Unit 15 — Eye & Ear · Question Bank

The eyeball is a three-layered sphere designed to focus light onto a neural receiver while protecting the delicate transducing apparatus inside. Walking outward to inward, you cross a tough fibrous capsule, a pigmented vascular middle coat, and finally the neural retina — each tunic specialised for a different supporting role in the act of vision. Junqueira treats these as three concentric coats with regional differentiations, and that is the easiest scaffolding for a structured answer.

Fibrous tunic — mechanical shell

The outermost coat gives the globe its shape and resists intraocular pressure. Posteriorly five-sixths of it is the sclera — dense irregular connective tissue, almost entirely type I collagen, opaque white because of its irregular fibre arrangement. It anchors the tendons of the six extraocular muscles and is pierced posteriorly by the optic nerve through a sieve-like region called the lamina cribrosa.

Anteriorly one-sixth becomes the cornea: avascular, transparent, and the dominant refractive surface of the eye (more powerful than the lens). Its five layers — non-keratinised stratified squamous epithelium, Bowman membrane, regularly arrayed collagen-lamellae stroma, Descemet membrane, and a single-layered endothelium — combine mechanical toughness with optical clarity. The endothelium actively pumps water into the aqueous, keeping the stroma slightly dehydrated and therefore clear.

Vascular tunic (uvea) — pigment and nutrition

The middle coat is loose, melanocyte-rich connective tissue laced with blood vessels. Posteriorly it forms the choroid, whose innermost capillary plexus (the choriocapillaris) feeds the avascular outer retina and RPE across Bruch membrane. Choroidal melanin absorbs stray light that has passed through the retina, preventing internal reflection.

Anteriorly the uvea thickens into the ciliary body — a wedge containing smooth ciliary muscle (parasympathetic CN III, for accommodation) and finger-like ciliary processes whose double epithelium (outer pigmented + inner non-pigmented) secretes aqueous humour. Suspensory zonular fibres run from the processes to the lens equator. In front of the lens, the uvea forms the iris, with a melanocyte-laden stroma, a sphincter pupillae (smooth muscle, parasymp, miosis), a radial dilator pupillae (myoepithelial, symp, mydriasis), and a posterior pigmented epithelium that blocks transmitted light.

Neural tunic (retina) — the photosensitive receiver

The innermost coat is an outpouching of the embryonic forebrain. It has two embryologically distinct sheets: an outer pigment epithelium (RPE) and a nine-layered inner neural retina containing photoreceptors, bipolar cells, ganglion cells, and lateral interneurons (horizontal, amacrine) with Müller glia. Signal flows photoreceptor → bipolar → ganglion → optic nerve (CN II). The retina thins anteriorly to a non-photosensitive pigmented strip overlying the ciliary body and posterior iris.

Refractive media — the optical path

Light traverses four transparent refractive media in series: the cornea, the aqueous humour of the anterior chamber, the lens (avascular, capsule + cuboidal epithelium + crystallin-packed fibres), and the gel-like vitreous body filling the posterior cavity behind the lens.

Clinical anchor

Glaucoma is fundamentally a disease of the iridocorneal angle — impaired aqueous drainage from the trabecular meshwork into the canal of Schlemm raises intraocular pressure and damages ganglion-cell axons at the optic disc. Cataract is opacification of crystallins in the lens. Choroidal neovascularisation through Bruch membrane underlies wet age-related macular degeneration. Each pathology maps onto one of the three tunics or the refractive media, making the tunic framework directly clinically useful.

The retina is the photosensitive innermost coat of the eye and developmentally an outpouching of the diencephalon. Microscopically it shows ten layers running from the outer choroidal aspect to the inner vitreal aspect, populated by four principal cell types in the signal pathway and three accessory cell types. Light, paradoxically, has to traverse the inner layers before reaching the photoreceptors at the back — an arrangement justified by the photoreceptors' dependence on the underlying pigment epithelium.

Pigment epithelium (RPE)

The outermost retinal layer is a single sheet of cuboidal cells densely packed with melanin granules and apical phagosomes. Tight junctions between RPE cells form the outer blood-retina barrier. Functionally the RPE phagocytoses worn-out outer-segment discs shed every morning by photoreceptors, regenerates 11-cis-retinal for the visual cycle, absorbs stray light that escapes the photoreceptors, and supplies oxygen and metabolites from the underlying choriocapillaris across Bruch membrane.

Photoreceptors — rods and cones

Photoreceptors are highly modified bipolar neurons with an outer segment (stacks of membranous discs carrying the visual pigment), an inner segment (mitochondria + protein synthesis machinery), a cell body in the outer nuclear layer, and a synaptic terminal contacting bipolar cells. Rods contain rhodopsin, are extremely light-sensitive, mediate scotopic (dim-light, monochrome) vision, and dominate the peripheral retina. Cones come in three subtypes (S, M, L) carrying different photopsins, mediate photopic and colour vision, and concentrate at the fovea where they give highest acuity.

Bipolar cells — the middle relay

Bipolar cells sit in the inner nuclear layer. Their outer dendrites synapse with photoreceptors in the outer plexiform layer; their inner axons synapse with ganglion cells in the inner plexiform layer. They are the obligatory intermediate neuron of the visual pathway — rods and cones never speak directly to ganglion cells.

Ganglion cells — output of the retina

Ganglion cells are multipolar neurons whose cell bodies form the ganglion cell layer. Their unmyelinated axons course in the nerve fibre layer, converge at the optic disc and form the optic nerve (CN II). Ganglion-cell loss at the optic disc is the histopathological substrate of glaucoma.

Accessory neurons and glia

Horizontal cells and amacrine cells provide lateral inhibition in the outer and inner plexiform layers respectively, sharpening contrast and motion detection. Müller cells are the principal retinal glia — they span the entire retinal thickness, form the external and internal limiting membranes, and provide structural, metabolic, and ionic homeostasis.

Specialised regions — macula, fovea, optic disc

The macula lutea is the yellow-pigmented central retina; at its centre is the fovea centralis, a small depression containing only cones with overlying bipolar and ganglion layers swept aside so that light strikes the cones directly — the anatomical basis of highest acuity. The optic disc, slightly nasal to the fovea, lacks photoreceptors and is the physiological blind spot.

Clinical anchor

Retinitis pigmentosa kills rods first and produces tunnel vision with night blindness. Age-related macular degeneration destroys cones at the fovea, robbing reading vision. Retinal detachment exploits the embryonic cleft between RPE and photoreceptors. Diabetic retinopathy progresses through microaneurysms, exudates, and neovascularisation driven by VEGF. Each disease maps directly onto a layer or cell type of this histological scheme.

The cornea is the transparent anterior one-sixth of the fibrous tunic. It is avascular, richly innervated, and contributes the greatest single refractive power to the optical system of the eye (about two-thirds of total dioptric power). Its histology — five precisely arranged layers from anterior to posterior — reconciles the conflicting requirements of mechanical toughness and perfect optical clarity.

Corneal epithelium

The outermost layer is non-keratinised stratified squamous epithelium, about five cell layers thick. Surface cells bear microvilli that anchor the precorneal tear film. Free nerve endings derived from the long ciliary nerves (ophthalmic division of CN V) make the cornea one of the most densely innervated tissues in the body, which is why even a tiny abrasion produces intense pain and reflex lacrimation. The epithelium regenerates rapidly from limbal stem cells at the corneo-scleral junction; destruction of the limbus (chemical burns) results in conjunctival overgrowth and corneal blindness.

Bowman membrane (anterior limiting lamina)

Beneath the epithelium lies a homogeneous, acellular layer of randomly oriented type I collagen fibrils and proteoglycans, about 8–14 µm thick. It is essentially a condensed superficial stroma. Because it contains no cells it does not regenerate after injury — any breach heals by scar, which can compromise vision if it overlies the visual axis.

Substantia propria (stroma)

The stroma constitutes about 90% of corneal thickness. It is built from 60–70 lamellae of type I collagen, each lamella running parallel to the corneal surface but at an angle to its neighbours, embedded in keratan-sulphate proteoglycan ground substance. Quiescent flat fibroblasts (keratocytes) sit between lamellae. Transparency depends on three structural facts: the collagen fibrils are uniformly thin (~25 nm), the spacing between fibrils is less than the wavelength of visible light, and the stroma is kept slightly dehydrated by the posterior endothelium — together these eliminate light scatter.

Descemet membrane (posterior limiting lamina)

Posterior to the stroma lies a thick, homogeneous layer of atypical type IV and type VIII collagen — the basement membrane of the corneal endothelium. Unlike most basement membranes, it is grossly visible on light microscopy. It thickens progressively from about 3 µm at birth to 10 µm in old age. In Wilson disease, copper deposits along its periphery as the Kayser-Fleischer ring.

Corneal endothelium

The innermost layer is a single sheet of squamous-to-low-cuboidal cells facing the anterior chamber. These cells contain abundant mitochondria and Na⁺/K⁺-ATPase, and they actively pump fluid out of the stroma into the aqueous humour, keeping the stroma dehydrated and therefore transparent. They are non-mitotic in adults; you are born with about 5000 cells/mm² and lose them slowly over life, surviving cells spreading to maintain coverage.

Clinical anchor

Failure of the endothelial pump in Fuchs endothelial dystrophy causes stromal oedema and morning corneal clouding, treated by Descemet membrane endothelial keratoplasty (DMEK). Keratoconus is a progressive non-inflammatory thinning of the central stroma that produces conical bulging and irregular astigmatism. Limbal stem cell deficiency follows chemical burns and contact-lens overuse and results in conjunctivalisation of the cornea. Corneal abrasions heal rapidly because of epithelial regeneration; deeper corneal ulcers that breach Bowman membrane scar.

The spiral organ of Corti is the auditory receptor of the inner ear — the place where mechanical vibration is converted into the electrical signal carried by the cochlear nerve. It is an epithelial ridge running the length of the spiral cochlear duct, resting on the basilar membrane within the endolymph of the scala media, and overhung by a gelatinous tectorial membrane. Two cell families build it: hair cells (the sensory transducers) and supporting cells (the architectural framework).

Location and fluid environment

The organ of Corti sits on the upper surface of the basilar membrane within the scala media (cochlear duct), bathed apically in endolymph (high K⁺, low Na⁺) produced by the stria vascularis. The base of the organ sits in perilymph-like fluid (cortilymph) communicating with the scala tympani below. This asymmetric ionic environment is essential for transduction.

Hair cells — inner and outer

There are two distinct populations. A single row of inner hair cells (about 3500 per ear) lines the medial side of the tunnel of Corti. These are the true sensory cells — they receive about 95% of cochlear afferent innervation and are responsible for the auditory signal transmitted to the brain. Three rows of outer hair cells (about 12,000 per ear) lie on the lateral side. They are electromotile: they change length in response to membrane voltage through a unique motor protein, prestin, embedded in their lateral wall. Their rapid length changes sharpen and amplify basilar-membrane motion — the cochlear amplifier — and explain the exquisite frequency selectivity of the ear.

Each hair cell carries an apical bundle of stereocilia (modified microvilli with an actin core, not true cilia) arranged in three rows of graded height. Tip-links of cadherin-23/protocadherin-15 connect the tips of shorter stereocilia to the sides of taller ones; when the bundle deflects toward the tallest row, tip-links pull open mechanotransduction channels and K⁺ flows in down its endolymph-driven gradient, depolarising the cell.

Supporting cells

The principal supports are the inner and outer pillar cells, which form the angular borders of the triangular tunnel of Corti running the length of the organ. Inner phalangeal cells cup the inner hair cells; outer phalangeal cells of Deiters cup the bases of outer hair cells and send a slender process upward whose flat top contributes to the reticular lamina — an apical mosaic that seals the organ and separates endolymph above from perilymph below. Border cells, Hensen cells, and Claudius cells make up the medial and lateral margins.

Tectorial membrane

An acellular gelatinous flap of collagen and tectorins extends laterally from the spiral limbus to overlie the stereocilia. The tallest stereocilia of outer hair cells are embedded in its undersurface; inner hair cell stereocilia float freely beneath it. When the basilar membrane vibrates upward, the tectorial membrane shears past the reticular lamina and the embedded outer hair cell stereocilia are bent — the trigger event of transduction.

Function — how sound becomes a signal

Stapes footplate motion at the oval window sets perilymph in motion, which deflects the basilar membrane up and down at the position tuned to that frequency (high frequencies near the stiff narrow base, low frequencies near the floppy wide apex — tonotopic place coding). The shearing tectorial membrane bends outer-hair-cell stereocilia, mechanotransduction channels open, K⁺ influx depolarises the cell, prestin contracts to amplify the local motion, and inner-hair-cell stereocilia are in turn deflected. Inner-hair-cell depolarisation triggers glutamate release at ribbon synapses onto bipolar neurons of the spiral ganglion, whose central processes form the cochlear division of CN VIII.

Clinical anchor

Presbycusis — age-related high-frequency hearing loss — reflects loss of outer hair cells at the basal cochlea. Aminoglycoside (gentamicin) and cisplatin ototoxicity selectively destroy outer hair cells; noise-induced hearing loss mechanically tears stereocilia bundles. Meniere disease is endolymphatic hydrops producing episodic vertigo, sensorineural hearing loss, and tinnitus. Cochlear implants bypass dead hair cells by stimulating spiral ganglion fibres directly.

The vestibular apparatus is the balance organ of the inner ear. It occupies the membranous labyrinth alongside the cochlea and shares its endolymph. Two kinds of receptor patch detect motion: the maculae of the utricle and saccule detect linear acceleration and the static pull of gravity, and the three cristae ampullares — one in the ampulla of each semicircular duct — detect angular (rotational) acceleration. Both rely on the same hair-cell mechanotransduction principle as the cochlea, with the mechanical stimulus altered by overlying gelatinous structures of different design.

Maculae — linear acceleration and gravity

Each macula is a roughly oval patch about 2–3 mm across, consisting of hair cells with apical stereocilia embedded in the otolithic (statoconial) membrane — a gelatinous glycoprotein layer studded with dense otoliths (otoconia), small crystals of calcium carbonate on a protein core. Because the otoliths are denser than the surrounding endolymph, gravity pulls them downward and any linear acceleration causes them to lag relative to the head. Either motion drags the gelatinous mat across the hair-cell apices, bending the stereocilia and modulating the receptor potential.

The two maculae are oriented at right angles to each other: the utricular macula lies horizontally and detects sideways tilt of the head and horizontal (forward/backward) linear acceleration; the saccular macula is oriented vertically and detects up-down tilt and vertical linear acceleration. A curved ridge called the striola divides each macula into two zones in which the hair cells are polarised in opposite directions, so any single head movement excites one population and inhibits the other.

Cristae ampullares — angular acceleration

Each of the three semicircular ducts (anterior, posterior, lateral) bears a swollen end called the ampulla, in whose floor sits a transverse ridge of hair cells — the crista ampullaris. The stereocilia of these hair cells project into a tall gelatinous cone called the cupula, which has the same density as endolymph and completely spans the lumen of the ampulla like a swinging door.

When the head rotates in the plane of a duct, the membranous duct turns with the skull but the endolymph inside lags by inertia. The relative flow of endolymph pushes the cupula and bends the embedded stereocilia. Because the three semicircular ducts lie in three orthogonal planes, the brain decodes rotation about any axis from the relative excitation across the six cristae (three per ear).

Vestibular hair cells

Vestibular hair cells come in two morphological types — type I (flask-shaped, surrounded by a chalice-like nerve terminal) and type II (cylindrical, with bouton-type endings) — but both share the same apical sensory apparatus. Each cell carries a bundle of stereocilia graded in height, plus a single true kinocilium on one side (a 9+2 cilium — this is what distinguishes vestibular from cochlear hair cells, which lose their kinocilium in adults). Deflection of the bundle toward the kinocilium opens transduction channels and depolarises the cell; deflection away hyperpolarises it — giving each cell directional sensitivity.

Innervation and central pathway

Hair cells synapse with the peripheral processes of bipolar neurons of the vestibular ganglion (Scarpa) in the internal acoustic meatus. Central processes form the vestibular division of CN VIII and project to the four vestibular nuclei in the brainstem, which then drive vestibulo-ocular, vestibulo-spinal, and vestibulo-cerebellar reflexes to keep the eyes, posture, and gaze stable during head motion.

Clinical anchor

Benign paroxysmal positional vertigo (BPPV) — the commonest cause of episodic vertigo — is caused by dislodged otoliths drifting into a semicircular duct (most often the posterior) where they cause inappropriate cupular deflection on head movement, and is fixed by the Epley repositioning manoeuvre. Meniere disease is endolymphatic hydrops producing the triad of vertigo, fluctuating sensorineural hearing loss, and tinnitus. Vestibular neuritis is an acute viral inflammation of the vestibular nerve producing severe vertigo with intact hearing.