Endocrine System: Structure and Function

Comparison of the Endocrine and Nervous Systems

The endocrine system and nervous system are the two major regulatory systems of the body, both essential for maintaining homeostasis. They communicate using chemical messengers, but differ in speed, duration, and mechanism of action.

• Nervous system: Uses neurons and neurotransmitters; effects are rapid and short-lived unless stimulation is repetitive.

• Endocrine system: Uses hormones secreted into the bloodstream; effects are slower to develop but longer-lasting.

Example: The nervous system can cause muscle contraction in milliseconds, while the endocrine system may take minutes to hours to alter metabolism or growth.

Hormone Pathways: How Endocrine Hormones Reach Target Cells

Endocrine hormones are secreted by endocrine cells, diffuse into interstitial fluid, enter blood capillaries, and are transported via the circulatory system to distant target cells.

• Hormones leave the bloodstream at capillaries near target tissues, diffuse into interstitial fluid, and bind to specific receptors on target cells.

• Only cells with the appropriate receptor can respond to a given hormone.

Types of Chemical Signals: Endocrine, Paracrine, and Autocrine

Chemical signaling in the body can be classified by the distance to the target cell:

• Endocrine: Hormone enters blood and affects distant target cells.

• Paracrine: Chemical affects nearby cells in the same tissue.

• Autocrine: Chemical affects the same cell that secreted it.

Memory aid: "Para" = nearby, "auto" = self, "endocrine" = distant (via blood).

Endocrine Organs and Tissues

Primary Endocrine Organs

Primary endocrine organs are ductless glands composed of glandular epithelial cells that secrete hormones directly into interstitial fluid for transport by the blood.

• Anterior pituitary gland: Located in the sphenoid bone of the skull.

• Thyroid gland: Anterior neck.

• Parathyroid glands: 3–5 small glands on the posterior thyroid.

• Adrenal cortices: On the superior surface of the kidneys.

• Endocrine pancreas: Left side of the abdominal cavity, mostly posterior to the stomach.

• Thymus: Superior mediastinum.

• Ovaries/Testes: Ovaries in the pelvic cavity; testes suspended below the pelvic cavity.

Secondary and Neuroendocrine Organs

• Secondary endocrine organs: Produce hormones but are primarily part of other systems (e.g., heart, kidneys, small intestine, adipose tissue).

• Neuroendocrine organs: Contain nervous tissue and secrete neurohormones (e.g., hypothalamus, pineal gland, adrenal medulla).

• Some cancer cells can act as secondary endocrine tissues, contributing to paraneoplastic syndrome.

Hormone Chemistry and Transport

Classes of Hormones

• Amino acid-based hormones: Derived from amino acids; most are hydrophilic. Exception: Thyroid hormone is amino-acid-derived but hydrophobic.

• Peptide/protein hormones: Chains of several to hundreds of amino acids; generally hydrophilic.

• Steroid hormones: Derived from cholesterol; hydrophobic and lipid-soluble. Produced by adrenal cortices, testes, and ovaries.

Hormone Transport in Blood

• Free hormones: Small, hydrophilic hormones travel unbound in plasma.

• Bound hormones: Hydrophobic hormones form complexes with plasma proteins, which:

  • Allow transport in watery blood

  • Provide a hormone reservoir

  • Reduce fluctuations in free hormone levels

  • Extend hormone half-life

• Example: Growth hormone is a large hydrophilic hormone that is also protein-bound.

Hormone Receptors and Target Cell Response

Target Cells and Receptors

• Only cells with the correct receptor can respond to a hormone.

• Receptors are specific proteins located in the plasma membrane, cytosol, or nucleus.

• Hydrophilic hormones: Bind to cell-surface (membrane) receptors.

• Hydrophobic hormones: Cross the plasma membrane and bind to intracellular receptors.

Regulation of Receptor Number

• Upregulation: Target cells increase receptor number, increasing sensitivity to a hormone.

• Downregulation: Prolonged high hormone levels cause target cells to decrease receptor number.

Mechanisms of Hormone Action

Hydrophilic Hormones: Second-Messenger Systems

Hydrophilic hormones cannot cross the plasma membrane and use second-messenger systems to transmit their signal inside the cell.

• The hormone (first messenger) binds to a membrane receptor.

• G-proteins are often involved in transmitting the signal to an effector enzyme.

• Adenylate cyclase–cAMP pathway:

  1. Hormone binds receptor; G-protein is activated.

  2. G-protein activates adenylate cyclase.

  3. Adenylate cyclase converts ATP to cAMP (second messenger):

  4. cAMP activates protein kinase A.

  5. Protein kinase A phosphorylates proteins, altering their activity.

• Other pathways involve phospholipase C, producing IP3 and DAG as second messengers.

• Some hydrophilic hormones affect ion channels directly or indirectly via G-proteins.

Hydrophobic Hormones: Intracellular Receptors

Hydrophobic hormones (steroids and thyroid hormone) diffuse across the plasma membrane and bind to intracellular receptors.

• The hormone-receptor complex binds to hormone-response elements on DNA.

• This alters the transcription of specific genes, changing protein synthesis.

Pathway: Hydrophobic hormone → enters cell → binds intracellular receptor → hormone-receptor complex → DNA → altered protein synthesis.

Effects of Hormone Action

• Stimulating secretion from endocrine or exocrine cells

• Activating or inhibiting enzymes

• Stimulating or inhibiting mitosis and/or meiosis

• Opening or closing ion channels, altering membrane potential

• Activating or inhibiting gene transcription

Hormone Interactions

• Complementary actions: Multiple hormones affect different target cells to achieve a common goal.

• Synergists: Hormones act on the same target cell to produce the same effect, with a combined effect greater than either alone.

• Antagonists: Hormones act on the same target cells but have opposite effects.

Hormone Elimination and Regulation

Hormone Half-Life and Elimination

• Half-life: The time required for plasma hormone concentration to decrease by half.

• Hydrophobic hormones have longer half-lives (often a week or more) due to protein binding.

• Hydrophilic hormones have shorter half-lives (seconds to minutes).

• Most hormones are eliminated by the kidneys (urine) or liver (enzymatic breakdown).

Regulation of Hormone Secretion

• Hormonal stimuli: Secretion in response to other hormones (e.g., GHRH stimulates GH; somatostatin inhibits GH).

• Humoral stimuli: Secretion in response to changes in blood/ECF composition (e.g., glucose, calcium).

• Neural stimuli: Secretion in response to nervous system signals (e.g., sympathetic stimulation of adrenal medulla).

• Some hormones are released cyclically under control of molecular clocks.

Negative Feedback Mechanisms

Most endocrine regulation uses negative feedback to maintain homeostasis.

1. Stimulus: A physiological variable deviates from its normal range.

2. Receptor: Endocrine cell receptors detect the deviation.

3. Control center: The endocrine cell or control center changes hormone secretion.

4. Effector/response: The hormone acts on target tissues to move the variable toward normal.

5. Return to normal: Receptors detect normalization; hormone secretion returns to baseline.

Note: Some rare endocrine feedback loops use positive feedback.

Summary Table: Endocrine vs. Paracrine vs. Autocrine Signaling

Quick Memorization List

• Endocrine = hormone enters blood to reach distant target

• Paracrine = nearby; autocrine = same cell

• Endocrine glands are ductless; exocrine glands use ducts

• Thyroid hormone is amino-acid-derived but hydrophobic

• Steroid hormones are cholesterol-derived and hydrophobic

• Hydrophilic hormones use membrane receptors; hydrophobic hormones use intracellular receptors

• Upregulation = more receptors; downregulation = fewer receptors

• cAMP = second messenger; hormone = first messenger

• G-protein → adenylate cyclase → cAMP → protein kinase A → phosphorylation

• Synergists = same effect together; antagonists = opposite effects

• Half-life = time for plasma concentration to fall by half

• Stimuli for secretion: hormonal, humoral, neural

• Negative feedback moves a regulated variable toward its normal range