Thyroid Hormones: Synthesis, Mechanism, and Functions

The Two Thyroid Hormones

Synthesis of Thyroid Hormones

1. Thyroglobulin (TG) synthesis - Follicular cells synthesize TG (a glycoprotein, MW ~335,000), which contains ~70 tyrosine residues. TG is extruded into the follicular lumen.
2. Iodide trapping - I⁻ is actively transported into follicular cells via the Na⁺/I⁻ symporter (NIS) against chemical and electrical gradients.
3. Oxidation of iodide - Thyroid peroxidase (TPO) and H₂O₂ oxidize I⁻ to active iodine (I₂), which enters the colloid.
4. Organification - Oxidized iodine binds tyrosine residues on TG, forming monoiodotyrosine (MIT) and diiodotyrosine (DIT).
5. Coupling - DIT + DIT → T4; DIT + MIT → T3.
6. Secretion - When stimulated by TSH, follicular cells endocytose TG from the colloid, lysosomes cleave T3/T4 from TG, and free hormones are released into the bloodstream.

Mechanism of Action

• T4 enters target cells and is converted to T3 by 5'-iodinase
• T3 enters the nucleus and binds a nuclear thyroid hormone receptor, which forms a heterodimer with the Retinoid X Receptor (RXR)
• This T3-receptor complex binds to thyroid hormone response elements (TREs) on DNA
• This stimulates transcription of many genes, leading to synthesis of new proteins including: Na⁺-K⁺ ATPase, cardiac myosin, β₁-adrenergic receptors, lysosomal enzymes, metabolic enzymes, and structural proteins
• These new proteins are responsible for the multiple physiological effects

• Some effects occur too rapidly to be explained by gene transcription
• Nongenomic actions occur at the plasma membrane, cytoplasm, and mitochondria
• Include regulation of ion channels and oxidative phosphorylation via cAMP and protein kinase cascades

Functions of Thyroid Hormones

1. Basal Metabolic Rate (BMR)

• Increase oxygen consumption in all tissues except the brain, gonads, and spleen
• Raise BMR by 60-100% above normal in excess
• Mechanism: upregulation of Na⁺-K⁺ ATPase activity → increased ATP hydrolysis → increased O₂ consumption and heat production
• This is why thyroid hormones are thermogenic

2. Metabolism

• Carbohydrates: Increase glucose absorption from GI tract, potentiate gluconeogenesis and glycogenolysis (via catecholamine and glucagon synergy)
• Fats: Stimulate lipolysis; increase fat mobilization and oxidation
• Proteins: Increase both protein synthesis and degradation - net effect is catabolic (decreased muscle mass), leading to negative nitrogen balance in excess
• Key enzymes induced: cytochrome oxidase, NADPH cytochrome C reductase, α-glycerophosphate dehydrogenase, malic enzyme

3. Cardiovascular System

• Increase cardiac output via:
  • ↑ Heart rate (chronotropy)
  • ↑ Stroke volume / contractility (inotropy)
• Mechanism: upregulation of β₁-adrenergic receptors in myocardium + synthesis of cardiac myosin and sarcoplasmic reticulum Ca²⁺ ATPase
• This explains why β-blockers (e.g. propranolol) control many symptoms of hyperthyroidism
• Also increase respiratory rate and ventilation to match increased O₂ demand

4. Growth and Development

• Synergize with growth hormone and somatomedins to promote bone formation
• Promote ossification, fusion of bone plates, and bone maturation
• In hypothyroidism: bone age lags chronologic age

5. Central Nervous System (CNS)

• Perinatal period: Essential for normal CNS maturation. Hypothyroidism here causes irreversible mental retardation (cretinism) - this is why neonatal thyroid screening is mandatory
• Adults:
  • Hypothyroidism → lethargy, slowed movement, somnolence, impaired memory, decreased mental capacity
  • Hyperthyroidism → hyperexcitability, hyperreflexia, irritability, tremor

6. Autonomic Nervous System

• Synergize with catecholamines - effects on BMR, heat production, heart rate, and stroke volume mirror adrenergic effects
• Sensitize tissues to sympathetic stimulation (via β₁ receptor upregulation)

Regulation (HPT Axis)

1. Hypothalamus releases TRH (thyrotropin-releasing hormone)
2. Anterior pituitary responds by releasing TSH (thyroid-stimulating hormone)
3. TSH stimulates T3/T4 synthesis and secretion
4. Negative feedback: Elevated T3/T4 inhibits both TRH and TSH secretion, maintaining homeostasis

• Cold exposure stimulates TRH → TSH → ↑ thyroid hormones (thermogenic adaptation)
• Fasting / low leptin → ↓ TRH → ↓ metabolic rate (energy conservation)
• Emotional stress / anxiety → acute ↓ TSH

Hyperthyroidism vs. Hypothyroidism at a Glance

• Guyton and Hall Textbook of Medical Physiology, Ch. 77
• Costanzo Physiology, 7th Ed., Ch. 9 (pp. 423-429)
• Sabiston Textbook of Surgery, Ch. 73

Glucocorticoids

Origin and Structure

Fig. 50-1 from Medical Physiology - Adrenal gland zones and their hormones

Adrenal Cortex Zonation (GFR Rule)

Synthesis

1. Cholesterol (from LDL or local synthesis) enters adrenal cells
2. Mitochondrial enzyme StAR (steroidogenic acute regulatory protein) transfers cholesterol into the inner mitochondrial membrane - this is the rate-limiting step
3. Cholesterol → Pregnenolone → Progesterone → 17α-hydroxyprogesterone → 11-deoxycortisol → Cortisol
4. Key enzymes: 21α-hydroxylase (most common site of congenital adrenal hyperplasia) and 11β-hydroxylase

• ~90% bound to corticosteroid-binding globulin (CBG / transcortin) - synthesized by the liver
• ~7% bound to albumin
• ~3-4% free (biologically active fraction)

• 11β-HSD1 (liver, adipose): reversible; converts cortisone ⇌ cortisol (locally regenerates active cortisol)
• 11β-HSD2 (kidney, colon): irreversible; converts cortisol → inactive cortisone, protecting mineralocorticoid receptors from cortisol's actions

Mechanism of Action

• Cortisol diffuses into the cell and binds the cytosolic glucocorticoid receptor (GR)
• The GR undergoes conformational change, releasing heat shock proteins, and the hormone-receptor complex translocates to the nucleus
• The GR binds glucocorticoid response elements (GREs) on DNA as a homodimer
• Activates or represses transcription of target genes (10-20% of all expressed genes are GR-regulated)
• Coactivators (with histone acetylase activity) and corepressors modulate transcriptional output

• Effects occurring within minutes (too fast for gene transcription)
• Mediated via membrane-associated GRs, G-protein-coupled receptors, and effects on ion channels
• Examples: rapid feedback suppression of ACTH, effects on glutamatergic neurons

Note: Cortisol can bind the mineralocorticoid receptor (MR) with similar affinity to aldosterone. In the kidney, 11β-HSD2 prevents this by inactivating cortisol locally, allowing aldosterone to act selectively.

Regulation: HPA Axis

Hypothalamus → CRH (corticotropin-releasing hormone)
      ↓
Anterior Pituitary → ACTH (derived from POMC precursor)
      ↓
Zona Fasciculata → Cortisol
      ↓ (negative feedback)
Inhibits both CRH and ACTH secretion

• Circadian rhythm: Cortisol peaks in early morning (~6-8 AM), troughs at midnight. Controlled by the suprachiasmatic nucleus.
• Stress override: Physical or emotional stress activates higher CNS centers to stimulate CRH release, overriding normal feedback
• Pulsatile secretion: ACTH and cortisol are secreted in episodic pulses (~15-18/day)
• Arginine vasopressin (AVP): A secondary ACTH secretagogue, important during dehydration and trauma

Physiological Functions of Glucocorticoids

1. Metabolic Effects

• Stimulate gluconeogenesis in the liver (upregulate PEPCK, glucose-6-phosphatase, glycogen synthase)
• Promote glycogen synthesis in the liver
• Inhibit glucose uptake in peripheral muscle and adipose tissue (anti-insulin effect → hyperglycemia)
• Net result: maintain blood glucose supply to the brain, especially in the fasting state

• Catabolic in muscle, lymphoid tissue, connective tissue, skin, and bone
• Promote protein breakdown and release of amino acids → used as gluconeogenic substrate
• Stimulate protein synthesis only in the liver
• Chronic excess: muscle weakness, skin thinning, osteoporosis

• Stimulate lipolysis via hormone-sensitive lipase
• Increased insulin (secondary to hyperglycemia) promotes lipogenesis simultaneously
• Net effect: fat redistribution - central (visceral) fat deposition, with peripheral fat loss (classic Cushingoid appearance: moon face, buffalo hump, truncal obesity, thin limbs)

2. Anti-inflammatory and Immunosuppressive Effects

• ↑ Neutrophils in circulation (from bone marrow mobilization + reduced margination)
• ↓ Lymphocytes (T and B cells), monocytes, eosinophils, basophils
• Peak effect at 6 hours, dissipates in 24 hours

• Inhibit production of inflammatory cytokines (IL-1, IL-2, IL-6, TNF-α, IFN-γ)
• Suppress arachidonic acid metabolism - inhibit phospholipase A2 (via annexin/lipocortin induction) → reduce prostaglandins, leukotrienes, and thromboxanes
• Suppress NF-κB (master regulator of inflammatory gene expression)
• Reduce capillary permeability and vasodilation (suppress histamine release from mast cells/basophils)
• Inhibit leukocyte adhesion molecule expression → prevent extravasation to sites of inflammation
• At high doses: reduce antibody production

3. Cardiovascular Effects ("Permissive" Actions)

• Permissive effect: Vascular smooth muscle responsiveness to catecholamines is diminished without cortisol - even physiological amounts restore normal vasomotor tone
• Increase blood pressure via enhanced vascular reactivity
• At high doses: some mineralocorticoid activity → sodium retention → hypertension

4. Renal Effects

• Cortisol deficiency → impaired GFR, augmented vasopressin secretion, inability to excrete a water load freely
• Physiological levels required for normal free water clearance

5. Central Nervous System Effects

• Adrenal insufficiency: slowed EEG alpha rhythm, depression
• Excess glucocorticoids: insomnia, euphoria (acute), then depression (chronic)
• Large doses: may raise intracranial pressure (pseudotumor cerebri)
• Hippocampal damage: genomic and nongenomic cortisol actions can be neurotoxic to hippocampal cells (which mediate negative feedback), contributing to a vicious cycle in chronic stress

6. Bone and Calcium Metabolism

• Inhibit osteoblast activity → reduced bone formation
• Increase osteoclast activity → increased bone resorption
• Inhibit calcium absorption from the gut (antagonize vitamin D)
• Long-term use → osteoporosis (major adverse effect of chronic steroid therapy)

7. Hematopoietic Effects

• ↑ Neutrophils, platelets, and red blood cells
• ↓ Lymphocytes, eosinophils, basophils, monocytes

8. Developmental / Fetal Effects

• Stimulate fetal lung maturation - induce surfactant production near term
• This is the basis for giving antenatal corticosteroids (betamethasone) to women at risk of preterm delivery
• Epigenetic effects on target genes: prenatal glucocorticoid exposure may have long-term consequences including predisposition to metabolic syndrome and depression

9. Effects on Other Endocrine Axes

• Chronic excess suppresses pituitary release of ACTH, GH, TSH, and LH
• Associated with peptic ulcer risk (possibly by suppressing local immune response to H. pylori)

Cushing Syndrome vs. Addison Disease

Common Synthetic Glucocorticoids (Relative Potencies)

• Medical Physiology (Boron & Boulpaep), Ch. 50, pp. 1495-1504
• Katzung's Basic and Clinical Pharmacology, 16th Ed., Ch. 39
• Goodman & Gilman's Pharmacological Basis of Therapeutics, Ch. 14

Physiology of the Hypothalamus

Overview

1. Homeostatic mechanisms - hunger, thirst, sexual desire, sleep-wake cycles
2. Endocrine control - via the pituitary gland
3. Autonomic control - cardiovascular, thermoregulatory, visceral
4. Limbic mechanisms - emotion, stress responses (mnemonic: HEAL)

Anatomy and Location

Fig. 41-2 - Structure of the hypothalamus showing key nuclei (Kandel, Principles of Neural Science)

• Anteriorly: Diagonal band of Broca
• Dorsally: Anterior commissure, bed nuclei of stria terminalis, zona incerta, thalamus
• Posteriorly: Ventral tegmental area and interpeduncular nucleus

Zones and Nuclei

Rostrocaudal Divisions (Three Regions)

Mediolateral Divisions

Key Hypothalamic Nuclei and Their Functions

Six Vital Physiological Functions

1. Blood Pressure and Electrolyte Composition

• Regulates thirst, salt appetite, and drinking behavior
• Autonomic control of vasomotor tone
• Releases vasopressin (ADH) from the paraventricular nucleus in response to increased plasma osmolality or decreased blood volume
• Key sensors: OVLT (vascular organ of the lamina terminalis), subfornical organ (SFO), and median preoptic nucleus (MnPO) - circumventricular organs lacking a blood-brain barrier, allowing direct sampling of blood osmolality

2. Energy Metabolism

• Arcuate nucleus is the primary integrator:
  • NPY/AgRP neurons: Orexigenic (stimulate feeding, reduce energy expenditure)
  • POMC/CART neurons: Anorexigenic (suppress feeding, increase energy expenditure)
• Leptin (from adipose tissue) inhibits NPY/AgRP and activates POMC neurons - signals satiety and adequate fat stores
• Ghrelin (from stomach) activates NPY/AgRP neurons - signals hunger
• Regulates glucocorticoids, growth hormone, and TSH release to coordinate metabolism

3. Thermoregulation

• Preoptic area (POA) is the primary thermostat
• Warm-sensitive neurons in POA → activated by heat → trigger sweating and cutaneous vasodilation
• Cold signals from skin and core → activate heat-conserving responses: vasoconstriction, shivering, brown fat thermogenesis
• There is no single "set point" neuron; temperature is controlled by a distributed settling point of multiple feedback loops

4. Reproductive and Sexual Behavior

• Medial preoptic nucleus: Controls male sexual behavior
• Ventromedial and ventral premammillary nuclei: Female reproductive behavior
• GnRH pulse generator in arcuate/preoptic area: Drives pulsatile LH and FSH release from pituitary → regulates menstrual cycle and spermatogenesis
• Hypothalamic gonadostat: Suppressed in childhood by high sensitivity to sex steroid feedback; "reset" at puberty

5. Defensive Behavior (Stress Response)

• Paraventricular nucleus (PVN): Releases CRH → ACTH → cortisol (HPA axis activation)
• Anterior hypothalamic area and dorsal premammillary nucleus: Fight-or-flight responses to threats
• Coordinates autonomic (sympathetic activation) and endocrine (glucocorticoid) responses to stress
• Chronic stress and hippocampal damage can impair negative feedback of cortisol, perpetuating HPA axis hyperactivity

6. Sleep-Wake Cycle

• Suprachiasmatic nucleus (SCN): Master circadian pacemaker; synchronized by light input via the retinohypothalamic tract
• Lateral hypothalamic area (LHA): Contains orexin/hypocretin neurons that project widely to promote arousal and stabilize wakefulness
  • Loss of orexin neurons → narcolepsy
• Tuberomammillary nucleus (TMN): Contains histaminergic neurons that promote wakefulness; blocked by antihistamines (causing sedation)
• Ventrolateral preoptic nucleus (VLPO): Sleep-promoting; inhibits arousal centers (histamine, orexin) via GABA and galanin - the "sleep switch"

Neuroendocrine Control of the Pituitary

Fig. 17.5 - The hypothalamic-pituitary portal system (Neuroanatomy through Clinical Cases)

Anterior Pituitary (Adenohypophysis) - Controlled by Portal Blood

Note: Dopamine is the only non-peptide hypothalamic factor. It tonically inhibits prolactin - so hypothalamic damage or dopamine antagonists (antipsychotics) cause hyperprolactinemia.

Posterior Pituitary (Neurohypophysis) - Direct Axonal Secretion

Autonomic Output

• Sympathetic output: PVN → intermediolateral cell column of spinal cord → sympathetic ganglia → fight-or-flight (↑ HR, ↑ BP, pupil dilation, sweating, gluconeogenesis)
• Parasympathetic output: Anterior hypothalamus → dorsal motor nucleus of vagus and sacral cord → rest-and-digest

Circumventricular Organs (CVOs)

Clinical Correlations

• Kandel et al., Principles of Neural Science, 6th Ed., Ch. 41
• Blumenfeld, Neuroanatomy through Clinical Cases, 3rd Ed., Ch. 17
• Kaplan & Sadock's Comprehensive Textbook of Psychiatry

Anterior and Posterior Pituitary Gland Physiology

Overview

Part I: Anterior Pituitary (Adenohypophysis)

The Hypothalamo-Hypophyseal Portal System

Fig. 390-1 from Harrison's Principles of Internal Medicine, 22nd Ed. - Pituitary axes and feedback loops

Five Cell Types and Their Hormones

LH, FSH, and TSH are all glycoproteins sharing a common α subunit - their specificity comes from unique β subunits.

Anterior Pituitary Hormones in Detail

1. ACTH (Adrenocorticotropic Hormone)

• Derived from: POMC (pro-opiomelanocortin, 266 aa precursor) - also yields β-lipotropin, β-endorphin, α-MSH, and CLIP
• Stimulated by: CRH (41 aa, from PVN), AVP, and pro-inflammatory cytokines (IL-6)
• Inhibited by: Glucocorticoids (negative feedback at pituitary and hypothalamus)
• Secretion pattern: Pulsatile with marked circadian rhythm - peaks early morning (6-8 AM), troughs at midnight; stress can override this completely
• Target: Zona fasciculata of adrenal cortex → cortisol synthesis and secretion
• Also stimulates: Adrenal androgen production (DHEA) and, in large amounts, aldosterone

2. Growth Hormone (GH)

• Structure: 191 amino acids, 22 kDa (main isoform); 5 genes in GH/PRL/hPL gene cluster
• Stimulated by: GHRH (44 aa, from arcuate nucleus), ghrelin (octanoylated gastric peptide), fasting, exercise, deep sleep (highest peaks at sleep onset), estrogen
• Inhibited by: Somatostatin (SRIF, from periventricular/preoptic area), IGF-1 (long-loop feedback), hyperglycemia (glucose load suppresses GH)
• Secretion: Pulsatile; highest levels during slow-wave sleep; declines markedly with age (~15% of pubertal levels in middle age); higher in women
• Direct actions:
  • Lipolysis and fatty acid oxidation
  • Anti-insulin effect (promotes insulin resistance in peripheral tissues)
  • Stimulates amino acid uptake and protein synthesis in muscle
• Indirect actions (via IGF-1):
  • Liver produces IGF-1 (insulin-like growth factor 1), which mediates most of the growth-promoting effects
  • Stimulates chondrocyte proliferation → linear bone growth
  • IGF-1 levels peak at age 16, then decline >80% across lifespan

3. Prolactin (PRL)

• Structure: 198 amino acids; weakly homologous to GH and hPL (from common ancestral gene)
• Unique regulatory feature: The only anterior pituitary hormone under predominantly tonic inhibitory control - dopamine from the arcuate nucleus tonically suppresses PRL release
  • This explains why pituitary stalk section or dopamine receptor antagonists (antipsychotics, metoclopramide) cause hyperprolactinemia
• Stimulated by: Estrogen (major physiological stimulus; explains rise in pregnancy), TRH, VIP, suckling (via afferent neural pathways), stress
• Inhibited by: Dopamine (D2 receptor-mediated; basis for bromocriptine/cabergoline therapy)
• Secretion: Pulsatile; highest levels during non-REM sleep (4-6 AM); normal adult levels: 10-25 µg/L (women), 10-20 µg/L (men); t½ ~50 min; rises ~10-fold during pregnancy
• Actions:
  • Initiates and maintains lactation (in conjunction with insulin, glucocorticoids, and estrogen)
  • Inhibits GnRH → suppresses LH/FSH → postpartum lactational amenorrhea (inhibits ovulation)
  • Stimulates mammary gland development
  • In males: inhibits testosterone production and libido if elevated

4. TSH (Thyroid-Stimulating Hormone)

• Structure: Glycoprotein with common α subunit + specific TSH-β subunit; 211 amino acids total
• Stimulated by: TRH (hypothalamic tripeptide: pyroGlu-His-Pro-NH₂)
• Inhibited by: T3/T4 (primary negative regulator), dopamine, somatostatin, glucocorticoids
• Secretion pattern: Pulsatile with modest amplitude; single measurement is adequate (unlike GH)
• Actions: Binds GPCR on thyroid follicular cells → stimulates T3/T4 synthesis and secretion; promotes follicular cell hypertrophy and hyperplasia (goiter when chronically elevated)
• Clinical note: In primary hypothyroidism (thyroid failure), loss of T3/T4 feedback → markedly elevated TSH; thyrotrope hyperplasia may enlarge the pituitary visibly on MRI

5. LH and FSH (Gonadotropins)

• Structure: Both are glycoproteins sharing the common α subunit; LH has a unique β subunit (204 aa), FSH has a unique β subunit (210 aa)
• Stimulated by: GnRH (pulsatile, from scattered preoptic/arcuate neurons; also called LHRH), activins, estrogen (positive feedback at mid-cycle for LH surge); GnRH pulsatility is controlled by kisspeptin neurons
• Inhibited by: Sex steroids, inhibin (from gonads)
• Secretion: Pulsatile; LH pulses tightly track GnRH pulses; pulse frequency and amplitude vary across menstrual cycle
• Key principle: Pulsatile GnRH stimulates LH/FSH; continuous GnRH exposure causes receptor downregulation and suppresses gonadotropin release (basis for GnRH agonist therapy in prostate cancer, endometriosis)
• Actions:
  • In females: FSH promotes follicular development; LH triggers ovulation and luteinization
  • In males: FSH drives spermatogenesis; LH stimulates Leydig cells → testosterone production

Hypothalamic Factors - Summary Table

Part II: Posterior Pituitary (Neurohypophysis)

The Two Posterior Pituitary Hormones

1. Vasopressin (ADH - Antidiuretic Hormone)

• Synthesized in: Mainly supraoptic nucleus (also PVN)
• Structure: Nonapeptide; in humans, position 8 is arginine (hence "arginine vasopressin", AVP)
• Stimuli for release:
  • ↑ plasma osmolality (detected by osmoreceptors in OVLT, SFO, and hypothalamus) - most sensitive stimulus (1% rise in osmolality is detectable)
  • ↓ blood volume / ↓ blood pressure (detected by carotid/aortic baroreceptors and cardiac volume receptors) - less sensitive but override osmotic signals in hemorrhage
  • Nausea, pain, stress, angiotensin II, nicotine
• Inhibited by: ↓ osmolality, ↑ blood volume, ethanol, ANP
• Receptors and actions:
  • V1 receptors (vascular smooth muscle): Vasoconstriction via Gq → phospholipase C → IP₃ → Ca²⁺
  • V2 receptors (renal collecting duct): Water reabsorption via Gs → adenylyl cyclase → cAMP → PKA → aquaporin-2 (AQP2) insertion into apical membrane
  • V2-like extrarenal receptors: Stimulate release of factor VIII and von Willebrand factor
• t½: ~15 minutes
• Clinical relevance:
  • Central (neurogenic) diabetes insipidus: ADH deficiency → dilute polyuria (>3 L/day), polydipsia; treated with desmopressin (DDAVP), a long-acting V2-selective synthetic analogue
  • SIADH (syndrome of inappropriate ADH): Excess ADH → hyponatremia, concentrated urine

2. Oxytocin

• Synthesized in: Mainly paraventricular nucleus (also SON)
• Structure: Nonapeptide; differs from vasopressin at positions 3 and 8
• Stimuli for release:
  • Cervical and vaginal stretch during labor (Ferguson reflex - positive feedback)
  • Nipple stimulation / suckling (neuroendocrine reflex)
  • Estrogen (sensitizes hypothalamic neurons)
• Actions:
  • Uterus: Stimulates myometrial contraction during labor; uterine sensitivity to oxytocin increases near term (↑ oxytocin receptors)
  • Mammary gland: Stimulates myoepithelial cell contraction → milk ejection ("let-down reflex")
  • CNS effects: Promotes social bonding, trust, maternal behavior, pair bonding; reduces anxiety
  • Cardiovascular: Minor vasodilatory effects at high doses
• Clinical relevance: Synthetic oxytocin (Pitocin) is used to induce or augment labor and to control postpartum hemorrhage (contracts uterine myometrium)

Feedback Control

Hypothalamus ─→ Releasing hormone ─→ Anterior pituitary ─→ Trophic hormone ─→ Target gland ─→ Peripheral hormone
    ↑_____________________________negative feedback___________________________________|
    ↑___________________negative feedback__________________________|

• Short-loop feedback: Pituitary hormones feed back to inhibit hypothalamic releasing factors
• Long-loop feedback: Peripheral hormones (cortisol, T3/T4, sex steroids, IGF-1) inhibit both pituitary and hypothalamic levels
• All pituitary axes except PRL are net stimulatory; PRL is under net tonic inhibition

Clinical Correlations

• Harrison's Principles of Internal Medicine, 22nd Ed., Ch. 390 (pp. 3031-3035)
• Kandel et al., Principles of Neural Science, 6th Ed., Ch. 41
• Katzung's Basic and Clinical Pharmacology, 16th Ed., Ch. 37

Auditory Pathway

Overview: The Relay Chain

ECOLI - Eighth nerve → Cochlear nucleus → Olivary complex → Lateral lemniscus → Inferior colliculus → (then MGN → Cortex)

Fig. 11.17 from Neuroscience: Exploring the Brain, 5th Ed. - The complete ascending auditory pathway

Step 1 - First-Order Neurons: Cochlea → CN VIII → Cochlear Nuclei

The Cochlea and Hair Cells

• Apex: Wide and floppy → responds best to low frequencies
• Base: Narrow and stiff → responds best to high frequencies

• Inner hair cells (IHCs): ~3,500 total; the primary sensory receptors - 95% of afferent fibers contact IHCs; each IHC contacts ~20 spiral ganglion fibers
• Outer hair cells (OHCs): ~12,000 total; act as a cochlear amplifier - they actively contract/elongate (electromotility) to amplify basilar membrane motion by up to 100-fold, greatly improving sensitivity and frequency selectivity

Spiral Ganglion and CN VIII

• Cell bodies of first-order neurons lie in the spiral ganglion (housed in the canal of Rosenthal)
• Peripheral processes synapse on hair cells; central processes form the cochlear nerve (division of CN VIII)
• CN VIII enters the brainstem at the pontomedullary junction (cerebellopontine angle)
• Each spiral ganglion neuron has a characteristic frequency - fires best to one frequency, with broader responses at higher intensities

Step 2 - Second-Order Neurons: Cochlear Nuclei

Critical clinical principle: The cochlear nuclei are the only stations in the auditory pathway that receive input from one ear only (ipsilateral). All higher auditory relay stations receive input from both ears. Therefore, a unilateral brainstem lesion above the cochlear nucleus level cannot produce monaural deafness - only lesions at or below the cochlear nucleus or cochlear nerve itself will cause single-ear deafness.

1. Dorsal acoustic stria - from DCN
2. Intermediate acoustic stria - from dorsal VCN
3. Ventral acoustic stria (trapezoid body) - from VCN; major decussation; crosses in the caudal pons

Step 3 - The Superior Olivary Complex (SOC)

Two mechanisms of sound localization:

• Sound from one side reaches the near ear slightly before the far ear (up to 600 µsec difference)
• Medial superior olive (MSO) neurons act as coincidence detectors - they fire maximally when inputs from both ears arrive simultaneously (Jeffress model)
• Humans can discriminate differences as small as 10 µsec, achieving ~1-2° precision in horizontal localization

• The head creates an acoustic "shadow" - the far ear receives a quieter sound
• Lateral superior olive (LSO) neurons compare the loudness of sound at each ear
• Excited by the ipsilateral ear, inhibited by the contralateral ear (via the medial nucleus of the trapezoid body, MNTB)

• Releases acetylcholine onto outer hair cells
• Suppresses OHC amplification - modulates gain of the cochlear amplifier (protective function, attention-related filtering)

Step 4 - Lateral Lemniscus

• Runs through the pons and midbrain tegmentum
• Contains axons from both cochlear nuclei (both direct and after relay in the SOC)
• Contains additional relay neurons: nuclei of the lateral lemniscus (NLL) (dorsal and ventral)
• The two lateral lemnisci are connected by the commissure of Probst
• All fibers converge onto the inferior colliculus

Step 5 - Third-Order Neurons: Inferior Colliculus (IC)

• Central nucleus: Strict tonotopic organization; the core auditory relay
• Pericentral nucleus: Part of the "belt" system; receives polymodal input

• Integrates inputs from both ears (bilateral)
• Processes complex sound features (duration, frequency modulation)
• Encodes sound location in space
• Generates auditory reflexes (e.g., startle response via connections to reticular formation)
• Projects to the superior colliculus (audiovisual integration for orienting responses)
• Sends efferents to the cerebellum

Step 6 - Fourth-Order Neurons: Medial Geniculate Nucleus (MGN) of Thalamus

Important: There are no commissural connections between the two medial geniculate nuclei (unlike all other auditory relay levels). This has clinical relevance - each MGN processes predominantly contralateral ear information.

Step 7 - Auditory Cortex

• Low tones → anterolateral
• High tones → posteromedial
• Neurons are arranged in isofrequency columns

• Like the visual system, auditory processing follows two streams:
  • "What" (ventral) stream: Temporal lobe - sound identification (what is it?)
  • "Where" (dorsal) stream: Parietal lobe - spatial localization (where is it?)
• Most neurons in AI respond to input from both ears, but strips of cells are excited by the contralateral ear and inhibited by the ipsilateral ear

• Left Wernicke's area: Language/speech processing (dominant)
• Right auditory cortex: Melody, pitch, sound intensity, prosody

Summary Table: Ascending Auditory Pathway

Bilateral Representation - Clinical Significance

• Cochlear nerve / cochlear nucleus lesion → ipsilateral deafness
• Unilateral brainstem lesion above cochlear nuclei → no monaural deafness (bilateral representation protects hearing)
• Unilateral temporal lobe lesion → subtle deficits (dichotic listening difficulty) - not frank deafness, because the opposite cortex still receives bilateral input
• Bilateral temporal lobe lesions → cortical deafness

Blood Supply Summary

Brainstem Auditory Evoked Potentials (BAEPs / ABR)

• Neuroscience: Exploring the Brain, 5th Ed., Ch. 11
• Ganong's Review of Medical Physiology, 26th Ed., Ch. 11
• Localization in Clinical Neurology, 8th Ed., Ch. 11 (pp. 818-821)