Endocrine System Quiz

Endocrine System Overview

• The endocrine system and nervous system are specialized for communication and coordination in humans.
• The endocrine system uses hormones, whereas the nervous system uses neurotransmitters.
• Four principal mechanisms of cellular communication exist: gap junctions, neurotransmitters, paracrines, and hormones.
• Endocrine system comprises glands, tissues, and cells that secrete hormones.
• Endocrinology studies this system and its disorders.
• Endocrine glands are traditional hormone sources.
• Hormones are chemical messengers transported by blood, stimulating physiological responses in distant target cells.

Endocrine vs. Exocrine Glands

• Exocrine glands have ducts, carrying secretions externally (e.g., digestive tract). Secretions have extracellular effects.
• Endocrine glands lack ducts; contain dense, fenestrated capillary networks facilitating hormone uptake. Secretions have intracellular effects.

Nervous vs. Endocrine Systems

• Speed and persistence of response differ significantly. Nervous system responds rapidly and effects are short-lived; endocrine system response is slow but effects can persist for extended periods.
• Adaptation to long-term stimuli differs. Nervous system quickly adapts, while endocrine system adapts more slowly.
• Effects are targeted and specific in the nervous system; general and widespread in the endocrine system.
• Several chemicals (e.g., norepinephrine, dopamine, ADH) function as both hormones and neurotransmitters, or have similar effects on target cells.
• The two systems interact; neurotransmitters can affect glands, and hormones affect neurons.
• Neuroendocrine cells share characteristics of both systems.
• Target cells are those possessing receptors for a hormone to respond to it.

The Hypothalamus

• Functions as a control center for primitive functions (childbirth, sex drive) and works with the pituitary gland.
• Connects to the pituitary via the infundibulum (a stalk).
• Located in the floor and walls of the third ventricle.
• Anterior pituitary: anterior ¾ of the pituitary; connected to hypothalamus by hypophysial portal system (primary-secondary capillaries).
• Posterior pituitary: posterior ¼ of pituitary; nerve tissue, not a true gland and stores/releases hormones from hypothalamus.
• Hypothalamic hormones affect the anterior pituitary (e.g., TRH, CRH, GnRH, GRH, PIH, somatostatin) and the posterior pituitary (OT, ADH).

Pituitary Gland

• Located in the sella turcica of the sphenoid bone.
• Shaped like a kidney bean.
• Composed of two structures (anterior and posterior pituitary) with independent origins and function.

Anterior Pituitary Hormones

• Six primary hormones are synthesized and secreted: FSH, LH, TSH, ACTH, PRL, and GH.
• Gonadotropins (FSH and LH) regulate gonads.
• TSH stimulates thyroid hormone secretion.
• ACTH stimulates adrenal cortex glucocorticoid secretion.
• PRL stimulates mammary gland milk synthesis.
• GH stimulates cell mitosis and differentiation.

Posterior Pituitary Hormones

• Two hormones produced in the hypothalamus and stored/released in posterior pituitary: ADH (vasopressin) and oxytocin.
• ADH increases water retention, reducing urine and preventing dehydration.
• Oxytocin is released during sexual arousal and orgasm, childbirth, and lactation, fostering bonding.

Control of Pituitary Secretion

• Rates vary; regulated by the hypothalamus, brain, and target organs.

Negative Feedback

• Increased target organ hormone levels inhibit hypothalamic and/or pituitary hormone release to maintain homeostasis.

Growth Hormone (GH)

• Widespread effects; boosts protein synthesis, lipid metabolism, and carbohydrate metabolism.
• Induces liver to release IGFs (insulin-like growth factors) to stimulate target cells.
• GH has a short half-life; IGFs have a longer half-life, prolonging action.
• Impacts bone growth, thickening, and remodeling, particularly during development.
• Secretion is high during initial sleep hours and can be triggered by exercise or hunger.
• GH levels decline with age.

Pineal Gland

• Located beneath the posterior end of the corpus callosum.
• Undergoes involution (shrinkage) after age 7.
• Synthesizes melatonin at night.
• Influences puberty timing and circadian rhythms.
• Seasonal affective disorder (SAD) symptoms are reduced by light exposure.

Thymus Gland

• Bilobed gland superior to the heart in the mediastinum; involved in the endocrine, lymphatic, and immune systems.
• Undergoes involution after puberty.
• Site of maturation for T cells.
• Secretes hormones (thymopoietin, thymosin, thymulin).

Thyroid Gland

• Largest purely endocrine gland.
• Two lobes and an isthmus below the larynx, dark reddish brown.
• Thyroid follicles contain colloid, surrounded by follicular cells (simple cuboidal epithelium).
• Secretes thyroxine (T4) and Triiodothyronine (T3) in response to TSH.
• Increases metabolic rate, oxygen consumption, heat production, appetite, growth hormone secretion, alertness, and reflex speed.
• Parafollicular cells secrete calcitonin to regulate blood calcium.

Parathyroid Glands

• Embedded in the posterior thyroid.
• Secrete parathyroid hormone (PTH) to increase blood calcium.
• Promotes calcitriol synthesis for calcium absorption.

Adrenal Glands

• Composed of cortex and medulla.
• Medulla: inner core, modified sympathetic neurons (chromaffin cells). Secretes epinephrine, norepinephrine, and dopamine, in response to stress.
  • Increases alertness, mobilizes energy, has a glucose-sparing effect, and has widespread impact on body function (blood pressure, heart rate, blood flow, metabolic rate).
• Cortex: secretes corticosteroids from three zones:
  • Zona glomerulosa: mineralocorticoids (aldosterone) regulates electrolyte balance, especially sodium content.
  • Zona fasciculata: glucocorticoids (cortisol) regulate metabolism of glucose and other fuels; promotes fat and protein catabolism and gluconeogenesis.
  • Zona reticularis: androgens for libido and prenatal male development.

Pancreatic Islets

• Endocrine tissue interspersed in exocrine pancreas.
• Alpha cells secrete glucagon (raises blood glucose).
• Beta cells secrete insulin (lowers blood glucose).
• Delta cells secrete somatostatin (partially suppresses glucagon and insulin).
• Pancreatic polypeptide (PP) acts as an inhibitor of digestion and absorption.
• Hyperglycemic hormones raise blood glucose (glucagon, GH, epinephrine, norepinephrine, cortisol, corticosterone)
• Hypoglycemic hormones lower blood glucose (insulin)

Gonads

• Ovaries and testes are both endocrine and exocrine organs.
• Ovarian hormones: Estrogen, progesterone, and inhibin.
• Testicular hormones: testosterone, androgens, estrogen, and inhibin.
• Gonads promote development of reproductive systems and regulate cycles (e.g., menstrual, pregnancy).

Other Tissues and Organs

• Various tissues besides traditional endocrine glands produce hormones (e.g., skin, liver, kidneys, heart, stomach, small intestine, adipose tissue, osseous tissue, placenta).

Hormone Chemistry

• Hormones are broadly classified into three groups: steroids, monoamines, and peptides.

Hormone Transport

• Hydrophilic hormones (peptides, monoamines) mix easily with blood plasma.
• Hydrophobic hormones (steroids, thyroid) bind to transport proteins, prolonging half-life in blood, protecting from breakdown, and ensuring only unbound hormones affect target cells.

Hormone Receptors and Mode of Action

• Target cells have specific receptors for hormones. Surface receptors for peptide and catecholamine hormones trigger intracellular second messenger systems; nuclear receptors for steroid and thyroid hormones affect gene expression.

Signal Amplification

• Hormone signals are potent; they can activate many enzyme molecules.

Modulation of Target Cell Sensitivity

• Receptor up-regulation increases sensitivity, receptor down-regulation reduces sensitivity.

Hormone Interactions

• Synergistic, permissive, and antagonistic interactions may occur between hormones affecting target cells.

Hormone Clearance

• Hormones are removed from the blood by the liver and kidneys to regulate blood levels.

Stress and Adaptation (General Adaptation Syndrome)

• Stress upsets homeostasis, triggering a response.
• Alarm reaction: characterized by elevated epinephrine and norepinephrine.
• Stage of resistance: dominated by cortisol to provide alternate fuels.
• Stage of exhaustion: occurs with prolonged stress.

Eicosanoids and Paracrine Signaling

• Paracrine and autocrine signaling involves local messengers (eicosanoids, histamine, nitric oxide). -Eicosanoids, like prostaglandins, regulate various physiological processes.
• NSAIDS inhibit eicosanoid synthesis.

Endocrine Disorders

• Hyposecretion and hypersecretion can result in various diseases (e.g., diabetes mellitus, pituitary, thyroid, parathyroid, adrenal disorders).

Pituitary Disorders

• Acromegaly (excess GH in adults), gigantism (excess GH in children), pituitary dwarfism (GH deficiency).

Thyroid and Parathyroid Disorders

• Hypothyroidism (low TH) and hyperthyroidism (high TH) manifest with different symptoms.
• Goiter: enlargement of thyroid gland, often due to iodine deficiency.
• Hypoparathyroidism and hyperparathyroidism cause imbalances in blood calcium.

Adrenal Disorders

• Cushing syndrome (excess cortisol), adrenogenital syndrome (excess adrenal androgens).

Diabetes Mellitus

• Caused by insufficient insulin or resistance to insulin. -Type 1 and type 2 are forms of the disease.
• Pathogenesis, chronicity, and treatment vary for the two forms.