cone photoresponse recovery

Definition

Target type: biologicalprocess

The processes required for a cone photoreceptor to recover, following light activation, so that it can respond to a subsequent light stimulus. Cone recovery requires the shutoff of active participants in the phototransduction cascade, including the visual pigment and downstream signal transducers. [GOC:gap, PMID:16039565, PMID:22802362]

Cone photoreceptor recovery is a complex process that restores the light sensitivity of photoreceptors after they have been exposed to light. It involves multiple steps, including:

1. **Photoisomerization of Retinal:** When light strikes a cone photoreceptor, it causes the retinal molecule within the rhodopsin protein to isomerize from its *cis* form to its *trans* form. This isomerization activates rhodopsin, initiating the visual cascade.

2. **Activation of Transducin:** Activated rhodopsin then binds to and activates a protein called transducin. This activation involves the exchange of GDP for GTP on the alpha subunit of transducin.

3. **Activation of Phosphodiesterase:** Activated transducin in turn activates a phosphodiesterase enzyme, which hydrolyzes cyclic GMP (cGMP) to GMP.

4. **Closure of Sodium Channels:** cGMP is normally bound to sodium channels in the plasma membrane of cone photoreceptors, keeping them open and allowing sodium ions to flow into the cell, thus maintaining the resting membrane potential. The decrease in cGMP concentration upon light stimulation causes these sodium channels to close, reducing sodium influx and hyperpolarizing the cell.

5. **Signal Amplification:** The visual cascade involves significant signal amplification. One molecule of rhodopsin can activate multiple molecules of transducin, each of which activates multiple molecules of phosphodiesterase. This amplification allows for a high sensitivity to light.

6. **Recovery:** Recovery from the activated state involves several mechanisms:
* **Rhodopsin Deactivation:** Rhodopsin is dephosphorylated and inactivated by rhodopsin kinase. This dephosphorylation prevents the rhodopsin from continuously activating transducin.
* **Transducin Deactivation:** The alpha subunit of transducin hydrolyzes GTP to GDP, becoming inactive and dissociating from the phosphodiesterase.
* **cGMP Regeneration:** Guanylate cyclase, an enzyme that synthesizes cGMP, is activated by the decrease in cGMP concentration. This increases cGMP levels, eventually reopening the sodium channels and restoring the resting membrane potential.
* **Retinal Regeneration:** The *trans* form of retinal is converted back to its *cis* form by retinal isomerase, enabling rhodopsin to be reactivated and the process to begin again.

7. **Adaptation:** The recovery process also involves adaptation to different levels of light. In bright light, the cones become less sensitive, while in dim light they become more sensitive. This adaptation is achieved through a variety of mechanisms, including changes in the rate of rhodopsin phosphorylation and dephosphorylation, the activity of guanylate cyclase, and the availability of cGMP.

Cone photoresponse recovery is a critical process for maintaining the visual system's ability to detect and respond to changes in light intensity. It ensures that the photoreceptors can rapidly return to their resting state, allowing them to respond to subsequent light stimuli.'
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