The Special Senses
Vision, smell, taste, hearing and equilibrium
By the end of this unit you can…
- ✓Describe the accessory structures and three tunics of the eye and the path of light
- ✓Explain focusing (refraction, accommodation) and common refractive errors
- ✓Compare rods and cones and describe phototransduction and the visual pathway
- ✓Describe the receptors and pathways for smell and taste
- ✓Describe the external, middle and internal ear and how hearing and equilibrium work
Key terms
Practice →1Eye: accessory structures
The special senses — vision, taste, smell, hearing and equilibrium — have receptors housed in complex sensory organs in the head. ~70% of the body's sensory receptors are in the eyes.
- Eyebrows & eyelids (palpebrae)
- Shade the eye and protect it; blinking (every 3–7 s) spreads tears. Eyelashes trigger reflex blinking.
- Conjunctiva
- Transparent mucous membrane lining the eyelids (palpebral) and covering the white of the eye (bulbar); produces lubricating mucus. Inflamed → conjunctivitis (“pinkeye”).
- Lacrimal apparatus
- Lacrimal gland (superolateral) makes tears containing lysozyme (antibacterial) and antibodies; tears drain via lacrimal canaliculi → lacrimal sac → nasolacrimal duct into the nasal cavity (why crying gives you a runny nose).
- Extrinsic eye muscles
- Six muscles move the eyeball (see table).
| Muscle | Moves the eye | Nerve |
|---|---|---|
| Lateral rectus | Laterally | Abducens (VI) |
| Superior oblique | Depresses and turns laterally | Trochlear (IV) |
| Medial rectus, superior rectus, inferior rectus, inferior oblique | Medially / up / down / up and laterally | Oculomotor (III) |
2Structure of the eyeball
| Tunic (layer) | Parts | Function |
|---|---|---|
| Fibrous (outer) | Sclera (white, posterior) and cornea (clear, anterior) | Protection, shape; the cornea is the main refractor of light (avascular; heals well; transplanted easily) |
| Vascular / uvea (middle) | Choroid (blood vessels, brown pigment absorbs light), ciliary body (ciliary muscles control lens shape; ciliary processes secrete aqueous humor), iris (colored; controls pupil size) | Nourishes; focuses; regulates light |
| Inner / retina | Pigmented layer + neural layer (photoreceptors → bipolar cells → ganglion cells) | Light detection |
Key retina features: the optic disc — where the optic nerve leaves — has no photoreceptors (blind spot). Lateral to it, the macula lutea contains the fovea centralis, a pit with only cones — the area of sharpest (highest acuity) vision.
Internal chambers: the posterior segment holds gel-like vitreous humor (keeps the retina pressed in place; lasts a lifetime). The anterior segment holds watery aqueous humor, constantly made by the ciliary processes and drained through the scleral venous sinus (canal of Schlemm). The lens is a flexible, avascular, biconvex structure held by suspensory ligaments (ciliary zonule).
3Focusing light
Light is bent (refracted) by the cornea, aqueous humor, lens and vitreous humor to focus on the retina. The image is upside down and reversed — the brain flips it.
- Distant vision (> 6 m)
- Ciliary muscles relax → suspensory ligaments taut → lens flattened. The eye is “at rest”.
- Close vision
- Three reflexes: accommodation (ciliary muscles contract → ligaments slacken → lens bulges, more refraction), pupil constriction (prevents divergent rays), and convergence (medial rotation of both eyes).
| Problem | What goes wrong | Correction |
|---|---|---|
| Myopia (nearsightedness) | Eyeball too long → distant objects focus in front of the retina | Concave (diverging) lenses |
| Hyperopia (farsightedness) | Eyeball too short → close objects focus behind the retina | Convex (converging) lenses |
| Astigmatism | Unequal curvature of cornea or lens → blurred images | Cylindrically ground lenses |
| Presbyopia | Lens loses elasticity with age → can't accommodate for near vision | Reading glasses (convex) |
4Photoreceptors & visual pathway
| Rods | Cones | |
|---|---|---|
| Sensitive to | Dim light — night and peripheral vision | Bright light — high-acuity color vision |
| Pigment | Rhodopsin | Three types: blue, green, red |
| Location | Periphery of retina | Concentrated in the fovea |
| Number | ~20× more numerous | Fewer |
| Image | Fuzzy, grayscale | Sharp, colored |
Phototransduction: light changes retinal (a vitamin A derivative) from its bent 11-cis form to straight all-trans form, activating the pigment. Unusually, photoreceptors are depolarized in the dark and hyperpolarize in light, which changes their release of glutamate onto bipolar cells; ganglion cells then fire action potentials in the optic nerve.
Light and dark adaptation: moving into bright light, rhodopsin is bleached — you're dazzled until cones take over. In the dark, rhodopsin must re-form (~20–30 min for full rod sensitivity).
- 1Retina photoreceptors → bipolar cells → ganglion cells, whose axons form the optic nerve (II).
- 2At the optic chiasma, fibers from the medial half of each retina cross to the opposite side.
- 3Optic tracts carry fibers from the lateral half of the same eye and medial half of the other eye.
- 4Most fibers synapse in the lateral geniculate nucleus of the thalamus.
- 5Optic radiation projects to the primary visual cortex in the occipital lobe.
5Smell & taste
Olfaction: the olfactory epithelium in the roof of the nasal cavity contains millions of olfactory sensory neurons — bipolar neurons whose olfactory cilia extend into the mucus. Odorants dissolve in mucus and bind receptors; axons pass through the cribriform plate of the ethmoid as the olfactory nerve (I) → olfactory bulb → olfactory tract → olfactory cortex and limbic system (why smells trigger emotions and memories). Olfactory neurons are among the few neurons that are replaced (every 30–60 days).
Taste (gustation): most of the ~10,000 taste buds are on the tongue in fungiform and vallate (circumvallate) papillae (filiform papillae have none — they give texture). Each taste bud has gustatory epithelial cells with microvilli (gustatory hairs) projecting through a taste pore, plus basal epithelial (stem) cells. Five basic tastes: sweet, sour, salty, bitter and umami (glutamate, savory).
Taste pathway: facial nerve (VII) (anterior two-thirds of the tongue), glossopharyngeal (IX) (posterior third) and vagus (X) (epiglottis, pharynx) → medulla → thalamus → gustatory cortex in the insula. ~80% of “taste” is really smell — why food is bland when you have a cold.
6The ear
| Region | Structures | Role |
|---|---|---|
| External (outer) ear | Auricle (pinna), external acoustic meatus (ceruminous glands), ends at the tympanic membrane (eardrum) | Hearing only — collects and funnels sound |
| Middle ear (tympanic cavity) | Air-filled; three auditory ossicles: malleus → incus → stapes (stapes footplate fits the oval window); round window; pharyngotympanic (auditory, Eustachian) tube connects to the nasopharynx to equalize pressure | Hearing — amplifies vibrations ~20× |
| Internal (inner) ear — bony labyrinth filled with perilymph; membranous labyrinth inside filled with endolymph | Cochlea (hearing), vestibule (static equilibrium), semicircular canals (dynamic equilibrium) | Hearing and balance |
7Hearing
The cochlea (snail-shaped) is divided into three chambers: scala vestibuli and scala tympani (perilymph) and the cochlear duct (endolymph) between them. The spiral organ (organ of Corti) — the receptor for hearing — rests on the basilar membrane; its hair cells have stereocilia embedded in the overlying gel-like tectorial membrane.
- 1Sound waves enter the external acoustic meatus and vibrate the tympanic membrane.
- 2Vibrations pass through the ossicles (malleus → incus → stapes) and are amplified.
- 3The stapes rocks the oval window, creating pressure waves in the perilymph of the scala vestibuli.
- 4Waves move the basilar membrane; hair cells bend against the tectorial membrane → depolarize → release neurotransmitter onto cochlear nerve fibers.
- 5Impulses travel via the cochlear nerve (VIII) → brain stem → thalamus → primary auditory cortex (temporal lobe). Pressure waves dissipate through the round window.
Pitch: high-frequency sounds vibrate the stiff, narrow basilar membrane near the oval window (base); low frequencies vibrate the wide, floppy end near the apex. Loudness = amplitude → more vigorous vibration → more hair cells firing faster.
| Deafness type | Cause |
|---|---|
| Conduction deafness | Sound vibrations can't reach the inner ear: impacted earwax, perforated eardrum, otitis media, otosclerosis (ossicles fuse) |
| Sensorineural deafness | Damage to hair cells, cochlear nerve or auditory cortex — e.g. loud noise exposure, aging (presbycusis). Cochlear implants can help |
8Equilibrium
The vestibular apparatus (vestibule + semicircular canals) senses head position and movement; signals travel via the vestibular nerve (VIII) to the brain stem and cerebellum.
- Maculae (static equilibrium)
- In the utricle (horizontal acceleration) and saccule (vertical acceleration). Hair cells are embedded in an otolith membrane studded with calcium carbonate crystals (otoliths); tilting the head lets gravity pull the otoliths, bending hair cells. Senses head position relative to gravity and linear acceleration.
- Crista ampullaris (dynamic equilibrium)
- In the ampulla of each semicircular canal (three canals in three planes). Hair cells are embedded in a gel cap, the cupula. When the head rotates, endolymph lags behind and bends the cupula. Senses rotational (angular) acceleration.