Sensory Organs

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Sensory organs are specialised structures that detect specific stimuli from the environment and transmit signals to the central nervous system for interpretation. In humans, the five classical sense organs are the eye (sight), ear (hearing and balance), nose (smell), tongue (taste), and skin (touch, temperature, pain). Each contains receptor cells or endings that are specific to one type of stimulus.

Key Sensory Organs and Their Receptors:

Sense Organ	Primary Receptors	Stimulus Detected
Eye	Photoreceptors (rods, cones)	Light (photons)
Ear	Mechanoreceptors (hair cells)	Sound waves, head movement
Skin	Thermoreceptors, mechanoreceptors, nociceptors	Temperature, pressure, pain
Nose	Olfactory receptors (chemoreceptors)	Volatile chemicals
Tongue	Gustatory receptors (chemoreceptors)	Dissolved chemicals

Human Eye — Key Facts:

• Diameter ≈ 2.4 cm; spherical structure
• Three layers: fibrous tunic (sclera + cornea), vascular tunic (choroid + ciliary body + iris), neural tunic (retina)
• Fovea centralis: Point of highest visual acuity; contains only cones
• Blind spot: Where optic nerve exits; no photoreceptors
• Ciliary muscles: Control lens shape (accommodation); attached via suspensory ligaments
• Aqueous humour: Clear fluid between cornea and lens; replaces every ~4 hours; intraocular pressure 10–21 mmHg
• Vitreous humour: Jelly-like substance behind lens; transparent; does not replenish

Human Ear — Key Facts:

• Outer ear: Pinna (collects sound) → external auditory canal → tympanic membrane (eardrum)
• Middle ear: 3 ossicles — malleus (hammer), incus (anvil), stapes (stirrup); amplify vibrations ×1.3
• Inner ear: Bony labyrinth — cochlea (hearing) + vestibular apparatus (balance)
• Eustachian tube: Connects middle ear to nasopharynx; equalises pressure
• Sound frequency range: 20 Hz to 20,000 Hz

⚡ Exam Tips:

• The human eye cannot focus objects closer than 25 cm — this is the near point of accommodation
• Presbyopia (age-related far-sightedness) occurs due to loss of lens elasticity
• Glaucoma results from increased intraocular pressure damaging the optic nerve
• The blind spot is at the optic disc — a classic NEET question

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Structure and Function of the Human Eye:

The eye is a fluid-filled spherical organ approximately 2.4 cm in diameter. Light enters through the transparent cornea (which provides ~65% of the eye’s refractive power), passes through the aqueous humour, the pupil (adjustable aperture controlled by iris muscles), the lens, and the vitreous humour, before reaching the retina.

Cornea: Avascular (no blood vessels) to maintain transparency. Receives oxygen directly from air. Composed of five layers: epithelium → Bowman’s membrane → stroma → Descemet’s membrane → endothelium. The endothelium pumps fluid out of the stroma to maintain clarity. If this pump fails, corneal oedema (swelling) occurs.

Iris: Coloured part of the eye; contains two sets of smooth muscle — circular (parasympathetic: constrict pupil) and radial (sympathetic: dilate pupil). Pupil diameter ranges from 2 mm (bright light) to 8 mm (dark).

Retina: Light-sensitive layer lining the back of the eye. Contains:

• Rods: ~120 million; sensitive to low light (scotopic vision); contain rhodopsin (visual purple); distributed mainly in peripheral retina; one type only (no colour discrimination)
• Cones: ~6 million; require bright light (photopic vision); three types (S, M, L cones) sensitive to blue (420 nm), green (534 nm), and red (564 nm) wavelengths respectively; concentrated at fovea

Visual Pathway: Photoreceptors → bipolar cells → ganglion cells → optic nerve → optic chiasma (nasal fibres cross) → optic tract → lateral geniculate nucleus (thalamus) → optic radiations → primary visual cortex (occipital lobe)

Accommodation: The process of changing lens shape to focus on near objects. Parasympathetic stimulation causes ciliary muscle contraction → zonular fibres slacken → lens becomes more convex (thicker). For distant objects, ciliary muscles relax → lens flattens.

Common Eye Defects:

Defect	Cause	Far Point	Near Point	Correction
Myopia (near-sightedness)	Elongated eyeball or steep cornea	< ∞	Normal	Concave lens (–)
Hypermetropia (far-sightedness)	Shortened eyeball or flat cornea	> ∞	> 25 cm	Convex lens (+)
Presbyopia	Loss of lens elasticity (age)	Normal	> 25 cm	Bifocal lens
Astigmatism	Irregular cornea curvature	—	—	Cylindrical lens
Cataract	Clouding of lens	—	—	Lens replacement surgery

The Human Ear — Structure and Hearing:

Sound is a pressure wave travelling through air (or water). The pinna funnels sound waves down the external auditory canal to the tympanic membrane, which vibrates. These vibrations are transmitted across the middle ear by the ossicular chain: the malleus is attached to the tympanic membrane, the incus connects malleus to stapes, and the stapes footplate sits in the oval window of the cochlea.

The ossicles act as a lever system, amplifying the force of vibration by approximately 1.3 times. Additionally, because the oval window is much smaller than the tympanic membrane, pressure is further amplified by a factor of about 17. This pressure amplification is essential for efficient stimulation of the cochlear fluids.

Cochlea — The Organ of Hearing: The cochlea is a coiled, fluid-filled tube divided into three compartments:

• Scala vestibuli: Filled with perilymph; receives vibrations from oval window
• Scala media (cochlear duct): Filled with endolymph; contains the organ of Corti
• Scala tympani: Filled with perilymph; ends at round window

The organ of Corti sits on the basilar membrane and contains hair cells (stereocilia) embedded in the tectorial membrane. Different frequencies of sound stimulate hair cells at different positions along the basilar membrane — high frequencies near the base, low frequencies at the apex (tonotopic organisation).

Vestibular Apparatus — Balance: The vestibular system comprises:

• Utricle and saccule: Detect linear acceleration and head position (gravity); contain macula (otolithic membrane + hair cells)
• Three semicircular canals: Detect rotational acceleration in three planes; each has an ampulla with a crista (hair cells) and cupula (gelatinous cap)

Common Ear Disorders:

• Otitis media: Middle ear infection; common in children; treated with antibiotics; severe cases require grommet insertion
• Ménière’s disease: Disorder of inner ear; triad of symptoms: episodic vertigo, fluctuating sensorineural hearing loss, and tinnitus (ringing)
• Presbycusis: Age-related hearing loss; progressive degeneration of hair cells in cochlea

⚡ Common Mistakes:

• Students confuse the functions of rods and cones: rods work in dim light but give no colour; cones need bright light but give colour vision
• The near point increases with age (presbyopia) because the lens becomes less elastic
• The Eustachian tube is normally closed but opens during swallowing/yawning to equalise pressure — this is why yawning “pops” your ears on an airplane

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🔴 Extended — Deep Dive (exam-level mastery)

For students preparing for top-rank selection.

Phototransduction Cascade — How Light Becomes a Neural Signal:

In the dark, rod cells maintain high cGMP levels, which keeps Na⁺ channels open (dark current). Na⁺ ions continuously enter the rod, depolarising it to about –40 mV. Glutamate is continuously released from the synaptic terminal, inhibiting bipolar cells.

When light strikes rhodopsin (a G-protein-coupled receptor consisting of 11-cis-retinal + opsin):

1. 11-cis-retinal absorbs a photon → isomerises to all-trans-retinal
2. Activated rhodopsin (R*) activates transducin (G-protein)
3. Transducin activates phosphodiesterase (PDE)
4. PDE hydrolyses cGMP to GMP
5. cGMP levels fall → Na⁺ channels close
6. Inward Na⁺ current stops → cell hyperpolarises to about –70 mV
7. Glutamate release decreases
8. ON-bipolar cells are depolarised (light response)

This is remarkable: the photoreceptor is inhibited by light — the opposite of most sensory cells. The neural signal is therefore a decrease in neurotransmitter release, not an increase.

Regeneration: All-trans-retinal is converted back to 11-cis-retinal by the RPE (retinal pigment epithelium) in the visual cycle, taking ~45 minutes.

Colour Vision — Trichromatic Theory: Three types of cone photopigments exist, each with a different opsin protein:

• S-cones (short wavelength): Peak absorption at 420 nm (blue-violet)
• M-cones (medium wavelength): Peak absorption at 534 nm (green)
• L-cones (long wavelength): Peak absorption at 564 nm (yellow-red)

Colour is determined by the relative activation of these three cone types. The brain computes colour from the difference between L and M signals (red-green axis) and a combined L+M signal (blue-yellow axis). This is why we cannot perceive reddish-green or bluish-yellow as distinct hues — they are opponent channels.

Types of Colour Blindness:

• Protanopia: Absence of L-cones (red blindness); red appears dark; common in ~1% of males
• Deuteranopia: Absence of M-cones (green blindness); green appears beige
• Tritanopia: Absence of S-cones (blue blindness); blue appears green

Red-green colour blindness is X-linked recessive, affecting ~8% of males but only ~0.5% of females.

Organ of Corti — Hair Cell Transduction: The stereocilia (60–80 per cell) are arranged in rows of increasing height. Mechanically-gated ion channels sit at the tips of the shorter stereocilia. When sound causes basilar membrane vibration, the stereocilia bend toward the tallest row, stretching a fine filament (tip link) that opens K⁺ channels.

Endolymph in scala media has an unusually high K⁺ concentration (~150 mM), so K⁺ influx depolarises the hair cell. This opens voltage-gated Ca²⁺ channels → Ca²⁺ influx → glutamate release onto VIII nerve afferents.

Hearing Loss Classification:

• Conductive: Problem in outer/middle ear (wax, otitis media, ossicle fixation); sound not transmitted efficiently; can often be surgically corrected
• Sensorineural: Problem in inner ear/cochlear nerve (noise damage, Ménière’s, age); hair cell loss is irreversible; treated with hearing aids or cochlear implants
• Mixed: Combination of both

Cochlear Implant: An electronic device that bypasses damaged hair cells. A microphone captures sound → processor encodes it → electrode array in cochlea directly stimulates auditory nerve fibres. Can restore functional hearing in profoundly deaf individuals.

Accommodation Reflex: When shifting gaze from a distant to a near object:

1. Convergence: Both eyes medially rotate (medial rectus muscles contract)
2. Constriction: Pupils constrict (circular muscles of iris contract)
3. Accommodation: Ciliary muscles contract → lens thickens (convex)

This three-part reflex is mediated by the parasympathetic pathway via the Edinger-Westphal nucleus and CN III. It can be used clinically to assess brainstem function in unconscious patients.

NEET High-Yield Pattern:

• Most common questions involve identifying the correct order of sound passage through the ear (tympanic membrane → malleus → incus → stapes → oval window → cochlea)
• The near point of the normal human eye is 25 cm
• Astigmatism is caused by irregular curvature of the cornea
• Rhodopsin is the photopigment in rods; decomposed by light into retinal + opsin
• The Eustachian tube connects the middle ear to the nasopharynx