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Detection of light by mammalsEdexcel A-Level Biology B: Revision notes

Section 1

Structure of the human retina

The retina is the light-sensitive layer at the back of the eye. Light passes through the ganglion cell layer and bipolar neurone layer before reaching the photoreceptors (rods and cones) at the back.

Signals pass photoreceptor, then bipolar neurone, then ganglion cell. Ganglion cell axons form the optic nerve.

Special regions:

  • Fovea: a small pit directly behind the lens with the highest density of cones and no rods
  • Blind spot: where the optic nerve leaves; no photoreceptors, so no vision

Behind the photoreceptors is a pigmented layer (choroid) that absorbs stray light.

Key termsretinaphotoreceptorfoveablind spotoptic nerve
Exam tip

Remember light passes through the ganglion and bipolar layers first; the signal travels in the opposite direction, photoreceptor to bipolar to ganglion.

Section 2

Rhodopsin and generating action potentials

Rhodopsin is the light-sensitive pigment in rods, made of opsin (a protein) and retinal. It is in the membranes of discs in the outer segment.

  1. In darkness, Na⁺ channels in the outer segment are open, so the rod is partly depolarised and releases an inhibitory transmitter onto the bipolar neurone
  2. Light is absorbed by rhodopsin, which breaks down (bleaches)
  3. The Na⁺ channels close, so the rod hyperpolarises
  4. Less inhibitory transmitter is released, so the bipolar neurone depolarises
  5. The bipolar neurone stimulates the ganglion cell, and if threshold is reached an action potential passes along the optic nerve

Rhodopsin is regenerated in the dark.

Key termsrhodopsinhyperpolarisationbleaching
Common mistake

Light does not depolarise the rod. Light hyperpolarises it. The ganglion cell is stimulated indirectly because the inhibitory transmitter is reduced.

Section 3

Rods and cones compared

Rods: contain rhodopsin; very sensitive, work in dim light; many rods synapse with one bipolar neurone (retinal convergence), so generator potentials sum and reach threshold; this gives low acuity; one type of pigment, so no colour vision.

Cones: need brighter light; three types with different pigments (red-, green- and blue-sensitive), giving colour vision; in the fovea each cone has its own bipolar neurone, giving high acuity.

Key termsretinal convergencevisual acuitycolour vision

Section 4

Distribution and light intensity

  • Fovea: densely packed cones, no rods: best detail and colour in bright light
  • Periphery: mostly rods: detects dim light and movement, with low acuity

This is why you see a faint star better by looking slightly to one side: the light then falls on rods in the periphery, not on the cones of the fovea.

The combination of rods and cones, in different regions, lets the eye work across a very wide range of light intensities.

Key termsfoveaperipheral retina
Exam tip

In an explanation of dim light vision, link three things: rhodopsin sensitivity, convergence and summation, and the cost in low acuity.

Must know

  • Retina: photoreceptors, bipolar neurones, ganglion cells, optic nerve; blind spot and fovea
  • Light breaks down rhodopsin, Na⁺ channels close, rod hyperpolarises, less inhibitory transmitter, ganglion cell action potential
  • Rods: sensitive, convergence, low acuity, periphery
  • Cones: colour, high acuity, fovea, bright light

That's the notes covered.

Carry on to the next subtopic.

Exam questions on Detection of light by mammals

  1. A student studies a section of a human retina. Light entering the eye passes through a layer of ganglion cells and a layer of bipolar neurones before reaching the photoreceptor cells at the back of the retina. The axons of the ganglion cells leave the eye together at one point as the optic nerve, and no photoreceptor cells are present at that point.
    Explain why a person cannot detect light that falls on the point where the optic nerve leaves the retina.2 marks
  2. Rod cells contain the pigment rhodopsin in the membranes of the discs in their outer segments. In darkness, Na⁺ channels in the membrane of a rod are open, so the rod is partly depolarised and continuously releases an inhibitory transmitter onto a bipolar neurone. When rhodopsin absorbs light it breaks down (bleaches), the Na⁺ channels close and the rod becomes hyperpolarised.
    Explain how the breakdown of rhodopsin leads to an action potential in a ganglion cell.2 marks
  3. A person walks from bright sunlight into a dim room. After a while she notices that she can see a faint light on the wall better if she looks slightly to one side of it than if she looks straight at it. In the fovea of the human retina there are about 150 000 cones per mm² and no rods. At 20° from the fovea there are about 4 000 cones per mm² and 150 000 rods per mm².
    Explain why the faint light is seen better when she looks slightly to one side of it.3 marks
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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).