Hormones and the Endocrine System: Structure, Function, and Regulation

Hormones: General Properties and Types

Introduction to Hormones

Hormones are chemical signals secreted by endocrine cells that regulate physiological processes throughout the body. They can act locally (autocrine or paracrine signaling) or be secreted into the circulatory system for long-distance communication. Only cells with the appropriate receptors can respond to a given hormone.

• Autocrine signaling: Hormone acts on the cell that secreted it.

• Paracrine signaling: Hormone acts on nearby cells.

• Endocrine signaling: Hormone travels through the bloodstream to distant target cells.

• Hormones regulate diverse responses, but generally act more slowly than the nervous system.

Pheromones

Pheromones are chemical signals released into the environment to communicate information between individuals, often affecting behavior or physiology.

• Examples: Synchronization of menstruation, territorial marking in animals, mate attraction in arthropods.

• Female silkworm moths secrete pheromones to attract males, which have sensitive antennae to detect the scent.

Endocrine Pathways

Types of Endocrine Pathways

Pathway	Example	Key Steps
Simple Endocrine	Low blood glucose	Stimulus → Endocrine cell → Target effector → Response
Simple Neuroendocrine	Suckling	Stimulus → Hypothalamus/pituitary → Target effector → Response
Hormone Cascade	Thyroid regulation	Stimulus → Hypothalamus → Pituitary → Endocrine gland → Target effector → Response

Increasing the number of steps allows for multiple levels of regulation.

Tropic vs. Non-Tropic Hormones

• Tropic hormones: Stimulate other endocrine glands to release their hormones (e.g., TSH, ACTH, FSH, LH).

• Non-tropic hormones: Directly affect target cells to induce physiological effects (e.g., prolactin, endorphins, growth hormone).

Hormone Receptors and Signal Specificity

Types of Hormone Receptors

• Transmembrane receptors: For hydrophilic hormones (peptides, proteins, amines).

• Cytoplasmic/nuclear receptors: For hydrophobic hormones (steroids).

Hormone Type	Examples	Receptor Location
Peptides/Proteins/Amines	Oxytocin, LH, FSH, GH, insulin, epinephrine	Plasma membrane
Steroids	Estrogen, testosterone, progesterone, cortisol	Cell nucleus/cytoplasm

Signal Specificity

• One chemical signal can produce different responses depending on the receptor type and cell type.

• Example: Epinephrine causes blood vessel constriction in the intestine (α receptor) and dilation in skeletal muscle (β receptor).

The Endocrine System of Humans

Major Endocrine Organs

• Pituitary gland (master gland)

• Thyroid

• Parathyroids

• Adrenals

• Pancreas

• Ovaries (female)

• Testes (male)

Pituitary Gland: The "Master Gland"

Posterior Pituitary: Neuroendocrine Pathway

• Hormones synthesized in the hypothalamus, released from nerve endings in the posterior pituitary.

• ADH (antidiuretic hormone/vasopressin): Regulates water reabsorption in the kidney.

• Oxytocin: Stimulates labor contractions and milk release.

ADH and Water Reabsorption

• Osmoreceptors in the hypothalamus trigger ADH release in response to increased blood osmolarity.

• ADH increases kidney permeability to water, promoting reabsorption and maintaining homeostasis.

• Ethanol blocks ADH release, leading to dehydration.

Anterior Pituitary: Hormone Cascade Pathway

• Secretes many hormones, both tropic and non-tropic.

• Tropic hormones regulate activity of other endocrine glands.

• Non-tropic hormones directly stimulate target cells.

• Secretion is often stimulated by releasing hormones from the hypothalamus.

Hormone	Target	Effect Type
FSH, LH	Testes/Ovaries	Tropic
TSH	Thyroid	Tropic
ACTH	Adrenal cortex	Tropic
Prolactin	Mammary glands	Non-tropic
MSH	Melanocytes	Non-tropic
Endorphin	Pain receptors	Non-tropic
Growth hormone	Liver, bones	Tropic & Non-tropic

Non-Tropic Hormones

Prolactin

• Stimulates breast development and milk production in mammals.

• Regulates fat metabolism and reproduction in birds; metamorphosis in amphibians; salt/water balance in fish.

Endorphins

• Natural opiates that bind to opioid receptors, reducing pain and producing euphoria.

• Released during stress and pain; mimicked by drugs like morphine.

Growth Hormone: Tropic and Non-Tropic Actions

Actions of Growth Hormone (GH)

• Non-tropic: Directly stimulates amino acid uptake and growth in all cells.

• Tropic: Stimulates liver cells to release insulin-like growth factors (IGFs), promoting bone and cartilage growth.

Growth Disorders

• Underproduction in childhood: Pituitary dwarfism

• Overproduction in childhood: Gigantism

• Overproduction in adulthood: Acromegaly (growth of face, hands, feet)

Negative Feedback Loops

Homeostatic Regulation

• Hormone release is often controlled by negative feedback to maintain internal stability (homeostasis).

• Example: Temperature regulation by a heater and thermostat.

Negative Feedback in Tropic Hormone Release

• External conditions stimulate hypothalamus to release releasing hormone.

• Anterior pituitary releases tropic hormone, which stimulates endocrine gland.

• Endocrine gland releases hormone, which produces physiological responses and inhibits further release via negative feedback.

Thyroid Regulation

TRH, TSH, and Thyroxine

• TRH (Thyrotropin-releasing hormone): Released from hypothalamus.

• TSH (Thyroid-stimulating hormone): Tropic hormone from anterior pituitary.

• Thyroxine (T3, T4): Thyroid hormones that increase basal metabolic rate, stimulate fat breakdown, protein synthesis, and heat production.

Forms of Thyroxine

• T3 (triiodothyronine): Three iodine atoms, more active form.

• T4 (thyroxine): Four iodine atoms, produced in greater quantity.

• T4 can be converted to T3 in blood or target cells.

• Iodine deficiency leads to production of inactive thyroxine, causing goiter.

Thyroid Disorders

• Hypothyroidism: Not enough active thyroxine; symptoms include goiter, low metabolism, sluggishness, and in children, cretinism (retarded growth and mental development).

• Hyperthyroidism: Too much active thyroxine; often due to autoimmune disease (Graves' Disease), causing high metabolism and enlarged thyroid.

Adrenal Hormones and Stress Response

Adrenal Gland Structure and Function

• Adrenal glands consist of the medulla (produces epinephrine and norepinephrine) and cortex (produces cortisol).

• Medulla is under nervous system control; cortex is regulated by ACTH from the anterior pituitary.

Fight or Flight Response

• Epinephrine (adrenaline) initiates rapid physiological changes: increased heart rate, blood pressure, metabolism, and blood flow to muscles.

Catecholamines: Epinephrine and Norepinephrine

• Derived from tyrosine (an amino acid).

• Epinephrine and norepinephrine act as neurotransmitters and hormones, regulating blood pressure and stress responses.

Receptors for Catecholamines

• Bind to GPCRs: α-adrenergic and β-adrenergic receptors.

• Norepinephrine acts mainly on α receptors; epinephrine acts on both α and β receptors.

• Beta blockers are drugs that inhibit β-adrenergic receptors, reducing effects of epinephrine (used in treating hypertension and anxiety).

Summary Table: Major Hormones and Their Functions

Hormone	Source	Target	Main Function
ADH	Posterior pituitary	Kidney	Water reabsorption
Oxytocin	Posterior pituitary	Uterus, mammary glands	Labor, milk release
TSH	Anterior pituitary	Thyroid	Stimulates thyroxine release
ACTH	Anterior pituitary	Adrenal cortex	Stimulates cortisol release
FSH/LH	Anterior pituitary	Gonads	Sex steroid production
Prolactin	Anterior pituitary	Mammary glands	Milk production
Growth hormone	Anterior pituitary	All cells, liver	Growth, IGF release
Epinephrine	Adrenal medulla	Various tissues	Fight or flight response
Cortisol	Adrenal cortex	Various tissues	Stress response, metabolism