The Endocrine System

Basics of Chemical Communication

  • Hormone Definition: A chemical messenger produced by cells of endocrine glands that enters the circulatory system and affects distant cells.

  • Types of Chemical Signals:

    • Autocrine: Released by cells and have a local effect on the same cell type from which the signals were released.

    • Paracrine: Released by cells and affect other cell types locally without being transported in the blood.

    • Neurotransmitter: Produced by neurons and secreted into extracellular spaces by presynaptic nerve terminals; travels short distances to influence postsynaptic cells.

    • Endocrine: Secreted into the circulatory system to affect target tissues located elsewhere in the body.

Characteristics and Functions of the Endocrine System

  • General Characteristics:

    • It is a body control system where regulation requires duration rather than speed.

    • Glands secrete chemical messengers (hormones) directly into the blood.

  • Hormone Specifics:

    • Produced in small quantities.

    • Transported via the circulatory system.

    • Acts on specific target tissues; target cells respond because they possess the correct receptors.

  • Patterns of Hormone Secretion:

    • Acute: Sudden release due to a specific stimulus, such as adrenaline released in response to stress.

    • Chronic: Small variations over long periods, such as thyroid hormones.

    • Episodic: Occurs in cycles, such as estrogen and progesterone during the menstrual cycle.

  • Ten Primary Functions of the Endocrine System:

    1. Metabolism.

    2. Control of food intake and digestion.

    3. Tissue maturation.

    4. Ion regulation.

    5. Water balance.

    6. Heart rate and blood pressure regulation.

    7. Control of blood glucose and other nutrients.

    8. Control of reproductive functions (gametogenesis and pregnancy).

    9. Uterine contractions and milk release.

    10. Immune system regulation.

Comparison of Nervous and Endocrine Systems

  • Similarities:

    • Both are associated with the brain (specifically the hypothalamus in the endocrine system).

    • Both may use the same chemical messenger as a neurotransmitter and a hormone (e.g., epinephrine).

    • The systems are cooperative; for example, the adrenal medulla is directly innervated by the nervous system.

  • Differences:

    • Mode of Transport: Nervous system uses axons; Endocrine system uses blood.

    • Speed of Response: Nervous responses are instant (milliseconds); Endocrine responses are delayed (seconds).

    • Duration of Response: Nervous responses are brief (milliseconds to seconds); Endocrine responses are long-lasting (minutes to days).

The Pituitary Gland and Hypothalamus

  • Interaction Hub: The hypothalamus and pituitary gland represent the site where the nervous and endocrine systems interact.

  • Pituitary Structure:

    • Posterior Pituitary (Neurohypophysis): An extension of the nervous system via the infundibulum. It secretes neuropeptides and is a direct extension of the hypothalamus.

    • Anterior Pituitary (Adenohypophysis): Develops from the embryonic oral cavity and secretes traditional hormones.

  • Hypothalamic Control Mechanisms:

    • Posterior Control: Hormones are produced in hypothalamic neurons and stored in the posterior pituitary. Axons form the hypothalamohypophysial tract. Action potentials trigger hormone release.

    • Anterior Control: Blood vessels form the hypothalamohypophysial portal system. The hypothalamus secretes releasing and inhibiting hormones to regulate anterior pituitary secretions.

Hypothalamic and Anterior Pituitary Hormones

  • Hypothalamic Releasing/Inhibiting Hormones:

    • Growth hormone-releasing hormone (GHRH): Increases GH secretion.

    • Growth hormone-inhibiting hormone (GHIH): Decreases GH secretion.

    • Thyrotropin-releasing hormone (TRH): Stimulates TSH secretion.

    • Melanocyte releasing hormone (MRH): Stimulates MSH secretion.

    • Corticotropin-releasing hormone (CRH): Stimulates ACTH secretion.

    • Gonadotropin-releasing hormone (GnRH): Stimulates secretion of LH and FSH.

    • Prolactin-releasing hormone (PRH): Increases prolactin secretion.

    • Dopamine (Prolactin-inhibiting hormone, PIH): Decreases prolactin secretion.

  • Anterior Pituitary Hormones:

    1. Growth Hormone (GH): Acts on most body cells to influence overall metabolism and growth; stimulates liver and skeletal muscle to produce IGF-1. Levels peak during deep sleep.

    2. Thyroid Stimulating Hormone (TSH): Stimulates thyroid to secrete T3T_3 and T4T_4.

    3. Adrenocorticotropic Hormone (ACTH): Stimulates adrenal cortex to secrete cortisol and aldosterone.

    4. Melanocyte-stimulating Hormone (MSH): Stimulates melanin production in skin cells; also plays roles in appetite and sexual behavior.

    5. Luteinizing Hormone (LH): Regulates gamete and reproductive hormone production.

    6. Follicle-stimulating Hormone (FSH): Regulates gamete production (Spermatogenesis in testes and Oogenesis in ovaries).

    7. Prolactin: Responsible for milk production (lactation) based on supply and demand.

  • Tropic vs. Non-tropic Hormones:

    • Tropic Hormones: Stimulate the secretion of other hormones from target endocrine tissues.

    • Non-tropic Hormones: Directly initiate a biological effect on non-endocrine tissues.

Posterior Pituitary Hormones

  • Antidiuretic Hormone (ADH / Vasopressin):

    • Stimulates reabsorption of sodium and water from the nephrons.

    • Results in less, more concentrated urine to conserve water.

    • Secrection is stimulated by decreased blood pressure (BP).

    • Acts as a vasoconstrictor to increase BP. Consumption of diuretics (tea, coffee, alcohol) inhibits this.

  • Oxytocin:

    • Stimulates uterine contractions during birth (Positive Feedback).

    • Triggers the "let down reflex" (ejection of milk) from the lactating breast.

Feedback Mechanisms in Hormone Secretion

  • Negative Feedback: The endocrine system's primary control mechanism. A hormone from a target endocrine cell travels to the hypothalamus/anterior pituitary to decrease the secretion of the tropic hormone, maintaining homeostasis.

  • Positive Feedback: A hormone from a target endocrine cell travels back to the hypothalamus/anterior pituitary to increase the secretion of the tropic hormone (e.g., oxytocin during labor).

The Thyroid and Adrenal Glands

  • Thyroid Gland Details:

    • Highly vascular; one of the largest endocrine glands.

    • The only gland that stores its hormones.

    • Composed of follicles with follicular cells surrounding thyroglobulin.

    • Requires Iodine and Tyrosine for the production of Triiodothyronine (T3T_3) and Tetraiodothyronine (T4T_4 or Thyroxine).

    • Functions: Increase metabolism of glucose, fat, and protein; increase body temperature; support normal growth.

  • Adrenal Gland Details:

    • Located near the superior poles of the kidneys.

    • Adrenal Cortex: Secretes Cortisol (glucocorticoid), Aldosterone (mineralocorticoid), and weak sex hormones (Androgens) in response to ACTH.

    • Adrenal Medulla: Secretes Epinephrine (Adrenaline) and Norepinephrine (Noradrenaline).

    • Secrection from the medulla is stimulated by stress, physical activity, and low blood glucose via the sympathetic autonomic nervous system. These hormones increase heart rate, contraction force, and cause vasoconstriction in skin and viscera.

The Endocrine Pancreas

  • Location: Retroperitoneal, located near the stomach and intestine.

  • Dual Function:

    • Exocrine: Produces pancreatic digestive juices.

    • Endocrine: Consists of pancreatic islets containing Alpha and Beta cells.

  • Hormone Regulation:

    • Beta Cells: Secrete Insulin after a meal to lower blood glucose.

    • Alpha Cells: Secrete Glucagon when fasting or hungry to increase blood glucose.