Physiology-March1

Endocrine system

A system of ductless glands that secrete hormones into the bloodstream to regulate metabolism, growth, reproduction, stress responses, and overall homeostasis.

Hormones

Chemical messengers released by endocrine glands into the blood supply that bind to specific target organs or cells to alter their function.

Endocrine vs. Exocrine

Endocrine glands secrete hormones directly into the blood to act intracellularly and change metabolism within cells, whereas exocrine glands use ducts to secrete onto epithelial surfaces such as skin or the GI tract for extracellular functions like digestion or cooling the body.

Nervous vs endocrine

The nervous system produces quick changes through rapid electrical signaling, while the endocrine system acts more slowly but produces longer lasting effects through hormonal signaling.

Neuroendocrine cells

Specialized cells that function as both neurons and endocrine cells; both neurotransmitters (NT) and hormones (HM) act as messengers, and their classification depends on where they are released.

Target organ/cell

The specific organ or cell that possesses receptors for a particular hormone and responds to its signaling.

Hypothalamus

A regulatory region of the brain that controls the pituitary gland by releasing hormones that stimulate or inhibit anterior pituitary secretion.

Hypothalamic hormones

Regulatory hormones produced by the hypothalamus that control the release of anterior pituitary hormones.

Thyrotropin-releasing hormone (TRH)

A hypothalamic hormone that stimulates the anterior pituitary to release thyroid stimulating hormone (TSH).

Corticotropin-releasing hormone (CRH)

A hypothalamic hormone that stimulates the anterior pituitary to release adrenocorticotropic hormone (ACTH).

Gonadotropin-releasing hormone (GnRH)

A hypothalamic hormone that stimulates the release of follicle-stimulating hormone (FSH) and luteinizing hormone (LH).

Growth hormone-releasing hormone (GHRH)

A hypothalamic hormone that stimulates the anterior pituitary to release growth hormone (GH).

Prolactin-inhibiting hormone

A hypothalamic hormone that inhibits the release of prolactin (PRL) from the anterior pituitary.

Somatostatin

A hypothalamic hormone that inhibits the release of growth hormone (GH) and thyroid stimulating hormone (TSH).

Posterior pituitary

The portion of the pituitary gland that does not synthesize hormones but stores and releases oxytocin and antidiuretic hormone produced in the hypothalamus.

Oxytocin

A hormone produced in the paraventricular nuclei of the hypothalamus and released by the posterior pituitary; functions include labor contractions, milk release, ejaculation, sperm transport, bonding, and sexual affection.

Antidiuretic hormone (ADH, Vasopressin)

A hormone produced in the supra optic nuclei of the hypothalamus and released by the posterior pituitary; promotes water retention and causes blood vessel constriction.

Anterior pituitary hormones

Hormones synthesized and secreted by the anterior pituitary that regulate growth, metabolism, and the function of other endocrine glands.

Follicle-stimulating hormone (FSH)

Stimulates estrogen production and follicle development in the ovary (which contains the ova) and stimulates sperm production in the testes.

Luteinizing hormone (LH)

Stimulates ovulation (egg release), corpus luteum development, progesterone secretion in females, and promotes testosterone release from the testes in males.

Thyroid stimulating hormone (TSH)

Stimulates thyroid growth and thyroid hormone secretion, which controls metabolic rate, body temperature, and nerve development.

Adrenocorticotropic hormone (ACTH)

Targets the adrenal cortex to secrete glucocorticoids, which regulate glucose, protein, and fat metabolism.

Prolactin (PRL)

Causes the breasts to produce milk.

Growth hormone (GH)

Promotes widespread growth throughout the body and exerts effects partly through insulin-like growth factors.

Trophic hormones

Hormones whose target organ is another endocrine organ, stimulating that gland to release its own hormones.

Pituitary hormone control

Hormone secretion is mostly regulated by negative feedback mechanisms and can occur at multiple levels including hypothalamus, pituitary, and target gland.

HPA axis

Hypothalamic-pituitary-adrenal axis that regulates the stress response through CRH, ACTH, and cortisol.

HPG axis

Hypothalamic-pituitary-gonadal axis that regulates reproductive hormones.

HPT axis

Hypothalamic-pituitary-thyroid axis that regulates thyroid hormone secretion.

Pituitary hormone release patterns

Hormones are not secreted at constant rates; GH is released mainly at night, and LH peaks mid-menstrual cycle.

Insulin-like growth factors (IGF I & II)

Hormones secreted by the liver in response to GH that have a longer half-life and mediate many growth hormone effects.

Half-life

The length of time a hormone remains active in circulation before being degraded.

Pineal gland

An endocrine gland that produces melatonin.

Melatonin

A hormone important in regulating circadian rhythms and seasonal biological cycles.

Seasonal affective disorder (SAD)

A condition characterized by depression, sleepiness, irritability, and carbohydrate craving, more common in regions with limited sunlight such as Alaska compared to Florida, and treated with 2–3 hours of bright light (phototherapy).

Thymus

An endocrine gland that produces thymopoietin and thymosin, which promote maturation of T-cells and support the immune system.

Thyroid gland

Produces thyroxine (T4), triiodothyronine (T3), and calcitonin.

Thyroid hormone (T3 and T4)

Increases metabolic rate and is important in non-shivering thermogenesis.

Calcitonin

Stimulates osteoblasts and inhibits osteoclasts, resulting in decreased blood calcium levels.

Parathyroid gland

Produces parathyroid hormone.

Parathyroid hormone (PTH)

Increases osteoclast activity, increases calcium absorption, and decreases calcium loss via urination, overall increasing blood calcium levels.

Adrenal glands

Consist of two sections—the medulla, which secretes epinephrine and norepinephrine, and the adrenal cortex, which secretes corticosteroids.

Adrenal cortex

Divided into three zones that produce distinct classes of corticosteroids.

Zona glomerulosa

Produces mineralocorticoids such as aldosterone.

Aldosterone

Regulates blood pressure and blood volume by controlling mineral and water balance.

Zona fasciculata

Produces glucocorticoids such as cortisol.

Cortisol

Promotes fat and protein catabolism, gluconeogenesis, stress adaptation, tissue repair, and anti-inflammatory effects; excessive levels suppress the immune system.

Zona reticularis

Produces gonadocorticoids such as DHEA.

DHEA (de-hydro-epi-andro-sterone)

A gonadocorticoid that can convert to testosterone and also contributes to estradiol production.

Pancreatic islets (islets of Langerhans)

Endocrine portion of the pancreas that produces glucagon and insulin.

Alpha cells

Secrete glucagon, which increases blood sugar through glycogenolysis and gluconeogenesis.

Beta cells

Secrete insulin, which decreases blood sugar and stimulates glycogenesis and lipogenesis.

Gonads

Reproductive endocrine organs consisting of ovaries and testes.

Ovaries

Produce estradiol and progesterone, which regulate the menstrual cycle and secondary sexual characteristics.

Testes

Produce testosterone, which regulates secondary sexual characteristics and libido.

Hormone chemical classes

Hormones are classified chemically as steroid hormones derived from cholesterol; monoamine (biogenic amines) and catecholamines; peptides; polypeptides; and glycoproteins.

Secretion control

Hormone release can be neural stimulated, hormonal stimulated (trophic hormones), or humoral stimulated such as low blood sugar triggering glucagon.

Hormone receptors

Receptors have high specificity; thyroid hormones and steroids bind nuclear receptors and act as transcription factors, while most other hormones bind membrane receptors and use second messengers such as cAMP, IP3, DAG, or tyrosine kinase pathways.

Hormone interactions

Hormones may act synergistically (additive effect), permissively (one enables another), or antagonistically (push-pull effect).

Stress

Defined as anything that upsets homeostasis.

General adaptation syndrome (GAS)

The three-stage response to stress consisting of alarm reaction, resistance, and exhaustion.

Alarm reaction

Initial stage of GAS where epinephrine frees up energy to fight or escape and raises blood pressure.

Resistance stage

Occurs after glycogen is exhausted; CRH-ACTH-cortisol pathway breaks down fat and proteins for alternative fuel, but excessive cortisol kills immature B and T cells and stops antibody production.

Exhaustion stage

Occurs when fat reserves are depleted, leading to rapid decline and death.

Eicosanoids

Paracrine and autocrine signaling molecules derived from arachidonic acid via lipoxygenase and cyclooxygenase pathways; include leukotrienes (mediate allergic and inflammatory responses), prostacyclin (inhibits blood clot and vasoconstriction), thromboxanes (promote clotting and constriction during injury), and prostaglandins (diverse effects); NSAIDs inhibit COX I and II.

Endocrine disorders

Result from hypo or hyper secretions; examples include ADH hyposecretion causing diabetes insipidus, hyposecretion of insulin causing diabetes mellitus (type I), hypersecretion often caused by tumors such as pheochromocytoma, Graves disease (autoantibodies mimic TSH), gigantism, pituitary dwarfism, acromegaly, myxedema, endemic goiter, hyperparathyroidism, Cushing syndrome, adrenogenital syndrome, and Addison disease.

Divisions of the Nervous System

The structural organization of the nervous system into the Central Nervous System (CNS) and Peripheral Nervous System (PNS).

Central Nervous System (CNS)

Composed of the brain and spinal cord; serves as the main integration and control center of the nervous system.

Peripheral Nervous System (PNS)

Consists of nerves that connect to the CNS and carry information to and from it.

Nerve tissue

Made up of neurons and neuroglial cells that work together to transmit and support nervous system signals.

Functions of the Nervous System

Includes sensory functions, integrative functions, and motor functions.

Sensory functions

Receptors detect internal changes and external stimuli and send information to the CNS.

Integrative functions

The CNS collects sensory information, processes it, and makes decisions about appropriate responses.

Motor functions

Effectors respond when stimulated by motor neurons, resulting in muscle contraction or gland secretion.

Neuron Structure

The structural components of a neuron include dendrites, a cell body, and an axon.

Dendrites

Short and highly branched nerve fibers that serve as the main receptive surfaces of a neuron; a neuron can have many dendrites.

Axon

A single nerve fiber that conducts nerve impulses away from the cell body; begins as a single fiber but may branch.

Myelin sheath

A lipid-protein insulating covering around larger axons that increases the speed of impulse conduction.

Schwann cells

Neuroglial cells in the PNS that wind around axons and form the myelin sheath, acting as an insulator.

Nodes of Ranvier

Narrow gaps in the myelin sheath between Schwann cells where nerve impulses are regenerated.

Structural Classification of Neurons

Neurons are classified structurally as unipolar, bipolar, or multipolar based on the number of processes extending from the cell body.

Unipolar neurons

Have a single nerve fiber that divides into two branches functioning as an axon and dendrite; commonly form ganglia.

Ganglia

Masses of nervous tissue composed primarily of neuron cell bodies in the PNS.

Bipolar neurons

Have two nerve fibers—one axon and one dendrite—and are found in the eyes, nose, and ears.

Multipolar neurons

Have one axon and many dendrites; represent most neurons in the brain and spinal cord.

Functional Classification of Neurons

Neurons are classified as sensory (afferent), interneurons, or motor (efferent) based on function.

Sensory (afferent) neurons

Carry nerve impulses from the PNS into the CNS; most are unipolar.

Interneurons

Located within the CNS; typically multipolar; link other neurons and transmit impulses from one part of the CNS to another.

Motor (efferent) neurons

Multipolar neurons that carry nerve impulses out of the CNS to effectors in the PNS; stimulate muscles to contract and glands to release secretions.

Functions of Neuroglia Cells

Neuroglial cells fill spaces, support neurons, provide structural frameworks, produce myelin, and carry on phagocytosis; in the CNS they greatly outnumber neurons.

Classification of Neuroglial Cells

Includes microglial cells, oligodendrocytes, astrocytes, ependymal cells, and Schwann cells.

Microglial cells

Support neurons and phagocytize bacterial cells and cellular debris in the CNS.

Oligodendrocytes

Form the myelin sheath in the CNS.

Astrocytes

Located between neurons and blood vessels; provide structural support, join structures, and regulate nutrient and ion concentration.

Ependymal cells

Form an epithelial-like membrane within the CNS associated with the meninges.

Cell Membrane Potential

The electrically charged state of a cell membrane due to unequal ion distribution, resulting in polarization.

Resting Membrane Potential

The membrane potential at rest, characterized by high Na+ concentration outside the cell, high K+ concentration inside, activity of the Na+/K+ pump, and a value of approximately -70 mV.

Na+/K+ pump

A membrane protein that helps Na+ leave and K+ enter the cell, maintaining resting membrane potential.

Threshold potential

The membrane potential of approximately -55 mV at which an action potential is triggered.

Depolarization

A change in membrane potential that makes the inside of the cell less negative, often due to Na+ channels opening and Na+ diffusing inward.