From Photons to Spikes: the Retina
Light activates a chemical pathway in rods and cones that closes cGMP-gated channels and hyperpolarises the cell. Retinal circuits process these graded signals before ganglion cells send spikes through the optic nerve. Opposing centre and surround effects make many cells sensitive to local contrast, while retinal output also carries light-level information.
What a learner can do afterwards
- Describe phototransduction from photon absorption to a change in membrane potential
- Explain how opposing centre and surround effects can reduce responses to uniform illumination and favour local contrast
- Relate the distribution of rods and cones to the difference between night and daylight vision
1 · Read
In darkness, cGMP keeps channels open in rods and cones so sodium and calcium enter. Potassium also leaves. This inward current keeps the membrane relatively depolarised and supports steady glutamate release. Light closes many cGMP-gated channels. Inward current falls while potassium continues to leave, so the cell hyperpolarises and releases less glutamate. Rods and cones signal with graded voltage changes, not optic-nerve spikes.
In a rod, a photon changes the shape of retinal within the pigment rhodopsin. This activates rhodopsin, then the G protein transducin. Transducin activates phosphodiesterase (PDE), which breaks down cGMP. Lower cGMP closes cGMP-gated channels and reduces inward current. Blocking PDE activation prevents this pathway's usual light-driven fall in cGMP.
The cornea and lens focus light on the retina, as in the eye lesson. In a direct cone pathway, a cone signals to a bipolar cell, then a ganglion cell. Ganglion cells generate action potentials. Their axons form the optic nerve and carry these spikes to the brain. At the optic disc, where these axons leave the eye, there are no photoreceptors. This creates a blind spot.
These boxes show signal order, not the positions or shapes of retinal cells.
A receptive field is the retinal area where light can change a cell's response. ON and OFF bipolar cells have different glutamate receptors. When light increases only in the field's centre, the photoreceptors there release less glutamate. An ON bipolar cell depolarises in response. An OFF bipolar cell hyperpolarises. The same decrease in transmitter has opposite effects in these two pathways.
Many bipolar and ganglion cells have receptive fields with a centre and an opposing surround. In an ON-centre bipolar cell, centre light promotes depolarisation, while surround light opposes it. Lighting both regions can produce a smaller response than centre-only light. Cancellation need not be complete. This favours local contrast without removing all light-level information or stopping every ganglion cell from firing.
Horizontal cells collect signals from nearby photoreceptors. Their feedback to photoreceptors helps create the opposing surround effect in many bipolar cells. This lateral route changes how centre signals reach a bipolar cell. Ganglion-cell axons, rather than horizontal cells, carry spikes through the optic nerve.
This is a functional signal sequence. It does not draw the feedback circuit's anatomy.
Rods are more sensitive to dim light than cones and support vision after dark adaptation. The foveal centre has densely packed cones and no rods. Rods are abundant outside it. Looking slightly beside a faint star moves its image onto rod-containing retina, which can make it easier to see. In brighter light, cones support colour and sharp foveal detail. Peripheral vision is not better for every task.
Light lowers cGMP and reduces photoreceptor glutamate release. Retinal circuits shape contrast signals. Ganglion cells send spikes. Rods support dim vision; cones support colour and detail.
Lesson, questions, worked explanations and revised flashcards written for this dataset. Biology for AP Courses and Introduction to Behavioral Neuroscience were consulted for factual background.
Three readings and six flashcards written for this dataset. Biology for AP Courses and Introduction to Behavioral Neuroscience were consulted for factual background.
More to read
Following a rod's response to light
A rod can signal without firing an action potential. Its membrane voltage changes by degrees, and its glutamate release changes with that voltage. Ganglion cells generate the spikes carried through the optic nerve later in the pathway.
To understand the rod's response, compare its currents before and after illumination. In darkness, cGMP keeps channels open that admit sodium and calcium. Potassium also flows out. When light reduces the inward current while potassium continues to leave, the membrane becomes more negative. This is hyperpolarisation, and it reduces glutamate release.
The link between light and channel closure is a chemical relay. A photon changes retinal within rhodopsin. Activated rhodopsin activates transducin, a G protein, which activates PDE. This enzyme breaks down cGMP. With less cGMP available, many cGMP-gated channels close.
A block in the relay changes the prediction. If transducin cannot activate PDE, that pathway cannot cause its usual light-driven fall in cGMP. This does not mean that every process involving cGMP has stopped.
Comparing light across a receptive field
A cell's receptive field is the retinal area where illumination can affect its response. For many bipolar and ganglion cells, the field contains a centre and a surround with opposing effects. Compare a bright spot restricted to an ON-centre bipolar cell's centre with light covering both regions. The broader illumination adds an opposing surround effect, so it can produce less depolarisation.
Horizontal cells help make this spatial comparison possible. They collect signals from nearby photoreceptors and provide feedback to photoreceptors. That feedback contributes to the surround's influence on the signal reaching a bipolar cell. It does not act as a switch that always cancels the centre completely.
ON and OFF bipolar cells also respond differently to the same transmitter change. Increased centre light reduces glutamate release from central photoreceptors. Different glutamate receptors make this depolarise ON bipolar cells and hyperpolarise OFF bipolar cells.
These circuits make local contrast more prominent before signals leave the eye. They do not remove every signal about overall illumination. Ganglion-cell output can continue under uniform light, and retinal output includes light-level information as well as contrast.
Fovea, peripheral retina and optic disc
Two retinal locations lack different kinds of receptors. The foveal centre has no rods, but it contains densely packed cones. The optic disc has neither rods nor cones. At the disc, ganglion-cell axons leave the eye to form the optic nerve. The absence of photoreceptors creates a blind spot there.
The rod-free foveal centre is useful for a different task. In brighter light, its cones support colour and sharp detail. Looking directly at an object places its image on this central region.
Under very dim conditions, rods are more sensitive than cones. Rods are abundant outside the foveal centre. After adapting to darkness, a person may detect a faint star more easily by looking slightly beside it. The image then falls on retina containing rods instead of the rod-free centre.
This benefit concerns sensitivity to faint light. It does not mean that peripheral vision gives finer detail or is better for every viewing task. Receptor type, retinal location and illumination all affect what can be seen.
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