neuroendocrine control is when the secretion of hormones is under the direct control of cells either
derived from, or part of, the nervous system. They are usually hormones that are secreted in response to
changes in the external environment, rather than the local environment.
A neurosecretory cell has a cell body that is larger than that of most other neural cells and can contain a
large number of secretory granules. The axon of such a cell is also usually packed with secretory
granules and characteristically does not form synapses with other nerve cells. While such cells may be
scattered diffusely inside any particular tissues, they can be aggregated together to form discrete,
highly-vascularised neurohaemal organs.
4.
1.
2.
What are the general properties of target organs and the physiological functions of (1)
posterior pituitary hormones and (2) anterior pituitary hormones? (pp. 641–643)
Hormones secreted by the posterior pituitary generally affect water balance and reproduction.
Hormones secreted by the anterior pituitary are protein-based hormones that have a very wide
range of functions. The glycoprotein hormones, thyroid-stimulating hormone (TSH), folliclestimulating hormone (FSH) and luteinising hormone (LH), together with one of the peptide
hormones, adrenocorticotrophic hormone (ACTH), stimulate the activity of other endocrine
glands. The remaining three hormones, growth hormone (GH), prolactin (PRL) and melanophorestimulating hormone (MSH), influence metabolism, growth, reproduction and water balance.
5. (a) What is the significance of dietary iodine to thyroid function? (pp. 645–646)
Without an adequate dietary supply of iodine, which forms part of thyroglobulin, a decrease in thyroid
hormone secretion occurs. This causes increased TSH secretion due to the lack of feedback inhibition
from the thyroid hormone, which in turn can cause a gross enlargement of the thyroid gland, called
goitre.
(b) What physiological processes do thyroid hormones influence? (pp. 645–646)
The physiological processes that thyroid hormones influence can be seen when their production is
insufficient. Lack of thyroid hormones impairs growth and development in young vertebrates (which is
called cretinism in humans) and can slow metabolism and nervous activity in adults, as well as causing
myxoedema, abnormal accumulation of water in subcutaneous tissues.
6.
(a) What is the functional difference between a mineralocorticoid and a glucocorticoid?
(pp. 646–647)
Mineralocorticoids influence Na+ and K+ balance, while glucocorticoids influence carbohydrate and
protein metabolism.
(b) What is the effect of ACTH on the adrenal gland? (pp. 646–647)
Adrenocorticotrophic hormone (ACTH) causes an increase in the secretion of glucocorticoids such as
cortisol and corticosterone from the adrenal gland.
7. (a) Where are the ultimobranchial and parathyroid glands? (p. 651)
The ultimobranchial and parathyroid glands are either associated with the thyroid gland, or are located
just posterior to it. In mammals, the ultimobranchial glands are incorporated into the thyroid glands and
are called thyroidal C-cells.
(b) Draw a flow diagram to show how the hormones they secrete interact to control blood
Ca2+ levels. (p. 651)
Parathyroids =
Ultimobranchial
glands =
parathyroid
hormone
(PTH)
calcitonin
increased plasma
Ca2+ via
mobilisation from
bone
reabsorption from
kidney
indirectly
increasing Ca2+
from gut
decreased plasma
Ca2+ via
increased bone
uptake
increased
secretion from
kidney
8.
What are the endocrine cell types in the islets of Langerhans and what are their functions?
(pp. 651–653)
There are three major endocrine cell types in the islets of Langerhans. These are the alpha cells, which
are responsible for the secretion of glucagon, the beta cells responsible for the secretion of insulin, and
the gamma cells responsible for the secretion of somatostatin. Insulin causes a cascade of reactions that
ultimately result in increased glucose utilisation and increased glucose storage as glycogen in liver and
muscle. Glucagon acts in opposition to insulin, causing a decrease in glucose oxidation in cells and an
increase in the breakdown of glycogen to glucose in the liver. The role of somatostatin is less well
understood but it appears to inhibit both alpha and beta cells, damping down any tendency for wild
oscillations in blood sugar levels.
9.
In considering control of moulting in insects, give an example of a neuroendocrine cascade.
(p. 656)
A neuroendocrine cascade is where a hormone, as well as having its own action on cells or tissues, has
an effect on other cells or tissues that play a role in the overall sequence of events. For example,
ecdysis-triggering hormone, as well as unlocking the sequence of behaviours that precede moulting,
stimulates the excitability of neurons producing eclosion hormone, which in turn increases the
excitability of neurons producing crustacean cardiactive peptide, which facilitates the commencement
of moulting in insects.
10. Draw a diagram to illustrate the hormonal control of ovulation in a mammal. (pp. 647–648)
This is actually a very difficult question to answer fully as the hormonal interactions involved are very
complex. It is dealt with only briefly in this chapter and so the diagram below would be considered by
many to be somewhat simplistic!
Hypothalamus
Gonadotrophin-releasing
hormone (GnRH)
Anterior pituitary
Luteinising hormone (LH), in
concert with FSH, causes
ovulation
Follicle-stimulating hormone (FSH)
causes ovarian follicles to proliferate
Ovary