Lecture Outline: Signals and the Endocrine System
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- Endocrine System vs. Nervous System
- Control Systems Comparison
- Body's two major control systems: Nervous system and Endocrine system.
- Fundamental difference: Nature of signals.
- Chemical Signals (Hormones vs. Neurotransmitters)
- Nervous System: Uses both electrochemical (action potentials) and chemical (neurotransmitters) signals.
- Endocrine System: Operates on chemical signals only (hormones).
- Neurotransmitters vs. Hormones:
- Can be the exact same chemical.
- Difference lies in release and travel:
- Neurotransmitters: Work locally, released into synaptic cleft, act on immediate cell.
- Hormones: Broadcast throughout the body, released by glands into bloodstream, reach all cells with blood supply.
- Receptor Binding: All chemical signals (neurotransmitters or hormones) require binding to a receptor for an effect.
- Hormone Action Mechanisms
- Steroid Hormones:
- Are lipids, modified cholesterol molecules.
- Can pass directly through phospholipid bilayer of cell and nuclear membrane.
- Receptors located inside the cell (cytoplasm or nucleoplasm).
- Binding causes receptor protein to change shape.
- Often leads to direct gene activation, creating mRNA for protein synthesis.
- Amino Acid-Based Hormones (Majority of Hormones):
- Not lipids, cannot enter the cell.
- Receptors located on the cell surface (membrane-bound proteins).
- Binding causes receptor shape change.
- Activates a second messenger system inside the cell (e.g., cyclic AMP).
- Hormone is the "first messenger," its job done upon binding; second messenger carries the signal internally.
- Major Endocrine Glands and Their Functions
- Hypothalamus and Pituitary Gland
- Pituitary Anatomy and Nomenclature:
- Pituitary gland (also called Hypophysis) is two organs in one.
- Anterior Pituitary (Adenohypophysis): Truly a gland.
- Posterior Pituitary (Neurohypophysis): Extension of the brain, not a gland, made of neurons.
- Hypothalamus-Pituitary Interaction:
- Hypothalamus (part of brain) and Pituitary (endocrine system) meet and overlap.
- Major way the two control systems control each other.
- Posterior Pituitary (Neurohypophysis) Hormones:
- Produces only two hormones.
- Hormones released directly from axon terminals of neurons starting in the hypothalamus.
- Chemicals are called neurohormones because they are released into the bloodstream, not a synaptic cleft.
- Hypothalamohypophysial tract: Bundle of axons connecting hypothalamus to neurohypophysis.
- Specific Neurohormones:
- Antidiuretic Hormone (ADH):
- Named for antagonizing diuresis (high volume dilute urine).
- Causes kidneys to produce small amount of concentrated urine to save water.
- Target tissue: Kidneys.
- Oxytocin:
- Involved in positive feedback (e.g., childbirth, uterine contractions).
- Operates on uterine muscles, causing contractions to push fetus.
- Also involved in milk ejection from mammary glands (different from milk production).
- Anterior Pituitary (Adenohypophysis) Hormones:
- Hormones are classified as releasing or inhibiting hormones.
- Effect is to cause or inhibit other glands from releasing their hormones.
- Do not directly cause ultimate effects; they control other glands.
- All are amino acid-based (peptides or proteins), cannot enter cells, bind to surface receptors.
- Specific Hormones:
- Growth Hormone (GH):
- Important in overall growth.
- Main targets: Bones and muscles.
- Important throughout life, especially during growth.
- Prolactin (PRL):
- Promotes lactation (production of milk).
- Causes mammary glands to create milk (stored, not released).
- Follicle-Stimulating Hormone (FSH) and Luteinizing Hormone (LH):
- Both classified as gonadotropins (act on gonads).
- Produced by both sexes, named for female functions.
- FSH: Stimulates follicle (part of ovary that becomes egg).
- LH: Involved in ovulation, corpus luteum production.
- In males, they perform different functions in testes.
- Thyrotropic Hormone (TH) (also Thyroid-Stimulating Hormone):
- Thyrotropic means acting on the thyroid.
- Target: Thyroid gland.
- Adrenocorticotropic Hormone (ACTH):
- Corticotropic means acting on the cortex.
- Target: Adrenal cortex (superficial part of adrenal glands).
- Thyroid and Parathyroid Glands
- Thyroid Gland Anatomy and Cells:
- Located in the neck, butterfly-shaped, wrapped around thyroid cartilage.
- Isthmus: Narrowing connecting the two "wings."
- Histology: Made of many follicles (sacks).
- Follicle cells (or follicular cells): Form walls of follicles, produce thyroid hormone.
- Parafollicular cells: Located between follicles, produce calcitonin.
- Thyroid Hormones:
- Thyroid Hormone (actually two hormones: T4 or Thyroxine, and T3 or Triiodothyronine).
- T4 has four iodines, T3 has three iodines.
- Exception among amino acid-based hormones: Hydrophobic, can pass through membranes and enter cells (like steroids).
- Main job: Regulate metabolic rate.
- Parathyroid Glands and Hormone:
- Usually four small glands embedded in the posterior thyroid gland.
- Named for being "by the thyroid," but have a completely different function.
- Release Parathyroid Hormone (PTH).
- Calcium Balance Regulation:
- PTH and Calcitonin have opposite effects. PTH is much more important.
- Parathyroid Hormone (PTH):
- Released when blood calcium level falls below normal.
- Tells osteoclasts (bone-breaking cells) to break down bone matrix (hydroxyapatite).
- Releases calcium from bone into blood, raising level back to normal.
- Essential for life (e.g., heart beating, signal molecule).
- Calcitonin:
- Released by parafollicular cells when blood calcium level is too high.
- Tells osteoblasts (bone-building cells) to build bone matrix.
- Takes calcium out of blood to make more bone mineral, lowering blood calcium.
- Adrenal Glands
- Adrenal Gland Anatomy:
- Two glands, named adrenal ("on the kidneys") for their placement, but functionally separate from kidneys.
- Composed of two functionally distinct parts: cortex (superficial) and medulla (deeper).
- Adrenal Medulla Hormones (Short-term Stress Response):
- Innervated directly by the nervous system (neurons from spinal cord).
- Releases epinephrine (also called adrenaline) and norepinephrine (also noradrenaline).
- Short-term stress hormones (fight-or-flight).
- Effects:
- Increase heart rate and blood pressure.
- Liver converts glycogen to glucose (for fuel).
- Dilation of bronchioles (easier airflow).
- Adrenal Cortex Hormones (Prolonged Stress Response):
- Stimulated by hormones, not neurons (humoral response).
- All are steroid hormones (lipids), can enter cells.
- Three layers, secreting different hormones:
- Outer layer: Mineralocorticoids (e.g., Aldosterone).
- Control mineral balance (sodium and potassium).
- Middle layer: Glucocorticoids (e.g., Cortisol).
- Control glucose level in blood (brain's primary fuel).
- Tell rest of body to use proteins and fats as fuel, sparing glucose for the brain.
- Prolonged stress suppresses immune system and can lead to other health issues.
- Deepest layer: Sex Hormones (androgens and estrogens).
- Produced here in addition to gonads.
- Pancreas
- Pancreas: Endocrine and Exocrine Functions:
- Dual organ: Part of endocrine system and digestive system.
- Location: Hairpin loop of the duodenum (first part of small intestine).
- Exocrine function: Produces digestive enzymes (pancreatic juice) released into small intestine via ducts. (Not hormones).
- Pancreatic Islets and Hormones:
- Pancreatic Islets (islands) are endocrine portions.
- Surrounded by exocrine cells.
- Two major types of endocrine cells:
- Alpha cells: Produce glucagon.
- Beta cells: Produce insulin.
- Insulin and glucagon are antagonistic (opposite effects) but both control blood glucose.
- Glucose Homeostasis:
- Glucagon (from alpha cells):
- Released when blood glucose level is too low (e.g., long time since eating).
- Travels to liver, tells liver to break down stored glycogen into glucose and release into bloodstream.
- Raises blood glucose level.
- Insulin (from beta cells):
- Released when blood glucose level is too high (e.g., after absorbing nutrients from a meal).
- Mainly acts on liver and skeletal muscles (not brain).
- Allows non-brain cells to take glucose from blood into themselves.
- Lowers blood glucose level.
- Brain does not require insulin to take up glucose.
- Other Endocrine Glands
- Pineal Gland:
- Part of the brain, cone-shaped, pea-sized.
- Releases melatonin.
- Regulates circadian rhythms (daily rhythms), important for sleep-wake cycles.
- Thymus:
- Located in the mediastinum (middle part of thorax).
- Huge in newborns, shrinks with age.
- Important for development of immunity.
- Releases chemicals for proper maturation of T-cells (T-lymphocytes), a type of white blood cell for foreign invaders.
- Gonads (Primary Sex Organs):
- Testes (male): Both exocrine (sperm) and endocrine (testosterone).
- Ovaries (female): Both exocrine (egg/ovum) and endocrine (estrogen, progesterone).
- Sex hormones (androgens, estrogens, progesterone) also produced in adrenal cortex.
- Androgen: General term for masculizing hormones (andro = man).
- Estrogen: Generates estrus ("heat").
- Progesterone: Promotes gestation/pregnancy (prevents spontaneous abortion).
- Hormonal Control and Interactions
- Renin-Angiotensin System:
- Complex signaling involving multiple organs.
- Triggered by decreased blood volume or pressure.
- Kidneys sense drop, release Renin (enzyme).
- Renin converts Angiotensinogen (from liver) to Angiotensin I.
- Angiotensin I travels to lungs, converted to Angiotensin II by Angiotensin-Converting Enzyme (ACE).
- Angiotensin II causes vasoconstriction, increasing blood pressure.
- Example of inter-system cooperation (urinary, digestive, respiratory, circulatory).
- Atrial Natriuretic Peptide (ANP): Released by heart atria, tells kidneys to release more sodium into urine, opposing this system.
- Stress Response (Short-term vs. Long-term):
- Short-term stress response (fight-or-flight):
- Involves adrenal medulla.
- Stimulated by neurons.
- Releases epinephrine and norepinephrine.
- Prolonged stress response:
- Involves adrenal cortex.
- Stimulated by hormones (humoral).
- Hypothalamus releases releasing hormones → Anterior Pituitary releases ACTH → Adrenal Cortex releases Glucocorticoids.
- Suppresses immune system; chronic stress has profound health effects.
- Interconnectedness of Systems:
- Endocrine glands are spread across many organ systems.
- Examples: Pancreas (endocrine/digestive), Gonads (endocrine/reproductive), Hypothalamus (nervous/endocrine), Thyroid (endocrine/respiratory system proximity).