1. Chemical signals in animals

2. Key concepts
Hormones are compounds produced in one part of the body and transported to another location to produce specific responses; small amount s can induce substantial responses.
Chemical signals produced by the body are mostly produced by glands.
Hormones either affect a target effector organ directly or via the signal transduction pathway.
Effects of hormones vary depending on whether they bind to a receptor in the plasma membrane or within the nucleus of a cell.
Action of hormones is controlled by feedback mechanisms.
Hormones are classified by mode of transport or through chemical composition.
Diseases in the endocrine system occur when levels of hormones released are at an abnormal level or when receptors of that particular hormone are damaged.

3. Endocrine system Hormone-secreting cells
Chemical signals secreted into body fluids, eliciting responses in target cells
Functions with NS in maintaining homeostasis
Effects are slower and longer-lasting

4. Maintenance of Homeostasis by the ES and NS
Neurosecretory cells – nerve cells that secrete hormones into bloodstream (eg. epinephrine)
Feedback regulation
Positive feedback
Suckling  release of oxytocin  milk secretion
Negative feedback
Control of blood calcium and glucose levels

5. Invertebrate Endocrine Systems
Hydra –control method of reproduction
Mollusks –controls egg-laying
Arthropods – extensive endocrine systems for molting and maturity
Ecdysone  molting and promotion of adult features
Brain hormone (BH)  regulates production of ecdysone
Juvenile hormone (JH)  retention of larval characteristics

6. Chemical signals and their modes of action
Hormones bind to specific receptor proteins on/within target cells
Signal transduction pathway is initiated
Target cell changes its behavior to produce a response
most peptides, proteins and glycoproteins
steroids and thyroid hormones

7. Importance of signal transduction pathways
Signal transduction pathways are important in two ways:
1) They enable different cells to respond to the same hormone in different ways. Acetylcholine can cause contraction or relaxation of different types of muscle cells because of different receptors. It can also bind to an endocrine cell and cause it to release its own hormones.
2) They can cause very little amounts of hormones to have a large effect. One act (the binding of 1 molecule of epinephrine to a receptor on a liver cell) can cause a chain reaction to happen that eventually leads to the conversion of 100,000,000 molecules of glycogen into glucose so that the cells have a fresh source of glucose for energy.
1) Different pathways in different cells  different responses to the same signal
2) Amplification of the single signal allows small amounts of hormones to have a large effect

8. Vertebrate endocrine systems
Hypothalamus integrates endocrine and nervous functions
Neurosecretory cells of hypothalamus produce trophic/tropic hormones
Stored and secreted by posterior pituitary
Affect anterior pituitary’s release of own hormones
Neurosecretory cells (neurons that can release hormones) found in the hypothalamus release trophic/tropic hormones. These hormones regulate the secretions of other endocrine glands.
The pituitary gland, also called the master gland, has two lobes. The anterior lobe secretes many hormones in response to the trophic hormones released by the hypothalamus (for example, the hypothalamus can release TRH, that stimulates the anterior lobe to produce TSH, that stimulates the thyroid gland to produce T3 and T4).
The hypothalamus also produces ADH and oxytocin which are stored and released by the posterior pituitary gland.

9. Please take note of the sources and actions of the following hormones
oxytocin – produced by hypothalamus, released by pituitary, “bonding” hormone, plays a role in childbirth (uterus contraction and milk production)
antidiuretic hormone (ADH) - produced by hypothalamus, released by pituitary, causes kidneys to retain water
growth hormone (GH) – produced by anterior pituitary, stimulates growth and metabolic functions
follicle stimulating hormone (FSH) – produced by anterior pituitary, stimulates ova and sperm production
luteinizing hormone (LH) – produced by anterior pituitary, stimulates ovaries and testes
T3 and T4 – produced by thyroid gland, stimulates metabolic processes
calcitonin – produced by thyroid gland, lowers amount of Ca in blood
parathyroid hormone (PTH) – produced by para thyroid gland, raises amount of Ca in blood
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

10. Please take note of the sources and actions of the following hormones
insulin – produced by pancreas, lowers amount of glucose in blood (body cells absorb glucose)
glucagon – produced by pancreas, raises amount of glucose in blood (glycogen in liver is converted into glucose)
epinephrine and norepinephrine – produced by adrenal medulla, raises blood glucose level, increases metabolic activities
androgens (e.g. testosterone) – produced by testes, supports sperm formation and maintains secondary characteristics [hairiness, lower-pitched voices, etc.]
estrogens (e.g. estradiol) – produced by ovaries, stimulate growth of endometrium and female characteristics [breast development, regulates menstrual cycle]
progesterone – produced by ovaries, maintains growth of endometrium
melatonin – produced by pineal gland, maintains biological rhythms
Table 45.1 (continued)
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

11. Feedback control loops: a closer look
Some examples of how hormones are regulated are exemplified by
The regulation of thyroid hormones – the hypothalamus secretes TRH in response to levels of TSH and T3 and T4. When the levels of these three hormones rise beyond a set point, they signal the hypothalamus to stop releasing TRH (negative feedback), causing it to stop releasing TRH.
The regulation of glucose in the blood – this is an example of how two hormones with opposite effects (insulin and glucagon) can act together to maintain blood glucose levels.
Regulation of thyroid hormones
Regulation of glucose in the blood

12. Hormones control the reproductive cycle of human females
The human female reproductive cycle exhibits a complex interaction of several hormones.
In the figure on the left, a shows what’s happening in the hypothalamus and pituitary gland, b shows levels of FSH and LH, c shows events in the ovary, d shows levels of estrogen and progesterone in the blood, and e shows what is happening to the walls of the uterus (endometrium). These events are plotted against a typical 28-day cycle.
Days The cycle starts on Day 0, the first day of menstration. During menstruation, the walls of the endometrium slough off resulting in blood flow. This can last up to 5 days. While that is happening in the uterus, the hypothalamus releases GnRH (gonadotropin-releasing hormone) that acts on the anterior pituitary causing it to release more FSH and LH. FSH and LH travel to the ovaries through the bloodstream where they cause one egg cell and its surrounding cells (called a follicle) to mature.
Days 6-13 – The growing follicle secretes estrogen in increasing amounts, reaching its peak at around Day 12. The extremely high levels of estrogen also stimulate the hypothalamus to produce more GnRH which leads to a spike in LH and FSH production by the anterior pituitary.
Day 14 – The peak in LH levels triggers the maturation of the follicle and the release of the mature egg cell from the ovary (ovulation).
Days – The follicle cells left behind in the ovary form a glandular body called the corpus luteum (yellow body) that continues to secrete progesterone and estrogen. These two hormones promote the thickening and vascularization of the endometrium wall, making conditions ideal for a growing embryo to implant itself into the wall and get nutrition from it via the placenta. High amounts of estrogen and progesterone also inhibit the anterior pituitary gland from producing FSH and LH to prevent the development and release of other egg cells. If fertilization does not happen, the corpus luteum degenerates. Deprived of a source of progesterone, the endometrium loses its blood supply and begins to slough off and the cycle begins again.

13. Performance-enhancing drugs