Visão aula 3 Transdução da luz nos fotorreceptores

Summary

In this lesson, Professor Aldo Lucion explains the mechanisms of light stimulus transmission in visual receptor cells, focusing on the role of photopigments, particularly rhodopsin. He describes how light alters rhodopsin’s structure, leading to a reduction in cyclic GMP levels, which ultimately causes the hyperpolarization of photoreceptor membranes, contrasting with the typical depolarization seen in other sensory receptors. The lesson covers the intricate processes of signal transduction, neurotransmitter release, and the essential role of calcium in resetting the system, providing a comprehensive overview of the cellular response to light in the retina.

Takeaways

• 😀 The process of visual signal transduction begins when light interacts with photoreceptor cells in the retina, specifically the rods.
• 😀 Rods contain the light-sensitive pigment rhodopsin, which plays a key role in converting light into an electrical signal.
• 😀 In the dark, rhodopsin is inactive, and ion channels in the photoreceptor cell membrane remain open, allowing sodium ions to enter and depolarize the cell.
• 😀 When light strikes the rod cells, rhodopsin undergoes a structural change from its cis form to the trans form, activating the pigment.
• 😀 This activation of rhodopsin leads to a reduction in intracellular cGMP levels, causing the ion channels to close and preventing the influx of sodium ions.
• 😀 As a result, the membrane of the photoreceptor becomes hyperpolarized, which is a key step in processing visual information.
• 😀 The hyperpolarization reduces the release of neurotransmitters (such as glutamate) from the rod cells, signaling the presence of light to the bipolar cells.
• 😀 This mechanism of hyperpolarization in response to light is unique compared to other sensory systems, where depolarization usually occurs.
• 😀 Calcium ions are crucial in restoring rhodopsin to its inactive state after the light stimulus has passed, preparing the system for the next visual stimulus.
• 😀 The ability of calcium ions to regulate the synthesis of cGMP and enhance the sensitivity of ion channels is essential for the ongoing function of the photoreceptor cells.

Q & A

What is rhodopsin and why is it important in visual signal transduction?

Rhodopsin is a photopigment found in the retina, particularly in the rods. It consists of a protein called opsin and retinal, a derivative of vitamin A. Rhodopsin is essential for converting light into an electrical signal, enabling the perception of visual stimuli.

What happens to rhodopsin when light hits the retina?

When light hits rhodopsin, retinal undergoes a structural change from its cis form to its trans form. This activation of rhodopsin triggers a biochemical cascade that ultimately leads to the reduction of cyclic GMP levels in the photoreceptor cells.

How does cyclic GMP affect ion channels in the dark?

In the dark, cyclic GMP maintains ion channels open in the photoreceptor membrane. These channels allow sodium ions to flow into the cell, causing the membrane to depolarize.

What happens to the photoreceptor membrane when light is absorbed by rhodopsin?

When light activates rhodopsin, cyclic GMP levels decrease, causing the ion channels to close. As a result, sodium ions are no longer able to enter the cell, which causes the photoreceptor membrane to hyperpolarize.

Why is hyperpolarization in photoreceptor cells counterintuitive?

Hyperpolarization is counterintuitive because most sensory systems, such as those for hearing or touch, involve depolarization in response to stimuli. However, in the visual system, light causes hyperpolarization of the receptor membrane.

How does the change in membrane potential affect neurotransmitter release?

When the photoreceptor membrane hyperpolarizes in response to light, it reduces the release of neurotransmitters, specifically glutamate, from the receptor cells. In contrast, in the dark, there is a tonic release of neurotransmitters.

What is the role of calcium in the recovery process of rhodopsin?

Calcium plays a critical role in the recovery process by helping to recombine opsin with retinal, returning rhodopsin to its inactive state. It also stimulates the synthesis of cyclic GMP and increases the sensitivity of ion channels to cyclic GMP.

What happens to the neurotransmitter release in the dark compared to light?

In the dark, there is continuous neurotransmitter release due to the depolarized state of the photoreceptor membrane. In contrast, exposure to light causes hyperpolarization, which reduces neurotransmitter release.

How does the conversion of retinal from cis to trans form impact the cell?

The conversion of retinal from its cis form to the trans form activates rhodopsin. This activation leads to a reduction in cyclic GMP levels, which then causes the ion channels in the photoreceptor membrane to close, resulting in hyperpolarization of the membrane.

What is the significance of cyclic GMP in the function of photoreceptor cells?

Cyclic GMP is crucial for keeping sodium channels open in photoreceptor cells. In the dark, high cyclic GMP levels allow sodium to enter the cell, causing depolarization. Light reduces cyclic GMP, leading to the closure of these channels and hyperpolarization of the cell membrane.