Eye Study Guide

📖 Core Concepts Eye – sensory organ that captures light and turns it into electrical signals transmitted to the brain via the optic nerve. Optical pathway – light → iris (pupil size) → cornea & lens (focus) → vitreous humour → retina (photoreceptors) → optic nerve → visual cortex. Photoreceptor types – Rods: highly sensitive, monochrome, dominate in dim light; Cones: colour‑detecting, require bright light, provide visual acuity. Eye classifications – Simple (non‑compound) eyes (single lens, one image) vs. Compound eyes (many ommatidia, multiple images). Visual acuity – ability to resolve fine detail, measured in cycles/degree (e.g., 20/20 ≈ 30 c/°, theoretical max ≈ 50 c/°). Colour vision – requires at least three cone types with narrow spectral sensitivities; colour perception = combined response of these cones. Opsins – light‑absorbing proteins; c‑opsins in ciliary (vertebrate) photoreceptors, r‑opsins in rhabdomeric (invertebrate) photoreceptors. Ecological eye adaptations – predator eyes: forward‑facing, high‑acuity fovea; prey eyes: laterally placed, wide field of view; deep‑sea: large/superposition eyes for maximal light capture. 📌 Must Remember Rod vs. Cone: rods = low‑light, no colour; cones = bright‑light, colour, high acuity. Iris function – muscles change pupil diameter to regulate light entry. Cornea + Lens supply ≈ 2/3 of refractive power; lens fine‑tunes focus. Compound eye resolution limit ≈ 1° per ommatidium; superposition eyes = higher sensitivity, lower resolution. PAX6 gene – master regulator of early eye development in all animals. Human cone peaks: L‑cone (560 nm), M‑cone (530 nm), S‑cone (430 nm). Cycles per degree ↔ Snellen: \( \text{Snellen denominator} \approx \frac{1800}{\text{c/°}} \). (e.g., 30 c/° → 20/20). UV protection – oil droplets absorb UV; lenses can become UV‑blocking. 🔄 Key Processes Phototransduction (rods & cones) Photon → photopigment (rhodopsin in rods, opsin‑cone pigment) → conformational change → cascade → hyperpolarization → reduced glutamate release → signal to bipolar cells. Focusing in vertebrate eye Light → cornea (≈ 43 D) → aqueous humour → lens (adjustable curvature) → vitreous humour → retina. Focusing in cephalopod eye Fixed‑focus lens → retina moves forward/backward (telescoping) to change focal plane. Image formation in compound eye Each ommatidium creates a tiny inverted image → brain stitches them into a mosaic; in superposition eyes, many facets converge light onto a single photoreceptor region. Colour perception Simultaneous activation of L, M, S cones → neural comparison → perceived hue. 🔍 Key Comparisons Rod vs. Cone → high sensitivity & monochrome vs. lower sensitivity & colour/acuity. Compound (Apposition) vs. Superposition → separate images, higher resolution vs. merged light, higher sensitivity in dim light. Vertebrate vs. Cephalopod eye → ciliary photoreceptors, focus by lens shape change vs. rhabdomeric photoreceptors, focus by retinal movement. Simple eye (pit) vs. Ocellus → shallow depression, can form rudimentary image vs. no lens, only light‑dark detection. UV‑detecting eye vs. UV‑blocked eye → oil droplets or transparent lens → UV reaches cones vs. UV‑absorbing lens/oil droplets → protection but loss of UV perception. ⚠️ Common Misunderstandings “All eyes see colour.” Only organisms with ≥ 3 cone types can discriminate colours; many insects and nocturnal mammals rely on rods. “More ommatidia = better vision.” Quantity improves field of view, but resolution still limited by inter‑ommatidial angle (1°). “Rods are just “dim‑light cones.” Rods use a different photopigment (rhodopsin) and have distinct signal pathways; they cannot be converted into cones. “The lens alone focuses.” Cornea provides the majority of refractive power; the lens fine‑tunes focus, especially in air. 🧠 Mental Models / Intuition “Camera vs. Mosaic” – Simple eyes act like a single‑lens camera; compound eyes act like a tiled mosaic of tiny cameras (ommatidia). “Light‑sensitivity trade‑off” – More light‑catching area (large aperture, superposition) → higher sensitivity but lower resolution; smaller aperture → sharper image, less light. “Colour as a three‑channel signal” – Think of cones as RGB sensors; the brain blends their outputs to produce the full colour spectrum. 🚩 Exceptions & Edge Cases Deep‑sea cephalopods – use reflective superposition eyes (mirror‑type) to maximize photon capture. Some birds & insects – possess a fourth cone type (UV) → tetrachromatic vision. Aquatic vertebrates – corneal refractive power drops in water; they rely more on lens curvature. Cephalopod photoreceptors – rhabdomeric, not ciliary, yet produce camera‑type images. 📍 When to Use Which Identify eye type → look for lens (simple) vs. many ommatidia (compound). Predict visual acuity → high‑acuity fovea → simple eye with dense cones; low‑acuity wide‑view → compound eye. Determine colour capability → count cone types (≥ 3 → colour vision). Assess UV sensitivity → presence of oil droplets or UV‑blocking lens decides whether UV reaches cones. 👀 Patterns to Recognize Forward‑facing + fovea → predator; lateral placement + wide field → prey. Large pupil + superposition eye → dim‑habitat species. Presence of PAX6 expression → early eye development stage. Higher rod density in periphery → motion detection & low‑light awareness. 🗂️ Exam Traps “All insects have compound eyes.” – Many insects also possess simple ocelli for light detection. “More cones = better colour vision.” – Without appropriate neural processing, extra cones may not expand colour range. “Superposition eyes give sharper images.” – They increase sensitivity, not resolution; resolution remains limited by facet size. “The lens is the only refractive element.” – The cornea contributes 2/3 of total refractive power in air‑borne eyes. “Rhodopsin = colour pigment.” – Rhodopsin is a rod pigment (monochrome), not involved in colour discrimination.