Endocrine System - Key Terms (Vocabulary Flashcards)

Communication by the Nervous and Endocrine Systems

• Neuron: basic signaling cell of nervous system.
• Nerve impulse: rapid electrical signal that propagates along a neuron.
• Neurotransmitter: chemical messenger released at synapses to influence target cells.
• Target cells: cells that respond to a particular hormone or neurotransmitter.
• Nervous system vs Endocrine system (overview):
  • Nervous system uses electrical signals and neurotransmitters to affect targeted, rapid responses.
  • Endocrine system uses hormones released into the bloodstream to affect many cells/tissues, often with slower onset but longer duration.
• Endocrine cells: specialized cells that secrete hormones directly into the bloodstream.
• Hormone in bloodstream: the circulating signal that reaches distant target cells.
• Key relationship: Endocrine vs Nervous system interactions and integration in maintaining homeostasis.

Endocrine Organs

• Pineal gland
• Hypothalamus
• Pituitary gland
• Thyroid gland
• Thymus
• Adrenal gland
• Pancreas
• Parathyroid glands
• Gonads:
  • Ovary (female)
  • Testis (male)
• Note: these organs collectively participate in endocrine signaling to regulate metabolism, growth, reproduction, and homeostasis.

Hypothalamus

• Located near lateral ventricle beneath overlying cortex.
• Role: primary regulator of the pituitary via releasing and inhibiting hormones.
• Visual context notes from slides: brain section orientation (e.g., brain stem and cerebellum removal, slight rotation for viewing).

Abbreviations (common hypothalamic–pituitary hormones)

• TRH: thyrotropin-releasing hormone
• CRH: corticotropin-releasing hormone
• GnRH: gonadotropin-releasing hormone
• GHRH: growth hormone-releasing hormone
• TSH: thyroid-stimulating hormone
• PRL: prolactin
• ACTH: adrenocorticotropic hormone
• FSH: follicle-stimulating hormone
• LH: luteinizing hormone
• GH: growth hormone
• PIH: prolactin inhibiting hormone (dopamine)

Embryonic Development of Hypophysis

• Timeline features: at 4 weeks, 8 weeks, 16 weeks.
• Key structures:
  • Neurohypophyseal bud (future posterior pituitary)
  • Hypophyseal pouch (future adenohypophysis/anterior pituitary)
• Connections:
  • Hypothalamus develops and connects to the pituitary via the infundibulum.
  • Pharynx and tongue development in relation to hypophyseal pouch.
• Post-embryonic arrangement: posterior lobe and anterior lobe derive from distinct embryonic origins but functionally coordinate via the hypothalamic hormones.

Pituitary Gland Anatomy and Hormones of Neurohypophysis

• Nuclei of hypothalamus involved in neurohypophysis:
  • Paraventricular nucleus
  • Supraoptic nucleus
• Optic chiasm and third ventricle in proximity.
• Floor of hypothalamus and stalk (infundibulum) connections.
• Neurohypophysis (posterior lobe) stores and releases:
  • Oxytocin
  • Antidiuretic hormone (ADH, also called vasopressin)
• Adenohypophysis (anterior lobe) includes Pars tuberalis and anterior lobe regions.
• Neurohypophyseal tract (hypothalamo-hypophyseal tract) transports hormones to posterior pituitary for release.

Hypothalamo-Hypophyseal Portal System

• Portal system: hypothalamus releases regulatory hormones that travel through the portal vessels to the anterior pituitary.
• Function: allows hypothalamic hormones to regulate anterior pituitary hormone secretion with high local concentration and minimal dilution.

Histology of Pituitary Gland

• Anterior pituitary (Adenohypophysis) features:
  • Chromophobes
  • Basophils
  • Acidophils
• Posterior pituitary (Neurohypophysis) features:
  • Unmyelinated nerve fibers
  • Glial cells (pituicytes)
• Cellular organization reflects two distinct origins and functions (neuroendocrine release vs trophic hormone secretion).

Anterior Pituitary Hormones – Axis concept

• Axis: describes how the hypothalamus–pituitary–target gland axis coordinates endocrine signaling.
• Anterior pituitary secretes multiple tropic and non-tropic hormones in response to hypothalamic releasing hormones.
• Conceptual note: the axis denotes integrated control among hypothalamus, pituitary, and peripheral endocrine glands.

Posterior Pituitary Hormones

• Hormones stored and released from the posterior lobe:
  • Oxytocin
  • Antidiuretic hormone (ADH)
• Organization referencing: median eminence, hypothalamo-hypophyseal tract, stalk, and posterior lobe.
• Interactions with hypothalamic nuclei (PVN and SON) drive release in response to physiological cues.

Feedback from Target Organs

• Negative feedback: rising target organ hormone levels inhibit release of tropic hormones (from hypothalamus and/or pituitary).
• Positive feedback: e.g., stretching of uterus increases oxytocin release, promoting more stretching until delivery.
• Purpose: maintain hormonal balance and prevent over/under production.

Pineal Gland

• Location: attached to roof of the third ventricle, beneath posterior end of corpus callosum.
• Post-puberty involution: shrinks ~75% by end of puberty; remaining tissue is small in adults.
• Function: may synchronize physiology with 24-hour circadian rhythms via melatonin synthesis from serotonin during the night; melatonin fluctuates with day length.

Thymus

• Site of maturation of T cells; important for immune defense.
• Secretes thymic hormones (thymopoietin, thymosin, thymulin) that stimulate development of other lymphatic organs and T-lymphocyte activity.

Thyroid Gland

• Hormones produced: thyroxine (T4) and triiodothyronine (T3).
  • T4 (tetraiodothyronine) contains 4 iodine atoms and is converted into T3.
• Primary actions: increases metabolic rate, O2 consumption, heat production (calorigenic effect), appetite, growth hormone secretion, alertness, and reflex speed.
• Parafollicular (C or clear) cells secrete calcitonin to lower blood calcium.
• Notable anatomical features: inferior thyroid vein, isthmus, thyroid cartilage; proximity to trachea and blood vessels.

Parathyroid Glands

• Secrete parathyroid hormone (PTH).
• Key actions: increase blood Ca^{2+} levels by promoting calcitriol synthesis, increasing Ca^{2+} absorption and calcium reabsorption, decreasing urinary excretion, and increasing bone resorption.
• Nearby anatomy: thyroid gland, esophagus, trachea, pharynx (posterior view).

Calcitriol Synthesis and Action

• Vitamin D pathway:
  • 7-dehydrocholesterol in skin exposed to ultraviolet light converts to Vitamin D3 (cholecalciferol).
  • Vitamin D3 is hydroxylated in liver to form calcidiol.
  • Calcidiol is hydroxylated in kidney to form calcitriol.
• Calcitriol effects: promotes bone resorption (in conjunction with PTH), reduces urinary Ca^{2+} excretion, and enhances intestinal absorption of Ca^{2+} and phosphate.

Steroid Hormones

• Steroids, peptides/glycoproteins, and monoamines (biogenic amines) are the three chemical classes of hormones.
• Examples and relationships:
  • Steroids: derived from cholesterol (e.g., cortisol, aldosterone, estrogen, testosterone).
  • Peptides/Glycoproteins: chains of amino acids (e.g., TSH, insulin).
  • Monoamines: derived from single amino acids (e.g., epinephrine, norepinephrine, thyroid hormones).
• Visual cue from slide shows steroid and amine examples linked to structures (e.g., cholesterol backbone for steroids).

Adrenal Gland

• Structure: adrenal cortex (outer) and adrenal medulla (inner) enclosed by a connective tissue capsule.
• Adrenal cortex zones (from outer to inner):
  • Zona glomerulosa
  • Zona fasciculata
  • Zona reticularis
• Adrenal medulla and its relationship to the sympathetic nervous system.
• Suprarenal vein drainage.

Pancreas

• Pancreatic tissue features:
  • Pancreatic islets (endocrine): alpha, beta, delta cells.
  • Beta cells: insulin
  • Alpha cells: glucagon
  • Delta cells: somatostatin
  • Exocrine pancreas: acinar cells and ducts for digestive enzymes.
• Pancreatic arrangement per slide (islet location amongst exocrine tissue).

Pancreatic Hormones

• Hyperglycemic hormones raise blood glucose: glucagon, growth hormone, epinephrine, norepinephrine, cortisol, corticosterone.
• Hypoglycemic hormone lowers blood glucose: insulin.

The Gonads

• Ovaries and testes function as both endocrine and exocrine glands.
  • Exocrine products: eggs and sperm (cytogenic glands).
  • Endocrine products: gonadal hormones (mostly steroids).
• Ovarian hormones: estradiol, progesterone, inhibin.
• Testicular hormones: testosterone, weaker androgens, estrogen, inhibin.

Endocrine Functions of Other Organs

• Skin, liver, kidneys, heart, stomach and small intestine (enteroendocrine cells), adipose tissue (leptin), osseous tissue (osteocalcin), placenta contribute to endocrine signaling with various hormones.

Regulation of Endocrine Hormone Action

• Three control mechanisms (humoral, neural, hormonal):
  • Humoral: secretion in response to blood composition (e.g., low Ca^{2+} triggers PTH release).
  • Neural: autonomic neurons stimulate endocrine glands (e.g., sympathetic input to adrenal medulla).
  • Hormonal: one gland's hormone stimulates another gland to release hormone.
• Example chains: hypothalamus releases hormones -> pituitary releases tropic hormones -> target glands secrete hormones.

Hormone Chemistry

• Three chemical classes summarized again:
  • Steroids
  • Peptides and glycoproteins
  • Monoamines (biogenic amines)
• Examples include testosterone, estradiol, insulin, angiotensin II, and thyroxine.
• Diagrammatic emphasis on chemical nature and receptor interactions.

Hormone Synthesis: Insulin

• Insulin biosynthesis process:
  • Begins as preproinsulin
  • Proinsulin forms after signal peptide removal
  • Connecting peptide (C-peptide) is removed to yield mature insulin
• The C peptide and insulin are produced in equal amounts within the beta cells of the pancreas.

Hormone Receptors

• Hormones only affect cells that have receptors for them.
• Receptors can be:
  • On the plasma membrane
  • In the cytoplasm
  • In the nucleus
• Receptors act as switches to turn on metabolic pathways when hormone binds.
• Typical target cells possess thousands of receptors for a given hormone.

Hormone Mode of Action

• Transport and receptor locations define mechanism:
  • Hydrophilic hormones: receptors on the plasma membrane; activate second-messenger systems.
  • Hydrophobic hormones: receptors in the nucleus; act as transcription factors.
• Modes summarized:
  • Free hormone (unbound) vs Bound hormone in circulation.
  • Activation of second-messenger pathways in target cells.
• Common second-messenger systems include cAMP and other G-protein coupled pathways.

Thyroid Hormone Effects

• Thyroid hormone forms: T4 (thyroxine) and T3 (triiodothyronine).
• T4 is converted to the more active T3 in target tissues.
• Transport and action involve thyroid-binding proteins in blood (e.g., TBG).
• Effects include broad regulation of metabolic rate, protein synthesis, and tissue-specific actions.

Hydrophilic Hormones Signaling Pathways

• Pathways include DAG/IP3 cascade driven by G protein-coupled receptors.
• Key components:
  • Hormone binds receptor
  • G protein activation
  • Phospholipase C activation
  • Production of DAG and IP3
  • DAG activates protein kinase C (PKC); IP3 elevates intracellular Ca^{2+}
• Consequences: activation of various metabolic and secretory processes.

Hormone Clearance

• Hormone signals must be terminated after their function.
• Major sites of degradation: liver and kidney.
• Excretion: via bile or urine.
• Metabolic clearance rate (MCR): rate of hormone removal from the blood.
• Half-life: time required to clear 50% of hormone from the blood.
• Conceptual relation: faster MCR -> shorter half-life.

Modulation of Target Cell Sensitivity

• Receptor density modulates response magnitude:
  • Up-regulation: increased receptor density -> stronger response and heightened sensitivity.
  • Down-regulation: reduced receptor density -> diminished response.

Hormone Interactions

• Cells may respond to multiple hormones with interaction effects:
  • Synergistic: multiple hormones produce greater combined effect.
  • Permissive: one hormone enhances the action of a second, later hormone.
  • Antagonistic: one hormone inhibits the action of another.

Stress and Adaptation

• Stress definition: any situation that disrupts homeostasis and threatens physical or emotional well-being.
• General adaptation syndrome (GAS): consistent response pattern to stress, typically involving elevated epinephrine and glucocorticoids (especially cortisol).
• Three stages:
  • Alarm reaction
  • Stage of resistance
  • Stage of exhaustion

Paracrine Secretions

• Paracrine messengers diffuse to nearby cells, not produced in neurons and not transported in blood.
• Examples:
  • Histamine: released from mast cells; causes relaxation of vascular smooth muscle.
  • Nitric oxide (NO): from endothelium; causes vasodilation.

Eicosanoids: A Family of Paracrine Secretions

• Leukotrienes, prostacyclin, thromboxanes, and prostaglandins are collectively eicosanoids with diverse roles in inflammation, vascular tone, and hemostasis.

Anti-inflammatory Drugs

• Cortisol and corticosterone are steroidal anti-inflammatory drugs (SAIDs); they inhibit inflammation and eicosanoid synthesis but may cause Cushing-like symptoms with prolonged use.
• NSAIDs (e.g., aspirin, ibuprofen, celecoxib/Celebrex) inhibit prostaglandin and thromboxane synthesis; useful for fever and thrombosis prevention.
• Paracetamol (acetaminophen) is not strongly anti-inflammatory; mainly antipyretic and analgesic.

Diabetes Mellitus

• Definition: most prevalent metabolic disease; disruption of metabolism due to hyposecretion or inaction of insulin.
• Symptoms: polyuria, polydipsia, polyphagia; elevated blood glucose, glucose in urine, ketones in urine.
• Transport maximum: limit to how fast glucose transporters can reabsorb glucose in kidneys.

Types of Diabetes Mellitus

• Type 1 (IDDM): 5–10% of cases in the US.
  • Treatment: insulin therapy (injections, pump, inhaler); glucose monitoring and diet control.
  • Pathophysiology: autoantibodies destroy pancreatic beta cells.
• Type 2 (NIDDM): 90–95% of diabetics.
  • Primary issue: insulin resistance (target cells fail to respond to insulin).
  • Treatment: weight loss and exercise; improves insulin sensitivity; some patients use oral agents to improve insulin secretion or sensitivity.

Pathology of Diabetes

• Fat catabolism increases free fatty acids and ketones in blood.

• Ketosis/ketonuria can lead to osmotic diuresis, loss of Na^+ and K^+, irregular heartbeat, and neurological issues.

• Ketoacidosis may occur as ketone bodies lower blood pH; can lead to deep, gasping breathing and coma.

• Vascular complications: retinal and kidney damage (especially type I); atherosclerosis risk increases for type II.

• Diabetic neuropathy: nerve damage due to poor blood flow; can cause erectile dysfunction, incontinence, poor wound healing, and loss of sensation.

• Notes for exam readiness:

  • Be able to identify major endocrine organs and their primary hormones/functions.
  • Distinguish hypothalamus, pituitary (anterior vs posterior) and their regulatory relationships.
  • Explain negative vs positive feedback in endocrine regulation and provide examples.
  • Describe the three chemical classes of hormones and give examples.
  • Summarize hormone action pathways (hydrophilic vs hydrophobic, receptor locations, second messengers).
  • Recall common disorders (diabetes types, ketoacidosis) and their pathophysiology.

$ext{Key equations / definitions (where applicable):}$

Metabolic clearance rate (MCR): rate of hormone removal from the blood.

  ext{Half-life} igl(t_{1/2}igr): time required to clear 50% of hormone from the blood.

• If concentration over time follows first-order decay: $C(t) = C<em>0 e^{-kt}$, then $t</em>{1/2} = \frac{\ln 2}{k}$.