Chapter 25 Endocrine Control (26)
Hormones and other signaling molecules are involved in integrating the activities of
individual cells that benefit the whole body, they are released by endocrine glands and
travel through the circulatory system to their target organs or tissues, where they have a
specific effect. Some hormones help the body to adjust to short-term changes in diet and
in levels of activity. Other hormones help spur long-term adjustments in cell activities
that bring about body growth, development, and reproduction. The endocrine (hormonal)
system acting in concert with the nervous system, plays a key role in homeostasis.
Pheromones are not hormones, since they are released by an animal species to be
intercepted by members of the same species to cause a specific behavioral response, such
as mating behavior. There is still controversy as to whether or not humans respond to
All of this material is relevant and testable but the material in section 25.7 on hormones
and sexual reproduction is in fact covered later on in chapter 26. Focus on the structures
and functions of the endocrine system, and the specific hormones that the glands secrete.
Ensure that you have a general understanding of the regulation of hormone action by
The endocrine system
Hormones are chemical messengers. They are secreted by endocrine (ductless) glands
where they travel in the circulation, the bloodstream, to target cells in the body where
they bind to specific receptors to cause a specific action.
The Hormone (endocrine) system and the nervous system are intricately connected and
affect each others properties and functions.
Homeostatic control may just involve one hormone in a negative feedback loop, but
regulation also occurs by means of at least two hormones that have opposite effects in
order to impose homeostatic control.
Sometimes a change in a property which is caused by hormones needs the action of two
or more hormones acting together (simultaneously) to cause that action, and sometimes
two or more hormones must act in sequence (called “priming”) for a property to be
changed by hormonal action.
There are two major chemical classes of hormones, they are either steroid based
hormones or they are peptide based hormones (There are also a few hormones that are of different chemical structure, such as the amine based hormones melatonin and
Steroid hormones are soluble in lipids; they have the ability to pass readily through the
lipids of a cell membrane, where they then bind to a receptor inside the cell to form a
steroid-protein complex that acts on DNA to cause gene activity. The gene activity will
produce a specific product in response to the steroid hormonal signal.
Peptide based hormones are not lipid soluble and do not readily move across cell
membranes, but instead they bind to specific receptors on the membrane surface, which
in turn transmit a signal to a second messenger system within the cell - this is often a
compound called cyclic AMP (cAMP).
The secondary messenger cAMP can then cause a wide variety of reactions in the cell
depending on the function of that cell. There are numerous other secondary messenger
systems within cells.
Peptide hormones tend to act in a manner that activates pre-existing enzymes inside their
target cells, and so peptide hormones are generally faster acting than steroid hormones,
which induce the slower process of gene activation and protein production.
The major link between the central nervous system (CNS ) and hormonal systems is the
hypothalamus of the forebrain which monitors body systems and directs release of
hormones by the pituitary gland. The pituitary gland is often called the Master Gland,
and is physically attached to the hypothalamus by a short stalk of tissue.
The hypothalamus is thus the major control agent of much of the hormonal system (but
not all hormone responses involve the hypothalamus).
The pituitary gland has two lobes:
The posterior lobe stores ADH and oxytocin, these hormones are produced by special
neurosecretory cells in the hypothalamus (the hypothalamus itself is therefore an
endocrine gland). ADH affects water uptake back to the body from the kidney, and
oxytocin induces mammary lactation (formation and release of milk) and uterine
contractions during childbirth.
The anterior lobe of the pituitary produces a range of hormones that are generally not
released except under the direction of specific releasing hormones secreted by the
hypothalamus; an example is Gonadotropin releasing hormone, which stimulates
release by the anterior pituitary of follicle stimulating (FSH) and luteinizing (LH)
hormones that are involved in the menstrual cycle.
Growth hormone (GH) is released by the anterior pituitary. Too little GH can result in
dwarfism, too much during the adolescent phase of growth (often a result of a benign
tumor of the pituitary) can result in gigantism, and continued excessive release of GH in an adult who has stopped growing can cause acromegaly, a massive overgrowth and
coarse thickening of facial and other bones.
The hypothalamus also produces inhibitor hormones that prevent release of hormones
by the anterior pituitary.
Hormonal control of physiological processes: An example of negative feedback
hormonal control is that of the interaction of insulin and glucagon to control the levels of
blood glucose (this is a hormonal system that is not controlled by the hypothalamus).