chaoter 16a

ENDOCRINE SYSTEM: OVERVIEW

• Course Information: BIOL 2301 ANATOMY AND PHYSIOLOGY II, Brazosport College, Joel Quiros, Ph.D.

16.1 – ENDOCRINE SYSTEM

Overview

• The endocrine system plays a critical role in coordinating and integrating body cell activity.
• It influences metabolic activities through chemical signals (hormones) that are released into the blood.

General Characteristics

• Origin of Terms:
    - Greek:
      - Endo: means within
      - Krino: means to secrete
      - Hormone: means to set in motion
• Comparison with Exocrine Glands:
    - Exocrine glands (e.g., tears, saliva, sweat) secrete products onto a membrane surface using ducts.

Hormone Production

• Hormones are produced in small amounts by collections of cells, secreted into interstitial spaces, and then enter the cardiovascular system to be transported to distant target tissues.
• Hormones act on specific target tissues, influencing their activity in a specific manner.

Hormonal Signal Characteristics

• Hormones can be described as Amplitude-modulated Signals:
    - The total signal produced can vary in concentration over periods (minutes to hours).
    - Responses:
      - Increased or decreased based on hormone concentration.
• Nervous System Characteristics:
    - Uses frequency-modulated signals via all-or-none potentials along axons.
      - Varies in frequency, affecting stimulus strength (low frequency = weak stimulus, high frequency = strong stimulus).
      - Effects last only milliseconds, with local actions regulating body structures.
• There is a recognized intimate relationship between the endocrine and nervous systems, although exceptions exist.

ENDOCRINE GLANDS AND FUNCTIONS

Endocrine Glands

• Major endocrine glands include:
    - Pituitary
    - Thyroid
    - Parathyroid
    - Adrenal
    - Pineal
• The hypothalamus acts as a neuroendocrine organ, providing hormone release alongside neural functions.
• Some organs have both exocrine and endocrine functions (e.g., pancreas, gonads, placenta).
• Other hormone-producing tissues include adipose cells, thymus, and specific cells in the walls of the small intestine, stomach, kidneys, and heart.

Broad Functions

• The endocrine system controls and integrates vital functions such as:
    - Reproduction
    - Growth and Development
    - Maintenance of electrolyte, water, and nutrient balance in blood
    - Regulation of cellular metabolism and energy balance
    - Mobilization of body defenses

CHEMICAL MESSENGERS OF THE ENDOCRINE SYSTEM

Types of Chemical Messengers

1. Autocrine
      - Secreted by cells in a local area; influences activities of the same cell type from which it was secreted.
2. Paracrine
      - Locally acting chemicals that affect cells other than those that secrete them.
3. Hormone
      - Secreted into blood/lymph by specialized cells; travels some distance to target tissues.
      - Autocrines and paracrines are considered local messengers and not part of the endocrine system.

16.2 - HORMONE CHEMICAL STRUCTURE

Hormonal Types

• Amino Acid–Based Hormones:
    - Includes amino acid derivatives, peptides, and proteins.
    - Generally water-soluble, can be stored and released later, and most are metabolized quickly resulting in a short half-life.
    - Thyroxin is an exception.
• Steroids:
    - Synthesized from cholesterol; includes gonadal and adrenocortical hormones.
    - Cannot be stored, thereby showing less fluctuation in bloodstream levels.
    - Examples: estrogen, progesterone, testosterone, aldosterone, cortisol.

HORMONE ACTION

Mechanisms of Hormone Action

• Hormones circulate systemically but only cells with specific receptors for that hormone are affected.
• Target Cells:
    - Tissues that have receptors for specific hormones.
• Hormones alter target cell activity through various mechanisms:
    - Alter plasma membrane permeability and/or membrane potential by opening or closing ion channels.
    - Stimulate synthesis of enzymes or other proteins.
    - Activate or deactivate enzymes.
    - Induce secretory activity.
    - Stimulate mitosis.

Hormonal Action Types

1. Water-Soluble Hormones (all amino acid–based except thyroid hormone):
      - Act on plasma membrane receptors using G protein second messengers.
      - Cannot enter the cell directly.
2. Lipid-Soluble Hormones (steroid and thyroid hormones):
      - Act on intracellular receptors that directly activate genes.
      - Can enter the cell.

16.4 - CONTROL OF HORMONE SECRETION RATE

Hormone Regulation

• Hormonal levels are primarily controlled by negative feedback systems:
    - Increased effects on target organs inhibit further hormone release, thus maintaining homeostasis.
    - May inhibit other hormones to prevent secretion.
• Positive Feedback Mechanisms:
    - Less common, they stimulate further secretion of the original hormone and promote further synthesis in addition to target cell stimulation.
• Hormone release can also be triggered by:
    - Endocrine gland stimuli
    - Nervous system modulation

ENDOCRINE GLAND STIMULI

Stimulation Types

• Hormones are synthesized and released in response to:
    1. Humoral Stimuli
    2. Neural Stimuli
    3. Hormonal Stimuli

HUMORAL STIMULUS

• Changes in blood levels of ions and nutrients directly stimulate hormone secretion.
• Example: Calcium Levels
    - A decrease in blood Ca2+ concentration stimulates parathyroid glands to secrete PTH (parathyroid hormone) which, in turn, causes Ca2+ concentration to rise.

NEURAL STIMULUS

• Nerve fibers stimulate hormone release:
    - Example: Stress/Exercise activates the sympathetic nervous system,
      - Releases epinephrine and norepinephrine from adrenal medulla during stressful situations, which declines once the stressor is removed.

HORMONAL STIMULUS

• Hormones stimulate release of other hormones:
    - Hypothalamic hormones kickstart the release of most anterior pituitary hormones, resulting in a feedback loop where hormones from target organs inhibit anterior pituitary hormone release.

NERVOUS SYSTEM MODULATION

• The nervous system can adjust hormone levels as needed:
    - Modifications can stimulate or inhibit endocrine glands.
    - It can override typical hormonal controls, particularly under severe stress (e.g., allowing blood glucose levels to rise by inhibiting insulin).

16.5 - TARGET CELL SPECIFICITY

Hormone Interaction with Target Cells

• Target Cells: Must possess specific receptors to bind hormones.
    - Eg. ACTH receptors are found only in adrenal cortex; however, thyroxin receptors are widespread.
• Activation Factors:
    1. Blood levels of hormone
    2. Relative number of receptors on/in target cell
    3. Affinity (strength) of binding between receptor and hormone

RECEPTOR MODULATION

• Hormone levels can influence receptor quantities:
    - Up-regulation: Increased receptors in response to low hormone levels, enhancing sensitivity.
    - Down-regulation: Decreased receptors in response to high hormone levels, reducing cell sensitivity to prevent overreacting.

HORMONE METABOLISM AND EXCRETION

Mechanism of Hormonal activity

• Hormonal activity is limited by destruction and elimination:
    - Half-Life: Time required for hormone concentration to reduce to half its original amount, varying from fractions of a minute to a week.

Removal Mechanisms

1. Excretion: Hormones are excreted via urine by the kidneys or by bile from the liver.
2. Metabolism: Enzymatically degraded in blood, liver, kidney, lungs, or target tissues with byproducts excreted in bile and urine.
3. Active Transport: Hormones are actively transported back into cells for later use as neurotransmitters or hormones.
4. Conjugation: Molecules are added to hormones mainly in the liver, typically reducing keyword activity and increasing excretion rates.

INTERACTION OF HORMONES AT TARGET CELLS

Types of Hormonal Interactions

• Permissiveness: One hormone is required for another to exert its effects (e.g., reproductive hormones need thyroid hormone).
• Synergism: More than one hormone produces similar effects on a target cell, amplifying the effect (e.g., glucagon and epinephrine both promoting liver glucose release).
• Antagonism: One or more hormones oppose the action of another hormone (e.g., insulin and glucagon).

16.6 - HYPOTHALAMUS AND PITUITARY GLAND

Hypothalamus

• The hypothalamus is referred to as the "master" gland; it is connected to the pituitary gland via the infundibulum.
• The pituitary gland secretes at least eight major hormones and consists of:
    - Posterior Pituitary: Composed of neural tissue that secretes neurohormones (classified as neurohypophysis).
    - Anterior Pituitary: Composed of glandular tissue (classified as adenohypophysis).

Posterior Pituitary and Hypothalamic Hormones

• Posterior pituitary contains axon terminals from hypothalamic neurons:
    - Paraventricular neurons: Produce oxytocin
    - Supraoptic neurons: Produce antidiuretic hormone (ADH)
• Oxytocin and ADH differ by two amino acids but both consist of nine amino acids.

Antidiuretic Hormone (ADH) / Vasopressin

• The hypothalamus contains osmoreceptors monitoring solute concentrations (normal range: 285-300 mOsm).
• High solute concentration triggers ADH secretion from the posterior pituitary.
• ADH acts on kidney tubules to enhance water absorption, leading to water retention and inhibition of urine formation.
• Release of ADH is also stimulated by pain, low blood pressure, and certain drugs, while it is inhibited by alcohol and diuretics.
• High concentrations result in vasoconstriction, hence also termed vasopressin.

Oxytocin

• Functions include:
    - Stimulating uterine contractions during labor.
    - Responsible for milk ejection during breastfeeding (by contracting smooth muscle around the alveoli in mammary glands).
    - Promotes maternal nurturing and bonding.
    - Triggers for release include stretching of the uterus, mechanical cervix stimulation, or breast stimulation during nursing.

Clinical Note: Homeostatic Imbalance 16.1

• Diabetes Insipidus: ADH deficiency due to hypothalamus or posterior pituitary damage; requires hydration.
• Syndrome of Inappropriate ADH Secretion (SIADH): Leads to fluid retention, headaches, and disorientation. Treatment involves fluid restriction and monitoring sodium levels.

ANTERIOR PITUITARY HORMONES

• Most anterior pituitary hormones are tropic hormones (tropins) that regulate secretion of other hormones.

Hormones Summary Table 16.2

1. Thyroid-Stimulating Hormone (TSH):
      - Structure: Glycoprotein, secreted by thyrotropic cells.
      - Regulation: Stimulated by TRH; inhibited by feedback from thyroid hormones.
      - Target: Thyroid gland.
      - Effects: Stimulates the release of thyroid hormones.
      - Effects of Hyposecretion: Cretinism in children; myxedema in adults.
      - Effects of Hypersecretion: Hyperthyroidism, effects akin to Graves' disease.

2. Adrenocorticotropic Hormone (ACTH):
      - Structure: Peptide, secreted by corticotropic cells.
      - Regulation: Stimulated by CRH; inhibited by feedback from glucocorticoids.
      - Target: Adrenal cortex.
      - Effects: Promotes the release of glucocorticoids and some mineralocorticoids, primarily.
      - Effects of Hyposecretion: Rare.
      - Effects of Hypersecretion: Cushing's disease.

3. Follicle-Stimulating Hormone (FSH):
      - Structure: Glycoprotein, secreted by gonadotropic cells.
      - Regulation: Stimulated by GnRH; inhibited by feedback from inhibin and gonadal hormones.
      - Target: Ovaries and testes.
      - Effects: In females, stimulates ovarian follicle maturation and estrogen production; in males, stimulates sperm production.
      - Effects of Hyposecretion: Failure of sexual maturation.
      - Effects of Hypersecretion: No significant effects.

4. Luteinizing Hormone (LH):
      - Structure: Glycoprotein, secreted by gonadotropic cells.
      - Regulation: Stimulated by GnRH; inhibited by feedback from estrogens, progesterone (females) and testosterone (males).
      - Target: Ovaries and testes.
      - Effects: In females, triggers ovulation and estrogen/progesterone production; in males, promotes testosterone production.
      - Effects of Hyposecretion: Similar to FSH.
      - Effects of Hypersecretion: Inappropriate milk production and cessation of menses in females; impotence in males.

5. Prolactin (PRL):
      - Structure: Protein, secreted by prolactin cells.
      - Regulation: Stimulated by decreasing PIH; enhanced by estrogens and breastfeeding.
      - Target: Breast secretory tissue.
      - Effects: Promotes lactation.
      - Effects of Hyposecretion: Poor lactation in nursing women.
      - Effects of Hypersecretion: Inappropriate milk production (galactorrhea); cessation of menses in females; impotence in males.

• Abbreviations for Regulatory Hormones:
    - GHRH: Growth hormone-releasing hormone
    - GHIH: Growth hormone-inhibiting hormone
    - TRH: Thyrotropin-releasing hormone
    - CRH: Corticotropin-releasing hormone
    - GnRH: Gonadotropin-releasing hormone
    - PIH: Prolactin-inhibiting hormone