Chapter 16: The Endocrine System

Comparison of the Endocrine and Nervous Systems

• The endocrine system interacts with the nervous system to coordinate and integrate the activity of most body cells.

• The study of hormones and endocrine organs is known as Endocrinology.

• Comparison between systems (Table 16.1):

  • Nervous System:

    • Initiates responses rapidly.

    • Proves short-duration responses.

    • Acts via action potentials and neurotransmitters.

    • Acts at specific locations determined by axon pathways.

    • Neurotransmitters act over very short distances.

    • Signal strength is coded by the frequency of action potentials.

  • Endocrine System:

    • Initiates responses slowly.

    • Provides long-duration responses.

    • Acts via hormones released into the blood.

    • Acts at diffuse locations—targets can be anywhere blood reaches.

    • Hormones act over long distances.

    • Signal strength is coded by the concentration of those specific hormones.

Major Processes and Gland Types

• Major processes controlled and integrated by the endocrine system include:

  • Reproduction.

  • Growth and development.

  • Maintenance of electrolyte, water, and nutrient balance of the blood.

  • Regulation of cellular metabolism and energy balance.

  • Mobilization of body defenses.

• Exocrine Glands:

  • Produce nonhormonal substances (e.g., sweat, saliva).

  • Possess ducts to carry secretions to a membrane surface (cutaneous or mucous).

• Endocrine Glands:

  • Produce hormones.

  • Ductless; they secrete hormones directly into the surrounding extracellular fluid.

  • Include the pituitary, thyroid, parathyroid, adrenal, and pineal glands.

• The Hypothalamus is considered a neuroendocrine organ.

• Organs containing endocrine tissue: Pancreas, gonads, and placenta.

• Other tissues producing hormones: Stomach, intestine, heart, kidneys, skin, thymus, bone, and adipose tissue.

Chemical Classification of Messengers

• Hormones: Long-distance chemical signals; travel in blood to reach target cells.

• Autocrines: Chemicals that exert effects on the same cells that secrete them.

• Paracrines: Locally acting chemicals that affect neighboring cells.

• Note: Autocrines and paracrines are "local" messengers and are not technically part of the endocrine system.

• Solubility and Structure: A hormone's chemical structure determines its water solubility, which dictates transportation in blood, degradation speed, and receptor type.

  • Amino Acid–Based Hormones: (Amino acid derivatives, peptides, and proteins). Most hormones fall into this class. They are water-soluble (except thyroxine) and cannot cross the plasma membrane.

  • Steroids: Synthesized from cholesterol. They are lipid-soluble, can cross the plasma membrane, and include gonadal and adrenocortical hormones.

Mechanisms of Hormone Action

• Hormones only affect target cells, which are tissue cells possessing specific receptors for that hormone.

• Hormones alter target cell activity by increasing or decreasing the rate of normal cellular processes.

• Changes produced in target cells:

  • Alteration of plasma membrane permeability or membrane potential (opening/closing ion channels).

  • Stimulation of enzyme or protein synthesis.

  • Activation or deactivation of enzymes.

  • Induction of secretory activity.

  • Stimulation of mitosis.

Second-Messenger Systems vs. Direct Gene Activation

• Water-Soluble Hormones (Amino acid–based except thyroid hormone): Act on plasma membrane receptors. Most are coupled via G proteins to second messengers.

• Lipid-Soluble Hormones (Steroid and thyroid hormones): Act on intracellular receptors that directly activate genes. They diffuse across the plasma membrane.

• Cyclic AMP (cAMP) Signaling Mechanism:

  1. Hormone (first messenger) binds to a receptor.

  2. Receptor activates a G protein.

  3. G protein activates (or inhibits) the effector enzyme, adenylate cyclase.

  4. Adenylate cyclase converts $ATP$ to $cAMP$ (second messenger).

  5. $cAMP$ activates protein kinases that phosphorylate other proteins.

  • Amplification Effect: One hormone molecule binding to a receptor can activate $10$ G proteins. Each G protein activates an adenylate cyclase enzyme. Each enzyme might produce $500$ molecules of $cAMP$, which each activate a protein kinase. Each kinase phosphorylates $500$ proteins. Result: $1$ hormone yields $2.5 \times 10^6$ phosphorylated proteins.

  • $cAMP$ is rapidly degraded by phosphodiesterase to stop the cascade.

• Direct Gene Activation:

  1. Lipid-soluble hormone diffuses into the cell and binds an intracellular receptor.

  2. The receptor-hormone complex enters the nucleus and binds to DNA.

  3. Binding initiates DNA transcription to produce $mRNA$.

  4. $mRNA$ is translated into specific proteins (metabolic, structural, or secretory).

Control of Hormone Release

• Blood levels of hormones are controlled by negative feedback mechanisms.

• Three types of endocrine gland stimuli:

  • Humoral Stimuli: Changing blood levels of ions and nutrients. E.g., low blood $Ca^{2+}$ concentration stimulates the parathyroid glands to secrete PTH (parathyroid hormone), which raises blood $Ca^{2+}$.

  • Neural Stimuli: Nerve fibers stimulate release. E.g., sympathetic nervous system fibers stimulate the adrenal medulla to secrete catecholamines.

  • Hormonal Stimuli: Hormones stimulate other endocrine organs. Hypothalamic hormones regulate the anterior pituitary, which in turn regulates other glands (the hypothalamic–pituitary–target endocrine organ feedback loop).

• Nervous System Modulation: The nervous system can override endocrine controls. For instance, severe stress causes the hypothalamus and sympathetic nervous system to override insulin, increasing blood glucose for "fight or flight."

Hormone Interactions and Dynamics

• Target Cell Activation factors:

  • Blood levels of the hormone.

  • Relative number of target cell receptors.

  • Affinity: Strength of binding.

• Receptor Regulation:

  • Up-regulation: Target cells add receptors in response to low hormone levels.

  • Down-regulation: Cells remove receptors in response to high hormone levels to prevent over-stimulation.

• Transport: Steroids and thyroid hormones are attached to plasma proteins; others circulate free.

• Half-life: Time required for blood hormone levels to decrease by half (ranges from a fraction of a minute to one week).

• Interactions at Target Cells:

  • Permissiveness: One hormone needs another to be present to work (e.g., reproductive hormones need thyroid hormone).

  • Synergism: More than one hormone produces the same effect, amplifying the response (e.g., glucagon and epinephrine both release glucose).

  • Antagonism: One hormone opposes another (e.g., insulin and glucagon).

The Hypothalamus and Pituitary Gland

• Connected via the infundibulum.

• Posterior Pituitary (Neurohypophysis):

  • Neural tissue that stores and secretes neurohormones produced in the hypothalamus.

  • Connected via the hypothalamic-hypophyseal tract (arises from paraventricular and supraoptic nuclei).

  • Oxytocin: Stimulates uterine contractions during labor and milk ejection during breastfeeding (positive feedback).

  • Antidiuretic Hormone (ADH/Vasopressin): Stimulated by high blood osmolarity (monitored by osmoreceptors). Signals kidneys to reabsorb water. Inhibited by alcohol and adequate hydration.

• Anterior Pituitary (Adenohypophysis):

  • Glandular tissue connected vascularly via the hypophyseal portal system.

  • Secretes six hormones: GH, TSH, ACTH, FSH, LH, and PRL.

Anterior Pituitary Hormones

• Growth Hormone (GH/Somatotropin):

  • Direct actions: Glucose-sparing; increases blood glucose; stimulates lipolysis in adipose tissue.

  • Indirect actions: Triggers liver, muscle, and bone to produce insulin-like growth factors (IGFs), which stimulate nutrient uptake and bone matrix deposition.

  • Regulation: Stimulated by GHRH (Growth hormone–releasing hormone); inhibited by GHIH (Somatostatin).

• Thyroid-Stimulating Hormone (TSH/Thyrotropin): Stimulates thyroid development and activity. Triggered by TRH (Thyrotropin-releasing hormone).

• Adrenocorticotropic Hormone (ACTH/Corticotropin): Stimulates the adrenal cortex to release glucocorticoids. Triggered by CRH (Corticotropin-releasing hormone).

• Gonadotropins (FSH and LH):

  • FSH: Stimulates gamete production.

  • LH: Promotes production of gonadal hormones ($testosterone$ in males; $estrogen/progesterone$ in females) and triggers ovulation.

• Prolactin (PRL): Stimulates milk production. Inhibited by PIH (Dopamine).

Homeostatic Imbalances of Pituitary Hormones

• ADH:

  • Diabetes insipidus: ADH deficiency; causes intense thirst and large urine output.

  • SIADH: Hypersecretion; causes fluid retention and brain swelling.

• GH:

  • Gigantism: Hypersecretion in children (height up to $8\,feet$).

  • Acromegaly: Hypersecretion in adults (thickened bones in hands/face).

  • Pituitary dwarfism: Hyposecretion in children (height up to $4\,feet$).

• ACTH: Hypersecretion leads to Cushing’s disease.

The Thyroid Gland

• Largest pure endocrine gland; butterfly-shaped.

• Follicular cells produce thyroglobulin; Colloid contains thyroglobulin with attached iodine.

• Thyroid Hormone (TH): Body's major metabolic hormone.

  • $T_4$ (Thyroxine): 2 tyrosine + 4 iodine atoms.

  • $T_3$ (Triiodothyronine): 2 tyrosine + 3 iodine atoms; $10 \times$ more active than $T_4$.

  • TH is lipid-soluble and travels bound to thyroxine-binding globulins (TBGs).

  • Calorigenic effect: Increases basal metabolic rate and heat production.

• Imbalances:

  • Myxedema: Hypothyroidism in adults; if due to iodine lack, a goiter (enlarged thyroid) may form.

  • Graves’ Disease: Hyperthyroidism; autoimmune antibodies mimic TSH. Symptoms include exophthalmos (bulging eyes).

• Calcitonin: Produced by parafollicular (C) cells. At high doses, it inhibits osteoclast activity and stimulates $Ca^{2+}$ uptake into bone.

Parathyroid Glands and Calcium

• Usually four glands on the posterior thyroid.

• Parathyroid Hormone (PTH): Most important hormone for $Ca^{2+}$ homeostasis.

  • Stimulates osteoclasts to digest bone matrix.

  • Enhances $Ca^{2+}$ reabsorption in kidneys.

  • Activates Vitamin D to increase intestinal absorption of $Ca^{2+}$.

• Imbalances:

  • Hyperparathyroidism: Causes soft/deformed bones and kidney stones.

  • Hypoparathyroidism: Causes hypocalcemia leading to tetany and convulsions.

Adrenal Glands

• Adrenal Cortex: Three layers producing corticosteroids:

  1. Zona glomerulosa: Mineralocorticoids (Aldosterone).

  2. Zona fasciculata: Glucocorticoids (Cortisol).

  3. Zona reticularis: Gonadocorticoids (Androgens).

• Aldosterone: Regulates $Na^+$ and $K^+$ levels. Reabsorbs $Na^+$ (and $H_2O$) while secreting $K^+$. Regulated by the renin-angiotensin-aldosterone system.

• Cortisol: Helps resist stress and maintain blood glucose through gluconeogenesis (synthesis of glucose from noncarbohydrates).

• Adrenal Medulla: Medullary chromaffin cells secrete epinephrine ($80\%$) and norepinephrine ($20\%$) during the fight-or-flight response.

• Imbalances:

  • Aldosteronism: Hypersecretion causing hypertension and edema.

  • Cushing’s Syndrome: Excess glucocorticoids; results in "steroid diabetes," muscle loss, and easy bruising.

  • Addison’s Disease: Deficits in glucocorticoids and mineralocorticoids; causes bronze skin and weight loss.

  • Adrenogenital Syndrome: Hypersecretion of gonadocorticoids causing masculinization (e.g., beard growth in females).

The Pancreas

• Has exocrine (acinar cells) and endocrine (pancreatic islets) functions.

• Glucagon: Produced by alpha ($\alpha$) cells. Hyperglycemic (raises blood glucose) via glycogenolysis and gluconeogenesis.

• Insulin: Produced by beta ($\beta$) cells. Hypoglycemic (lowers blood glucose). Enhances membrane transport of glucose into cells. Inhibits glycogen breakdown.

• Diabetes Mellitus (DM):

  • Type 1: Hyposecretion of insulin.

  • Type 2: Hypoactivity of insulin.

  • Signs: Polyuria (excess urine), polydipsia (thirst), polyphagia (hunger).

  • Ketoacidosis: Result of using fats for fuel, leading to ketones and lowered blood pH.

• Hyperinsulinism: Causes hypoglycemia, disorientation, and even death.

Hormones from Other Organs

• Adipose Tissue: Leptin (satiety), Resistin (insulin antagonist), Adiponectin (insulin sensitizer).

• Heart: Atrial Natriuretic Peptide (ANP); reduces blood pressure and $Na^+$ concentration.

• Kidneys: Erythropoietin (red blood cell production), Renin (enzyme for blood pressure regulation).

• Skeleton: Osteocalcin (prods pancreas for insulin).

• Skin: Cholecalciferol (precursor to active Vitamin D, calcitriol).

• Thymus: Thymosins/thymopoietins (T lymphocyte development).

• Gastrointestinal Tract: Gastrin, ghrelin, secretin, and cholecystokinin (CCK).

Developmental and Aging Aspects

• Endocrine glands arise from all three germ layers.

• Environmental Pollutants: Pesticides, arsenic, and dioxin can disrupt hormone function, particularly sex hormones, thyroid hormones, and glucocorticoids.

• Aging effects:

  • GH declines (muscle atrophy).

  • Estrogen (females) and testosterone (males) decline.

  • Glucose tolerance deteriorates.

  • TH declines, lowering basal metabolic rate.