General Principles of Reception and Response in Animals

Lessons

Senior 6 · Biology

General Principles of Reception and Response in Animals

5 lessons

Lesson 3 of 5

The eye: structure, the retina and colour vision

30 min

Seeing a faint star

On a clear night far from town lights, astronomers have a trick: to see a very faint star, look slightly to one side of it. Stare straight at it and it vanishes. The explanation lies in the retina, which has two kinds of photoreceptor arranged very differently. In Senior 3 you learned the parts of the eye, how it focuses and how its defects are corrected. This lesson reviews that structure briefly and then studies the retina at A-level: how a rod turns light into a nerve signal, and why rods and cones give such different vision.

Definition

Photoreceptor: a receptor cell that responds to light. The human retina has two kinds, rods and cones.

Accommodation: the change in the shape, and so the focusing power, of the lens that keeps the image of an object focused on the retina as the object's distance changes.

Rhodopsin: the light-sensitive pigment in rods, made of the protein opsin joined to retinal, a molecule made from vitamin A.

Visual acuity: the ability to see two points close together as separate points; the detail an eye can resolve.

Convergence (retinal): several receptor cells passing their signals to a single bipolar neurone.

Structure of the eye

Part	Function
Cornea	Transparent front of the sclera; does most of the refraction (bending) of light
Aqueous and vitreous humours	Transparent fluid and jelly that keep the eyeball's shape and refract light
Iris and pupil	Circular and radial muscles of the iris change the size of the pupil and so control how much light enters
Lens, ciliary body and suspensory ligaments	The ciliary muscle and ligaments change the lens's shape to fine-tune focusing (accommodation)
Sclera	Tough, white outer coat that protects the eye and gives muscles attachment
Choroid	Dark, with blood vessels that supply the retina; its pigment absorbs stray light so it is not reflected inside the eye
Retina	Contains the photoreceptors (rods and cones) and the neurones that process their signals
Fovea	Small pit in the retina packed with cones only; gives the sharpest, colour vision
Blind spot	Where the optic nerve leaves; it has no photoreceptors

Accommodation, in brief. For a near object the ciliary muscle contracts, the suspensory ligaments go slack and the elastic lens becomes fatter and refracts light more. For a distant object the ciliary muscle relaxes, the ligaments are pulled taut and the lens becomes thinner. The pupil reflex works through two antagonistic muscles: in bright light the circular muscles contract and the radial muscles relax, so the pupil narrows; in dim light the opposite happens.

The retina: rods, cones and convergence

Surprisingly, light must pass through two layers of neurones, the ganglion cells and the bipolar cells, before it reaches the rods and cones at the back of the retina. Behind them, a pigmented layer absorbs light that has passed through. The axons of the ganglion cells run across the front of the retina and leave together as the optic nerve, which is why there is a blind spot.

Feature	Rods	Cones
Pigment	Rhodopsin, one kind	Iodopsin, in three kinds, one in each type of cone
Number in each human eye	About 120 million	About 6 million
Where found	Throughout the retina except the fovea; most numerous towards the edges	Concentrated in the fovea; few towards the edges
Connection to neurones	Many rods share one bipolar cell (convergence)	At the fovea, one cone to one bipolar cell
Sensitivity to light	Very high: work in dim light	Low: need bright light
Visual acuity	Low	High
Colour vision	No: give black, white and grey only	Yes

Why rods are sensitive. Rhodopsin is broken down by very little light, and the small generator potentials of many rods are added together in one bipolar cell (spatial summation). Together they reach the threshold of the bipolar cell even in dim light.

Why cones give sharp vision. Each fovea cone has its own line to the brain, so two points of light falling on neighbouring cones send impulses along two separate ganglion cells and are seen as two points. Signals from neighbouring rods are merged in one bipolar cell, so the brain cannot tell which rod was stimulated.

How a rod responds to light

In the dark, rhodopsin is intact. Sodium ion channels in the rod's membrane are open, Na⁺ flows in, and the rod is partly depolarised. It continually releases a neurotransmitter (glutamate) that inhibits its bipolar cell.
Light is absorbed by retinal, which changes shape from cis-retinal to trans-retinal. Rhodopsin breaks into opsin and retinal: it is bleached.
Through a cascade of reactions, this closes the Na⁺ channels. The rod becomes hyperpolarised (more negative inside) and releases less neurotransmitter.
Freed from inhibition, the bipolar cell depolarises; if threshold is reached, the ganglion cell sends impulses along the optic nerve.
Rhodopsin is rebuilt from opsin and retinal using ATP. This takes time: after moving from bright sunlight into a dark room, full dark adaptation takes about 20 to 30 minutes.

Because retinal is made from vitamin A, a diet lacking vitamin A slows the rebuilding of rhodopsin and causes night blindness.

Colour vision: the trichromatic theory

There are three kinds of cone, each with a different form of iodopsin. Their peak sensitivities are in the blue (about 420 nm), green (about 530 nm) and red (about 560 nm) parts of the spectrum. Any colour is seen as a particular ratio of stimulation of the three types: yellow light, for example, stimulates red- and green-sensitive cones about equally. Red–green colour blindness, which is inherited on the X chromosome, results when one type of cone pigment is missing or faulty.

Worked example 1: sensitivity and summation

Illustrative values: in dim light, each stimulated rod or cone changes the transmitter it releases enough to cause a 2 mV depolarisation of its bipolar cell. A bipolar cell must be depolarised by 5 mV to pass the signal on. Three rods share one bipolar cell; each cone has its own. Will either kind of cell signal in dim light?

Rods: three rods add their effects in one bipolar cell: $3 \times 2\,\mathrm{mV} = 6\,\mathrm{mV}$ , which is above 5 mV. The signal passes on: the light is seen.
Cones: each bipolar cell receives only $2\,\mathrm{mV}$ , below 5 mV. No signal: in dim light we cannot see with our cones, so we see no colour.

Interpretation: convergence buys sensitivity at the cost of detail. This is why colours fade at dusk and why the world looks grey by moonlight.

Worked example 2: the faint star

Explain why a faint star disappears when you look straight at it but reappears when you look slightly to one side.

Looking straight at the star focuses its image on the fovea, which contains only cones.
Cones have a high threshold and no convergence, so the dim light cannot bring their bipolar cells to threshold. The star "vanishes".
Looking to one side moves the image onto the retina around the fovea, where rods are plentiful.
Many rods converge on each bipolar cell and their generator potentials summate, so threshold is reached and the star is seen, although without colour and with less detail.

Warning

Common mistakes

"The lens does most of the focusing." The cornea does most of the bending; the lens fine-tunes the focus.
"For near objects the ciliary muscle relaxes." It contracts for near objects; the ligaments then go slack and the lens fattens.
"Light makes a rod depolarise." A rod hyperpolarises in light, unlike most receptors.
"Rods are for black-and-white, cones for colour", and nothing more. In an exam also explain sensitivity (convergence, summation) and acuity (one-to-one connections).

CrashCourse: "Vision: Crash Course Anatomy & Physiology #18". Watch for the layers of the retina and how rods and cones differ.

✦

Practice: the eye and the retina

Name the part of the eye that (a) does most of the refraction, (b) absorbs stray light, (c) contains only cones.
Describe the changes in the ciliary muscle, the suspensory ligaments and the lens when you look up from a book to a distant hill.
Explain why there is a blind spot in each eye.
Give three differences between rods and cones.
Explain what is meant by the "bleaching" of rhodopsin.
Explain why a person who walks from sunlight into a dark classroom at first cannot see, but can see after some minutes.
Explain why rods give greater sensitivity but lower visual acuity than cones.
Suggest why a deficiency of vitamin A affects vision in dim light first.

Show answersHide answers

(a) Cornea. (b) Choroid (with the pigmented layer of the retina). (c) Fovea.
The ciliary muscle relaxes; the suspensory ligaments are pulled taut; the lens is pulled thinner and refracts light less.
Where the axons of the ganglion cells leave the eye as the optic nerve there is no room for rods or cones, so light falling there is not detected.
Any three: rods contain rhodopsin, cones iodopsin; rods are most numerous towards the edges of the retina, cones at the fovea; several rods converge on one bipolar cell, fovea cones connect one-to-one; rods are more sensitive; cones give higher acuity; only cones give colour vision.
Light changes retinal from the cis to the trans form, so rhodopsin breaks down into opsin and retinal and can no longer absorb light until it is rebuilt.
Bright light has bleached most of the rhodopsin, and the cones cannot work in dim light. Over about 20 to 30 minutes, rhodopsin is resynthesised from opsin and retinal using ATP, and the rods become sensitive again (dark adaptation).
Many rods converge on one bipolar cell, so their small generator potentials summate and reach threshold in dim light (sensitivity); but the brain cannot tell which of the rods was stimulated, so detail is lost (low acuity). Fovea cones connect one-to-one, so each sends a separate signal (high acuity) but each needs bright light to reach threshold.
Retinal, part of rhodopsin, is made from vitamin A. Without enough, rhodopsin is not rebuilt, so the rods, which are needed in dim light, fail first: night blindness.

Key points

The cornea and lens focus light on the retina; accommodation changes the lens shape; the iris controls the pupil.
Light passes through ganglion and bipolar cells to reach the rods and cones; the optic nerve leaves at the blind spot.
Rods: rhodopsin, convergence, summation, high sensitivity, low acuity, no colour. Cones: three iodopsins, one-to-one connections at the fovea, high acuity, colour, need bright light.
Light bleaches rhodopsin, closing Na⁺ channels; the rod hyperpolarises and stops inhibiting its bipolar cell. Rhodopsin resynthesis gives dark adaptation.

Sources and credits

7.4.6: Vision, from Biology 2e by OpenStax (Mary Ann Clark, Matthew Douglas, Jung Choi), via LibreTexts Biology, licensed under CC BY 4.0. Used for the structure of the eye and retina, phototransduction and the trichromatic theory. Adapted by Ganzaa.
Photo: Fundus photograph of normal left eye, Mikael Häggström, CC0 1.0, via Wikimedia Commons.
Video: "Vision: Crash Course Anatomy & Physiology #18", CrashCourse, embedded from YouTube.

Figures 1 and 2, the tables, the worked examples (with illustrative values) and the practice questions were written by Ganzaa.

1Seeing a faint star

2Structure of the eye

3The retina: rods, cones and convergence

4How a rod responds to light

5Colour vision: the trichromatic theory

Seeing a faint star

Structure of the eye

The retina: rods, cones and convergence

How a rod responds to light

Colour vision: the trichromatic theory