Hormonal Pathways and Control Mechanisms

Introduction to Hormonal Control

• Hormones are chemical messengers regulating physiological processes.
• Pathways involve complex feedback mechanisms to maintain homeostasis.

Feedback Control of Tropic Hormones

• Tropic Hormones: Regulate other endocrine glands.
• Negative Feedback (Most Common): Response counteracts stimulus (e.g., high T3/T4 inhibits TRH/TSH).
• Positive Feedback (Less Common): Response amplifies stimulus (e.g., Oxytocin during childbirth).
• Hierarchical Control: Hypothalamus → Pituitary → Target Gland.

Pineal Gland & Thymus Hormones

• Pineal Gland:
  • Melatonin: Regulates circadian rhythms, sleep-wake cycle.
• Thymus:
  • Thymosin, Thymulin, Thymopoietin: T-lymphocyte maturation, immune system development.

Thyroid Gland Hormones

• Thyroxine (T4) & Triiodothyronine (T3):
  • Functions: Regulate metabolic rate, growth, development, protein synthesis.
  • Regulation: TRH-TSH-T3/T4 axis with negative feedback.
• Calcitonin:
  • Function: Lowers blood calcium levels.

Parathyroid Gland Hormones

• Parathyroid Hormone (PTH):
  • Functions: Increases blood calcium levels (hypercalcemic hormone).
  • Stimulates osteoclast activity, enhances calcium reabsorption in kidneys, activates vitamin D.
  • Regulation: Inversely related to blood calcium levels.

Pancreatic Hormones

• Insulin (Beta cells):
  • Functions: Lowers blood glucose by promoting cellular uptake and utilization.
• Glucagon (Alpha cells):
  • Functions: Increases blood glucose by promoting glycogenolysis and gluconeogenesis.
• Somatostatin (Delta cells): Inhibits insulin and glucagon release.

Adrenal Gland Hormones

• Adrenal Cortex:
  • Mineralocorticoids (Aldosterone): Na⁺ retention, K⁺ excretion, BP regulation.
  • Glucocorticoids (Cortisol): Glucose metabolism, anti-inflammatory, stress response.
  • Androgens: Secondary sex characteristics.
• Adrenal Medulla:
  • Epinephrine (Adrenaline) & Norepinephrine (Noradrenaline): Fight-or-flight response.

Gastrointestinal Tract Hormones

• Gastrin: Stimulates gastric acid secretion.
• Secretin: Stimulates pancreatic bicarbonate secretion, inhibits gastric acid.
• GIP (Glucose-dependent Insulinotropic Peptide): Stimulates insulin release.
• CCK (Cholecystokinin): Stimulates gallbladder contraction, pancreatic enzyme secretion.

Gonadal Hormones

• Male (Testes):
  • Testosterone: Male sex characteristics, spermatogenesis.
  • Inhibin: Inhibits FSH release.
• Female (Ovaries):
  • Estrogen: Female sex characteristics, endometrial proliferation.
  • Progesterone: Pregnancy maintenance, mammary gland development.
  • Inhibin: Suppresses FSH release.

Mechanisms of Hormone Action

• cAMP-Mediated Pathway (Water-soluble hormones):
  • Hormone binds to cell surface receptor → Activates G-protein → Activates adenylyl cyclase → Forms cAMP (second messenger) → Activates PKA → Cellular response.
• Steroid Hormone Mechanism (Lipid-soluble hormones):
  • Hormone crosses cell membrane → Binds to intracellular receptor → Hormone-receptor complex enters nucleus → Binds to HREs on DNA → Regulates gene transcription → Protein synthesis → Cellular response.

Comparison of Mechanisms