Hormones: Definitions and Classification | Zoology Optional Notes for UPSC PDF Download

Hormones: Expanded Definitions and Classification

Evolving Definitions of Hormones

1. Traditional Hormones:

   • Origin: Glands such as adrenals, ovaries, parathyroids, pituitary, testes, and thyroid.
   • Definition: Initially, these were considered the primary physiologically relevant hormones.

2. Contemporary Understanding:

   • Definition Expansion: Today, any substance carrying a signal to induce cellular changes is termed a hormone.
   • Classification: Endocrine, paracrine, and autocrine hormones based on their action and target distance.

Classification of Hormones

1. Endocrine Hormones:

   • Secretion: Directly into the bloodstream.
   • Travel: Over considerable distances through circulation.
   • Target: Distant cells expressing cognate receptors.

2. Paracrine Hormones:

   • Secretion: From a cell.
   • Travel: Short distances to interact with receptors on neighboring cells.

3. Autocrine Hormones:

   • Secretion: By the same cell.
   • Action: Functions as the target for that hormone.

Hormone Secretion

1. Endocrine Hormones:

   • Secretion: Directly into the bloodstream.

2. Exocrine Hormones:

   • Secretion: Directly into a duct.

Cascade System of Hormonal Signaling

1. Signal Pathway:

   • Origin: Often begins with the brain.
   • Transmission: Electrical pulse (action potential) or chemical signals.
   • Limbic System: In many cases, signals are forwarded to the limbic system.
   • Gland Activation: Subsequently reaches the hypothalamus, pituitary, or other glands secreting the final hormone.

2. Hormone Impact:

   • Target Cells: Affects various target cells.
   • Magnitude: Often proportional to the number of cognate receptors.

Hormone Stability

1. Stability Consideration:
   • Long-Distance Travel: Endocrine hormones may be more stable, traveling long distances to target cells.
   • Short-Distance Effects: Autocrine hormones, acting over short distances, may be less stable.

This expanded understanding of hormones reflects their diverse mechanisms and properties, playing a crucial role in maintaining homeostasis in organisms.

Classification of Hormones

Classification Based on Chemical Structure

• Characteristics:

  • Water-soluble.
  • Consist of 3 to over 200 amino acid residues.

• Examples:

  • Hypothalamus and pituitary hormones.
  • Insulin and glucagon of the pancreas.

• Synthesis:

  • Ribosomal synthesis as longer precursor proteins (prohormones).
  • Cleaved to form active peptides during secretion.

2. Amino Acid Derivatives

• Characteristics:
  • Small, water-soluble compounds with amino groups.
• Examples:
  • Adrenaline (adrenal medulla).
  • Thyroid hormones.
• Catecholamine Hormones:
  • Epinephrine and norepinephrine (catecholamines) synthesized from tyrosine.
  • Function as neurotransmitters in the brain and neural tissues.
  • Also synthesized and secreted by adrenal glands.
  • Act through surface receptors, mediating physiological responses to stress.

3. Steroid Hormones

• Characteristics:

  • Fat-soluble (lipophilic).
  • Derived from cholesterol.

• Examples:

  • Adrenal cortical hormones.
  • Androgens (male sex hormones).
  • Estrogens (female sex hormones).

• Synthesis:

  • Derived from cholesterol in various endocrine tissues.
  • Travel through the bloodstream bound to carrier proteins.
  • Corticosteroid hormones produced in the adrenal cortex.

• Types:

  • Glucocorticoids:
    • e.g., Cortisol.
    • Primarily affect carbohydrate metabolism.
  • Mineralocorticoids:
    • e.g., Aldosterone.
    • Regulate concentrations of electrolytes (K⁺¹, Na⁺¹, Ca⁺², Cl^–) in the blood.
  • Androgens and Estrogens:
    • Synthesized in testes and ovaries.
    • Affect sexual development, behavior, and various reproductive and nonreproductive functions.

Mechanism of Hormone Action

Hormones can be categorized based on the mechanism of action, leading to two groups: Group I and Group II, determined by the location of hormone receptors.

Group I Hormones

1. Characteristics:
   • Lipophilic and pass through the lipophilic plasma membrane.
   • Interact with receptors located intracellularly (cytosol or nucleus).
2. Examples:
   • Steroid hormones (e.g., adrenal and gonadal hormones).
   • Thyroid hormones.

Mechanism of Action at Systolic Nuclear Level

• Hormones, such as steroid and thyroid hormones, bind with receptors inside the cell.
• Because these hormones are lipid-soluble, they readily cross the cell membrane.
• Receptors for steroid hormones are mainly in the cytoplasm, and receptors for thyroid hormones are in the nucleus.

Group II Hormones

1. Characteristics:
   • Water-soluble and do not penetrate the lipophilic cell membrane.
   • Receptors located on the outer surface of the target cell (cell surface receptors).

Mechanism of Action at Cell Membrane Receptor Level

• Receptors for Group II hormones are located on the outer surface of the target cell.
• Hormones that bind to surface receptors communicate their action through second messengers.
• The binding of hormones to receptors induces a conformational change, activating G-proteins and second messenger systems.

Second Messenger Systems

1. c-AMP (Cyclic Adenosine Monophosphate):

   • Activated by G-protein.
   • Hormones like epinephrine, glucagon, calcitonin, PTH utilize this second messenger.

2. Phosphatidylinositol/Calcium Second Messenger:

   • Specific G-protein activates phospholipase C.
   • Breakdown of phospholipids produces Inositol triphosphate (IP3) and Diacylglycerol (DAG).
   • IP3 liberates stored intracellular calcium ions, acting as a third messenger.
   • DAG activates enzyme kinase C, influencing physiological processes.

Glands and Hormone Secretion

• Definition:
  • An organ synthesizing substances like hormones for release into the bloodstream (endocrine gland) or body cavities or surfaces (exocrine gland).
• Endocrine Glands:
  • Pituitary Gland secretes hormones such as growth hormone, prolactin, adrenocorticotropic hormone, thyroid-stimulating hormone, luteinizing hormone, and oxytocin.
  • Hypothalamus Gland secretes thyrotrophic-releasing hormone, growth hormone-releasing hormone, corticotrophin-releasing hormone, gonadotropin-releasing hormone.
  • Thymus secretes humoral factors crucial for a healthy immune system during puberty.