HORMONES

Coordinating Systems in Mammals

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• Mammals possess two main coordinating systems:

  • Nervous System: Rapid response.

  • Endocrine System: Slow response.

• Both systems interact to maintain a constant internal environment and respond to varying external conditions.

• Example of chemical messengers: Adrenaline (hormone) and Acetylcholine (neurotransmitter).

Differences Between Nervous and Endocrine Systems

Nervous System

• Parts: Central Nervous System (CNS), Peripheral Nervous System (PNS)

• Stimuli: Internal

• Mode: Electrical impulses via neurons

• Speed: Fast

• Coverage: Localized target area

• Duration: Temporary

• Actions: Can be voluntary or involuntary

Endocrine System

• Parts: Glands (ductless)

• Stimuli: Internal, External

• Mode: Chemical substances released into the bloodstream

• Speed: Slow

• Coverage: Widespread

• Duration: Long-lasting

• Actions: Involuntary

Endocrine and Exocrine Systems

• Endocrine System:

  • Secretes products (hormones) directly into the bloodstream (ductless).

  • Examples of glands: pituitary, pineal, thyroid, adrenal, islets of Langerhans.

• Exocrine System:

  • Secretes products into ducts that empty into body cavities or surfaces (e.g., digestive enzymes, sweat).

Categories of Hormones

• Functions:

  • Reproduction

  • Growth

  • Homeostasis

• Hormone secretion is regulated to avoid over/under production.

Characteristics of Hormones

• Regulating chemicals produced by endocrine glands, carried in blood to target cells.

• Types include steroids, proteins, and amino acids; all are characterized by:

  • Ability to pass easily through capillaries.

  • Low molecular weight.

  • Effective at low concentrations.

  • Non-antigenic.

Hypothalamus and Pituitary Gland

• Hypothalamus: Link between nervous and endocrine systems, controls hormone secretion from pituitary gland.

• Pituitary Gland Parts:

  • Anterior and posterior pituitary

Hormones Released by Anterior Pituitary Gland

• Hormones include:

  • Thyroid-Stimulating Hormone (TSH)

  • Follicle-Stimulating Hormone (FSH)

  • Luteinising Hormone (LH)

  • Adrenocorticotrophic Hormone (ACTH)

  • Growth Hormone (GH)

  • Prolactin (PRL)

Functions of Key Hormones

Thyroid-Stimulating Hormone (TSH)

• Stimulates thyroid to release Thyroxine (T4) and Triiodothyronine (T3), regulates metabolism, weight, energy.

Follicle-Stimulating Hormone (FSH)

• Stimulates follicle development in ovaries & spermatogenesis in males.

Luteinising Hormone (LH)

• Stimulates ovulation and development of the corpus luteum in females; stimulates testosterone production in males.

Adrenocorticotropic Hormone (ACTH)

• Stimulates hormone release from adrenal cortex.

Growth Hormone (GH)

• Stimulates growth, protein synthesis, and fat metabolism.

• Inhibited by somatostatin (SST).

Prolactin (PRL)

• Stimulates milk synthesis in mammary glands.

Posterior Pituitary Gland

• Does not synthesize hormones, contains axons from hypothalamus.

• Releases:

  • Antidiuretic Hormone (ADH)

  • Oxytocin

Effects of Key Hormones

Antidiuretic Hormone (ADH)

• Stimulates water reabsorption in kidneys, increases blood pressure.

Oxytocin

• Stimulates uterine contractions during childbirth, milk release.

Hormones from Thyroid, Adrenal, and Pancreas

Thyroid Glands

• Produce T4 and T3, regulate metabolic rate, growth, and temperature.

Adrenal Cortex

• Releases aldosterone (regulates minerals) and glucocorticoids (regulate metabolism).

Pancreas

• Produces insulin (lowers blood sugar) and glucagon (raises blood sugar).

Hormonal Imbalances and Diseases

• Homeostasis maintained by hypothalamus; negative feedback controls hormone release.

• Conditions:

  • Insulin Over secretion: Leads to hypoglycemia.

  • Insulin Under secretion: Causes diabetes mellitus.

  • Thyroxine Over secretion: Causes hyperthyroidism.

  • Thyroxine Under secretion: Leads to hypothyroidism.

  • Growth hormone disruptions: Gigantism and dwarfism.

Mechanism of Hormone Action on Target Cells

General Mechanisms

• Lipid-soluble hormones: Diffuse through the plasma membrane and bind to receptors in the cytoplasm.

• Water-soluble hormones: Bind to membrane-bound receptors, activate second messengers.

Specific Mechanisms

• Steroid hormones: Bind to nuclear receptors, affecting gene transcription.

• Peptide hormones: Trigger second messenger systems.

Control and Coordination in Plants

• Plants grow and develop through meristem cells at tips of roots and shoots.

• Growth regulated by plant hormones:

  • Auxin

  • Gibberellins

  • Cytokinins

  • Abscisic acid

  • Ethylene

Auxin Functions

• Involved in cell elongation, gravitropism, and phototropism.

• Affects root development and apical dominance.

Gibberellin Functions

• Involved in seed germination, fruit maturation, and stem elongation.

Abscisic Acid

• Acts as a stress hormone, regulates stomatal closure.

Ethylene

• Promotes fruit ripening and stress responses.