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Special Senses Physiology - Complete Viva Guide

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🟠 PART 1: SENSE OF HEARING (Auditory Physiology)

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1. Anatomy of the Ear - Quick Overview

• Outer ear - pinna + external auditory meatus + tympanic membrane
• Middle ear - ossicles (malleus, incus, stapes) + Eustachian tube
• Inner ear - cochlea (hearing) + vestibular apparatus (balance)

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2. Sound Conduction to the Cochlea

1. Area ratio - tympanic membrane (55 mm²) vs stapes faceplate (3.2 mm²) = 17-fold force increase
2. Lever ratio - ossicular lever = 1.3-fold force increase
3. Combined: ~22 times more force on cochlear fluid than on tympanic membrane
4. Matching efficiency = 50-75% for frequencies 300-3000 Hz

Viva Q: "What happens if ossicles are missing?" - Hearing decreases by 15-20 dB (drops from medium voice to barely perceptible).

• Loud sounds → reflex contraction of tensor tympani (CN V) and stapedius (CN VII)
• Reduces sound transmission by 30-40 dB
• Protects inner ear from loud, prolonged noise
• Does NOT protect from sudden sounds (reflex delay ~40-160 ms)

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3. The Cochlea

• Scala vestibuli and scala tympani communicate at the helicotrema at the apex
• Round window at base of scala tympani - bulges out when oval window moves in

• Endolymph = high K⁺, low Na⁺ (like intracellular fluid) - secreted by stria vascularis
• Perilymph = high Na⁺, low K⁺ (like CSF/extracellular fluid)
• Endocochlear potential = +80 mV (scala media is electrically positive)

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4. Basilar Membrane and Place Coding of Frequency

• Contains 20,000-30,000 basilar fibers
• Fibers are short and stiff at base, long and flexible at apex
• Base responds to HIGH frequency sounds
• Apex responds to LOW frequency sounds
• This is called tonotopic organization

• Sound enters scala vestibuli → creates a traveling wave along basilar membrane
• Wave amplitude peaks at a specific location depending on frequency
• High freq (20,000 Hz) → peak near oval window (base)
• Low freq (20 Hz) → peak near helicotrema (apex)
• This "place principle" is the basis of frequency discrimination

Viva Q: "How does the ear discriminate frequencies?" - By the tonotopic organization of the basilar membrane; each frequency maximally displaces a specific location.

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5. Organ of Corti & Hair Cell Transduction

• Sits on basilar membrane inside scala media
• Contains inner hair cells (IHCs) and outer hair cells (OHCs)
• ~3,500 IHCs (single row) - primary sensory cells (90-95% of auditory nerve fibers)
• ~12,000 OHCs (3 rows) - amplification role (electromotility via prestin)
• Tectorial membrane lies above and contacts the stereocilia of OHCs

1. Basilar membrane vibrates → stereocilia of hair cells deflect
2. Deflection toward tallest stereocilia = depolarization (K⁺ influx through tip links)
3. Deflection away = hyperpolarization
4. Depolarization → Ca²⁺ influx → neurotransmitter (glutamate) release → spiral ganglion nerve fires
5. Note: K⁺ enters because endolymph K⁺ is high AND the endocochlear potential (+80 mV) drives K⁺ in

Viva Q: "Why is K⁺ the main ion for hair cell depolarization?" - Because endolymph has high K⁺ concentration AND the endocochlear potential provides a large electrochemical driving force.

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6. Auditory Neural Pathway

• Signal crosses at the cochlear nucleus/superior olive - so each cortex receives BILATERAL input
• Superior olivary nucleus is important for sound localization (compares timing/intensity between ears)
• Inferior colliculus: reflex responses to sound
• Auditory cortex: frequency discrimination, pattern recognition, sound localization

Viva Q: "Why does unilateral cortical lesion NOT cause complete deafness in one ear?" - Because of bilateral representation at all levels above the cochlear nucleus.

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7. Loudness and the Decibel Scale

• Sound intensity range: 1 trillion-fold (from softest whisper to loudest noise)
• Amplitude of basilar membrane movement: 1 million-fold
• Brain perception: approximately 10,000-fold (compresses the range)
• 1 bel = 10-fold increase in sound energy
• 1 decibel (dB) = 0.1 bel = 1.26-fold increase in actual energy
• Threshold of hearing: ~0 dB; painful levels: ~130-140 dB

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8. Types of Deafness

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🟡 PART 2: VESTIBULAR APPARATUS (Equilibrium)

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1. Structure

• Contain hair cells with otoliths (calcium carbonate crystals) on a gelatinous matrix
• Detect linear acceleration and gravity (head position)
• Utricle - horizontal acceleration; Saccule - vertical acceleration

• Three canals in three planes (horizontal, anterior/superior, posterior)
• Each has an ampulla containing the crista ampullaris (hair cells + cupula)
• Detect angular acceleration (rotational movement)

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2. Mechanism

• Otoliths are denser than surrounding fluid
• During head tilt or linear acceleration, otoliths shift, bending stereocilia
• Depolarization when stereocilia bend toward kinocilium

• Rotation causes endolymph to lag behind due to inertia
• Endolymph movement deflects cupula → bends hair cells
• Horizontal canal: head rotation left → left canal excited, right canal inhibited
• Nystagmus: the fast phase is toward the excited side

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3. Vestibular Pathways

• Cerebellum (flocculus/nodulus - flocculonodular lobe)
• Spinal cord via vestibulospinal tract (postural reflexes)
• CN III, IV, VI nuclei via MLF (medial longitudinal fasciculus) → vestibulo-ocular reflex
• Cortex (conscious awareness of movement/position)

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🟢 PART 3: SENSE OF TASTE (Gustation)

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1. Five Primary Taste Modalities

Viva Q: "Which taste has the lowest threshold?" - Bitter (most sensitive, threshold 0.000008 M for quinine) - protective against toxins.

Viva Q: "Is fat a primary taste?" - Emerging evidence suggests yes (a 6th modality), but not yet fully confirmed.

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2. Taste Buds

• Located in papillae on the tongue, soft palate, pharynx, and epiglottis
• Total ~10,000 taste buds in humans
• Types of papillae:
  • Fungiform - scattered on anterior 2/3 of tongue
  • Circumvallate (vallate) - large, 9-12 in a V-shaped row at back of tongue - most taste buds
  • Foliate - folds on lateral tongue edges
  • Filiform papillae have NO taste buds (tactile only)
• Each taste bud contains 40-60 taste receptor cells (TRCs) + basal cells
• Life span of taste cells: ~10 days (constantly replaced from basal cells)
• Microvilli (taste hairs) project through taste pore to contact saliva

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3. Mechanism of Taste Receptor Stimulation

• Salty: Na⁺ enters directly through amiloride-sensitive ENaC channels → depolarization
• Sour: H⁺ directly blocks K⁺ channels (or enters via H⁺ channels) → depolarization

• Sweet & Umami: T1R family (T1R2+T1R3 for sweet; T1R1+T1R3 for umami)
• Bitter: T2R family (~30 different receptors)
• → Gα activates → phospholipase Cβ2 → IP3 → Ca²⁺ from ER → activation of TRPM5 channel → depolarization

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4. Taste Pathway (Neural Transmission)

Viva Q: "What nerve carries taste from anterior 2/3 of tongue?" - Chorda tympani (branch of facial nerve CN VII). Note: general sensation from anterior 2/3 is lingual nerve (V3).

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5. Taste Adaptation

• Taste sensation adapts (decreases) rapidly if continuous stimulation occurs
• Adaptation mostly occurs centrally (in the brain), not peripherally

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🔵 PART 4: SENSE OF SMELL (Olfaction)

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1. Olfactory Epithelium

• Located in superior nasal cavity (superior turbinate + superior septum)
• Total surface area: ~5 cm² in humans (much less developed than in animals)
• Contains:
  • Olfactory receptor cells (~100 million) - bipolar neurons derived from CNS
  • Sustentacular cells - supporting cells
  • Basal cells - stem cells (replace olfactory neurons every 4-8 weeks)
  • Bowman's glands - secrete mucus

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2. Structure of Olfactory Receptor Cell

• Bipolar neuron with a peripheral dendrite ending in an olfactory knob
• Knob bears 4-25 olfactory cilia (up to 200 μm long) - these are the sensory surfaces
• Cilia project into mucus layer
• Central axon → forms olfactory nerve (CN I) → passes through cribriform plate → olfactory bulb

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3. Mechanism of Olfactory Transduction

1. Odorant molecule dissolves into nasal mucus
2. Binds to olfactory receptor protein (GPCR - 7-transmembrane domain) on cilia
3. Receptor coupled to G-protein (Golf)
4. Alpha subunit activates adenylyl cyclase
5. ATP → cAMP increases
6. cAMP opens cyclic-nucleotide-gated (CNG) Na⁺ channels
7. Na⁺ influx → depolarization → action potential → travels along axon to olfactory bulb

Viva Q: "What second messenger mediates olfactory transduction?" - cAMP (via Gs-type Golf protein and adenylyl cyclase).

Viva Q: "How many olfactory receptor genes are there?" - ~1000 different olfactory receptor genes in rodents; humans have ~350 functional genes (largest gene family in the genome).

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4. Olfactory Bulb

• Glomeruli - spherical structures where olfactory nerve axons synapse with mitral cells and tufted cells
• One glomerulus receives input from olfactory cells expressing the SAME receptor type
• This convergence (1000s of similar receptors onto one glomerulus) increases sensitivity
• Granule cells provide lateral inhibition (sharpening odor discrimination)

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5. Olfactory Pathways - UNIQUE compared to other senses

• Prepiriform cortex (primary olfactory cortex)
• Periamygdaloid cortex (amygdala)
• Entorhinal cortex

• Hippocampus - memory (smell strongly evokes memories - "Proust effect")
• Hypothalamus - feeding, emotional responses
• Limbic system - emotional and behavioral responses

Viva Q: "Why does smell trigger emotional memories so strongly?" - Olfactory signals project directly to amygdala and hippocampus without a thalamic relay, unlike all other senses.

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6. Olfactory Adaptation

• Adaptation is very rapid (within seconds to minutes)
• Occurs both at receptor level (cAMP pathway desensitizes) and centrally
• Centrifugal control: Granule cells in olfactory bulb receive feedback from higher centers → inhibit mitral/tufted cells → sharpen odor discrimination

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7. Threshold and Discrimination

• Humans can distinguish ~10,000 different odors (possibly more)
• Olfactory threshold is very low for certain substances (e.g., mercaptans at 1 part in 25 billion)
• Olfactory membrane can detect even a few molecules of some substances

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⚡ HIGH-YIELD VIVA QUESTIONS & ANSWERS

• Salty: Na⁺ via ENaC
• Sour: H⁺ direct
• Sweet/bitter/umami: GPCR → cAMP/IP3

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📋 QUICK-RECALL SUMMARY TABLE

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