Premed · Premed · General Biology 2
Lecture 19: Hormones and the Endocrine System
General Biology II — Organismal, Evolution & Ecology
Learning Objectives
By the end of this lecture, students will be able to:
- Compare the endocrine and nervous systems as mechanisms of communication and control
- Classify hormones by chemical type and describe their mechanisms of action
- Describe the structure and function of the hypothalamic-pituitary axis
- Identify the major endocrine glands, their hormones, and target effects
- Explain feedback regulation of hormone secretion
- Describe hormonal regulation of blood glucose and calcium homeostasis
Lecture Content
I. Endocrine vs. Nervous System Signaling
The endocrine and nervous systems are the two great communication networks of the animal body, and they work in concert to coordinate virtually every physiological process. The nervous system operates with speed and precision: electrical impulses (action potentials) race along neurons and deliver signals to specific target cells via synapses, producing responses within milliseconds that typically last only briefly. The endocrine system, by contrast, communicates through chemical messengers called hormones, which travel through the bloodstream to reach target cells bearing the appropriate receptors. Endocrine signaling is generally slower in onset, more widespread in its reach, and longer-lasting in its effects than neural signaling.
The two systems overlap in important ways. Neuroendocrine cells are neurons that release hormones directly into the blood, and the hypothalamus serves as the critical bridge between the nervous and endocrine systems. Chemical signaling itself comes in several forms. Endocrine signaling involves hormones that travel through the blood to act on distant target cells. Paracrine signaling involves local chemical messengers -- such as histamine and prostaglandins -- that act on nearby cells. Autocrine signaling occurs when a cell responds to signals it has itself released. Neurocrine (neuroendocrine) signaling involves neurons that release hormones into the bloodstream, as the hypothalamic neurons do with ADH and oxytocin.
II. Hormone Types and Mechanisms of Action
A. Water-Soluble Hormones
Peptide and protein hormones -- including insulin, glucagon, ADH, and growth hormone -- along with amino acid derivatives such as epinephrine, are hydrophilic molecules that cannot cross the lipid bilayer of the plasma membrane. Instead, they bind to cell-surface receptors and activate intracellular signal transduction cascades. A common pathway involves the hormone binding its receptor, which activates a G protein, which in turn activates adenylyl cyclase. This enzyme converts ATP to cyclic AMP (cAMP), a second messenger that activates protein kinase A, ultimately producing the cellular response. Other second messengers include IP3, DAG, and calcium ions. Because these hormones work through pre-existing signaling machinery rather than requiring new gene expression, their effects are rapid in onset but short in duration. Water-soluble hormones are typically synthesized in advance and stored in secretory vesicles, ready for release by exocytosis when the appropriate stimulus arrives.
B. Lipid-Soluble Hormones
Steroid hormones -- derived from cholesterol and including cortisol, aldosterone, testosterone, estrogen, and progesterone -- along with thyroid hormones, are hydrophobic molecules that can diffuse directly across the plasma membrane. Once inside the cell, they bind to intracellular receptors, often located in the nucleus. The hormone-receptor complex acts as a transcription factor, binding to specific DNA sequences and altering gene expression. Because this mechanism requires the synthesis of new proteins, lipid-soluble hormones produce effects that are slower in onset but longer in duration than those of water-soluble hormones. In the bloodstream, these hydrophobic hormones are transported bound to carrier proteins such as sex hormone-binding globulin. Thyroid hormones represent an interesting exception: though derived from the amino acid tyrosine, they are lipid-soluble, enter cells, and bind nuclear receptors in the manner of steroid hormones.
III. The Hypothalamic-Pituitary Axis
The hypothalamus is the master regulator of the endocrine system, integrating neural input from the brain with hormonal output to the body. It exerts its control largely through the pituitary gland, which in turn governs many other endocrine glands.
A. Posterior Pituitary (Neurohypophysis)
The posterior pituitary is not a true gland -- it does not synthesize hormones of its own. Rather, it stores and releases two hormones that are actually produced by neurosecretory cells in the hypothalamus. Antidiuretic hormone (ADH, or vasopressin) promotes water reabsorption in the kidney collecting ducts and causes vasoconstriction. Oxytocin stimulates uterine contractions during labor, triggers milk ejection (the let-down reflex) during breastfeeding, and plays roles in social bonding.
B. Anterior Pituitary (Adenohypophysis)
The anterior pituitary is a true endocrine gland that synthesizes and secretes its own hormones. It is controlled by releasing and inhibiting hormones produced by the hypothalamus and delivered through the hypophyseal portal system -- a specialized portal blood network that connects the hypothalamus directly to the anterior pituitary. The anterior pituitary produces several major hormones. Growth hormone (GH, or somatotropin) stimulates growth of bone and cartilage, promotes protein synthesis, and triggers the release of insulin-like growth factor 1 (IGF-1) from the liver. Its secretion is regulated by growth hormone-releasing hormone (GHRH, stimulatory) and somatostatin (inhibitory) from the hypothalamus. Excess GH in childhood produces gigantism, while excess in adults causes acromegaly; deficiency in childhood results in pituitary dwarfism. Thyroid-stimulating hormone (TSH) drives the thyroid gland to produce T3 and T4. Adrenocorticotropic hormone (ACTH) stimulates the adrenal cortex to produce cortisol. Follicle-stimulating hormone (FSH) promotes gamete production in both sexes. Luteinizing hormone (LH) triggers ovulation and stimulates sex hormone production. Prolactin (PRL) stimulates milk production in the mammary glands.
<image>A diagram of the hypothalamic-pituitary axis. Panel A: The hypothalamus is shown at the top with neurosecretory cells. Two pathways are illustrated: (1) Posterior pituitary pathway — axons of hypothalamic neurons extend down into the posterior pituitary, where they release ADH and oxytocin directly into the blood. (2) Anterior pituitary pathway — hypothalamic neurons release releasing/inhibiting hormones into the hypophyseal portal system (a capillary network connecting the hypothalamus to the anterior pituitary); these hormones stimulate or inhibit the anterior pituitary cells to secrete GH, TSH, ACTH, FSH, LH, and prolactin into the general circulation. Panel B: A cascade diagram showing the hypothalamus-anterior pituitary-target gland axis with negative feedback loops. Example: hypothalamus releases TRH → anterior pituitary releases TSH → thyroid releases T3/T4 → T3/T4 feeds back negatively to inhibit TRH and TSH release. Similar cascades shown for CRH-ACTH-cortisol and GnRH-FSH/LH-sex steroids.</image>
IV. Major Endocrine Glands and Their Hormones
A. Thyroid Gland
The thyroid gland is a butterfly-shaped organ located in the anterior neck. It produces T3 (triiodothyronine) and T4 (thyroxine), hormones that increase basal metabolic rate and are essential for normal growth and development, particularly neural development in infants and children. Synthesis of T3 and T4 requires dietary iodine. Deficiency of thyroid hormones (hypothyroidism) causes low metabolism, weight gain, and fatigue; in infants, severe deficiency produces cretinism, characterized by intellectual disability and stunted growth. Excess thyroid hormone (hyperthyroidism) causes elevated metabolism, weight loss, heat intolerance, and anxiety -- Graves' disease, the most common cause, is an autoimmune condition. Iodine deficiency itself leads to goiter, an enlargement of the thyroid gland. The thyroid also contains parafollicular cells (C cells) that secrete calcitonin, a hormone that lowers blood calcium by inhibiting osteoclast activity and promoting calcium deposition in bone.
B. Parathyroid Glands
Four small parathyroid glands are embedded in the posterior surface of the thyroid. They produce parathyroid hormone (PTH), which raises blood calcium through three coordinated actions: stimulating osteoclasts to release calcium from bone, increasing calcium reabsorption in the kidneys, and stimulating the production of active vitamin D, which in turn enhances calcium absorption from the intestines. PTH and calcitonin function as an antagonistic pair, working in opposition to maintain calcium homeostasis within a narrow range.
C. Adrenal Glands
The adrenal glands sit atop each kidney and consist of two functionally distinct regions. The adrenal cortex (the outer region) produces steroid hormones in three categories: mineralocorticoids such as aldosterone, which regulate sodium and potassium balance and blood pressure (discussed in Lecture 18); glucocorticoids such as cortisol, which mediate the stress response by increasing blood glucose through gluconeogenesis and exerting anti-inflammatory and immunosuppressive effects; and androgens such as DHEA, which serve as precursors to sex hormones. Chronic elevation of cortisol produces Cushing's syndrome, characterized by obesity, hypertension, hyperglycemia, and immunosuppression. The adrenal medulla (the inner region) produces the catecholamines epinephrine (adrenaline) and norepinephrine, which orchestrate the fight-or-flight response -- increasing heart rate, blood pressure, and blood glucose while dilating airways and redirecting blood to skeletal muscles. This catecholamine response is rapid and short-lived, complementing the slower but more sustained stress response mediated by cortisol.
D. Pancreas (Endocrine Function)
The endocrine pancreas consists of the islets of Langerhans, clusters of hormone-secreting cells scattered among the exocrine tissue. Beta cells secrete insulin, the hormone that lowers blood glucose. Insulin stimulates glucose uptake by muscle and adipose cells through insertion of GLUT4 transporters into their membranes, promotes glycogenesis (the conversion of glucose to glycogen in liver and muscle), stimulates lipogenesis and protein synthesis, and inhibits gluconeogenesis. Alpha cells secrete glucagon, which raises blood glucose by stimulating glycogenolysis (breakdown of glycogen to glucose in the liver) and gluconeogenesis (synthesis of glucose from amino acids and glycerol). Insulin and glucagon are antagonistic hormones that together maintain blood glucose within a narrow range of approximately 70-110 mg/dL. Diabetes mellitus arises when this regulation fails. In type 1 diabetes, autoimmune destruction of beta cells eliminates insulin production, requiring lifelong insulin injections. In type 2 diabetes -- the more common form, strongly associated with obesity -- cells become resistant to insulin's effects despite its continued production.
E. Gonads
The testes produce testosterone, which drives male secondary sex characteristics, spermatogenesis, and muscle growth. The ovaries produce estrogen, which drives female secondary sex characteristics, regulates the menstrual cycle, and supports bone health, along with progesterone, which prepares the endometrium for pregnancy and helps maintain pregnancy once established.
F. Pineal Gland
The pineal gland produces melatonin, a hormone that regulates circadian rhythms and the sleep-wake cycle. Melatonin secretion increases in darkness and is suppressed by light, providing the body with a chemical signal of nighttime.
V. Feedback Regulation
Most hormones are regulated by negative feedback. As a hormone's level rises, it inhibits the signals that stimulate its own release; as the level falls, that inhibition is relieved and secretion resumes. Cortisol, for example, feeds back to inhibit the release of both CRH from the hypothalamus and ACTH from the anterior pituitary, preventing excessive cortisol production. Positive feedback, though rare in endocrine regulation, drives processes that must proceed rapidly to completion. During labor, oxytocin stimulates uterine contractions, which push the baby against the cervix and stimulate the release of even more oxytocin, creating a self-amplifying cycle that continues until delivery. Another example occurs during the menstrual cycle, when rising estrogen levels, rather than inhibiting LH as they do at lower concentrations, trigger a dramatic LH surge that induces ovulation.
<image>A paired diagram comparing blood glucose regulation by insulin and glucagon. Left side (high blood glucose after a meal): Beta cells in the islets of Langerhans detect high glucose and release insulin into the blood. Arrows show insulin's effects: promotes glucose uptake by muscle and fat cells (GLUT4 inserted into membranes), stimulates glycogen synthesis in the liver, and inhibits gluconeogenesis. Blood glucose returns to normal. Right side (low blood glucose between meals): Alpha cells detect low glucose and release glucagon. Arrows show glucagon's effects: stimulates glycogen breakdown (glycogenolysis) in the liver and stimulates gluconeogenesis, releasing glucose into the blood. Blood glucose returns to normal. A central homeostatic set point (~90 mg/dL) is indicated, with negative feedback loops from normalized glucose levels back to the secreting cells.</image>
