Ophthalmology

Introduction to Ophthalmology • Physiology of sight • Emergency-medicine learning resource Physiology of Sight: How the Eye and Brain Produce Vision Sight is an active neural process, not simply the passage of light through the eye. The cornea and lens focus photons on the retina; photoreceptors convert light into changes in membrane potential; retinal circuits extract contrast, colour, movement and spatial detail; the optic pathways carry coded signals to the brain; and visual cortex integrates them with attention, memory and movement. Understanding these steps helps clinicians localise visual loss and recognise emergencies. Clinical safety note: Sudden monocular or binocular visual loss, a new field defect, flashes/floaters with a curtain, painful loss of vision, acute diplopia, an abnormal pupil with neurological symptoms or visual loss after trauma requires urgent assessment. Physiology explains patterns but never replaces examination. Learning objectives Explain how light is focused on the retina and converted into neural signals. Describe rod and cone phototransduction, dark/light adaptation and visual pigment regeneration. Explain retinal circuits for contrast, edges, movement, colour and spatial detail. Trace the visual pathway from retina to visual cortex and relate lesions to field defects. Explain accommodation, convergence, pupillary reflexes, binocular vision and depth perception. Relate physiology to common symptoms, examination findings and emergency presentations. 1. From photons to perception: the sequence Optical capture: light enters through the tear film, cornea, aqueous, pupil, lens and vitreous. Focusing: corneal and lenticular refraction form an inverted image on the photoreceptor layer. Phototransduction: rods and cones convert photons into graded electrical responses. Retinal computation: bipolar, horizontal, amacrine and ganglion cells compare and transform signals. Transmission: ganglion-cell action potentials travel through the optic nerve, chiasm, tract and radiations. Cortical analysis: visual cortex extracts orientation, motion, colour and location; association areas identify objects and guide action. Conscious perception: attention, memory and multisensory context determine what the person reports seeing. A failure at any level may produce visual symptoms. Corneal/lens problems usually reduce clarity; macular disease impairs central detail; optic-nerve disease affects colour and central vision; retinal or vascular disease may cause field loss; pathway or cortical lesions produce characteristic binocular defects. 2. Optical physiology Refraction and image formation Refraction occurs when light changes speed and direction at an interface between materials with different refractive indices. The curved air–tear–cornea interface supplies most of the eye’s refractive power. The lens changes shape to adjust focus for near objects. The retinal image is inverted and reversed; the brain interprets it in relation to body and environmental coordinates. Optical element Physiological role Common disturbance Tear film Smooth first refractive surface and nourishes the corneal epithelium. Dry-eye instability causes fluctuating blur and glare. Cornea Major fixed refractive power; transparent protective barrier. Scar, oedema, keratitis or irregularity causes blur and photophobia. Pupil Controls light entry and depth of focus; smaller aperture reduces aberrations. Abnormal size/reactivity may indicate ocular or neurological disease. Lens Variable refractive power through accommodation. Cataract, dislocation or refractive error degrades the image. Vitreous Transparent medium maintaining the posterior optical path. Blood, cells or opacities cause floaters and haze. Retina Converts the focused image into neural information. Detachment, oedema, haemorrhage or degeneration causes visual loss. Resolution and visual acuity Visual acuity depends on optical clarity, photoreceptor spacing, retinal circuitry, contrast, illumination and intact visual pathways. The fovea has a high density of cones, small receptive fields and minimal overlying tissue, supporting the highest spatial resolution. Peripheral retina has larger receptive fields and more convergence, improving sensitivity and motion detection at the expense of fine detail. Contrast sensitivity is separate from high-contrast acuity. Cataract, corneal haze, glaucoma and retinal disease can reduce contrast before a standard chart detects major change. 3. Photoreceptors and visual pigments Feature Rods Cones Best light level Dim light (scotopic vision). Daylight/bright light (photopic vision). Location Abundant in peripheral retina; absent from the foveal centre. Concentrated in the fovea and present throughout the retina. Resolution Lower spatial resolution because many rods converge onto downstream cells. High resolution, especially in the fovea. Colour Monochromatic. Colour discrimination through different opsins. Temporal response Slower integration; good sensitivity. Faster responses; supports detail and motion in bright light. Clinical association Night blindness and peripheral field problems in rod disease. Central acuity and colour abnormalities in cone/macular disease. Rhodopsin in rods and cone photopigments consist of an opsin protein bound to 11-cis-retinal, a vitamin-A-derived chromophore. Light changes 11-cis-retinal to all-trans-retinal, initiating a cascade and requiring enzymatic recycling through the RPE. 4. Phototransduction step by step Dark state: high cyclic GMP keeps cation channels open in the photoreceptor outer segment. The cell is relatively depolarised and releases glutamate continuously. Photon absorption: a photon changes the chromophore configuration within rhodopsin or a cone pigment. Transducin activation: activated pigment stimulates the G-protein transducin. Phosphodiesterase activation: transducin activates phosphodiesterase, which lowers cyclic GMP. Channel closure: cation influx decreases, the photoreceptor hyperpolarises and glutamate release falls. Retinal interpretation: ON and OFF bipolar pathways respond differently to the change in glutamate; lateral circuits enhance edges and contrast. Recovery and adaptation: pigment is phosphorylated/inactivated, cyclic GMP is restored and calcium-dependent feedback adjusts sensitivity. Photoreceptors use graded potentials rather than action potentials. Retinal ganglion cells are the first major output neurons to generate action potentials, which preserve timing and encode visual information along the optic nerve. 5. Retinal information processing Retinal circuit Physiological role Why it matters ON/OFF bipolar channels Separate increases and decreases in light intensity. Allows the visual system to detect changes rather than only absolute brightness. Horizontal cells Provide lateral inhibition and surround comparison. Enhances edges, contrast and spatial resolution. Amacrine cells Shape timing, motion, transient responses and rod pathways. Supports movement detection and adaptation. Ganglion-cell receptive fields Centre–surround organisation; different types encode detail, motion, colour and contrast. Creates parallel information streams to the brain. Retinal pigment epithelium Supports photoreceptor renewal, pigment recycling, metabolic exchange and barrier function. RPE dysfunction can damage photoreceptors and macular vision. Retinal processing reduces redundant information and emphasises change, boundaries and biologically important signals. This is why a uniform surface is less visually informative than an edge, moving

Introduction to Ophthalmology • Human eye anatomy and physiology • Emergency-medicine learning resource Review of the Anatomy and Physiology of the Human Eye The eye is a specialised sensory organ that converts light into neural signals and protects, focuses and transmits visual information to the brain. Emergency clinicians need more than a list of parts: the location, blood supply, innervation and function of each structure explains the pattern of pain, visual loss, pupil abnormality, eye movement disorder and injury. Clinical safety note: Sudden visual loss, painful red eye, chemical or penetrating trauma, new flashes/floaters with a field defect, proptosis with fever, or a pupil/eye-movement abnormality with neurological signs needs urgent assessment. Anatomy helps localisation but does not replace examination and referral. Learning objectives Describe the coats, chambers, optical media, retina, optic nerve, orbit and adnexa of the eye. Relate each structure to its function and major blood/nerve supply. Explain aqueous humour circulation, intraocular pressure and drainage. Explain accommodation, tear-film physiology, phototransduction and colour vision. Trace the visual pathway from photoreceptors to the visual cortex. Relate anatomical lesions to common patterns of visual loss, pupil change and diplopia. Apply the anatomy to eye trauma, glaucoma, retinal disease, optic-nerve disease and emergency triage. 1. Position, orientation and basic dimensions Each globe lies in a pyramidal bony orbit with the apex posteriorly and the base anteriorly. The visual axis points toward the object of fixation; the orbital axes diverge slightly. The globe is roughly spherical, but its anterior transparent cornea protrudes from the scleral coat. The optic nerve exits posteromedially, and the macula lies temporal to the optic disc. Orientation term Meaning in eye examination Example Anterior/posterior Toward the cornea/front or optic nerve/back. Anterior chamber; posterior pole. Nasal/temporal Toward the nose or the temple. Temporal visual field is seen by the nasal retina. Superior/inferior Upward or downward. Superior oblique; inferior retinal detachment. Medial/lateral Toward or away from the midline. Medial canthal tendon; lateral rectus. Central/peripheral Near the fovea/optic disc or toward the retinal edge. Central scotoma; peripheral field loss. 2. The three coats of the eyeball Coat Main components Main functions Fibrous coat Cornea anteriorly and sclera posteriorly, meeting at the limbus. Protection, shape, muscle attachment and major refraction through the cornea. Vascular coat (uvea) Iris, ciliary body and choroid. Blood supply, aqueous production, pupil regulation, accommodation and nutrition of outer retina. Neural coat Retinal pigment epithelium and neurosensory retina. Phototransduction, signal processing and transmission into the optic nerve. 3. Fibrous coat: cornea and sclera Cornea The cornea is transparent, avascular and richly innervated. It provides approximately two-thirds of the eye’s refractive power because the air–tear–cornea interface has a large change in refractive index. Corneal transparency depends on regular stromal collagen, relative dehydration maintained by endothelial ion pumps, and an intact epithelium and tear film. Corneal layer (anterior to posterior) Structure and function Clinical relevance Epithelium Stratified, non-keratinised, rapidly renewing barrier. Abrasion, punctate epithelial disease and recurrent erosion cause pain and fluorescein staining. Bowman layer Acellelar anterior stromal zone. Scarring may be permanent if breached. Stroma Organised collagen lamellae, keratocytes and water; most of corneal thickness. Oedema, infection or scar reduces transparency and vision. Descemet membrane Basement membrane produced by endothelial cells. Can detach or rupture in trauma and disease. Endothelium Single layer of non-regenerating cells maintaining stromal deturgescence with active pumps. Cell loss causes corneal oedema; contact-lens and intraocular surgery can stress it. Corneal sensation is mainly via the ophthalmic division of the trigeminal nerve (V1), through the nasociliary nerve and long ciliary nerves. This explains severe pain from superficial injury and the importance of the corneal blink reflex. Sclera and limbus Sclera: dense, opaque collagenous tissue forming most of the outer wall; posteriorly continuous with the dural sheath of the optic nerve and pierced by vessels, nerves and the optic nerve. Episclera: vascular connective tissue superficial to sclera; inflammation produces episcleritis. Limbus: transition between clear cornea and white sclera. It contains conjunctival epithelium, palisades of Vogt, Schlemm canal and the trabecular outflow region. Open-globe injury: a full-thickness wound of cornea or sclera. Avoid pressure, tonometry, forced eyelid opening or removal of embedded objects; place a rigid shield and refer urgently. 4. Uvea: iris, ciliary body and choroid Iris and pupil The iris is a contractile diaphragm in front of the lens. Its central opening, the pupil, regulates retinal illumination and contributes to depth of focus. The sphincter pupillae constricts the pupil under parasympathetic control; the radial dilator pupillae enlarges it under sympathetic control. Pathway Route and action Clinical clue Parasympathetic constriction Retinal light signal → pretectal area → bilateral Edinger–Westphal nuclei → CN III → ciliary ganglion → short ciliary nerves → sphincter pupillae. Loss causes a large poorly reactive pupil; compare direct and consensual responses. Sympathetic dilation Hypothalamus → descending fibres → ciliospinal centre → superior cervical ganglion → internal carotid plexus → V1/nasociliary pathway → long ciliary nerves → dilator pupillae. Interruption causes miosis and sometimes mild ptosis (Horner syndrome). Ciliary body Ciliary processes: capillary-rich folds that secrete aqueous humour through the non-pigmented epithelium. Ciliary muscle: has longitudinal, radial and circular fibres; contraction relaxes zonular tension and allows the lens to become rounder for near focus. Pars plicata and pars plana: anterior folded and posterior flatter regions; the pars plana is used for some vitreoretinal surgical access. Choroid The choroid is a pigmented vascular layer between sclera and retina. Its choriocapillaris supplies the outer retina, especially photoreceptors, which have high metabolic demand. Choroidal haemorrhage, inflammation or detachment can threaten the retina. 5. Chambers, aqueous humour and intraocular pressure The anterior chamber lies between cornea and iris; the posterior chamber lies between iris, lens and ciliary body. Both contain aqueous humour. Aqueous is continuously produced, circulates through the pupil and leaves mainly through the trabecular meshwork into Schlemm canal and episcleral veins, with an additional uveoscleral route. Step Physiology Clinical link Production Active secretion by non-pigmented ciliary epithelium; ultrafiltration and diffusion contribute. Ciliary-body drugs can reduce production. Flow Posterior chamber → pupil → anterior chamber; convection is influenced by