Comparison of Control by the Nervous and Endocrine Systems

Overview of Regulatory Systems

The nervous and endocrine systems coordinate the functions of all body systems, acting together as a neuroendocrine system. Each system uses different mechanisms to regulate physiological processes.

• Nervous System: Controls body actions through nerve impulses and neurotransmitters, causing rapid responses such as muscle contraction or gland secretion.

• Endocrine System: Regulates body activities by releasing hormones (mediator molecules) into the bloodstream, affecting metabolism, growth, development, and reproduction.

• Interconnection: The nervous system can stimulate or inhibit hormone release, and hormones can influence nerve impulse generation.

Table Purpose: Table 18.1 compares the characteristics of the nervous and endocrine systems (e.g., speed, duration, target cells).

Endocrine Glands

Types and Functions of Glands

• Exocrine Glands: Secrete products into ducts that carry secretions to target sites (e.g., sweat, salivary glands).

• Endocrine Glands: Secrete hormones into interstitial fluid, which diffuse into capillaries and are transported by blood. Major endocrine glands include the pituitary, thyroid, parathyroid, adrenal, pancreas, kidneys, gastrointestinal organs, and pineal glands.

Hormone Activity

The Role of Hormone Receptors

• Hormones affect only target cells with specific protein or glycoprotein receptors.

• Down-regulation: Excess hormone decreases receptor number.

• Up-regulation: Hormone deficiency increases receptor number.

• Clinical Application: Synthetic hormones can block natural hormone receptors (used as drugs).

Circulating and Local Hormones

• Circulating Hormones (Endocrines): Travel in blood to distant target cells.

• Local Hormones: Act without entering the bloodstream.

  • Paracrines: Affect neighboring cells.

  • Autocrines: Affect the same cell that secreted them.

Chemical Classes of Hormones

• Lipid-Soluble Hormones: Steroids, thyroid hormones, nitric oxide.

• Water-Soluble Hormones: Amines, peptides, proteins, glycoproteins, eicosanoids.

Hormone Transport in the Blood

• Water-soluble hormones: Circulate freely in plasma.

• Lipid-soluble hormones: Bind to transport proteins, which make them temporarily water-soluble, slow their loss in urine, and provide a reserve in blood.

• Clinical Application: Protein and peptide hormones (e.g., insulin) are destroyed by digestive enzymes and must be injected.

Mechanisms of Hormone Action

General Principles

• The response to a hormone depends on both the hormone and the target cell; different cells may respond differently to the same hormone.

Action of Lipid-Soluble Hormones

• Bind to receptors inside target cells.

• Activated receptors alter gene expression, leading to new protein synthesis and physiological responses.

Action of Water-Soluble Hormones

• Bind to plasma membrane receptors (first messenger).

• Activate G-proteins, which stimulate adenylate cyclase to convert ATP to cyclic AMP (cAMP, second messenger).

• cAMP activates protein kinases, which phosphorylate enzymes to produce physiological responses.

• Phosphodiesterase inactivates cAMP, terminating the response.

Equation:

Hormonal Interactions

• Permissive Effect: One hormone enhances the effect of another.

• Synergistic Effect: Two hormones act together for a greater effect.

• Antagonistic Effect: One hormone opposes the action of another.

Control of Hormone Secretions

Regulation Mechanisms

• Hormones are released in short bursts, maintaining homeostasis.

• Secretion is controlled by nervous signals, chemical changes in blood, or other hormones.

• Most hormone secretion is regulated by negative feedback.

Hypothalamus and Pituitary Gland

Structure and Function

• Hypothalamus: Major link between nervous and endocrine systems.

• Pituitary Gland (Hypophysis): Located in the sella turcica of the sphenoid bone; divided into anterior (adenohypophysis), posterior (neurohypophysis), and pars intermedia.

Anterior Pituitary (Adenohypophysis)

• Receives blood via the hypothalamic portal system.

• Cell types and hormones:

  • Somatotrophs: Human growth hormone (hGH)

  • Thyrotrophs: Thyroid-stimulating hormone (TSH)

  • Gonadotrophs: Follicle-stimulating hormone (FSH), Luteinizing hormone (LH)

  • Lactotrophs: Prolactin (PRL)

  • Corticotrophs: Adrenocorticotropic hormone (ACTH), Melanocyte-stimulating hormone (MSH)

• Secretion is regulated by hypothalamic hormones and negative feedback.

Key Hormones and Actions

• hGH: Stimulates synthesis of insulin-like growth factors (IGFs) for growth and metabolism.

• TSH: Stimulates thyroid hormone production.

• FSH: Initiates follicle development and estrogen secretion in females; stimulates sperm production in males.

• LH: Triggers ovulation and progesterone secretion in females; stimulates testosterone secretion in males.

• PRL: Initiates and maintains milk secretion.

• ACTH: Stimulates glucocorticoid secretion from adrenal cortex.

• MSH: Increases skin pigmentation (role in humans unclear).

Posterior Pituitary (Neurohypophysis)

• Stores and releases hormones produced by the hypothalamus: oxytocin (OT) and antidiuretic hormone (ADH).

• Oxytocin: Stimulates uterine contractions and milk ejection.

• ADH: Promotes water reabsorption in kidneys, reducing urine volume; controlled by blood osmotic pressure.

Thyroid Gland

Structure and Hormones

• Located below the larynx; consists of right and left lobes.

• Follicular cells secrete thyroxine (T4) and triiodothyronine (T3); parafollicular cells secrete calcitonin (CT).

Thyroid Hormone Synthesis

• Involves iodide trapping, thyroglobulin synthesis, oxidation, iodination, coupling, colloid digestion, and hormone release.

• Transported in blood by thyroxine-binding globulin (TBG).

Actions and Regulation

• Regulate oxygen use, basal metabolic rate, cellular metabolism, and growth.

• Secretion controlled by iodine levels and negative feedback involving hypothalamus and anterior pituitary.

• Calcitonin: Inhibits osteoclasts, lowering blood calcium; secretion controlled by blood calcium levels.

Parathyroid Glands

Structure and Function

• Located on the posterior thyroid; principal cells produce parathyroid hormone (PTH).

• PTH increases blood calcium, decreases blood phosphate, and promotes calcitriol formation for GI absorption of minerals.

• Secretion regulated by blood calcium via negative feedback (not involving pituitary).

Adrenal Glands

Structure

• Located superior to kidneys; consist of outer cortex and inner medulla.

Adrenal Cortex

• Three zones:

  • Zona glomerulosa: Mineralocorticoids (e.g., aldosterone) regulate sodium and potassium.

  • Zona fasciculata: Glucocorticoids (e.g., cortisol) regulate metabolism, stress resistance, and immune response.

  • Zona reticularis: Androgens (minimal effect in males, more in females).

• Mineralocorticoid secretion controlled by renin-angiotensin pathway and potassium levels.

• Glucocorticoid secretion controlled by CRH and ACTH.

Adrenal Medulla

• Chromaffin cells secrete epinephrine and norepinephrine, producing sympathetic responses under stress.

• Release is controlled by autonomic nervous system innervation.

Pancreatic Islets

Structure and Cell Types

• Located posterior to the stomach; both endocrine (islets of Langerhans) and exocrine (acini) functions.

• Cell types:

  • Alpha cells: Secrete glucagon (raises blood glucose).

  • Beta cells: Secrete insulin (lowers blood glucose).

  • Delta cells: Secrete somatostatin (inhibits insulin and glucagon).

  • F-cells: Secrete pancreatic polypeptide (regulates digestive enzyme release).

• Glucagon and insulin secretion regulated by negative feedback.

Ovaries and Testes

Hormones and Functions

• Ovaries: Produce estrogens and progesterone (female sexual characteristics, reproductive cycle, pregnancy, lactation), inhibin, and relaxin.

• Testes: Produce testosterone (male sexual characteristics, reproductive function) and inhibin.

Pineal Gland and Thymus

Pineal Gland

• Located in the brain; secretes melatonin in a diurnal rhythm linked to the light-dark cycle.

• Overproduction of melatonin is associated with seasonal affective disorder (SAD); bright light therapy can help.

Thymus

• Secretes hormones (thymosin, thymic humoral factor, thymic factor, thymopoietin) that promote T cell proliferation and maturation for immunity.

Other Endocrine Tissues, Eicosanoids, and Growth Factors

Other Endocrine Tissues

• Many tissues not classified as endocrine glands secrete hormones (see Table 18.11 for summary).

Eicosanoids

• Prostaglandins (PGs) and leukotrienes (LTs) act as paracrines and autocrines, altering second messenger production (e.g., cAMP).

• NSAIDs (e.g., aspirin, ibuprofen) inhibit prostaglandin synthesis, reducing inflammation.

Growth Factors

• Stimulate cell growth and division (e.g., EGF, PDGF, FGF, NGF, TAFs, IGF, cytokines).

The Stress Response

General Adaptation Syndrome (GAS)

• Triggered by extreme, unusual, or prolonged stressors (e.g., heat, cold, infection, emotional stress).

• Eustress: Productive stress; Distress: Harmful stress.

Stages of Stress Response

• Alarm Reaction (Fight or Flight): Initiated by hypothalamic nerve impulses to the sympathetic nervous system and adrenal medulla; increases circulation and energy production.

• Resistance Reaction: Initiated by hypothalamic hormones (CRH, GHRH, TRH); leads to ACTH release and adrenal cortex hormone secretion for long-term stress adaptation.

• Exhaustion: Results from depletion of resources, leading to organ dysfunction or death if stress persists.

Stress and Disease

• Chronic stress increases risk for diseases (e.g., ulcers, hypertension, asthma, arthritis, depression).

• Interleukin-1 (IL-1) links stress and immunity by stimulating ACTH secretion.

Developmental Anatomy of the Endocrine System

Embryological Origins

• Pituitary gland: Anterior from oral ectoderm (Rathke’s pouch), posterior from neuroectoderm.

• Thyroid: Endodermal outgrowth from pharynx.

• Parathyroid: Endodermal outgrowths from pharyngeal pouches.

• Adrenal cortex: Intermediate mesoderm; medulla: neural crest (ectoderm).

• Pancreas: Endodermal outgrowth from foregut.

• Pineal gland: Ectodermal outgrowth from diencephalon.

• Thymus: Endoderm of third pharyngeal pouch.

Aging and the Endocrine System

Age-Related Changes

• hGH production decreases; gonadotropins and TSH increase; ACTH unchanged.

• Thyroid output decreases; thymus atrophies at puberty; adrenal glands produce less cortisol and aldosterone.

• Pancreas releases insulin more slowly; receptor sensitivity declines.

• Ovaries shrink and become unresponsive; testosterone decreases but is less problematic.

Disorders: Homeostatic Imbalances

Pituitary Gland Disorders

• Pituitary Dwarfism: hGH hyposecretion in childhood.

• Giantism: hGH hypersecretion in childhood.

• Acromegaly: hGH hypersecretion in adulthood.

• Diabetes Insipidus: ADH hyposecretion; causes excessive dilute urine and dehydration.

Thyroid Gland Disorders

• Congenital Hypothyroidism (Cretinism): Hyposecretion in infancy.

• Myxedema: Hypothyroidism in adults.

• Graves’ Disease: Most common hyperthyroidism; autoimmune.

• Goiter: Enlarged thyroid gland.

Parathyroid Gland Disorders

• Hypoparathyroidism: Causes muscle tetany.

• Hyperparathyroidism: Causes osteitis fibrosa cystica (bone weakening).

Adrenal Gland Disorders

• Cushing’s Syndrome: Hypersecretion of cortisol.

• Addison’s Disease: Hyposecretion of glucocorticoids and aldosterone.

• Pheochromocytomas: Benign medullary tumors causing prolonged fight-or-flight response.

Pancreatic Disorders

• Diabetes Mellitus: Inability to produce or use insulin.

  • Type I: Absolute insulin deficiency.

  • Type II: Down-regulation of insulin receptors.

  • Hyperinsulinism: Excess insulin causes hypoglycemia and possible insulin shock.

Summary Table: Major Endocrine Glands, Hormones, and Actions

Additional info: Some details, such as the full content of referenced tables and figures, were inferred or summarized based on standard Anatomy & Physiology knowledge.