Study Notes on Chemical Coordination and Integration

CHEMICAL COORDINATION AND INTEGRATION

Overview of Coordination in the Body

  • The neural system provides rapid point-to-point coordination among organs.

    • This coordination is fast but short-lived.

  • The endocrine system complements the neural system by providing long-lasting regulation through hormones.

    • Hormones are required as nerve fibers do not innervate all cells continuously.

  • Together, the neural and endocrine systems regulate physiological functions in the body.

19.1 ENDCRINE GLANDS AND HORMONES

  • Endocrine glands are ductless and their secretions are called hormones.

  • Classical definition of hormones: Chemicals produced by endocrine glands, released into the blood, and transported to target organs.

  • Current scientific definition: Hormones are non-nutrient chemicals that act as intercellular messengers, produced in trace amounts.

    • This definition includes new molecules aside from those from organized endocrine glands.

  • Invertebrates have simple endocrine systems with few hormones; vertebrates have a wide range of hormone-producing chemicals.

19.2 HUMAN ENDOCRINE SYSTEM

  • Composed of various endocrine glands and hormone-producing cells/tissues throughout the body.

  • Major endocrine glands include:

    • Pituitary

    • Pineal

    • Thyroid

    • Adrenal

    • Pancreas

    • Parathyroid

    • Thymus

    • Gonads (testis in males, ovary in females)

  • Other organs such as the gastrointestinal tract, kidney, and heart also produce hormones.

19.2.1 The Hypothalamus

  • Basal part of the diencephalon, forebrain.

  • Regulates various body functions via neurosecretory cells called nuclei that produce hormones.

  • Two types of hormones produced:

    • Releasing hormones: Stimulate secretion of pituitary hormones.

    • Example: Gonadotrophin Releasing Hormone (GnRH) stimulates gonadotrophins.

    • Inhibiting hormones: Inhibit secretion of pituitary hormones.

    • Example: Somatostatin inhibits growth hormone release.

  • Hypothalamic hormones reach the pituitary through a portal circulatory system, impacting anterior pituitary function.

  • The posterior pituitary is directly regulated by the hypothalamus.

19.2.2 The Pituitary Gland

  • Located in a bony cavity called sella turcica, connected to hypothalamus by a stalk.

  • Divided into two parts:

    • Adenohypophysis (anterior pituitary):

    • Produces hormones such as:

      • Growth Hormone (GH): Stimulates body growth; excess leads to gigantism; deficiency causes pituitary dwarfism.

      • Prolactin (PRL): Regulates mammary gland growth and milk production.

      • Thyroid Stimulating Hormone (TSH): Stimulates synthesis and secretion of thyroid hormones.

      • Adrenocorticotrophic Hormone (ACTH): Stimulates steroid hormone secretion from adrenal cortex.

      • Luteinizing Hormone (LH) and Follicle Stimulating Hormone (FSH): Control gonadal activity, impacting sexual function.

    • Neurohypophysis (posterior pituitary):

    • Stores and releases hormones like:

      • Oxytocin: Stimulates uterine contractions and milk ejection.

      • Vasopressin (Antidiuretic Hormone, ADH): Promotes water reabsorption in kidneys; deficiency causes Diabetes Insipidus.

19.2.3 The Pineal Gland

  • Located on the dorsal side of the forebrain.

  • Secretes melatonin, which regulates:

    • Circadian rhythms (sleep-wake cycle).

    • Various bodily functions including metabolism, pigmentation, and menstrual cycles.

19.2.4 The Thyroid Gland

  • Composed of two lobes on either side of the trachea, connected by isthmus.

  • Structure includes follicles and stromal tissues.

  • Produces two main hormones:

    • Thyroxine (T4) and Triiodothyronine (T3).

    • Iodine is essential for their synthesis; deficiency causes hypothyroidism and goitre.

    • Hypothyroidism leads to developmental issues like cretinism in infants and irregular menstrual cycles in adult women.

    • Hyperthyroidism caused by excess T4/T3 leads to abnormal physiological functions, such as Graves’ disease characterized by exophthalmic goitre.

  • Regulates metabolic processes, affecting carbohydrate, protein, and fat metabolism, along with water and electrolyte balance.

  • Also secretes thyrocalcitonin (TCT), which lowers blood calcium levels.

19.2.5 The Parathyroid Gland

  • Four glands located posteriorly to the thyroid gland.

  • Secretes parathyroid hormone (PTH), which aids in:

    • Increasing blood calcium levels by:

    • Stimulating bone resorption.

    • Enhancing renal absorption of calcium.

    • Increasing dietary calcium absorption.

  • Acts as a hypercalcemic hormone alongside TCT to maintain calcium homeostasis.

19.2.6 The Thymus

  • Located between the lungs and behind the sternum.

  • Functions primarily in the immune system development.

  • Secretes thymosins, crucial for:

    • Differentiation of T-lymphocytes for cell-mediated immunity.

    • Production of antibodies for humoral immunity.

  • Degeneration occurs with age, reducing thymosin production and weakening immune responses.

19.2.7 The Adrenal Gland

  • One pair located atop each kidney.

  • Composed of:

    • Adrenal Medulla: Secretes epinephrine (adrenaline) and norepinephrine (noradrenaline), known as catecholamines.

    • Released in response to stress (fight or flight), raising heartbeat and respiration rates, dilating pupils, and stimulating glycogen breakdown.

    • Adrenal Cortex: Has three layers:

    • Zona reticularis (inner)

    • Zona fasciculata (middle)

    • Zona glomerulosa (outer)

    • Secretes corticoids:

      • Glucocorticoids (mainly cortisol): Affect carbohydrate metabolism, anti-inflammatory reactions, and immune response suppression.

      • Mineralocorticoids (mainly aldosterone): Regulate electrolyte and water balance.

      • Androgens: Influence reproductive growth during puberty.

19.2.8 The Pancreas

  • A composite gland acting as both exocrine and endocrine.

  • Endocrine function via Islets of Langerhans, containing:

    • α-cells: Secrete glucagon, increasing blood sugar levels.

    • β-cells: Secrete insulin, which lowers blood sugar levels by promoting glucose uptake.

    • Glucagon acts mainly on the liver to stimulate glycogenolysis and gluconeogenesis.

  • Insulin deficiency or resistance leads to diabetes mellitus, characterized by hyperglycemia.

19.2.9 The Testis

  • Located within the scrotal sac, serving as both a primary sex organ and endocrine gland.

  • Composed of seminiferous tubules and interstitial tissue containing Leydig cells.

  • Produces androgens, particularly testosterone:

    • Regulates development and maturation of male reproductive organs and secondary sexual characteristics.

    • Influences sexual behavior and enhances muscular growth and protein metabolism.

19.2.10 The Ovary

  • Located in the abdomen, functioning as the primary female sex organ.

  • Produces estrogens and progesterone:

    • Estrogens: Support growth of female sex organs and characteristics.

    • Progesterone: Essential for maintaining pregnancy and stimulating mammary gland development.

19.3 HORMONES OF HEART, KIDNEY, AND GASTROINTESTINAL TRACT

  • Hormones are also secreted by non-endocrine tissues:

    • Heart: Atrial natriuretic factor (ANF) lowers blood pressure by causing vascular dilation.

    • Kidney: Erythropoietin stimulates red blood cell formation (erythropoiesis).

    • Gastrointestinal Tract: Secretes hormones such as:

    • Gastrin: Stimulates gastric acid and pepsinogen secretion.

    • Secretin: Activates water and bicarbonate ion secretion from the pancreas.

    • Cholecystokinin (CCK): Stimulates pancreatic enzyme secretion and bile from the gall bladder.

    • Gastric Inhibitory Peptide (GIP): Inhibits gastric secretion and motility.

  • Other non-endocrine tissues secrete growth factors, essential for tissue growth and repair.

19.4 MECHANISM OF HORMONE ACTION

  • Hormones exert effects on target tissues by binding to specific proteins called hormone receptors.

  • There are two types of receptors:

    • Membrane-bound receptors: Found on the cell membrane; too large to enter the cell directly. They generate second messengers (e.g., cyclic AMP, IP3, Ca²⁺) that modify cellular activity.

    • Example: Protein hormones interact with membrane-bound receptors.

    • Intracellular receptors: Located within the cell, usually nuclear. They bind hormones that can pass through cell membranes and regulate gene expression.

    • Example: Steroid hormones and thyroid hormones.

  • Each receptor is specific to one hormone, forming a hormone-receptor complex that initiates biochemical changes in target tissues, influencing metabolism and physiological functions.

SUMMARY

  • Hormones provide coordination and regulation in the body, influencing metabolism and growth.

  • Key components of the endocrine system:

    • Hypothalamus, Pituitary, Pineal, Thyroid, Adrenal, Pancreas, Parathyroid, Thymus, Gonads.

  • Each gland and hormone plays crucial roles in maintaining physiological balance and facilitating bodily functions.

Exercises

  1. Define the following:

    • Exocrine gland

    • Endocrine gland

    • Hormone

  2. Diagrammatically indicate the location of various endocrine glands.

  3. List the hormones secreted by:

    • Hypothalamus

    • Pituitary

    • Thyroid

    • Parathyroid

    • Adrenal

    • Pancreas

    • Testis

    • Ovary

    • Thymus

    • Atrial wall of the heart

    • Kidney

    • Gastrointestinal tract

  4. Fill in the blanks regarding hormones and their target glands.

  5. Write short notes on specific hormones such as PTH, thyroid hormones, thymosins, androgens, estrogens, insulin, and glucagon.

  6. Provide examples of specific hormonal actions and deficiency impacts, like hyperglycemic and hypoglycemic hormones.

  7. Discuss the mechanism of action of FSH and similarities among hormone actions.

  8. Match specified hormones with their corresponding sources or functions as indicated in the provided table format.