Visual Pathway: Eye Anatomy, Optics, Phototransduction & Refractive Errors

Objectives of the Unit

  • Trace the visual pathway from cornea to visual cortex.
  • Understand how refraction and mechanical eye control shape the retinal image.
  • Preview: Current lecture covers eye optics & phototransduction; next lecture covers retina → cortex pattern processing.

Clinical Reference: Visual Acuity Charts

  • Snellen chart (US/Canada): 20 / 20 denotes normal acuity.
  • Metric equivalent (UK/AU): 6 / 6.
    • Example: 6 / 4 means the person sees at 6 m what a typical observer must approach to 4 m (better acuity).

External Eye Anatomy

  • Sclera: tough, white, protective shell.
  • Cornea: transparent, continuous with conjunctiva; first & strongest refracting surface.
  • Iris: pigmented ring; houses sphincter & dilator muscles that set pupil size.
  • Pupil: aperture for incoming light.
  • Conjunctiva & Tears
    • Conjunctiva lines eyelids & scleral surface.
    • Lacrimal glands secrete tears → washing, antimicrobial & antifungal action.

Extra-Ocular Muscles & Mechanics

  • Recti: lateral, medial, superior, inferior (prod. horizontal & vertical gaze).
  • Obliques: superior (via trochlea pulley) & inferior — add torsion.
  • Abducens (CN VI) controls lateral rectus; Oculomotor (CN III) controls most others including medial rectus & ciliary muscle.
  • Trochlea pulley reverses pull of superior oblique → eye moves down & in on contraction.

Internal Chambers & Media

  • Anterior chamber: aqueous humour (continually produced & drained → turnover). Imbalance ⇢ glaucoma.
  • Posterior/Vitreous chamber: gelatinous vitreous humour (static; age-related debris → "floaters").
  • Lens (crystalline) held by Zonules of Zinn (suspensory ligaments).
    • Far vision: lens flattened by zonular tension.
    • Near vision: ciliary muscles contract → zonular slack → lens fattens.

Retinal Landmarks

  • Optic disc (blind spot): exit/entry for optic nerve & central retinal vessels. No photoreceptors.
  • Macula lutea (yellow spot): vascular-sparse zone for high acuity.
    • Fovea: central pit rich in cones; highest spatial & colour resolution.

Visual Axis & Light Path

  • Central rays pass almost unbent → direct line cornea → pupil → lens → fovea.
  • Peripheral rays require greater refraction → converge at common focal point.
  • Cornea supplies ~⅔ total optical power; lens fine-tunes for accommodation.

Pupillary Light Reflex

  • Reflexive, consensual, bilateral constriction to bright light.
  • Pathway: Retina → Pretectal nuclei → Edinger-Westphal (CN III parasympathetic) → iris sphincter.
  • Clinical red flag: anisocoria (unequal pupils) may signal neurological or vascular emergency.

Physical Nature of Light

  • Wave described by wavelength (λ) & amplitude.
  • Electromagnetic spectrum span; human vision limited to 400 nm≤λ≤700 nm400\,\text{nm} \le \lambda \le 700\,\text{nm}.
    • Shorter λ (UV) → higher energy, DNA damage potential; useful for sterilization.
    • Longer λ (IR) perceived as heat.

Basic Optics & Refraction

  • Outcomes for incident light:
    1. Reflection (angle of incidence = angle of reflection) – basis of most visible objects.
    2. Absorption – e.g.
      choroid pigmented layer prevents back-scatter.
    3. Refraction – direction change when entering medium with different refractive index (n).
  • Lens types
    • Convex (converging): bends peripheral rays inward.
    • Concave (diverging): spreads rays.
  • Refractive Power (Diopters)D=1f (meters)D=\frac{1}{f\,(\text{meters})}
    • Human eye: cornea-to-fovea distance f≈0.024 mf \approx 0.024\,\text{m} → Dcornea≈42 DD_{cornea}\approx 42\,\text{D}.
    • Lens can add ≈ 16–17 D when fully rounded.

Accommodation: Far vs Near

  • Far field (>9 m)
    • Lens flat; cornea alone focuses image on retina.
  • Near field (

Refractive Errors (Ametropias)

  • Emmetropia: perfect focus on retina.
  • Myopia (nearsighted)
    • Eye too long or lens too strong → focus in front of retina.
    • Correction: concave (-D) lenses.
  • Hyperopia (farsighted)
    • Eye too short or lens too weak → focus behind retina.
    • Correction: convex (+D) lenses.
  • Astigmatism
    • Cornea/lens aspherical; different meridians have different power → multiple focal points.
    • Correction: cylindrical lenses.
  • Presbyopia (age-related accommodative loss)
    • Lens stiffens, ciliary muscle weaker → near tasks hard; onsets ≈ 40–50 yr.
    • Reading (+) adds, bifocals/trifocals.
  • Cataract
    • Lens proteins opacify (age, UV, genetics) → scatter light; surgical lens replacement common.

Retinal Topography & Visual Field Mapping

  • Azimuth 0° = gaze centre; ±90° at temporal extremes.
  • Binocular overlap ≈ ±20–25° around midline.
  • Blind spot ≈ 15° nasal to fovea for each eye; filled by contralateral eye input.

Photoreceptor Distribution

  • Rods
    • Peak density peripheral; absent at fovea.
    • Very light-sensitive; low spatial acuity; monochromatic.
  • Cones
    • Concentrated at fovea; sparse periphery.
    • Need bright light; high acuity; basis for colour.
  • Trade-off: Periphery = better night detection; Central = detailed, colour vision.

Retinal Laminar Architecture & Foveal Specialization

  • Light passes (vitreous →) ganglion cell layer → inner plexiform → bipolar cells → outer plexiform → photoreceptor outer segments.
  • Fovea: inner layers pushed aside → minimal scatter & vascular obstruction, maximising photon capture.

Molecular Basis of Phototransduction

  • Key molecule: Rhodopsin (rods) & cone opsins embedded in stacked disks.
  • Photon → 11-cis-retinal → all-trans-retinal isomerization.
    • Conformational shift closes cGMP-gated Na+Na^+/Ca2+Ca^{2+} channels.
    • Dark: channels open → depolarised (~-40 mV) → constant glutamate release.
    • Light: hyperpolarisation → glutamate release decreases; signal relayed to bipolar cells.
  • Only first step light-dependent; cascade greatly amplifies signal.

Cone Opsins & Colour Coding

  • Three cone opsins with distinct λ-max:
    • S-cones (short): ≈ 420 nm ("blue").
    • M-cones (medium): ≈ 530 nm ("green").
    • L-cones (long): ≈ 560 nm ("red").
  • Rods peak ≈ 500 nm.
  • Perception of colour emerges centrally (LGN & cortex) by comparing relative cone activations.
  • Genetic notes:
    • L & M opsins X-linked → ≈ 10 % males dichromatic ("red-green" colour blindness).
    • Some females heterozygous for L/M variants → potential tetrachromacy (4 opsins).

Vision vs Ambient Illumination

  • Photopic (daylight): only cones; full colour & high acuity.
  • Mesopic (dusk): rods + cones; colour desaturates.
  • Scotopic (night): rods only; monochrome, low acuity.
  • Clinical tie-in: Macular (foveal) damage forces reliance on rod-rich periphery → loss of colour & fine detail.

Ethical & Clinical Connections

  • Unequal pupils (anisocoria) warrant urgent neuro/vascular evaluation.
  • UV protection (sunglasses) may reduce cataract risk & retinal damage.
  • Early detection of macular degeneration crucial; wet AMD treated with anti-VEGF to curb neovascularisation.
  • Vision correction (glasses, contacts, refractive surgery) fundamentally manipulates external optical power to counter ametropias.
  • Cataract extraction + intra-ocular lens implantation restores transparency & often reduces refractive error.

Key Numbers & Formulae to Remember

  • Cornea–fovea distance ≈24 mm(0.024 m)\approx 24\,\text{mm} (0.024\,\text{m}).
  • Corneal power Dc≈42 DD_{c}\approx 42\,\text{D}.
  • Total relaxed eye ≈58 D\approx 58\,\text{D} (cornea + lens minimal).
  • Accommodation adds up to ≈17 D\approx 17\,\text{D} in youth.
  • Diopter: 1 D=1 / 1 m1\,\text{D}=1\,/\,1\,\text{m} focal length (e.g., 2 D lens ⇒ f=0.5 mf=0.5\,\text{m}).

Study Hints & Integrations

  • Link optics concepts to camera lenses (aperture = pupil; shutter speed not present – photoreceptors integrate continuously).
  • Recall autonomic parallels: pupillary reflex akin to acoustic middle-ear reflex (both protect transducers).
  • Compare photoreceptor depolarise-in-dark strategy to vestibular hair cells (also tonic release).
  • Reuse diopter math when analysing corrective lenses in lab problems.
  • Map cranial nerves: CN II (optic) for afferent light, CN III (oculomotor) for efferent pupil & accommodation, CN VI (abducens) for lateral gaze.