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Rods, rhodopsin and the optic neurone

Coordination, Response and Gene Technology · Impulses, synapses and receptors · note 6 of 7

Spec 8.8
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Rods, rhodopsin and the optic neuroneSpec 8.8

In short

Rod cells in the retina contain rhodopsin, made of opsin and retinal. In the dark, cation channels are open, Na⁺ enters and the rod is depolarised. Light splits rhodopsin, the channels close, the rod hyperpolarises and releases less transmitter, so a bipolar neurone triggers action potentials in the optic neurone.

Rod cells in the retina detect light, particularly in dim conditions. Their outer segment contains the pigment rhodopsin, made of a protein, opsin, bound to retinal, a derivative of vitamin A.

In the retina, the rods synapse with bipolar neurones, which in turn synapse with the optic neurones whose axons form the optic nerve. The key point is that light causes hyperpolarisation, not depolarisation, of the rod cell.

Section of the retina showing light passing through optic neurone and bipolar neurone to a rod cell with its outer segment, plus panels for a rod in the dark and in the light. (opens full size in a new tab)
Light passes through the optic neurone and bipolar neurone to the rod cell; in the light the rod hyperpolarises.
  1. In the dark: cation channels in the rod membrane are open, so Na⁺ continuously enters. The rod is partly depolarised (about –40 mV) and releases an inhibitory transmitter (glutamate) onto the bipolar neurone, so the bipolar neurone is hyperpolarised and does not pass a signal on.
  2. In the light: absorbing light changes retinal from its cis form to its trans form, so rhodopsin breaks down into opsin and retinal (bleaching). Opsin activates a cascade of reactions in the membrane (a fall in cyclic GMP, which normally keeps the channels open), so the cation channels close.
  3. Na⁺ can no longer enter, but the sodium–potassium pump keeps removing it and K⁺ continues to leave, so the rod becomes more negative: hyperpolarisation (about –70 mV).
  4. The hyperpolarised rod releases less inhibitory transmitter, so the bipolar neurone is depolarised and releases transmitter onto an optic neurone.
  5. The optic neurone reaches threshold and action potentials pass along the optic nerve to the visual cortex of the brain.

Rods are very sensitive because one photon can close many channels. Several rods synapse with one bipolar neurone, which makes rods sensitive in dim light but gives low visual acuity, and they do not distinguish colours.

Common mistake:

The rod cell itself does not fire action potentials. The action potentials are formed in the optic neurone, after the bipolar neurone is depolarised.

Written and checked against the Edexcel International A Level Biology (YBI11) specification · Updated October 2026

Frequently asked questions

How does an action potential work?

An action potential is a brief reversal of the potential across an axon membrane. Voltage-gated sodium channels open at threshold, so Na⁺ enters and the membrane depolarises. The sodium channels then close and potassium channels open, so K⁺ leaves and the membrane repolarises. Local circuits depolarise the next region, so the impulse travels.

Why do myelinated axons conduct impulses faster?

Myelin insulates the axon so ions cross the membrane only at the nodes of Ranvier. The action potential jumps from node to node by local circuits, which is called saltatory conduction. Less of the membrane has to depolarise, so conduction reaches about 100 m s⁻¹, much faster than the 0.5 to 2 m s⁻¹ in an unmyelinated axon.

How does acetylcholine transmit a signal across a synapse?

An impulse opens calcium channels in the presynaptic knob, and calcium ions make vesicles release acetylcholine by exocytosis. Acetylcholine diffuses across the cleft and binds to receptors on the postsynaptic membrane, opening sodium channels. Sodium ions enter and depolarise it. Acetylcholinesterase then breaks the acetylcholine down.

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