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PY11.1-7 | Special Senses — Part 2

Physiology of Hearing, Pathophysiology of Deafness, Hearing Tests — PY11.4

Physiology of Hearing — From Sound to Perception

Figure: Physiology of Hearing, Pathophysiology of Deafness, Hearing Tests — PY11.4

Sound is a pressure wave (compression-rarefaction) in air. Hearing converts this mechanical energy into electrical nerve signals.

Step-by-step mechanism:

1. Sound waves → collected by pinna → travel down EAC → vibrate tympanic membrane
2. Tympanic membrane vibration → ossicular chain (malleus → incus → stapes footplate) → pressure waves in perilymph of scala vestibuli (via oval window)
3. Pressure waves travel up scala vestibuli → through helicotrema → down scala tympani → dissipated at round window (secondary tympanic membrane) into middle ear
4. Perilymph waves → basilar membrane vibration

Place Principle (Tonotopy) — Bekesy's discovery (Nobel Prize 1961):
• Different sound frequencies cause maximum vibration at different points along the basilar membrane
• High frequency (e.g., 20,000 Hz): maximum vibration at BASE (near oval window) — basilar membrane narrow and stiff
• Low frequency (e.g., 20 Hz): maximum vibration at APEX (helicotrema) — basilar membrane wide and flexible
• Think: bass notes are low frequency — they make the apex vibrate. Treble notes are high — base of cochlea.
Mnemonic: "High frequency → BASE; Low frequency → APEX" — "High-BASE, Low-APEX"

1. Basilar membrane vibration → hair cell stereocilia bend against tectorial membrane
2. Bending → mechanically-gated K⁺ channels (tip links between stereocilia) open → K⁺ influx from endolymph (high K⁺) → depolarisation (unique: K⁺ depolarises because endolymph is the extracellular fluid here)
3. Depolarisation → Ca²⁺ influx → neurotransmitter (glutamate) release at base of hair cell → auditory nerve (cochlear branch of CN VIII) action potentials
4. Auditory nerve → cochlear nuclei (medulla) → superior olivary complex → lateral lemniscus → inferior colliculus (midbrain) → medial geniculate nucleus (thalamus) → primary auditory cortex (Heschl's gyrus — superior temporal gyrus, Area 41/42)

Note: At the superior olivary complex, fibres from both ears converge → binaural hearing → allows sound localisation

Normal hearing range: 20 Hz – 20,000 Hz; most sensitive at 1,000–3,000 Hz (speech frequencies)
Threshold of hearing: 0 dB SPL (softest audible sound); threshold of pain: 120–130 dB

Pathophysiology of Deafness

Deafness (hearing loss) is one of the most common disabilities worldwide. Two fundamental types:

1. Conductive Deafness (Conductive Hearing Loss)
• Definition: Defect in transmission of sound from the external environment to the cochlea
• Site of lesion: Outer ear or middle ear
• Causes:
- Wax impaction (ceruminous occlusion) — very common in India
- Perforated tympanic membrane (trauma, infection)
- Otitis media (middle ear infection — very common in children)
- Otosclerosis: abnormal bone growth fixing the stapes footplate → cannot vibrate
- Ossicular chain disruption (trauma)
• Bone conduction is PRESERVED — sound still reaches cochlea via bone
• Treatment: Often surgical or mechanical (hearing aids)

2. Sensorineural Deafness (SNHL — Sensorineural Hearing Loss)
• Definition: Defect in the cochlea (hair cells), auditory nerve, or central auditory pathway
• Site of lesion: Cochlea, CN VIII, or brain
• Causes:
- Presbycusis: age-related hearing loss — outer hair cell degeneration starting at basal (high-frequency) end → characteristically lose HIGH frequency hearing first
- Noise-induced hearing loss: chronic loud noise → OHC damage — occupational hazard (construction workers, factory workers, DJs) and now smartphone earphones
- Ototoxic drugs: Aminoglycosides (gentamicin, streptomycin), loop diuretics (furosemide), cisplatin — damage OHC especially at base
- Acoustic neuroma (vestibular schwannoma): benign tumour on CN VIII → unilateral SNHL + tinnitus
- Viral labyrinthitis (measles, mumps — a key reason for MMR vaccination)
- Congenital: maternal rubella, genetic (connexin 26 mutation — most common genetic deafness)
• Bone and air conduction both reduced
• Treatment: Hearing aids, cochlear implants (for severe/profound SNHL)

Hearing Tests

Bedside Tuning Fork Tests (clinical exam staple):

Rinne's Test (tests one ear at a time):
• 512 Hz tuning fork placed on mastoid (bone conduction, BC) then in front of ear (air conduction, AC)
• Normal: AC > BC (Rinne positive) — air conduction is better
• Conductive deafness: BC > AC (Rinne negative) — bone bypasses the defect
• SNHL: AC > BC (Rinne positive) — but both reduced in absolute terms

Weber's Test (compares the two ears):
• Tuning fork placed on vertex of skull (midline)
• Normal: Heard equally in both ears (midline)
• Conductive deafness: Lateralises to the DEAF ear (sound appears louder on the blocked side)
• SNHL: Lateralises to the BETTER ear (good cochlea picks it up better)

Mnemonic for Weber's: "Conducts to the same; Nerve goes the other way" — conductive deafness lateralises to affected ear; SNHL lateralises to healthy ear.

Audiometry: Standard hearing test using pure tones at different frequencies and intensities
• Air conduction and bone conduction thresholds plotted on an audiogram
• Air-bone gap: AC worse than BC → conductive component
• Audiometry is the definitive test; tuning forks are screening tools

Tympanometry: Tests eardrum and middle ear compliance (used to diagnose middle ear effusion in children)

Auditory Brainstem Response (ABR/BERA): Objective test — measures brainstem electrical responses to sound; used for infant screening and diagnosing retrocochlear (nerve/brainstem) pathology

Part 4: Functional Anatomy of the Eye, Visual Pathway, and Pupillary Reflex — PY11.5

Functional Anatomy of the Eye

Figure: Part 4: Functional Anatomy of the Eye, Visual Pathway, and Pupillary Reflex — PY11.5

The eye is the optical and transduction apparatus for vision. It consists of:

Outer coat (fibrous layer):
• Sclera (posterior 5/6): tough white opaque coat — maintains eye shape, site of extraocular muscle attachment
• Cornea (anterior 1/6): transparent, avascular, accounts for 70% of total refractive power of the eye (~43 diopters)
- Avascular: receives nutrition from aqueous humour and tears
- Densely innervated by CN V (trigeminal) → corneal reflex (blink)

Middle coat (uveal tract / vascular layer):
• Choroid: highly vascular, provides nutrition to outer retina; contains melanin (absorbs stray light)
• Ciliary body: contains ciliary muscle (smooth muscle) + produces aqueous humour (via ciliary processes)
- Ciliary muscle contraction → suspensory ligaments (zonules) relax → lens becomes more convex → accommodation for near vision
• Iris: muscular diaphragm controlling pupil size
- Sphincter pupillae (circular muscle, parasympathetic CN III): pupil constriction (miosis)
- Dilator pupillae (radial muscle, sympathetic): pupil dilation (mydriasis)

Inner coat — The Retina:
• The neural layer — where light is converted to electrical signals
• Two regions of functional importance:
- Fovea centralis (centre of macula): highest cone density, maximum visual acuity, no rods
- Optic disc (blind spot): where optic nerve exits; no photoreceptors → blind spot
• Two types of photoreceptors:
- Rods: ~120 million; for scotopic (dim light) vision; one type of photopigment: rhodopsin (opsin + retinal)
- Cones: ~6 million; for photopic (bright light) and colour vision; concentrated in fovea
- Three types: S-cones (blue, 420 nm), M-cones (green, 530 nm), L-cones (red, 560 nm)

Refractive Media:
Light passes through: Cornea → Aqueous humour → Lens → Vitreous humour → Retina
• Aqueous humour: produced by ciliary body, drains via canal of Schlemm at angle of anterior chamber
- Blockage → raised intraocular pressure (IOP) → glaucoma → optic nerve damage
• Lens: biconvex, avascular, no nerves, changes shape during accommodation
- Normally transparent; clouding → cataract (most common cause of reversible blindness in India)
• Vitreous humour: gel-like, fills posterior chamber, maintains eye shape; not replenished

Visual Pathway

This is the single most important topic in the special senses for the Physiology examination.

Light → Photoreceptors → Bipolar cells → Retinal Ganglion Cells → Optic Nerve

Step-by-step pathway:

1. Optic nerve (CN II): axons of retinal ganglion cells from each eye
2. Optic chiasma: where partial decussation occurs
- Nasal (medial) fibres CROSS to the opposite side
- Temporal (lateral) fibres DO NOT cross (remain ipsilateral)
- Why this matters: The nasal retina sees the temporal (outer) visual field; temporal retina sees the nasal (central) visual field
3. Optic tract: post-chiasmal; contains fibres from both eyes but for the SAME visual field
- Right optic tract: temporal fibres from right eye + nasal fibres from left eye → carries LEFT visual field information
- Left optic tract: carries RIGHT visual field information
4. Lateral Geniculate Nucleus (LGN) of thalamus: primary relay station
- 6 layers (laminar organisation)
- Maintains retinotopic (point-to-point) map
5. Optic radiations (geniculocalcarine tract): fan out around lateral ventricle
- Meyer's loop (lower fibres, temporal lobe): carry superior visual field from upper retina
- Upper fibres (parietal lobe): carry inferior visual field
6. Primary visual cortex (V1): Area 17 (striate cortex) — calcarine sulcus, occipital lobe

Visual Field Defects — Localising Lesions (High Exam Value)

Site of Lesion	Visual Field Defect	Cause Example
Right optic nerve	Right monocular blindness (complete loss in one eye)	MS, orbital tumour
Optic chiasma (central)	Bitemporal hemianopia (tunnel vision)	Pituitary macroadenoma
Right optic tract	Left homonymous hemianopia	MCA stroke, trauma
Right temporal lobe (Meyer's loop)	Left superior quadrantanopia ("pie in the sky")	Temporal lobe lesion
Right parietal lobe (upper radiation)	Left inferior quadrantanopia ("pie on the floor")	Parietal lobe lesion
Right occipital cortex (V1)	Left homonymous hemianopia with macular sparing	PCA stroke (macula has dual supply)

Mnemonic for chiasmal lesion: "Pituitary P-resses the chiasma from below → B-itemporalB-lindness" — P-B rule

Light Reflex and Pupillary Reflex

Direct light reflex: Shine light in right eye → right pupil constricts (direct response)
Consensual light reflex: Shine light in right eye → left pupil ALSO constricts (consensual)

Pathway of light reflex:
• Afferent: Retina → Optic nerve → Optic chiasma → BOTH pretectal nuclei (via brachium of inferior colliculus) — note: NOT through LGN or cortex
• Efferent: Pretectal nucleus → BOTH Edinger-Westphal nuclei (CN III, parasympathetic) → ciliary ganglion → sphincter pupillae → pupil constriction

Since each pretectal nucleus connects to BOTH Edinger-Westphal nuclei → both pupils respond when one eye is illuminated (consensual reflex)

Relative Afferent Pupillary Defect (RAPD) / Marcus Gunn pupil:
• When light shone in affected eye (e.g., optic neuritis, severe retinal damage) → pupils dilate (paradoxical) because the afferent signal is weaker
• Tested with the swinging flashlight test — gold standard for optic nerve disease

Accommodation Reflex (Near Reflex Triad):
When focusing on a near object → three simultaneous responses:
1. Accommodation: Lens becomes more convex (ciliary muscle contracts)
2. Convergence: Both eyes turn medially (medial recti contract)
3. Miosis: Pupils constrict (reduces aberrations, increases depth of focus)
All three mediated by CN III.

Part 5: Image Formation, Refractive Errors, Colour Vision — PY11.6 & PY11.7

Physiology of Image Formation — PY11.6

How the Eye Forms an Image

The eye is a camera — it uses a lens system to focus parallel rays of light onto the retina.

Key optical concepts:
• Refraction: Bending of light as it passes between media of different refractive indices
• Refractive index: Cornea (1.376), aqueous humour (1.336), lens (1.4 core, 1.386 average), vitreous (1.336)
• Principal focus: Point where parallel light rays converge after passing through the refracting system
• Focal length: Distance from the lens to the principal focus
• Power of lens = 1/focal length (in metres) = Diopters (D)

Total refractive power of the eye: ~58–60 D
• Cornea: ~43 D (fixed)
• Lens: ~15–17 D at rest, up to ~30 D when fully accommodated

Accommodation — Focusing on Near Objects

Accommodation is the ability of the eye to increase its refractive power for near vision.

Mechanism:
1. Near object → blurred image on retina → visual cortex detects blur
2. Cortex signals → CN III → ciliary muscle contracts (circular/sphincter-like muscle)
3. Ciliary muscle contraction → suspensory ligaments (zonules) RELAX → lens released from tension
4. Lens elastic capsule allows lens to become more convex → increased refractive power → sharp near image

Remember: Ciliary muscle CONTRACTS → Zonules RELAX → Lens MORE CONVEX (opposite of what you might expect!)

Amplitude of accommodation = difference between near point and far point power
• Young adults: can accommodate up to ~14 D
• By age 40–45: presbyopia — lens loses elasticity → cannot accommodate → reading glasses needed

Near point: Closest point of clear vision (~7 cm at age 20)
Far point: Furthest point of clear vision (at infinity for an emmetrope)

Refractive Errors (Ametropia)

Emmetropia = normal refraction; parallel light focuses exactly on retina with ciliary muscle relaxed.

1. Myopia (Short-sightedness / Nearsightedness)
• Definition: Parallel light from a distant object focuses IN FRONT OF (anterior to) the retina
• Cause: Eyeball too long (axial myopia — most common) OR cornea/lens too curved (refractive myopia)
• Clinical: Clear near vision, blurred distance vision
• Correction: Concave (diverging) lens — spreads rays so they focus further back onto retina
• Prevalence in India: Rapidly increasing — now epidemic in urban youth (reduced outdoor activity, increased screen time)

2. Hypermetropia (Long-sightedness / Farsightedness)
• Definition: Parallel light focuses BEHIND the retina
• Cause: Eyeball too short OR cornea/lens not curved enough
• Clinical: Blurred near vision (worse), blurred distance vision (in severe cases)
• Correction: Convex (converging) lens — converges rays before entering eye
• Mild hypermetropia is common in children and often corrects as eye grows

3. Astigmatism
• Definition: Unequal curvature of the cornea (or lens) in different meridians → cannot focus a point as a point; instead, it forms a line or oval
• Cause: Cornea is not perfectly spherical — like a rugby ball instead of a football
• Clinical: Blurred and distorted vision at all distances
• Correction: Cylindrical lens (corrects one meridian only)

4. Presbyopia
• Definition: Age-related loss of accommodation due to loss of lens elasticity
• Age of onset: Typically >40 years
• Clinical: Difficulty reading small print (holds book at arm's length — classic sign)
• Correction: Convex (converging) reading glasses or bifocal/progressive lenses

Physiological Principles of Management
• Spectacles: External lenses that pre-correct the incoming light
• Contact lenses: Same principle, placed directly on cornea
• Refractive surgery (LASIK, PRK): Uses laser to reshape the cornea → permanently change its curvature
- LASIK: Most common; creates a flap, reshapes stroma, replaces flap
- Mechanism: Removing corneal tissue flattens it (corrects myopia) or steepens peripheral cornea
• Intraocular lens (IOL) implant: For cataract surgery — the cloudy lens is replaced with a synthetic lens of calculated power

Colour Vision and Colour Blindness — PY11.7

Trichromatic Theory (Young-Helmholtz Theory)
• The retina has three types of cones, each maximally sensitive to a different wavelength:
- S-cones (Short wavelength): Blue — peak ~420 nm
- M-cones (Medium wavelength): Green — peak ~530 nm
- L-cones (Long wavelength): Red — peak ~560 nm
• All colour perception arises from the ratio of stimulation of these three cone types
• White light stimulates all three equally; pure wavelengths stimulate them in different ratios

Opponent Colour Theory (Hering)
• Complements trichromatic theory — explains colour at the ganglion cell/LGN level
• Three opponent pairs: Red-Green, Blue-Yellow, Black-White
• Ganglion cells are excited by one colour of a pair and inhibited by the other
• Explains afterimages: stare at red → see cyan (opposite) in afterimage
• Both theories are needed for complete explanation — trichromatic at receptor level; opponent at ganglion/cortical level

Photopigments and Transduction
• Each photopigment = opsin protein + retinal (vitamin A aldehyde — link to Biochemistry!)
• Rhodopsin (in rods) = scotopsin + 11-cis retinal
• Light → 11-cis retinal converts to all-trans retinal → conformational change in opsin → activates transducin (G-protein) → activates phosphodiesterase → decreases cGMP → cGMP-gated channels close → Na⁺ influx stops → hyperpolarisation (rods hyperpolarise in light, unlike most receptors)

Note: Rods HYPERPOLARISE in response to light — the opposite of what you might expect. In darkness, rods have a constant "dark current" of Na⁺ → are slightly depolarised.

Colour Blindness (Colour Vision Deficiency)
• Affects ~8% of males, ~0.5% of females
• Inheritance: X-linked recessive — hence much more common in males
- Mother is usually a carrier (normal vision): X^B X^b
- Sons have 50% chance of being colour-blind: X^b Y
- Daughters are colour-blind only if homozygous (X^b X^b) — much rarer

Types:
• Anomalous trichromacy (most common — ~75% of colour-blind people): all 3 cone types present but one has shifted peak sensitivity
- Protanomaly (shifted L-cone), Deuteranomaly (shifted M-cone — most common overall), Tritanomaly (shifted S-cone — very rare, autosomal)
• Dichromacy (one cone type missing):
- Protanopia: missing L-cones (red-blind) — cannot distinguish red from green
- Deuteranopia: missing M-cones (green-blind) — most common dichromacy
- Tritanopia: missing S-cones (blue-blind) — rare, autosomal
• Achromatopsia: ALL cones absent — only rod vision; complete colour blindness; extremely rare

Ishihara colour plates: Standard screening test (can detect red-green colour blindness reliably; less sensitive for blue-yellow).

Applied implications in India:
• Colour blindness disqualifies candidates from certain careers: Armed Forces (combat roles), Railways (signal detection), Civil Aviation (pilot licence)
• Patients should be counselled early — at school entrance examination
• No treatment — but assistive technology (EnChroma glasses) can help anomalous trichromats

Dark Adaptation
• Moving from bright light → dark → vision initially very poor → gradually improves
• Two phases: Cone adaptation (rapid, first 5–7 min); Rod adaptation (slow, full adaptation by ~30–45 min)
• Mechanism: Regeneration of rhodopsin from all-trans retinal back to 11-cis retinal (requires Vitamin A)
• Impaired dark adaptation = night blindness (nyctalopia) — most commonly due to Vitamin A deficiency in India