The Endocrine System Flashcards
Exocrine glands
Release ENZYMES to the external environment through ducts. Include sweat, oil, mucus, and digestive glands.
Endocrine glands
Release HORMONES directly into body fluids. The effects of the endocrine system tend to be slower, less direct, and longer lasting than the nervous system. Endocrine hormones do not move directly to their target tissue, but are released into general circulation.
Receptors
All hormones act by binding to protein receptors. Hormones NEED a receptor, either on the cell membrane or inside the cell.
Each receptor is highly specific for its hormone. Some hormones have receptors on all cells, others have receptors only on specific tissues.
3 types of hormones
- Peptide
- Steroid
- Tyrosine
Peptide hormones
Derived from peptides.
Precursor is made in the rough ER and cleaved in the ER lumen, where it becomes a pro hormone. From there, it goes to the Golgi, where it gets cleaved again and sometimes paired with carbs to become it’s final form. The Golgi (post office!) packages the hormones into secretory vesicles and releases them via exocytosis.
Peptide hormones are water soluble: move freely through the blood, but can’t get through the membrane, so they use a membrane receptor.
Effector
The target cell of the hormone, the cell that the hormone is meant to affect.
Target cell
The cell that the hormone is meant to affect
Intracellular second messenger
One affect of a hormone binding to a receptor maybe to activate an intracellular second messenger such as cAMP, cGMP, or calmodulin.
These chemicals are called second messengers because the hormone is the original, or first messenger, to the cell.
The second messenger activates or deactivates enzymes and or ion channels and often creates a cascade of chemical reactions that amplifies the effect of the hormone. The cascade is one way that a small concentration of hormone can have a significant effect.
Review:
The primary messenger binds to membrane-bound receptors which activates a second messenger. The second messenger triggers a cellular response. Peptide hormones typically utilize a secondary messenger system
The anterior pituitary hormones
FLAGTOP. FSH, LH, ACTH, HGH, TSH, Prolactin
All water soluble.
The posterior pituitary hormones
ADH and oxytocin.
Both water-soluble.
The pancreatic hormones
Glucagon and insulin
Both water-soluble.
Steroid hormones
Derived from and often chemically similar to cholesterol. Formed in a series of steps mainly taking place in the smooth ER and the mitochondria.
Steroids are lipids and typically require a protein transport molecule, carrier protein, in order to dissolve into the bloodstream.
Being lipid soluble, steroids diffuse through the cell membrane of their effector. Once inside the cell, they combined with a receptor in the cytosol, which transports the steroid into the nucleus.
Steroids act at the transcription level.
The typical effect of a steroid hormone is to increase certain membrane or cellular proteins within the factor.
Important steroid hormones for the MCAT
- Glucocorticoids and mineral corticoids of the adrenal cortex- cortisol and aldosterone
- The gonadal hormones- estrogen, progesterone, and testosterone.
(Note that estrogen and progesterone are also produced by the placenta.)
Steroid hormones come only from the adrenal cortex, the gonads, or the placenta.
Tyrosine derivatives
- The thyroid hormones: T3 and T4
T3 and T4 are lipid soluble and must be carried in the blood by plasma protein carriers. They are slowly released to their targets and bind to receptors inside the nucleus. Thyroid hormones increase the transcription of large numbers of genes in nearly all cells of the body.
- The catecholamines formed in the adrenal medulla: epinephrine and norepinephrine
Epinephrine and norepinephrine are water-soluble and dissolve in the blood. They bind to receptors on the target tissue and act mainly through the second messenger cAMP.
All tyrosine derivative hormones are formed by enzymes in the cytosol or on the rough ER
Endocrine glands and negative feedback
Endocrine glands tend to over-secrete their hormones. Some aspect of their effect on the target tissue will inhibit the secretion. The control point of the feedback is the conduct of the effector, not the concentration of hormone. In other words, the gland lags behind the effector.
So if an MCAT question indicates that the patient has high blood glucose, and asks whether high levels of insulin or high levels of glucagon would be expected, the correct answer is the hormone that is responding to the condition, not creating it, in this case- insulin.
Anterior pituitary
Located in the brain beneath the hypothalamus. The hypothalamus controls the release of the interior pituitary hormones. These hormones are carried to the capillary bed of the interior pituitary by small blood vessels. The release of these hormones is, in turn, controlled by various nervous signals throughout the nervous system.
The interior pituitary releases six major hormones and several minor hormones. All of these are peptide hormones.
Hypothalamus
Controls the release of the anterior pituitary hormones with releasing and inhibitory hormones of its own.
Human growth hormone (hGH)
Peptide which stimulates growth in almost all cells of the body. Stimulates growth by increasing episodes of mitosis, increasing cell size, increasing the rate of protein synthesis, mobilizing fat stores, increasing the use of fatty acids for energy, and decreasing the use of glucose.
This is accomplished by increasing amino acid transport across the cell membrane, increasing translation and transcription, and decreasing the breakdown of protein and amino acids.
Adrenocorticotropic hormone (ACTH)
A peptide which stimulates the adrenal cortex to release glucocorticoids via the second messenger system using cAMP. Release of ACTH is stimulated by many types of stress. Glucocorticoids are stress hormones.
Thyroid stimulating hormone (TSH)
A peptide which stimulates the thyroid to release T3 and T4 be at the second messenger system using cAMP. TSH increases thyroid cell size, number, and the rate of secretion of T3 and T4. It is important to note that T3 and T4 concentrations have a negative feedback effect on TSH release, both at the anterior pituitary and the hypothalamus.
Prolactin
A peptide which promotes lactation by the breasts. The reason that milk is not normally produced before birth is due to the inhibitory effects of the production by progesterone and estrogen.
Although the hypothalamus has a stimulatory effect on the release of all other interior pituitary hormones, it mainly inhibits the release of prolactin. The act of suckling, which stimulates the hypothalamus to stimulate the interior pituitary to release prolactin, inhibits the menstrual cycle. It is not known whether or not this is directly due to prolactin. The milk production effect of prolactin should be distinguished from the milk ejection affective oxytocin.
Posterior pituitary
Composed mainly of support tissue for nerve endings extending from the hypothalamus. The hormones oxytocin and ADH are synthesized in the neural cell bodies of the hypothalamus, and transported down axons the posterior pituitary were there released into the blood. Both oxytocin and ADH are small polypeptides.
Antidiuretic hormone (ADH)
AKA vasopressin. A small peptide hormone which causes the collecting ducts of the kidney to become permeable to water, reducing the amount of urine and concentrating the urine. Since fluid is reabsorbed, ADH also increases blood pressure. Coffee and beer are ADH blockers that increase volume.
Oxytocin
A small peptide hormone that increases uterine contractions during pregnancy, and causes milk to be ejected from the breasts.