Integrative Physiology: The Endocrine System Study Notes

Introduction to Endocrinology

• Function and Purpose of Hormones

  • Hormones act as cell-to-cell communication molecules, integral to physiological responses.

  • Affect physiological activities such as rates of enzymatic reactions, transport of ions, and gene expression.

• Classification, Structure, and Synthesis of Hormones

  • Hormones can be classified into:

  • Peptide Hormon

  • Steroid Hormones

  • Amine Hormones

  • Hormone synthesis involves pre-hormones, pro-hormones, and active hormones, predominantly produced within specific glands.

• Pathways of Nervous to Endocrine Regulation

  • Involves integration of signals from the nervous system that lead to hormone release.

• Effects of Hormone Interactions

  • Interactions can be synergistic, permissive, or antagonistic, influencing target cell response.

• Pathologies of the Endocrine System

  • Disorders may arise from either hypersecretion (too much hormone) or hyposecretion (too little hormone).

Anatomy Summary: Hormones

• Table Overview—Location, Hormones, Primary Targets, and Main Effects:

  • Pineal Gland:

  • Hormone: Melatonin

  • Primary Targets: Brain, other tissues

  • Main Effects: Regulates circadian rhythms, immune function, acts as an antioxidant.

  • Adrenal Cortex:

  • Hormones: Cortisol, Aldosterone, Sex Hormones

  • Primary Targets: Many tissues, kidneys, blood vessels

  • Main Effects: Stress response, regulates blood pressure, metabolism.

  • Stomach and Small Intestine:

  • Hormones: Gastrin, Cholecystokinin, Secretin

  • Primary Targets: GI tract and pancreas

  • Main Effects: Aid in digestion and nutrient absorption.

  • Endocrine Cells: Secrete insulin, glucagon, etc., regulating glucose metabolism.

Chemical Regulating Systems

• Hormones:

  • Defined as cell-to-cell communication molecules.

  • Made in glands or specific cells, released into the blood and travel to distant tissues where they bind to specific receptors.

  • They exert physiological responses generally at very low concentrations.

• Pheromones:

  • Defined as organism-to-organism communication signals, often influencing social behaviors.

Hormones: Function

• Control of enzymatic reactions and transport:

  • Hormones adjust enzymatic rates and facilitate transport of ions across cell membranes.

  • Regulate gene expression and protein synthesis.

• Binding to Target Cell Receptors:

  • Hormones bind specifically to receptors on target cells to exert their effects.

• Half-life:

  • Indicates the length of time a hormone remains active in the body, influencing dosage and responsiveness.

Hormones: Classification

• Peptide or Protein Hormones

• Steroid Hormones

• Amine Hormones

Hormones: Peptides or Proteins

• Preprohormone: Large inactive form; the precursor of all peptide hormones.

• Prohormone: Active form resulting from post-translational modification.

• Peptide Hormone-Receptor Complex:

  • Involves signal transduction, activating physiological responses upon binding to the surface receptors of target cells.

Table 7-1: Comparison of Peptide, Steroid, and Amino Acid-Derived Hormones

Hormones: Peptide Hormone Synthesis, Packaging, and Release

1. Preprohormone is synthesized in ribosomes, directed into the ER lumen.

2. Signal sequence is removed to create an inactive prohormone.

3. Prohormone is processed in the Golgi apparatus, packaged in secretory vesicles.

4. Active peptide hormones and fragments are produced and released via exocytosis into the extracellular space for circulation.

Peptide Hormone-Receptor Complex

• Hormones bind to surface receptors, activating second messenger systems that elicit cellular responses.

Steroid Hormones: Features

• Derivatives of cholesterol that are lipophilic and can easily cross cell membranes.

• Typically bind to intracellular receptors in the cytoplasm or nucleus to regulate gene expression and protein synthesis.

• Examples include cortisol, estrogen, and testosterone.

Endocrine Control

• Hormonal secretion is regulated at three levels:

  1. Hypothalamic Stimulation - Central nervous system controls.

  2. Pituitary Stimulation - Regulated by hypothalamic hormones.

  3. Endocrine Gland Stimulation - Controlled by pituitary hormones.

Thyroid Gland Overview

• Structure: Butterfly-shaped gland with follicular and parafollicular cells.

• Follicular Cells: Produce T3 (triiodothyronine) and T4 (thyroxine).

• Parafollicular Cells: Produce calcitonin.

Thyroid Hormone Functions and Regulation

• Functions:

  • Affect metabolic rate, enhance catecholamine actions, and support growth and development.

• Regulation:

  1. Low levels of T3/T4 stimulate release of TRH and subsequently TSH.

  2. Elevated levels of T3 inhibit TRH and TSH via negative feedback.

Calcium Homeostasis

• Parathyroid Gland Function: Releases PTH to regulate calcium levels in the bloodstream.

  • PTH increases blood calcium by mobilizing it from bones, increasing renal reabsorption, and stimulating calcitriol production.

• Calcitonin: Released to lower blood calcium by inhibiting resorption of bone and promoting excretion in the kidneys.

Adrenal Glands

• Comprised of the cortex (outer) and medulla (inner).

• Cortex: Synthesizes glucocorticoids (cortisol), mineralocorticoids (aldosterone), and sex steroids (androgens).

• Medulla: Produces catecholamines such as epinephrine and norepinephrine, involved in the stress response.

Endocrine Pathologies

• Examples include:

  • Hypersecretion: Excess hormone production due to tumors (e.g., Cushing’s Syndrome).

  • Hyposecretion: Insufficient hormone levels leading to conditions like diabetes.

• Receptor Abnormalities: Can cause issues with hormone response, as seen in conditions like hyperinsulinemia or testicular feminization syndrome.

Screening Tests for Cushing's Syndrome

• Dexamethasone suppression test: Determines cortisol secretion levels.

• Assessment of plasma cortisol with specific values indicating pathology based on the response to dexamethasone.