All subjects · Biology · WBI15 (Unit 5)

How rod cells detect light

Rod cells in the retina respond to dim light. Unusually, they are depolarised in the dark and hyperpolarised in the light.

In the dark

Rod cells contain the pigment rhodopsin, made of the protein opsin joined to retinal (in its cis form).

In the dark, Na+ channels in the rod's outer segment are open, so Na+ flows in (and is pumped out of the inner segment). The rod is depolarised, at about -40 mV.

The depolarised rod continually releases the neurotransmitter glutamate, which inhibits the bipolar neurone it synapses with. So no impulses are sent to the brain.

In the light

1. Light is absorbed by rhodopsin. Retinal changes shape from the cis to the trans form.

2. Rhodopsin breaks down into opsin and trans-retinal (bleaching).

3. Opsin triggers a series of reactions that close the Na+ channels.

4. Na+ is still pumped out, so the inside becomes more negative: the rod is hyperpolarised (about -70 mV).

5. The rod stops releasing glutamate, so the bipolar neurone is no longer inhibited. It depolarises, and an action potential is generated in a ganglion cell and sent along the optic nerve to the brain.

Rhodopsin is re-formed from opsin and retinal using ATP. This takes time, which is why your eyes take several minutes to adjust to the dark.

Sensitivity

Several rods synapse with one bipolar neurone, so small amounts of light on many rods add together to trigger a response. This makes rods very sensitive in dim light, but gives low visual acuity (detail). Rods contain only one pigment, so they do not detect colour.

Worked example

Explain how light falling on a rod cell leads to an impulse in the optic nerve.

  1. Light converts cis-retinal to trans-retinal; rhodopsin breaks down.
  2. Opsin causes Na+ channels to close, so the rod hyperpolarises.
  3. Glutamate release stops, so the bipolar cell is no longer inhibited and depolarises.
  4. An action potential is set up in the ganglion cell and travels along the optic nerve.

Answer: Light hyperpolarises the rod, which removes inhibition of the bipolar cell, leading to an action potential.

Key idea: Dark: Na+ channels open, rod depolarised (about -40 mV), releases inhibitory glutamate. Light: cis-retinal → trans-retinal, rhodopsin bleaches, Na+ channels close, rod hyperpolarises, glutamate stops, bipolar cell depolarises, optic nerve impulse. Many rods per bipolar cell: sensitive but low acuity.

Check yourself

  1. A rod cell changes from -40 mV in the dark to -70 mV in bright light. By how many mV has it hyperpolarised?
    Show answer

    30 mV. -40 - (-70) = 30 mV more negative.

  2. What happens to retinal when it absorbs light?
    • It joins opsin to make rhodopsin
    • It opens Na+ channels
    • It is converted to glutamate
    • It changes from the cis form to the trans form
    Show answer

    It changes from the cis form to the trans form. The shape change triggers rhodopsin breakdown.

  3. In the dark, what is the state of the Na+ channels in a rod cell's outer segment?
    • Closed, so the rod is hyperpolarised
    • Open, so Na+ flows in and the rod is depolarised
    • Blocked by glutamate
    • Absent
    Show answer

    Open, so Na+ flows in and the rod is depolarised. The dark current depolarises the rod.

  4. What does a rod cell do in the dark that prevents impulses being sent to the brain?
    • It stops releasing all chemicals
    • It releases acetylcholine onto the optic nerve
    • It releases glutamate, which inhibits the bipolar neurone
    • It absorbs all the light
    Show answer

    It releases glutamate, which inhibits the bipolar neurone. Glutamate inhibits the bipolar cell.

  5. What happens to a rod cell's membrane potential in light?
    • It becomes hyperpolarised
    • It becomes depolarised
    • It reaches +40 mV
    • It does not change
    Show answer

    It becomes hyperpolarised. Na+ channels close, so the inside becomes more negative.

  6. Why are rods very sensitive in dim light?
    • Each rod has its own neurone to the brain
    • Rods contain three pigments
    • Several rods connect to one bipolar neurone, so their effects add together
    • Rods are found only at the fovea
    Show answer

    Several rods connect to one bipolar neurone, so their effects add together. Summation from many rods triggers a response.

  7. Why does it take several minutes to see well after entering a dark room?
    • Rhodopsin must be re-formed from opsin and retinal
    • Glutamate must be made from scratch
    • The pupil cannot dilate
    • Rods need to divide
    Show answer

    Rhodopsin must be re-formed from opsin and retinal. Bleached rhodopsin takes time to regenerate.

Practise with instant feedback

Sources

Last updated 7 November 2026. Spotted a mistake? Tell us.