Special Senses — revision sheet

The special senses — vision, hearing, equilibrium, olfaction and gustation — use highly specialised receptor organs concentrated in the head and carried by cranial nerves. Each converts a specific physical or chemical stimulus (photons, pressure waves, head acceleration, odorants, tastants) into graded receptor potentials that the brain decodes into perception.

Axial length of adult eye: ≈24 mmTotal refractive power of eye: ≈60 D (cornea ≈43 D, lens ≈15–20 D)Photoreceptors per retina: ≈92 million rods, ≈4.6 million conesOptic nerve axons: ≈1.2 million per nerveNormal intraocular pressure: 10–21 mmHgAqueous humour production: ≈2–3 µL/minHuman hearing range: ≈20 Hz–20 kHz (most sensitive ≈1–4 kHz)Cochlear hair cells: ≈3 500 inner (1 row), ≈12 000 outer (3 rows)Endocochlear potential: ≈+80 mV (endolymph K⁺ ≈150 mmol/L)Middle-ear pressure gain: ≈22× (≈25–30 dB)Functional odorant receptor genes: ≈350–400Taste buds (adult): ≈2 000–5 000; 50–100 cells each, renewed every ≈8–12 days

Vision — eyeball

Eye (eyeball) Sensory organ: The paired, roughly spherical organ of vision (≈24 mm in diameter) housed in the orbit, which focuses light onto the retina and converts it into neural signals carried by the optic nerve.

Vision — accessory structures

Orbit Region: The pyramidal bony cavity (≈30 mL) formed by seven bones that houses and protects the eyeball, extraocular muscles, nerves, vessels, lacrimal gland and orbital fat.
Eyelids Organ part: Mobile musculofibrous folds that protect the anterior eye, spread the tear film by blinking and regulate light entry; each is stiffened by a tarsal plate containing meibomian glands.
Conjunctiva Membrane: A thin, transparent mucous membrane lining the inner surface of the eyelids (palpebral) and reflected onto the sclera (bulbar) up to the limbus, enclosing the conjunctival sac.
Lacrimal apparatus Gland: The system that produces tears (lacrimal gland) and drains them (puncta → canaliculi → lacrimal sac → nasolacrimal duct → inferior nasal meatus).
Extraocular muscles Muscle group: Six skeletal muscles — four recti and two obliques — that rotate the eyeball about three axes so the two eyes move together (conjugate gaze) and keep images on both foveae.

Vision — fibrous tunic

Sclera Organ part: The tough, opaque white outer coat forming the posterior five-sixths of the fibrous tunic; it maintains the shape of the globe and anchors the extraocular muscles.
Cornea Organ part: The transparent, avascular, densely innervated front window of the eye; with the tear film it provides ≈2/3 of the eye's refractive power (≈43 D).
Corneoscleral limbus Organ part: The 1–2 mm-wide transition zone between the cornea and sclera, which houses the corneal epithelial stem cells and the aqueous outflow apparatus (trabecular meshwork and canal of Schlemm).

Vision — aqueous outflow

Trabecular meshwork Tissue: A spongy, triangular filter of collagen beams lined by endothelium-like cells in the iridocorneal angle, through which most aqueous humour passes to reach the canal of Schlemm; it is the main site of outflow resistance.
Canal of Schlemm Vessel: A circular, endothelium-lined channel encircling the limbus that collects aqueous humour from the trabecular meshwork and passes it to the episcleral veins.

Vision — vascular tunic

Uvea (vascular tunic) Organ part: The pigmented, vascular middle coat of the eye, comprising (posterior → anterior) the choroid, ciliary body and iris.
Choroid Organ part: The thin, highly vascular and pigmented posterior part of the uvea, between sclera and retina, whose capillary layer (choriocapillaris) nourishes the photoreceptors and retinal pigment epithelium.
Ciliary body Organ part: A ring-shaped thickening of the uvea between the choroid and iris that contains the ciliary muscle (accommodation) and the ciliary processes (aqueous humour production), and anchors the zonule of the lens.
Ciliary zonule Ligament: A ring of fine, fibrillin-rich suspensory fibres connecting the ciliary body to the lens capsule; their tension flattens the lens for distance vision.
Iris Organ part: The pigmented, contractile diaphragm of the uvea in front of the lens; its two smooth muscles alter the diameter of the central aperture, the pupil, under autonomic control.
Pupil Space: The central round aperture of the iris (≈2–8 mm) through which light enters the posterior eye; its size is set by the balance of parasympathetic (constrictor) and sympathetic (dilator) tone.

Vision — refractive media

Lens Organ part: A transparent, avascular, biconvex structure suspended behind the iris by the zonule; it contributes ≈15–20 D of refraction and changes shape to focus on near objects (accommodation).

Vision — chambers & humours

Anterior chamber Space: The aqueous-filled space between the posterior surface of the cornea and the anterior surface of the iris and lens, with the drainage (iridocorneal) angle at its periphery.
Posterior chamber Space: The small, ring-shaped aqueous-filled space behind the iris and in front of the lens and zonule, into which the ciliary processes secrete aqueous humour.
Aqueous humour Fluid: A clear, protein-poor fluid secreted by the ciliary processes (≈2–3 µL/min) that fills the anterior and posterior chambers, nourishes the avascular cornea and lens, and maintains intraocular pressure.
Vitreous body Fluid: A transparent hydrogel (≈4 mL, ≈80% of the eye's volume) filling the vitreous chamber between the lens and retina, which transmits light and supports the retina.

Vision — retina

Retina Organ part: The light-sensitive inner tunic of the eye — an outgrowth of the brain — in which photoreceptors transduce light and a three-neuron chain with lateral interneurons processes the image before ganglion-cell axons leave as the optic nerve.
Retinal pigment epithelium Tissue: A single layer of hexagonal pigmented cells between the photoreceptors and Bruch membrane that sustains photoreceptor function: it recycles visual pigment, phagocytoses shed outer segments, absorbs stray light and forms the outer blood–retina barrier.
Rods and cones (photoreceptors) Receptor: The light-sensitive neurons of the retina: ≈92 million rods for dim-light (scotopic), achromatic vision and ≈4.6 million cones for bright-light (photopic), high-acuity colour vision.
Phototransduction Physiological process: The G-protein cascade in rod and cone outer segments by which absorption of a photon closes cGMP-gated cation channels and hyperpolarises the photoreceptor, reducing its glutamate release.
Macula lutea Organ part: An oval, yellowish area ≈5–6 mm across at the posterior pole, temporal to the optic disc, specialised for central high-acuity and colour vision; the fovea lies at its centre.
Fovea centralis Organ part: A small pit (≈1.5 mm) at the centre of the macula containing tightly packed cones and no rods; the point of sharpest vision, on which the eye fixates.
Optic disc Organ part: The pale oval area ≈1.5–1.8 mm across, ≈3–4 mm nasal to the fovea, where ganglion-cell axons converge and leave the eye as the optic nerve; it has no photoreceptors and forms the physiological blind spot.

Vision — neural pathways

Visual pathway Neural pathway: The pathway from retina to cortex: optic nerve → optic chiasm (nasal fibres cross) → optic tract → lateral geniculate nucleus → optic radiation → primary visual cortex (V1), so each hemisphere sees the opposite half of the visual field.
Pupillary light reflex Physiological process: Constriction of both pupils when light is shone into one eye — direct (same eye) and consensual (other eye) responses — mediated by the optic nerve (afferent) and the parasympathetic fibres of the oculomotor nerve (efferent).
Accommodation (near) reflex Physiological process: The "near triad" triggered when gaze shifts from a distant to a near object: convergence of the eyes, accommodation (increased lens curvature) and pupillary constriction.

Hearing & balance — overview

Ear Sensory organ: The paired organ of hearing and equilibrium, divided into the external ear (collects sound), middle ear (amplifies and transmits it via the ossicles) and inner ear (transduces sound and head motion in the fluid-filled labyrinth).

Hearing & balance — external ear

External ear Organ part: The auricle and external acoustic meatus, which collect sound, funnel it to the tympanic membrane, amplify speech frequencies by resonance and protect the eardrum.
Auricle Organ part: The visible, shell-shaped part of the external ear: a single plate of elastic cartilage covered by tightly adherent skin, except for the cartilage-free lobule.
External acoustic meatus Organ part: The S-shaped canal (≈2.5 cm) from the concha to the tympanic membrane: its lateral third is cartilaginous and wax-producing, its medial two-thirds bony.
Tympanic membrane Membrane: A thin (≈0.1 mm), oval, cone-shaped membrane (≈9–10 mm across) separating the external acoustic meatus from the middle ear; it vibrates with sound and transmits motion to the malleus.

Hearing & balance — middle ear

Middle ear (tympanic cavity) Space: A narrow, air-filled, mucosa-lined cavity in the temporal bone between the tympanic membrane and the inner ear; it contains the ossicles and two muscles, and connects to the nasopharynx (auditory tube) and the mastoid air cells.
Malleus Bone: The largest and most lateral auditory ossicle ("hammer"), whose handle is embedded in the tympanic membrane and whose head articulates with the incus.
Incus Bone: The middle auditory ossicle ("anvil"), linking the malleus to the stapes; its long process ends in the lenticular process, which articulates with the head of the stapes.
Stapes Bone: The smallest bone in the body ("stirrup", ≈3 mm, ≈3 mg); its footplate fits into the oval window and transmits ossicular vibration directly to the perilymph of the inner ear.
Oval window Organ part: A kidney-shaped opening in the medial wall of the middle ear, closed by the stapes footplate and annular ligament, through which sound energy enters the perilymph of the vestibule and scala vestibuli.
Round window Organ part: An opening below and behind the oval window, closed by a flexible membrane (secondary tympanic membrane), at the base of the scala tympani; it bulges outwards to release pressure when the stapes moves inwards.
Auditory tube Duct: The ≈36 mm canal connecting the middle ear to the nasopharynx, which equalises middle-ear pressure with the atmosphere and drains mucus; its lateral third is bony and medial two-thirds cartilaginous.

Hearing & balance — inner ear

Inner ear (labyrinth) Organ part: A system of fluid-filled channels in the petrous temporal bone: a bony labyrinth (vestibule, semicircular canals, cochlea) containing perilymph, enclosing a membranous labyrinth containing endolymph and the sensory epithelia for hearing and balance.
Perilymph Fluid: The sodium-rich, extracellular-like fluid filling the bony labyrinth around the membranous labyrinth — including the scala vestibuli and scala tympani — and bathing the basolateral surfaces of the hair cells.
Endolymph Fluid: The potassium-rich, sodium-poor fluid filling the membranous labyrinth; secreted mainly by the stria vascularis, it bathes the stereocilia of hair cells and, in the cochlea, carries a +80 mV endocochlear potential.
Cochlea Sensory organ: The snail-shaped part of the inner ear (2½–2¾ turns) in which sound-induced fluid waves displace the basilar membrane tonotopically and stimulate the hair cells of the spiral organ.
Cochlear duct (scala media) Duct: The triangular, endolymph-filled spiral tube of the membranous labyrinth between the scala vestibuli and scala tympani; it houses the spiral organ on its floor and the stria vascularis on its outer wall.
Basilar membrane Membrane: The fibrous membrane forming the floor of the cochlear duct and supporting the organ of Corti; its graded width and stiffness make each place along the cochlea resonate best at a particular frequency (tonotopy).
Spiral organ (organ of Corti) Receptor: The sensory epithelium of hearing on the basilar membrane, with one row of ≈3 500 inner hair cells (the true sensory cells) and three rows of ≈12 000 outer hair cells (the cochlear amplifier), covered by the tectorial membrane.
Hair cells Receptor: The mechanoreceptor cells of the cochlea and vestibular organs: epithelial cells topped by a staircase bundle of stereocilia whose deflection opens mechanically gated cation channels.
Vestibule of the inner ear Organ part: The central oval chamber of the bony labyrinth (≈4–5 mm), between the cochlea and semicircular canals; it contains the utricle and saccule (otolith organs), which sense gravity and linear acceleration.
Utricle Receptor: The larger otolith organ of the membranous labyrinth, lying in the elliptical recess of the vestibule; its roughly horizontal macula senses head tilt and horizontal linear acceleration.
Saccule Receptor: The smaller, globular otolith organ in the spherical recess of the vestibule; its vertically oriented macula senses vertical linear acceleration and head tilt in the sagittal plane.
Semicircular canals Receptor: Three mutually perpendicular bony canals (anterior, posterior, lateral) containing membranous ducts whose ampullae house the cristae ampullares, which detect angular (rotational) acceleration of the head.

Hearing & balance — neural pathways

Auditory pathway Neural pathway: The multi-synaptic, largely bilateral pathway from the cochlea to the cortex: cochlear nerve → cochlear nuclei → superior olivary complex → lateral lemniscus → inferior colliculus → medial geniculate nucleus → primary auditory cortex (transverse temporal gyri).
Vestibular pathways and the vestibulo-ocular reflex Neural pathway: Vestibular nerve fibres project to four vestibular nuclei and the cerebellum, which drive the vestibulo-ocular reflex (gaze stability), vestibulospinal reflexes (posture) and conscious perception of motion via the thalamus and cortex.

Smell — olfaction

Olfactory epithelium Tissue: A specialised, yellowish-brown pseudostratified epithelium in the roof of the nasal cavity containing olfactory receptor neurons, supporting (sustentacular) cells and basal stem cells, with mucus-secreting olfactory (Bowman) glands beneath.
Olfactory receptor neurons Receptor: Bipolar primary sensory neurons of the olfactory epithelium — the only neurons directly exposed to the external environment and among the few continually replaced in adults — each expressing one type of odorant receptor.
Olfactory bulb Brain region: The oval, layered forebrain structure resting on the cribriform plate, where olfactory nerve axons synapse in glomeruli onto mitral and tufted cells, whose axons form the olfactory tract.
Olfactory pathway Neural pathway: Olfactory receptor neurons → olfactory bulb → olfactory tract → (lateral olfactory stria) → primary olfactory cortex (piriform cortex, amygdala, entorhinal cortex) — the only sensory pathway that reaches the cortex without first relaying in the thalamus.

Taste — gustation

Taste buds Receptor: Onion-shaped clusters of 50–100 taste receptor, supporting and basal cells embedded in the epithelium of the tongue papillae, soft palate, pharynx and epiglottis, opening to the surface by a taste pore.
Taste receptor cells Cell: The specialised epithelial cells of taste buds that transduce sweet, umami and bitter (type II, via G-protein-coupled receptors), sour (type III, via the OTOP1 proton channel) and salty (via ENaC) stimuli into transmitter release onto gustatory nerve fibres.
Lingual papillae Organ part: Projections of the mucosa on the dorsum of the anterior two-thirds of the tongue — filiform (no taste buds), fungiform, foliate and circumvallate (with taste buds) — that give the tongue its rough texture and carry most taste buds.
Gustatory pathway Neural pathway: Taste fibres of the facial (VII), glossopharyngeal (IX) and vagus (X) nerves → rostral nucleus of the solitary tract → ventral posteromedial (VPM) thalamus → primary gustatory cortex in the anterior insula and frontal operculum.

Mnemonics

Extraocular muscle innervation — LR6SO4R3: Lateral Rectus — CN 6 (abducens); Superior Oblique — CN 4 (trochlear); the Rest — CN 3 (oculomotor: superior, inferior and medial rectus, inferior oblique, and levator palpebrae superioris).
Actions of SR, IR, SO, IO — "RAD SIN": Recti ADduct (superior and inferior rectus), Superiors INtort (superior rectus and superior oblique). By exclusion the obliques abduct and the inferiors extort. Test vertical recti in abduction and obliques in adduction ("look in, then up/down for obliques").
Ten layers of the retina (outer → inner): "Really Patient Ophthalmologists Often Observe Inner Images Giving New Insight": RPE, Photoreceptor layer, Outer limiting membrane, Outer nuclear, Outer plexiform, Inner nuclear, Inner plexiform, Ganglion cell layer, Nerve fibre layer, Inner limiting membrane.
Auditory pathway — "E COLI MA": Eighth nerve → Cochlear nuclei → superior Olivary complex → Lateral lemniscus → Inferior colliculus → Medial geniculate nucleus → Auditory cortex (transverse temporal gyri).
Bones of the orbit: "My Zany Sister Finds Excellent Lemon Pies": Maxilla, Zygomatic, Sphenoid, Frontal, Ethmoid, Lacrimal, Palatine.
Caloric testing — COWS: Cold Opposite, Warm Same: the fast phase of nystagmus beats away from a cold-irrigated ear and towards a warm-irrigated ear (in a conscious patient with an intact brainstem).
Ossicles lateral → medial: "Many Infants Scream": Malleus (hammer) → Incus (anvil) → Stapes (stirrup).
Taste nerves — "7, 9, 10": Anterior two-thirds of the tongue → VII (chorda tympani); posterior third including the circumvallate papillae → IX; epiglottis and pharynx → X. All converge on the nucleus of the solitary tract.

From the Corpus anatomy atlas — for education only.