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PY11.1-7 | Special Senses — Part 1

Part 1: Smell (Olfaction) — PY11.1

What is smell?

Figure: Part 1: Smell (Olfaction) — PY11.1

Olfaction (olfacere = Latin, to smell) is the chemical sense that detects airborne volatile molecules. It is phylogenetically the oldest sense — even primitive vertebrates have olfactory systems. In humans, it is intimately connected to emotion and memory (olfactory signals reach the limbic system directly — the only sense without a thalamic relay).

The Olfactory Receptor Cells

The olfactory receptor cells (ORCs) are located in the olfactory epithelium — a small patch (2.5 cm²) in the roof of the nasal cavity (superior concha and nasal septum). The olfactory epithelium contains three cell types:

• Olfactory receptor cells (bipolar neurons) — the actual smell sensors
• They are modified bipolar neurons — part receptor, part neuron
• Their dendrites project as olfactory cilia into the nasal mucus layer
• Their axons pass upward through the cribriform plate of the ethmoid bone to form Cranial Nerve I (olfactory nerve)
• Unique: they are the only neurons that regenerate in the adult human brain (lifespan ~60 days)

• Supporting (sustentacular) cells — provide structural and metabolic support
• Basal cells — stem cells that regenerate new receptor cells

Mechanism of Olfactory Transduction

Transduction = converting a chemical signal into an electrical nerve signal.

1. Odorant molecule binds to olfactory receptor protein (G-protein coupled receptor — GPCR) on olfactory cilia
2. G-protein (Golf) activates adenylyl cyclase → increases cAMP
3. cAMP opens cyclic nucleotide-gated (CNG) channels → Na⁺ and Ca²⁺ influx
4. Depolarisation → generator potential → action potential in the axon
5. Axons travel via CN I → olfactory bulb (first relay) → olfactory tract → primary olfactory cortex (piriform cortex, uncus of temporal lobe)

Key: No thalamic relay — olfactory signals reach the cortex directly. This is unique among sensory systems.

Mnemonic for olfactory pathway: "Nose → Bulb → Tract → Cortex (No Thalamus!)" — NB-TC-No T

Olfactory Adaptation

Adaptation = the decrease in sensitivity after continuous exposure to a smell. You stop noticing your own perfume after a few minutes. Mechanism: receptor cell desensitisation (phosphorylation of the receptor by CaM kinase) and central habituation.

Applied Aspects (PY11.1)

Anosmia = absence of smell
• Causes:
- Head injury: shearing of olfactory nerve filaments as they pass through the cribriform plate (most common traumatic cause)
- Nasal polyps, rhinitis (most common cause in clinical practice — the blocked nose of Rohan's cold)
- COVID-19: damage to olfactory epithelium/supporting cells (anosmia was a key diagnostic clue in 2020–21)
- Frontal lobe tumour (Foster Kennedy syndrome: ipsilateral anosmia + optic atrophy, contralateral papilloedema)

Hyposmia = decreased smell; Parosmia = distorted smell; Cacosmia = perceiving pleasant smells as foul

Clinical test: Ask patient to identify common odours (coffee, soap) with each nostril separately while the other is occluded. Use non-pungent odours (avoid ammonia — it stimulates CN V not CN I).

Why biryani tastes bland with a cold: Flavour = taste + smell + texture. When the nasal cavity is blocked, odorant molecules cannot reach the olfactory epithelium. You taste (sweet, salt, sour, bitter, umami) but you cannot smell → food seems bland. Smell contributes approximately 75–80% of what we perceive as 'flavour'.

Part 2: Taste (Gustation) — PY11.2

What is taste?

Figure: Part 2: Taste (Gustation) — PY11.2

Gustation (gustare = Latin, to taste) detects chemical substances dissolved in saliva. It is a contact chemical sense (unlike smell, which is distance). The primary function is to distinguish nutritious food from toxic substances.

The Five Basic Tastes

Modern taste physiology recognises five primary taste qualities:

Taste	Stimulus	Biological function	Example
Sweet	Sugars, saccharin	Detect energy source	Glucose, sucrose
Sour	H⁺ ions (acids)	Detect spoiled food	Lemon, tamarind
Salty	Na⁺ ions	Detect electrolytes	Table salt
Bitter	Alkaloids, toxins	Warn against poisons	Coffee, neem
Umami	Glutamate	Detect protein	Paneer, tomato, mushroom

Note: "Pungent" (chilli) and "astringent" are NOT true tastes — they are pain and tactile sensations mediated by CN V.

Taste Receptor Cells and Taste Buds

Taste buds are the taste organs. They are oval structures embedded in the tongue and, to a lesser extent, the soft palate, pharynx, and epiglottis.

• Each taste bud contains 50–100 taste receptor cells (also called gustatory receptor cells)
• Taste receptor cells are NOT neurons — they are modified epithelial cells
• They have microvilli (taste hairs) projecting through the taste pore into saliva
• They synapse with sensory nerve fibres at the base
• Lifespan: ~10 days (constantly replaced — one reason why burning your tongue recovers)

Papillae — the bumps on your tongue that house taste buds:
• Fungiform papillae: mushroom-shaped, scattered on anterior 2/3 of tongue; each has 3–5 taste buds
• Circumvallate (vallate) papillae: large, ring-like, in a V-shape at the junction of anterior 2/3 and posterior 1/3; 100–300 taste buds each — most sensitive
• Foliate papillae: leaf-like folds on lateral tongue edges
• Filiform papillae: thread-like, NO taste buds — only tactile sensation (the rough texture of tongue)

Mechanism of Taste Transduction

Different tastes use different receptor mechanisms:

• Salt: Na⁺ enters taste cells directly through ENaC (epithelial Na⁺ channels) → depolarisation
• Sour: H⁺ blocks K⁺ channels AND enters cells → depolarisation
• Sweet, Bitter, Umami: Use GPCRs (G-protein coupled receptors) → activate phospholipase C → IP₃ → Ca²⁺ release → neurotransmitter release
• Sweet: T1R2+T1R3 receptor heterodimer
• Umami: T1R1+T1R3 receptor heterodimer
• Bitter: T2R family (25+ receptor types — why bitters are diverse)

Taste Pathways

This is one of the most tested topics in Physiology Viva:

• Anterior 2/3 of tongue: Chorda tympani branch of Facial nerve (CN VII) → geniculate ganglion → enters the brainstem via facial nerve
• Posterior 1/3 of tongue: Glossopharyngeal nerve (CN IX)
• Epiglottis and pharynx: Vagus nerve (CN X) (small contribution)

All three converge on the Nucleus of Tractus Solitarius (NTS) in the medulla → thalamus (VPMpc nucleus) → primary gustatory cortex (in the parietal operculum / insula)

Mnemonic: "Taste path: 7-9-10 → NTS → Thalamus → Taste Cortex" — "Sev-nine-ten NTS TaTa"

Applied Aspects (PY11.2)

Ageusia = absence of taste; Hypogeusia = decreased taste; Dysgeusia = distorted taste

Clinical causes:
• Damage to CN VII (chorda tympani) — e.g., Bell's palsy → loss of taste on anterior 2/3
• Zinc deficiency: Taste receptor cell turnover depends on zinc. Common in India — malnutrition, dialysis patients. Treat with zinc supplementation.
• COVID-19: Dysgeusia/ageusia (along with anosmia) was a cardinal symptom
• Drugs: Metronidazole (metallic taste), ACE inhibitors (dysgeusia), chemotherapy
• Radiation to head/neck: damages taste buds (temporary — they regenerate)
• Burning mouth syndrome: Dysgeusia + burning pain (complex, poorly understood)

Taste threshold: Bitter has the lowest threshold (most sensitive) — protective, since most toxins are bitter.

Elderly patients: Taste sensitivity decreases with age (presbygeusia) — taste buds reduce in number. This explains why elderly patients often over-salt food and may lose appetite.

Part 3: Ear — Functional Anatomy, Auditory Pathways, Vestibular System and Equilibrium — PY11.3 & PY11.4

Overview of the Ear

The ear serves two entirely separate functions using the same fluid-filled inner ear structure:
1. Hearing — detection and analysis of sound vibrations (cochlea)
2. Equilibrium/Balance — detection of head position and movement (vestibular apparatus)

Functional Anatomy of the Ear

The ear is divided into three compartments:

OUTER EAR (External Ear)
• Pinna (auricle): cartilaginous funnel that collects sound waves and helps with sound localisation
• External auditory canal (EAC): 2.4 cm long S-shaped canal — cartilaginous outer 1/3, bony inner 2/3
- Contains ceruminous glands (wax) and hair follicles
- Wax protects against insects and humidity — do not attempt to remove with cotton buds
• Tympanic membrane (eardrum): thin, cone-shaped membrane separating outer and middle ear
- Pearly grey, translucent; the handle (umbo) of the malleus is attached to its inner surface
- Vibrates in response to sound waves — the first mechanical transducer

MIDDLE EAR (Tympanic Cavity)
• Air-filled space in the temporal bone
• Contains the ossicular chain — three smallest bones in the human body:
1. Malleus (hammer): handle attached to tympanic membrane; head articulates with incus
2. Incus (anvil): connects malleus to stapes
3. Stapes (stirrup): footplate sits in the oval window — transmits vibrations to inner ear
• The ossicular chain amplifies sound by ~22 times (area ratio of tympanic membrane to oval window = ~17:1; lever ratio of ossicles = ~1.3:1; total ≈ 22×)
• Eustachian tube (pharyngotympanic tube): connects middle ear to nasopharynx
- Normally closed; opens during swallowing, yawning, Valsalva manoeuvre
- Equalises air pressure between middle ear and atmosphere
- Clinical: Blockage → middle ear effusion (glue ear), eustachian tube dysfunction → barotrauma (aeroplane ear)
• Two muscles protect against loud sounds:
- Tensor tympani (CN V3): pulls malleus medially → stiffens tympanic membrane
- Stapedius (CN VII): pulls stapes posteriorly → stiffens ossicular chain
- Together they form the acoustic reflex (stapedius reflex): protect against sounds >80 dB

INNER EAR (Labyrinth)

The inner ear is a complex fluid-filled system housed in the petrous temporal bone:

• Bony labyrinth: rigid channels filled with perilymph (ionic composition like plasma — high Na⁺, low K⁺)
• Membranous labyrinth: delicate tubes within the bony labyrinth, filled with endolymph (unique: high K⁺, low Na⁺ — like intracellular fluid)
• Endolymph is produced by the stria vascularis of the cochlea

The inner ear consists of:

For HEARING — The Cochlea:
• Snail-shaped, coiled 2¾ turns, contains the organ of Corti
• Three fluid-filled scalae (chambers):
- Scala vestibuli (upper): perilymph
- Scala media/cochlear duct (middle): endolymph — contains the organ of Corti
- Scala tympani (lower): perilymph
• Scala vestibuli and scala tympani communicate at the helicotrema at the apex

The Organ of Corti — the actual hearing organ:
• Sits on the basilar membrane
• Contains hair cells — the mechanoreceptors of hearing
- Inner hair cells (IHC): ~3,500; the actual afferent receptors (95% of auditory nerve fibres go to IHC)
- Outer hair cells (OHC): ~12,000; amplify the basilar membrane vibration (cochlear amplifier)
• Stereocilia (hair-like projections) of hair cells are embedded in the tectorial membrane

For EQUILIBRIUM — The Vestibular Apparatus:

Semicircular canals (3 pairs — anterior, posterior, lateral):
• Detect angular acceleration (rotational movements of the head)
• Each canal has an ampulla containing the crista ampullaris with hair cells
• Hair cells embedded in a gelatinous cupula
• Rotation → endolymph movement → cupula deflection → hair cell stimulation → signal to brain
• The three canals are perpendicular to each other → detect rotation in any plane

Otolith organs (saccule + utricle):
• Detect linear acceleration and static head position (gravity)
• Contain the macula with hair cells covered by otolithic membrane + otoliths (calcium carbonate crystals = "ear stones")
• Utricle: horizontal linear acceleration (e.g., car starting)
• Saccule: vertical linear acceleration (e.g., lift/elevator)

Vestibular Pathways and Equilibrium

Vestibular signals (via CN VIII vestibular division) → Vestibular nuclei (4 nuclei in medulla/pons) → multiple connections:

1. Vestibulospinal tract → spinal cord motor neurons → postural muscle tone adjustments
2. Medial longitudinal fasciculus (MLF) → CN III, IV, VI nuclei → vestibuloocular reflex (VOR) — keeps eyes stable during head movement
3. Cerebellum (via vestibulocerebellar tract) → fine-tuning of balance
4. Thalamus → cortex → conscious awareness of position
5. Autonomic nuclei → nausea, vomiting (motion sickness — why we feel sick on a boat)