1. Hormones and the endocrine system

2. In addition to electrical signals in the nervous
system, chemical signals in the endocrine system
are a primary means by which anatomical,
physiological, and behavioral changes are regulated
in humans.
The chemical signals in the endocrine system are
hormones.

3. Chemical communication requires both a cell (or
group of cells) that produces and releases the
hormone and a cell (or group of cells) with an
appropriate receptor that receives the hormone.
The interaction between the hormone and the
receptor activates mechanisms within the target
cell that lead ultimately to anatomical, physiological,
or behavioral changes.

4. Humans produce a wide array of hormones that
regulate a large number of changes or responses.
Most hormones in humans are produced by
endocrine glands, which are aggregations of cells
that secrete hormones into the extracellular fluid,
from which they enter the blood. After entering
the blood, the hormones may be transported
throughout the body.
The major human endocrine glands are shown in
Figure 41.5.

6. * Some hormones in humans are secreted by endocrine cells that are not
part of discrete endocrine glands.

7. The endocrine system contains both lipid-soluble
and water-soluble hormones, which differ in how
they bind to receptors in or on the target cells.

8. Lipid-soluble hormones
Lipid-soluble hormones include the steroid
hormones.
They pass readily through the lipid-rich plasma
membrane that surrounds the target cell, and bind
to receptors in the cytoplasm or nucleus of the cell.

9. In most cases, lipid-soluble hormones stimulate the
synthesis of new kinds of proteins by altering gene
expression in the target cell.
The synthesis of new kinds of proteins leads
ultimately to anatomical, physiological, or behavioral
changes.
As an example, the mechanism of action of one
lipid-soluble hormone (cortisol) is illustrated in
Figure 15.8.

11. * Because lipid-soluble hormones stimulate the synthesis of new kinds of proteins,
rather than serving to modify existing proteins, their actions are slower and last
longer than those of water-soluble hormone.

12. Examples of lipid-soluble hormones include:
1) cortisol
2) androgens, especially testosterone
3) estrogens, especially estradiol, and progesterone

13. * Cortisol is produced by the adrenal glands specifically
the adrenal cortex (The adrenal glands are located above
the kidneys)
It is important in regulating the body’s response to stress
It causes a decrease in the metabolism of glucose in most
cells, except the brain and muscles, and an increase in the
metabolism of proteins and fats
It also blocks immune system reactions.

14. 2) Androgens are produced by the testes of males
They stimulate ted development and maintenance of male sexual characteristics and
behavior.
They also stimulate sperm production.

15. 3) Estrogen and progesterone are produced by the ovaries
of females.
They stimulate the development and maintenance of female
sexual characteristics and behavior.

16. Water-soluble hormones
Water-soluble hormones do not pass readily
through the lipid-rich plasma membrane that
surrounds the target cell.
Instead, they bind with receptors on the surface of
the target cell.
The receptors are glycoprotein complexes.

17. The receptors have a binding domain that projects
beyond the outside of the plasma membrane and a
catalytic domain that projects into the cytoplasm
of the cell.
Directly or indirectly, most receptors initiate
cellular responses by activating protein kinases,
which catalyze the transfer of phosphate groups
from ATP to specific proteins.

18. Phosphorylation of specific proteins causes them to
become activated or de-activated, thereby leading
ultimately to anatomical, physiological, or behavioral
changes.
As an example, the mechanism of action of one
water-soluble hormone (epinephrine) is illustrated in
Figure 15.18.

22. * By activating or de-activating existing proteins,
many water-soluble hormones produce rapid changes or
responses (in contrast to most lipid-soluble hormones).

23. Examples of water-soluble hormones include:
1) epinephrine (or adrenaline)
2) insulin
3) luteinizing hormone and follicle-stimulating
hormone

24. * 1) Epinephrine is produced by the adrenal glands specifically the adrenal
medulla.
It is important in regulating the body’s immediate response to stress (the so-
called “fight or flight” reactions).
It causes increases in heart rate, breathing rate, blood pressure, and blood
sugar concentration.
It also divert blood flow to active skeletal muscles.

25. * 2) Insulin is produced by the pancreas gland. (The
pancreas gland is located just below the stomach)
It stimulates the uptake and metabolism of glucose, and
increases the conversion of glucose to glycogen and fat.

26. * 3) Luteinizing hormone and follicle-stimulating
hormone are produced by the anterior pituitary gland.
( The pituitary gland is located in a depression at the
bottom of the skull just over the back of the roof of the
mouth.)
Luteinizing hormone stimulates the production and
release of androgens by the testes.
Luteinizing hormone and follicle-stimulating hormone
stimulate the production and release of estrogens and
progesterone by the ovaries.

27. * Luteinizing hormone and follicle-stimulating hormone are
also known as gonadotropins.

28. Interaction between the nervous and endocrine
systems
As we discussed previously, the hypothalamus is
part of the brain (specifically the diencephalon),
and is thus part of the nervous system.
As illustrated in Figure 41.5, the hypothalamus also
functions as a key endocrine gland.

29. The hypothalamus is connected to the anterior
pituitary gland by a specialized set of portal blood
vessels, as illustrated in Figure 41.7.
Neurohormones synthesized in neurons in the
hypothalamus are secreted near capillaries that
give rise to the portal blood vessels.
The neurohormones are then transported in the
blood vessels to the anterior pituitary gland, where
they regulate secretion of the so-called tropic
hormones.

30. *Tropic hormones produced by the anterior
pituitary gland are hormones that control the
activities of other endocrine glands.

32. As one example, gonadotropin-releasing hormone is
a neurohormone produced by the hypothalamus.
Gonadotropin-releasing hormone stimulates the
production and release of the two tropic hormones,
luteinizing hormone and follicle-stimulating
hormone, by the anterior pituitary gland.

33. In turn, luteinizing hormone stimulates the
production and release of androgens by the testes
(in males), and luteinizing hormone and follicle-
stimulating hormone stimulate the production and
release of estrogens and progesterone by the
ovaries (in females).

34. As a second example, corticotropin-releasing
hormone is a neurohormone produced by the
hypothalamus.
Corticotropin-releasing hormone stimulates the
production and release of the tropic hormone,
corticotropin, by the anterior pituitary gland.
In turn, corticotropin stimulates the production and
release of cortisol by the adrenal cortex.

35. Multiple negative feedback loops may regulate the
“chain of command” from the hypothalamus to the
anterior pituitary gland to other endocrine glands,
as illustrated in Figure 41.8.
For example, when sufficient cortisol reaches the
anterior pituitary gland in the circulating blood, it
inhibits the further release of corticotropin.
In addition, sufficient cortisol reaching the
hypothalamus inhibits the further release of
corticotropin-releasing hormone.

36. In some cases, a tropic hormone also exerts
negative feedback control on the hypothalamus,
inhibiting the production of the corresponding
releasing hormone.

38. *With respect to sexuality, puberty is initiated by a reduction in the
sensitivity of hypothalamic neurons to negative feedback control by
gonadotropins and sex hormones.
As a result, more gonadotropin-releasing hormone is produced by the
hypothalamic neurons, resulting in higher levels of gonadotropins and
this in sex hormones (androgens or estrogen/progesterone)
The increase in sex hormones leads, in turn, to the profound anatomical
and physiological changes associated with sexual maturation.

39. Recommended reading: pp. 1054-1078