Endocrine system

Endocrine System

A. Hormones = chemical messengers carried by blood to non-adjacent "target" cells.

1. Overview of hormone functions

- Regulation of growth and development.
- Homestatic control (regulation of internal environment; parameters maintained w/in relatively narrow limits)
- Control of reproductive system processes (ovulation, mestruation, maintanence of pregnancy)
- Effects on Behavior (modification, modulation, initiation of specific patterns)

2. Endocrine glands, endocrine cells and some neurons secrete hormones into tissue fluid from which they diffuse into capillaries. Sources of hormones in humans = the endocrine system [Fig. 37.2]
3. Target cells = cells with receptors for a given chemical messenger that can alter their activities in response to messenger.

- Hormones are transported throughout the body by the blood, but they only elicit responses in target cells.
- Receptors may be on cell surface (non-steroid hormones) or internal to cell in cytoplasm or nucleus (steroid hormones) [cf. Figs. 37.3 and 37.4]

B. Types of Hormones [Table 37.1]

There are two main classes (chemical groups) into which hormones fall:

1. steriods (in vertebrates sythesized from cholesterol by adrenal cortex, testis, ovary and placenta)

[examples: cortisol, estradiol]

2. non-steroid

- amines [epinephrine and norepinephrine]
- peptides [oxytocin, ADH]
- proteins [growth hormone, insulin]
- glycoproteins [FSH, TSH]

C. How hormones work in target tissues.

1. Steriod hormones and thyroid hormones (gene activation) [Fig. 37.3]:

a. Relatively small, lipid soluble. Therefore, diffuse freely into and out of cells.
b. Affect target cells that have receptor proteins in their cytoplasm (or nucleus)
c. Hormone-receptor complex moves into nucleus (if not already there). (Current evidence is that this is an active tranport process and that only target cells have transport mechanism)
d. Complex binds to DNA and stimulates transcription of specific genes (synth. of mRNA) --> protein synthesis.
e. Full effect of hormone manifested over minutes to days.

2. Peptide/protein hormones (second messenger mechanism) [Fig. 37.4]:

a. Do not diffuse into cells.
b. Target cells have specific receptor for hormone on cell surface.
c. Hormone binds to receptor and causes a second messenger to be released within the cell.
d. Second messenger can be

- cyclic nucleotide: c-AMP (rx. inside cell catalyzed by adenylate cyclase) --> many c-AMP molecules
- Ca ion; binding of hormone causes Ca channels in cell membrane to open --> influx of many Ca ions. Ca ions bind to protein = calmodulin.

e. c-AMP molecules or Ca-calmodulin complexes activate many enzymes (= cascade of reactions) --> rapid response to hormone (seconds to minutes).

D. Examples of hormone control

1. Secretion of ADH (antidiuretic hormone) by pitutitary. [Fig. 37.5]

- dehydration (low blood pressure) --> secretion of ADH --> increased retention of water (more water reabsorbed by kidneys).
- cell bodies in region of brain called hypothalamus synthesize ADH. ADH moves down axons to posterior pitutitary; accumulates in axon terminals; released by action potentials. ADH enters bloodstream. = Neurohormone [What are the target cells for ADH?]
- What stops secretion of ADH by posterior pituitary? [As more water is retained by body, blood pressure rises and production of ADH falls off. = Negative feedback]

2. Stimulation of Cortisol Secretion by Adrenal Cortex [Fig. 37.10]

Low blood sugar --> Hypothalamus secretes CRH (corticotropin-releasing hormone) --> Anterior pituitary cells (certain cells only) secrete ACTH (adrenocorticotropic hormone) --> Adrenal cortex secretes cortisol (and other glucocorticoids)

[cortisol prevents sugar uptake by muscle cells]

- Negative Feedback control pathways:

- cortisol inhibits ACTH secretion by adrenal cortex
- cortisol also inhibits CRH secretion by hypothal.

3. Role of Pancreatic Islets [Fig. 37.9]:

- Islets of Langerhans = clusters of endocrine-secreting cells disbursed throughout pancreas. Alpha cells secrete glucagon; beta cells secrete insulin

a. Insulin lowers blood glucose level by stimulating cells (esp. skeletal muscle and fat cells as well as liver cells) to take up glucose; stimulates muscles and liver to store glucose as glycogen.
b. Glucagon raises blood glucose level by stimulating liver to convert glycogen to glucose and by stimulating the conversion of fatty acids and amino acids to glucose.
c. Secretion of insulin and glucagon is controlled directly by the concentration of glucose in the blood. [Normal "fasting level" = 90mg/100ml blood]
d. Insulin-glucagon system is fast-acting for keeping blood glucose level tightly regulated. (Brain cells cannot utilize other nutrients as fuel.)
e. Diabetes mellitus:

- Type II = maturity onset: Insulin receptors cannot bind hormone.
- Type I = juvenile onset = insulin-dependent: Decrease in the number of beta cells --> insulin deficiency. Requires daily injections of insulin.

4. Role of Adrenal Medulla

- Medulla is central portion of adrenal gland which produces epinephrine and norepinephrine [cortex produces cortisol (discussed earlier) along with mineralocorticoids and androgen]

a. Medulla develops from neural tissue and its secretion is controlled by sympathetic nerves.
b. Trigger for increased output is perception of a "threatening" situation resulting in fear or anger. Brain integrates information and stimulates output of medulla directly via sympathetic nerves (ACh stimulates output of epinephrine and norepi.)
c. Effects of epineph., norepineph.

-vasodilation of vessels to brain, muscles, heart
-vasoconstriction of vessels to kidney, skin (paling)
-heart rate speeds up
-mental alertness increases
-fatty acid and glucose levels in blood increase
-muscles contract more strongly

E. Human Endocrine System (similar in most vertebrates)

1. Principal organs (Fig. 37.2):

-Hypothalamus, Pituitary gland (two lobes), Pineal gland, Thyroid gland, Parathyroid glands (4), Thymus gland, Adrenal glands, Pancreas (islets of Langerhans), Ovaries or Testes.

2. Role of Hypothalamus and Pituitary.

- Hypothalamus links nervous and endocrine systems.
- Hypothalamic neurons secrete hormones that regulate the release of hormones from the anterior lobe of the pituitary gland. Pituitary hormones, in turn, control activity of other endocrine glands.
[note: An endocrine gland can be a target organ for hormones secreted by other endocrine glands]
- The pituitary gland has two lobes:

a. Posterior Lobe [Fig. 37.5] contains endings of neurosecretory cells whose cell bodies are located in the hypothalamus.

- peptide hormones secreted = oxytocin and ADH. (These hypothalamic hormones are released directly into the systemic circulation where they act on the kidneys (ADH) or on mammary glands and the uterus (oxytocin)).

b. Anterior Lobe [Fig. 37.6]lu contains endocrine cells that secrete prolactin, growth hormone, and several trophic hormones that stimulate other endocrine glands (see Table 37.2).

-Hypothalamus secretes releasing and release-inhibiting hormones that control the anterior lobe of the pituitary.
- cells of anterior lobe are targets for specific releasing and/or release-inhibiting hormones.
- special portal system: capillaries in hypothalamus pick up hypothalamic hormones ("brain hormones") and carry them to a second capillary bed in the anterior lobe.