Clinical Case Analysis: Right Hemiplegia in a Hypertensive 60-Year-Old

i) Probable Diagnosis

• 60-year-old hypertensive - hypertension is the single most important risk factor for stroke
• Acute onset (2 hours) - sudden neurological deficits are characteristic of vascular events
• Right upper + lower extremity weakness - contralateral hemiplegia (left hemisphere lesion affecting the right side of the body)
• Increased tone, exaggerated tendon jerks, extensor plantar response - the classic triad of UMN (pyramidal tract) involvement

ii) Reason for Increased Muscle Tone and Exaggerated Tendon Jerks (Spasticity and Hyperreflexia)

Spasticity (Increased Muscle Tone)

• Loss of the dorsal reticulospinal tract (inhibitory) removes the brake on spinal motor neurons.
• The medial reticulospinal and vestibulospinal tracts, which facilitate extensor tone, are relatively spared.
• The imbalance results in unopposed excitatory drive on alpha motor neurons in the spinal cord's anterior horn.
• Additionally, there is disinhibition of gamma (fusimotor) motor neurons, which increase the sensitivity of muscle spindles - causing an exaggerated response to muscle stretch.

"Increased tone and hyperreflexia do not occur in experimental animals when a selective lesion is made in the corticospinal tract alone. It has therefore been hypothesized that spasticity is caused by damage to descending inhibitory pathways that travel closely with the corticospinal tract." - Neuroanatomy through Clinical Cases, 3rd Ed.

"The heightened stretch reflexes (tendon jerks) of the spastic state may be a 'release' phenomenon - the result of interruption of descending inhibitory pathways... mediated through disinhibition of spindle efferents (increased tonic activity of gamma motor neurons)." - Adams and Victor's Principles of Neurology, 12th Ed.

iii) Why the Plantar Response is Altered (Babinski Sign / Extensor Plantar Response)

Normal Plantar Response

Extensor Plantar Response (Babinski Sign)

• The UMN normally exerts tonic descending inhibition over a primitive flexion-withdrawal (nociceptive) reflex arc in the spinal cord.
• When the UMN pathway is damaged (stroke), this inhibitory control is removed.
• The plantar reflex is then "released" - it reverts to the primitive spinal reflex pattern: extension of the great toe as part of a generalized leg flexion (withdrawal) synergy.
• As Adams and Victor note: "The extension movement of the great toe is a component of a larger synergistic flexion or shortening reflex of the leg - a protective (nocifensive) response."
• This sign is present normally in infants (before corticospinal myelination is complete) and reappears in adults only when corticospinal pathways are damaged.

iv) The Corticospinal Tract - Course and Description

Diagram

Origin

• Arises from the primary motor cortex (Brodmann area 4) in the precentral gyrus - >50% of fibers
• Additional fibers from premotor/supplementary motor cortex (area 6) and somatosensory cortex (areas 3, 1, 2, 5, 7)
• Special large neurons called Betz cells (giant pyramidal cells, layer V) contribute approximately 3% of fibers - the largest neurons in the human nervous system
• ~1 million axons total in the medullary pyramid (far exceeding the ~25,000-35,000 Betz cells)

Course (Step-by-Step)

Key Facts

• The lateral corticospinal tract is the only direct long-fiber connection between the cerebral cortex and the spinal cord
• "Pyramidal tract" strictly refers to the portion traveling through the medullary pyramid
• The broader term "upper motor neuron" also includes indirect pathways: rubrospinal, reticulospinal, vestibulospinal, tectospinal tracts
• The UMN is a two-neuron pathway: Upper motor neuron (cortex to anterior horn) → Lower motor neuron (anterior horn to muscle)

Summary Table: UMN vs LMN Signs

Clinical Case: Right Hemiplegia in a 60-Year-Old Hypertensive

i) Probable Diagnosis

• 60-year-old + hypertension = strongest risk factor for stroke
• Acute onset (2 hours) = vascular event (ischaemic or haemorrhagic stroke)
• Right upper + lower limb weakness = the left motor cortex/left corticospinal tract controls the right side of the body (crossed pathway)
• Increased tone + exaggerated jerks + extensor plantar = classic triad of UMN (pyramidal tract) damage

ii) Reason for Increased Muscle Tone and Exaggerated Tendon Jerks

The Normal State

What Happens After a Stroke

1. Loss of dorsal reticulospinal inhibition → removes the "brake" on anterior horn motor neurons
2. Disinhibition of gamma (fusimotor) motor neurons → increases muscle spindle sensitivity → any stretch triggers an exaggerated response
3. Relative preservation of the medial reticulospinal and vestibulospinal tracts (which facilitate extensor tone) → these now act unopposed
4. Net result: anterior horn alpha motor neurons become hyperexcitable → increased resting tone (spasticity) + exaggerated tendon jerk reflexes (hyperreflexia)

"The heightened stretch reflexes of the spastic state may be a 'release' phenomenon - the result of interruption of descending inhibitory pathways... mediated through disinhibition of spindle efferents (increased tonic activity of gamma motor neurons) and through loss of reticulospinal and vestibulospinal influences on alpha motor neurons."

• Adams and Victor's Principles of Neurology, 12th Ed.

"Increased tone and hyperreflexia do not occur when a selective lesion is made in the corticospinal tract alone. Spasticity is caused by damage to descending inhibitory pathways that travel closely with the corticospinal tract."

• Neuroanatomy through Clinical Cases, 3rd Ed.

iii) Why the Plantar Response is Altered (Extensor Plantar Response / Babinski Sign)

Normal Plantar Response

In This Patient

Mechanism

• The UMN normally exerts tonic inhibition over a primitive nociceptive flexion-withdrawal reflex arc in the spinal cord
• When the corticospinal pathway is damaged, this inhibitory control is lost
• The plantar reflex reverts to the primitive spinal reflex pattern: the great toe extends as part of a generalised leg flexion/withdrawal synergy (also called the "triple flexion response" - hip, knee, ankle dorsiflex)
• From a physiological standpoint, toe extension IS part of limb withdrawal from a noxious stimulus - the dorsum of the great toe moves away from the ground/stimulus
• This response is normal in infants (corticospinal tract not yet myelinated) and reappears in adults only when the corticospinal pathway is damaged

"The extension movement of the great toe is a component of a larger synergistic flexion reflex of the leg - a protective (nocifensive) response. The Babinski sign is the most reliable indication of an upper motor neuron lesion."

• Adams and Victor's Principles of Neurology, 12th Ed.

iv) The Corticospinal Tract

Diagram 1 - Overview of the Two-Neuron Motor Pathway

Diagram 2 - Full Course from Cortex to Spinal Cord

Origin

Step-by-Step Course

• Face fibers → anterior part of posterior limb
• Arm/hand fibers → middle part
• Leg/foot fibers → posterior part

• Alpha motor neurons directly (especially for fine, skilled movements of distal muscles)
• Spinal interneurons (for most proximal muscle movements)

Summary Table

Key Terminology Distinctions

• Corticospinal tract = fibers from cortex to spinal cord
• Pyramidal tract = strictly the portion running through the medullary pyramid
• Upper motor neuron (UMN) = broader term including all descending pathways (corticospinal + rubrospinal + reticulospinal + vestibulospinal + tectospinal)
• These terms are not interchangeable, though often loosely used as such in clinical practice

UMN vs LMN Signs - Summary Table

The Lesion is on the LEFT Side

The Corticospinal Tract Crosses (Decussates)

• Left motor cortex → controls → Right side of body
• Right motor cortex → controls → Left side of body

Applying to This Patient

Simple Rule to Remember

"The brain lesion is ALWAYS on the OPPOSITE side to the body weakness in UMN lesions above the pyramidal decussation."

• Symptoms → Right side of body
• Lesion → Left cerebral hemisphere (most likely left internal capsule, posterior limb)

Corrected Diagnosis

Why LEFT Side?

LEFT Motor Cortex  →  Decussates at Medulla  →  Controls RIGHT body

Corrected Answer for All Parts

Key Rule

Body signs = RIGHT side Brain lesion = LEFT side (Because the tract crosses at the medullary pyramidal decussation)

Clinical Case Analysis: Right Hemiplegia in a 60-Year-Old Hypertensive

i) Probable Diagnosis

The corticospinal tract crosses at the pyramidal decussation in the lower medulla - so the LEFT motor cortex controls the RIGHT side of the body. A lesion in the LEFT hemisphere causes RIGHT-sided deficits.

ii) Reason for Increased Muscle Tone and Exaggerated Tendon Jerks

Normal State

After Left-Sided Stroke

"The heightened stretch reflexes of the spastic state may be a 'release' phenomenon - the result of interruption of descending inhibitory pathways... mediated through disinhibition of spindle efferents (increased tonic activity of gamma motor neurons)."

• Adams and Victor's Principles of Neurology, 12th Ed.

iii) Why the Plantar Response is Altered

Normal Plantar Response (in a healthy adult)

In This Patient - RIGHT Foot Shows Babinski Sign

• Dorsiflexion (upward extension) of the great toe
• Fanning (abduction) of the remaining toes

Mechanism

• The LEFT corticospinal tract normally sends tonic inhibitory signals down to suppress a primitive nociceptive flexion-withdrawal reflex arc in the right spinal cord
• The LEFT-sided stroke interrupts this inhibitory control
• The right-sided plantar reflex reverts to the primitive spinal pattern - the great toe extends as part of a generalised leg withdrawal synergy (triple flexion: hip + knee flex, ankle dorsiflexes)
• This response is normal in infants (before corticospinal myelination is complete) and reappears in adults only when the LEFT corticospinal pathway is damaged, causing the sign to appear on the RIGHT (contralateral) foot

"The Babinski sign is the most reliable indication of an upper motor neuron lesion."

• Adams and Victor's Principles of Neurology, 12th Ed.

iv) The Corticospinal Tract - Full Description with Diagrams

Diagram 1 - Two-Neuron Motor Pathway Overview

Diagram 2 - Complete Course from Cortex to Spinal Cord

Origin of the Corticospinal Tract

Step-by-Step Course

• Face fibers → anterior part of posterior limb
• Arm/hand fibers → middle part
• Leg/foot fibers → posterior part

• ~75-80% of fibers cross to the contralateral (RIGHT) side
• Remaining ~20-25% stay ipsilateral
• Upper limb fibers cross more rostrally than lower limb fibers
• This is why a LEFT brain lesion causes RIGHT body signs

• Alpha motor neurons directly (fine, skilled distal muscle movements)
• Spinal interneurons (proximal muscle movements)

Course Summary Table

Key Terminology

UMN vs LMN Signs - Summary Table

Cerebellar Dysfunction in a 10-Year-Old Boy: Ataxic Gait and Dysarthria

Diagnosis

i) Neurons of the Cerebellar Cortex - Types and Locations

The Three Layers

Layer 1: Molecular Layer (Outermost)

Layer 2: Purkinje Cell Layer (Middle)

Layer 3: Granular Layer (Innermost - deepest)

Memory Aid for Inhibitory vs Excitatory

"All axons projecting UPWARD are excitatory (mossy fibers, climbing fibers, granule cell parallel fibers). All axons projecting DOWNWARD are inhibitory (Purkinje cells, stellate cells, basket cells, Golgi cells). Outputs of deep cerebellar nuclei are always excitatory."

• Neuroanatomy through Clinical Cases, 3rd Ed.

ii) Physiological Significance of the Cerebellar Neuronal Circuit

Circuit Diagram

Two Major Input Systems

• Originates from: spinal cord (spinocerebellar tracts), pontine nuclei (corticopontine fibers), vestibular nuclei
• Mossy fibers synapse on granule cells in cerebellar glomeruli
• Granule cell axons → parallel fibers → weakly excite MANY Purkinje cells (divergent, widespread effect)
• Also excite basket cells, stellate cells, and Golgi cells via parallel fibers

• Originates exclusively from: inferior olivary nucleus of the medulla
• Each climbing fiber winds around the Purkinje cell dendrites like a vine
• Each Purkinje cell receives input from only ONE climbing fiber
• A single climbing fiber action potential powerfully excites a single Purkinje cell - produces complex spikes
• Thought to act as an error signal for motor learning (comparison of intended vs actual movement)

The Core Circuit - Step by Step

INPUTS
  ↓
Mossy fibers ──→ Granule cells ──→ Parallel fibers
                                        ↓           ↓           ↓
                                  Purkinje     Basket/     Golgi cells
                                  cell(+)     Stellate(-)      (-)
                                                  ↓               ↓
                                         Inhibits adjacent   Inhibits granule
                                         Purkinje cells      cells (feedback)
                                   ↓
Climbing fibers ──────────────→ Purkinje cell (powerful +)

PURKINJE CELL AXON (GABA - inhibitory)
  ↓
DEEP CEREBELLAR NUCLEI (also receive + collaterals from mossy/climbing fibers)
  ↓
EXCITATORY OUTPUT → Thalamus → Motor Cortex
                  → Brainstem

Physiological Significance of Each Circuit Element

Overall Physiological Role

iii) Functional Divisions of the Cerebellum

Diagram

Division 1: Vestibulocerebellum (Archicerebellum)

Division 2: Spinocerebellum (Paleocerebellum)

Division 3: Cerebrocerebellum / Pontocerebellum (Neocerebellum)

Functional Divisions Summary Table

Connecting Back to This Patient

• Unstable gait (ataxia) - primarily vestibulocerebellum and spinocerebellum dysfunction
• Difficulty with speech (dysarthria) - primarily cerebrocerebellum (dentate nucleus) involvement
• Both features together point to diffuse or midline cerebellar dysfunction

Clinical Case: Parkinson's Disease in a 60-Year-Old Male

i) Probable Diagnosis

ii) Pathophysiology of Parkinson's Disease

Anatomy of the Basal Ganglia

Normal Basal Ganglia Circuit

Cortex (Glu+) → Striatum (D1 receptors)
  → Inhibits GPi (GABA-)
  → GPi inhibition REDUCED → Thalamus disinhibited
  → Thalamus (Glu+) → Motor Cortex ACTIVATED → MOVEMENT

Cortex (Glu+) → Striatum (D2 receptors)
  → Inhibits GPe (GABA-)
  → GPe inhibition REDUCED → STN activated
  → STN (Glu+) → Activates GPi
  → GPi (GABA-) → Inhibits Thalamus
  → Motor Cortex SUPPRESSED → LESS MOVEMENT

• D1 receptors on direct pathway neurons → Dopamine excites → more movement
• D2 receptors on indirect pathway neurons → Dopamine inhibits → less suppression → more movement
• Net effect: dopamine facilitates movement through both pathways

Pathophysiology in Parkinson's Disease

• Direct pathway LESS active → GPi NOT inhibited → GPi remains tonically overactive → Thalamus over-inhibited → Motor cortex under-activated

• Indirect pathway MORE active → GPe MORE inhibited → STN disinhibited and overactive → STN over-stimulates GPi → GPi even MORE overactive → Thalamus even more inhibited

• Excessive GPi/SNr output → Thalamus excessively inhibited → Motor cortex under-activated
• Clinical manifestation: Akinesia, bradykinesia, difficulty initiating movement (hypokinesia)

Step-by-Step Summary Table

"Dopamine denervation leads to increased firing of neurons in the STN and GPi, excessive inhibition of the thalamus, reduced activation of cortical motor systems, and the development of parkinsonian features."

• Harrison's Principles of Internal Medicine, 22nd Ed.

iii) Physiological Basis of Hyperkinesia in Parkinson's Disease

Normal Balance

In Parkinson's Disease

1. Nigrostriatal dopaminergic system
2. Intrastriatal cholinergic system
3. GABAergic system (striatum → globus pallidus and substantia nigra)

Resting Tremor (Hyperkinetic Feature)

• Caused by rhythmic, alternating 8-Hz contractions of antagonist muscles
• Due to uncoordinated oscillating activity in the basal ganglia - thalamo-cortical loop in the absence of dopaminergic stabilisation
• Tremor is present at rest (when the motor cortex is not actively driving the muscles)
• Disappears with voluntary movement because cortical activation temporarily overrides the oscillating basal ganglia circuit
• This is called "tremor at rest" or "pill-rolling tremor" - the classic feature in this patient

Why it stops with movement

iv) Two Drugs Used to Reduce Hypokinesia

1. Levodopa (L-DOPA) + Carbidopa (Sinemet)

• Mechanism: Levodopa is a precursor to dopamine; it crosses the blood-brain barrier (dopamine itself cannot) and is decarboxylated to dopamine in the brain, replenishing dopamine in the striatum
• Carbidopa inhibits peripheral DOPA decarboxylase, preventing conversion of L-DOPA to dopamine outside the brain - increases CNS delivery and reduces peripheral side effects (nausea, cardiac effects)
• Most effective drug for hypokinesia and bradykinesia in PD
• First drug identified for treating a specific neurotransmitter deficiency

2. Dopamine Agonists (e.g., Pramipexole, Ropinirole, Bromocriptine)

• Mechanism: Directly stimulate dopamine receptors (D1 and D2) in the striatum, bypassing the need for dopamine synthesis by degenerated SNc neurons
• Used as monotherapy (especially in younger patients to delay levodopa complications) or in combination with levodopa
• Also include apomorphine (rapid-acting injectable dopamine agonist)

Additional drug classes used: MAO-B inhibitors (selegiline, rasagiline - prevent dopamine breakdown), COMT inhibitors (entacapone - block L-DOPA breakdown, allowing more to reach brain), Amantadine (increases dopamine release).

v) Type of Rigidity in Parkinson's Disease vs UMN Rigidity

Rigidity in Parkinson's Disease

"The rigidity is different from spasticity because motor neuron discharge increases to both the agonist and antagonist muscles. Passive motion of an extremity meets with a plastic, dead-feeling resistance... called lead pipe rigidity. Sometimes a series of 'catches' takes place during passive motion (cogwheel rigidity), but the sudden loss of resistance seen in a spastic extremity is absent."

• Ganong's Review of Medical Physiology, 26th Ed.

• Loss of dopamine → increased GPi output → thalamic inhibition → loss of descending cortical inhibition of spinal stretch reflexes
• Also: dopamine-acetylcholine imbalance → hyperexcitability of spinal interneurons
• Both alpha and gamma motor neurons are overactive → tone increased in ALL muscle groups

Rigidity in UMN Lesion (Spasticity)

Comparison Table: Parkinson's Rigidity vs UMN (Spastic) Rigidity

Summary - Key Distinction

Parkinson's rigidity = plastic, uniform, lead-pipe + cogwheel quality; both agonist and antagonist equally stiff; NO clasp-knife; tremor at rest disappears with movement.

UMN spasticity = velocity-dependent, clasp-knife quality; anti-gravity muscles predominantly; WITH exaggerated reflexes and Babinski sign; tremor absent.

Placental Hormones

1. Human Chorionic Gonadotropin (hCG)

• α subunit is identical to that of LH, FSH, and TSH (shared)
• β subunit is unique to hCG and confers its specificity

• Rescues the corpus luteum from luteolysis - prevents its regression after ovulation
• Stimulates corpus luteum to continue secreting progesterone and estrogen for the first 6-8 weeks (until placenta takes over)
• Stimulates testosterone production by Leydig cells in the male fetus (essential for male genital development)
• Acts on the same receptor as LH
• Small amounts also secreted by various gastrointestinal tumors → used as a tumor marker

2. Placental Progesterone

• Inhibits uterine (myometrial) contractions - maintains uterine quiescence throughout pregnancy
• Maintains the decidua (decidual reaction of endometrium)
• Prevents prostaglandin production by the uterus (which would otherwise stimulate contractions)
• Promotes differentiation and development of the mammary glands
• Suppresses maternal immune response to prevent rejection of the fetus
• ~2/3 enters maternal circulation, ~1/3 enters fetal circulation

3. Placental Estrogen

• Fetal adrenal cortex and fetal liver produce DHEAS and 16α-hydroxy-DHEAS
• Placenta converts these precursors into estrogens (aromatization)
• Neither fetus nor placenta can complete the synthesis alone

• Promotes growth of the uterus (myometrial hypertrophy)
• Promotes development of the mammary glands (ductal proliferation)
• Increases sensitivity of the uterus to oxytocin near term
• Promotes sodium and water retention
• Increases hepatic protein synthesis (binding proteins, clotting factors)

4. Human Chorionic Somatomammotropin (hCS) / Human Placental Lactogen (hPL)

• Acts as a "maternal growth hormone of pregnancy"
• Promotes lipolysis → raises maternal free fatty acids → provides fuel for maternal tissues
• Decreases maternal glucose utilization (anti-insulin/diabetogenic effect) → spares glucose for the fetus
• Nitrogen, potassium, and calcium retention
• Lactogenic - prepares mammary glands for lactation (though milk secretion requires prolactin)
• Suppresses maternal GH secretion from pituitary (replaces it functionally)

5. Other Placental Hormones

Summary Table

Ovarian Changes in the Menstrual Cycle

Hormonal Control - Overview

Hormone Levels Throughout the Cycle

Phase 1: Follicular Phase (Days 1-14)

Overview

Step-by-Step Ovarian Events

• As corpus luteum regresses at the end of the previous cycle, estrogen and progesterone fall sharply
• This removes negative feedback on the hypothalamus and anterior pituitary
• FSH rises slightly → recruits a cohort of primordial follicles for development

• Primordial follicles consist of a primary oocyte arrested in prophase I surrounded by a single layer of flat (squamous) follicle cells
• FSH stimulates follicle cells to become cuboidal → forming the primary follicle
• The oocyte grows and secretes the zona pellucida (ZP glycoproteins)
• Surrounding stromal tissue differentiates into theca interna (inner, vascularised, androgen-producing) and theca externa (outer, fibrous)

• FSH stimulates granulosa cell proliferation (only cells with FSH receptors)
• Granulosa cells secrete follicular fluid which accumulates in spaces between them
• Spaces coalesce to form the antrum - a single fluid-filled cavity
• This stage is called the secondary (antral) follicle
• The oocyte is still arrested in prophase I of meiosis I

• LH acts on theca interna cells → converts cholesterol to androgens (androstenedione, testosterone)
• FSH acts on granulosa cells → stimulates aromatase → converts androgens to estrogens (estradiol)
• Rising estradiol provides negative feedback on the pituitary (keeps FSH/LH low initially)

• From the recruited cohort, one dominant follicle is selected (has the most FSH receptors and most granulosa cells → produces most estradiol)
• All other follicles undergo atresia (apoptotic degeneration) - only one reaches maturity per cycle
• The dominant follicle develops into the mature Graafian follicle (up to 20mm diameter)
• Graafian follicle has: large antrum, prominent theca interna, granulosa cells, cumulus oophorus (stalk connecting oocyte to granulosa), corona radiata (granulosa cells immediately surrounding oocyte)

• The enlarged Graafian follicle produces a sharp surge in estradiol (≥200 pg/mL)
• At this critical threshold, estradiol switches from negative to positive feedback on the pituitary
• This triggers the LH surge (and a smaller FSH surge) - the ovulatory surge

Ovulation - Day 14

1. LH surge (36-48 hours before) triggers resumption of meiosis in the oocyte
2. Primary oocyte completes meiosis I → secondary oocyte + first polar body
3. Oocyte arrests again in metaphase II
4. Prostaglandins, proteolytic enzymes, and collagenase digest the follicular wall
5. Stigma (thinned spot on follicle wall) ruptures
6. Secondary oocyte (surrounded by corona radiata) is expelled into the peritoneal cavity
7. Fimbriae of the fallopian tube sweep the oocyte into the tube

• Cervical mucus increases in volume, becomes watery and spinnbarkeit (stretchable) - maximally penetrable by sperm
• Estradiol levels briefly fall immediately after ovulation

Regardless of total cycle length, ovulation always occurs 14 days before menses (the luteal phase is constant; the follicular phase varies in length).

Phase 2: Luteal Phase (Days 14-28)

• After the oocyte is expelled, the remaining follicular wall collapses and is vascularised
• Under continued LH influence, granulosa and theca cells undergo luteinization:
  • Granulosa lutein cells (large, yellow, lipid-rich) → produce progesterone (major product) and estrogen
  • Theca lutein cells → produce androgens (precursors for estrogen)
• The structure is called the corpus luteum (yellow body, due to lutein/carotenoid pigments)

• Progesterone rises sharply → peaks at Day 21
• Estradiol has a second (smaller) rise
• Inhibin A - suppresses FSH
• These hormones provide negative feedback on hypothalamus and pituitary → FSH and LH remain low (preventing new follicle development)

• Basal body temperature rises 0.2-0.5°C (due to progesterone raising hypothalamic temperature set point) - used clinically to confirm ovulation
• Cervical mucus becomes thick, scanty, and impenetrable to sperm

End of Cycle: Corpus Luteum Regression (Days 24-28)

• Corpus luteum has a lifespan of 10-12 days after ovulation
• Without hCG from an implanting embryo, the corpus luteum undergoes luteolysis (regression)
• Progesterone and estrogen levels fall sharply
• Loss of hormonal support causes endometrial ischemia → menstruation (Day 1 of next cycle)
• The regressed corpus luteum leaves a white fibrous scar - the corpus albicans
• Falling estrogen and progesterone remove negative feedback → FSH begins to rise → next cohort of follicles is recruited → new cycle begins

• Implanting trophoblast produces hCG from Day 6-8 post-fertilization
• hCG acts like LH → rescues the corpus luteum from luteolysis
• Corpus luteum continues to produce progesterone for the first 6-8 weeks of pregnancy (corpus luteum of pregnancy)
• After 8 weeks, the placenta takes over progesterone production

Summary Table: Ovarian Changes Throughout the Cycle

Two-Cell, Two-Gonadotropin Theory (Key Concept)

LH → Theca interna cells → Androgens (androstenedione)
                                     ↓
FSH → Granulosa cells → Aromatase → ESTRADIOL

Why is Vertigo Common in Ear Diseases?

Definition of Vertigo

The Core Anatomical Reason

Anatomy of the Vestibular Apparatus

1. Three Semicircular Canals

• Horizontal (lateral), Superior (anterior), Posterior
• Arranged in three perpendicular planes to detect rotational (angular) acceleration in any direction
• Each canal ends in an ampulla containing the crista ampullaris (the sensory receptor)
• The crista contains hair cells embedded in a gelatinous mass called the cupula

• When the head rotates, the bony canal moves but endolymph lags behind due to inertia
• The endolymph deflects the cupula → bends hair cell cilia
• Bending toward the kinocilium → depolarisation → increased firing in vestibular nerve
• Bending away from kinocilium → hyperpolarisation → decreased firing

2. Utricle and Saccule (Otolith Organs)

• Contain the macula - a patch of hair cells embedded in a gelatinous membrane bearing otoconia (calcium carbonate crystals/stones)
• Utricle: detects horizontal linear acceleration and head tilt
• Saccule: detects vertical linear acceleration

Left-Right Pairing - The Key to Understanding Vertigo

• Left and right semicircular canals are paired and respond symmetrically
• At rest, both sides fire at a baseline rate; the equal signals cancel → no sensation of movement
• When the head moves normally, both sides change firing rates in a mirror-image, balanced way

Why Inner Ear Disease Causes Vertigo

The Fundamental Mechanism: Asymmetric Vestibular Signals

"With inner ear disease, the resting discharge or the discharge stimulated by motion can be altered in ONE ear. This alteration produces asymmetric responses and results in the perception of vertigo."

• Rosen's Emergency Medicine

Left vestibular input  = Right vestibular input
        ↓
Brain perceives: NO movement (balanced signals)

Left vestibular input ≠ Right vestibular input
        ↓
MISMATCH → Brain interprets difference as rotation
        ↓
Vertigo (false sensation of spinning)

Neural Pathways That Explain All the Symptoms of Vertigo

Maintenance of Equilibrium: The Three-System Model

1. Vestibular system (inner ear)
2. Visual system (eyes)
3. Proprioceptive system (joints, muscles, skin)
         ↓
All three integrated at CEREBELLUM (via vestibular nuclei)

"Any disease that causes a MISMATCH of information from any two of these three systems may give rise to symptoms of vertigo."

• Rosen's Emergency Medicine

How Nystagmus Arises (A Specific Sign of Inner Ear Disease)

• Asymmetric stimulation of medial and lateral rectus muscles of the eye
• Unopposed activity causes a slow drift of eyes toward the side of the diseased ear
• The cerebral cortex rapidly corrects this with a fast snap back
• This cycle repeats → nystagmus (slow phase toward diseased side, fast phase toward healthy side)

"By convention, the direction of nystagmus is denoted by the direction of the FAST (cortical correction) component."

Common Ear Diseases Causing Vertigo

Summary

1. The vestibular apparatus (the balance organ) is physically located inside the inner ear, sharing the same bony labyrinth, fluid spaces, and nerve as the cochlea
2. Any inner ear disease disrupts vestibular hair cell function → creates asymmetric input from the two ears
3. The brain interprets this asymmetry as movement → generates the false sensation of vertigo
4. Connections to brainstem, cerebellum, eye muscles, spinal motor neurons, and the autonomic system produce the full clinical syndrome: vertigo + nystagmus + unsteadiness + nausea/vomiting