12.5 The Retina

Three short movements: the case, the anatomyThe study of the structure of the human body. and physiologyThe study of how the body functions., then the tie back to Stina. Each links to the scene where you’ll see it move.

I · Wired backwards

The retina is the neural tunic, and in section A cut or slice of the body or an organ for study. it is a stack. From the vitreous inward: the <strong>axons of the ganglion cells</strong> on the surface, then the <strong>ganglion cells</strong> themselves, then the <strong>bipolar cells</strong>, and at the very back, pressed against a dark <strong>pigment epithelium</strong> and the choroid, the <strong>photoreceptors</strong> — rods and cones, with their light-catching outer segmentsThe subdivisions of lung lobes, supplied by tertiary bronchi. pointing <em>away</em> from the light. the textbook says it plainly: a significant amount of light is absorbed by the overlying layers before it reaches the receptorsProteins located on the surface or inside cells that bind specific molecules (e.g., neurotransmitter. Two places solve that. At the exact center of the retina, the <strong>fovea</strong>, the supporting cells and vessels are swept aside and only photoreceptors remain — cones, packed, each with its own private line out — so <strong>visual acuity</strong> is greatest there, and most of what the eye does with its muscles is keep the thing you care about on it. And where the ganglion axons gather and leave, the <strong>optic disc</strong>, there are no photoreceptors at all: a <strong>blind spot</strong> in each eye that the brain paints over so thoroughly you need a trick to find it. Now the cells. <strong>Rods</strong> — roughly 120 million of them — crowd the periphery, work in dim light, and give no color. <strong>Cones</strong> — about 6 million — pack into the fovea, need bright light, and give color. (The counts are standard anatomy, not the textbook; the arrangement is.) Both are built the same way: an inner segment with the nucleusThe control center of the cell that contains DNA and directs cellular activities. and ordinary organellestructures within a cell that perform specialized functions., and an <strong>outer segment</strong> stuffed with membrane — stacked discs in a rod, folds in a cone — carrying the pigment. A rod’s pigment is <strong>rhodopsin</strong>, peaking at 498 nm, so sensitive that a single photon can produce a spike in the rod’s ganglion cell. Cones come in three kinds with three <strong>opsins</strong>, peaking near 420, 534 and 564 nm — blue, green, red — and color is the brain comparing how hard each kind is driven: a bright blue at 450 nm drives the blue cones hard, the green ones a little, the red ones barely. In sunlight rhodopsin is bleached continuously and cones do the seeing; in a dark room only rods work, and the world is in grayscale — if you think you can see color in the dark, your brain is remembering it. Wiring finishes the division of labor. In the fovea each cone has its own bipolar and ganglion cell, <strong>one to one</strong>; in the periphery up to <strong>fifty</strong> rods converge on one ganglion cell. Convergence buys sensitivity and spends acuity, which is why you see a faint star by looking slightly beside it, and read a page by looking straight at it.

See it move: Scene 1 · Wired Backwards · Scene 2 · Rods and Cones

II · Switched on in the dark

Here is the factA statement based on direct observation that is repeatedly confirmed. the chapter turns on, and the one every textbook under-explains. A photoreceptor is <strong>depolarized in the dark</strong> and steadily releasing transmitter; light <em>hyperpolarizes</em> it and it releases <em>less</em>. the textbook gives the chain to the edge of that and stops: an <strong>opsin</strong> is a membrane protein carrying a cofactorA non-protein substance (often a metal ion) that helps enzymes function., <strong>retinal</strong>, made from vitamin A (which is why a diet without it dims the night first); in the dark retinal sits bent, <strong>11-cis-retinal</strong>; a photon straightens it to <strong>all-trans-retinal</strong> — <em>photoisomerization</em>; the changed opsin activates a <strong>G protein</strong>; and the G proteinA signaling protein activated by neurotransmitter receptors that triggers internal cellular response “changes the membrane potential of the photoreceptor, which then releases less neurotransmitterChemicals that transmit signals across synapses..” The middle of that sentence is standard physiology the textbook leaves out, so take this next part as an addition to it: in the dark a messenger called <strong>cGMP</strong> holds sodium(Na⁺): Major ECF cation; important for fluid balance, nerve function. channelsProtein passages in the cell membrane that allow specific molecules to pass through. open in the outer segment, and that inward “dark current” keeps the cell depolarized and dripping glutamate. The G protein (<em>transducin</em>) switches on an enzyme that destroys cGMP, the channels close, the cell hyperpolarizes, and the glutamate drip slows. Less transmitter is the signal. So the receptor potentialA graded potential produced at the receptor membrane in response to a stimulus. here is graded and <em>negative</em>, and neither the photoreceptor nor the bipolar cell ever fires an action potential; they pass graded changes along, and the first spike in the whole system belongs to the <strong>ganglion cell</strong>. The bent retinal has to be rebuilt before the opsin can work again — <strong>bleaching</strong> — which is why a flash leaves an afterimage in negative, why a dark room takes minutes to come back after bright light (the pigments are being re-cocked), and why rhodopsin, bleached all day in sunlight, hands the day shift to the cones.

See it move: Scene 3 · Switched On in the Dark

III · The relay and the crossing

The relay is three cells, and the middle one is a callback: photoreceptor → <strong>bipolar cell</strong> → <strong>ganglion cell</strong>. In Nervous Tissue we sorted neuronsThe functional cells of the nervous system that transmit signals. by shape and called the bipolar form rare, a shape you meet almost nowhere except the special senses — here it is, doing the job it was named for. Standard texts add <strong>amacrine cells</strong> at the inner synaptic layer, sharpening the message before the ganglion cell fires (and standard texts add <em>horizontal cells</em> at the outer layer). Ganglion-cell axons run along the surface, gather at the disc, and leave as the <strong>optic nerve</strong>, cranial nerve II — brain tissue, oligodendrocyte myelin, meninges(singular: meninx) – Protective membranes surrounding the spinal cord and brain. and all. A few ganglion cells are themselves <em>photosensitive</em> and never report to the seeing part of the brain at all: they drive the pupillary reflex and set the body’s clock through the <strong>suprachiasmatic nucleus</strong> of the hypothalamusA small but vital brain region controlling hormones, temperature, and autonomic functions., which is why the retina is the organ that knows what time it is. Now the rule. At the <strong>optic chiasm</strong>, under the hypothalamus and just above the pituitary, <strong>the axons from the medialToward the midline of the body (nasalTwo small rectangular bones forming the bridge of the nose.) half of each retina cross; the axons from the lateralAway from the midline of the body. (temporal) half do not</strong>. Because the lens inverts the image, the nasal retina sees the <em>outer</em> half of the world on its side — so after the chiasm each <strong>optic tract</strong> carries the <em>opposite</em> half of the visual field from both eyes: everything to your left lands on your right brain. The tract’s main stop is the <strong>lateral geniculate nucleus</strong> of the thalamus, then the <strong>optic radiation</strong> to the <strong>primary visual cortex</strong> in the occipitalPosterior and base of the skull; curves under to form the back of the head. lobe; a branch peels off to the <strong>superiorAbove or toward the upper part of the body. colliculus</strong> of the midbrainThe upper portion of the brainstem involved in visual and auditory processing., which turns your eyes and headRounded proximal end that fits into the acetabulum of the hip bone. toward something that moved. The geometry predicts the lesions. Cut one optic nerve: one eye blind. Press on the middle of the chiasm — a pituitary tumor from below does exactly this — and only the crossing fibers suffer, so each eye loses its <em>outer</em> field: <strong>bitemporal hemianopia</strong>, a patient who walks into door frames; Be careful: this is not tunnel vision, because the top and bottom of the field survive. Cut one optic tract or one occipital lobe and the same half of the world disappears from both eyes. Beside the primary cortex sits the <strong>visual association area</strong>, where seeing becomes recognizing — the Brain module gives it two jobs, spatial recognition and facial recognition — and a person with a damaged fusiform gyrus can have perfect acuity and fail to know their own mother’s face. The pathway from Stina’s retina to her occipital lobe is untouched; the eye in front of it kept inflaming for twenty years (The Eye, 12.4). On this map, both are information: a process that reaches a drain at the front of the eye and never the nerve behind it is choosing its addresses.

See it move: Scene 4 · The Three-Cell Relay · Scene 5 · The Crossing

5 scenes. Each one has a short animation or a slide, something to explore, a true/false spot-check of your thinking, and a quick check. Every scene gives you an evidence card for your board.

Every scene gave you an evidence card, and the chart adds more. Sort each card into the column it supports: A visual system intact from retina to cortex, Neither on its own, or Vision is simply lucky, and could go next. Some cards can honestly go in more than one place — that’s allowed, and the feedback tells you which cards decide the case.

Time to decide. Look back at your bet, then build your verdict from statements that hold up — no writing needed.