The Eye: Retina Function and Structure

Rhodopsin is a light-sensitive pigment found in rod cells of the retina. It's responsible for vision in low light conditions.

When light hits rhodopsin, it breaks down into retinal and opsin. This process is called bleaching.

Retinal is a light-absorbing molecule that changes shape when it absorbs light. Opsin is a protein that helps retinal bind to it.

Metarhodopsin II is an activated form of rhodopsin that signals the brain to perceive light.

Rhodopsin activation leads to a decrease in sodium permeability in the rod cell membrane, resulting in hyperpolarization.

When rhodopsin breaks down, retinal isomerase converts all-trans retinal back into 11-cis retinal, allowing it to re-combine with opsin and reform rhodopsin.

The inner segment of the rod cell pumps sodium ions out and potassium ions in, creating a negative potential inside the cell.

The outer segment of the rod cell, where the discs containing rhodopsin are located, is specialized for light reception.

The state of a neuron when it is more negative inside compared to the outside, due to the movement of ions.

A molecule that acts as a messenger inside cells, controlling the opening and closing of sodium channels in photoreceptor cells.

A change in the electrical potential of a photoreceptor cell (rod or cone) in response to light stimulation.

The process by which photoreceptor cells in the retina become more negative as light intensity increases.

The amount of time it takes for the photoreceptor to reach its peak hyperpolarization after exposure to light.

The relationship between the intensity of light striking the photoreceptor and the magnitude of its hyperpolarization.

Specialized cells in the retina that are responsible for detecting light and converting it into electrical signals.

A pump in photoreceptor cells that maintains the concentration of sodium and potassium ions, keeping the cell in a stable state.

Hydrolysis of cGMP by phosphodiesterase removes the splinting effect, allowing sodium channels to close and reducing sodium ion flow.

The closure of hundreds of sodium channels per activated rhodopsin molecule results in a significant decrease in sodium ion flow, which excites the rod cell, causing it to signal to the brain.

Rhodopsin kinase inactivates activated rhodopsin (metarhodopsin II), reversing the cascade and restoring the rod cell to its original resting state with open sodium channels.

A single photon of light can trigger a cascade of events that amplifies the signal and leads to the movement of millions of sodium ions. This mechanism explains the high sensitivity of rods in low-light conditions.

Cones are less sensitive to light than rods, requiring a higher intensity of light for efficient signaling. This allows for color vision in brighter environments.

The three types of cones (red, green, blue) have peak absorbencies at different wavelengths (445, 535, and 570nm respectively), which correlates with their peak light sensitivity. This allows the brain to differentiate colors based on the relative activation of these cones.

Exposure to bright light depletes the photochemicals in rods and cones, and retinal is converted into vitamin A. These changes reduce sensitivity, but adjust the eye to brighter conditions.

The sensitivity of photoreceptor cells can be automatically regulated by changes in light levels. This allows the eye to adapt to a wide range of light intensities, from very dim to very bright.

The ability to distinguish different colors is based on how intensely light of various wavelengths stimulates the three different types of cones (blue, green, and red).

A genetic disorder that affects color perception. It prevents individuals from distinguishing certain colors, most commonly red and green.

The gene responsible for the production of a specific type of cone (like a red cone) is located on the X chromosome.

Females carry two X chromosomes, so even if one X chromosome has a faulty gene, the other X chromosome can usually provide a normal gene for the corresponding cone.

Males have only one X chromosome, so if this chromosome contains a faulty gene for a certain cone, they cannot see the color corresponding to that cone.

The type of light that stimulates each cone to its greatest extent, meaning it absorbs the most of that specific light wavelength.

The amount of light that a cone absorbs, expressed as a percentage of its maximum absorption.

The measure of how much each cone is stimulated by a specific wavelength of light, expressed as a percentage of its maximum stimulation.

Electrotonic conduction allows graded signals, meaning the signal strength can vary based on the intensity of the stimulus. This is different from action potentials, which are all-or-none events.

Lateral inhibition enhances contrast by inhibiting neighboring neurons, making the activated area stand out more. It works by horizontal cells connecting photoreceptors and bipolar cells.

Horizontal cells connect laterally between photoreceptors and bipolar cells, always sending inhibitory signals.

The horizontal cell's inhibitory signal enhances contrast by suppressing the activity of neighboring neurons. This allows the brain to perceive edges and details more clearly.

The two types of bipolar cells respond differently to photoreceptor stimulation. One depolarizes, the other hyperpolarizes. This difference arises from the direct or indirect reception through horizontal cells, influencing the electrical signal.

The difference in bipolar cell responses allows for both positive and negative signals to be transmitted to the brain. This creates a richer representation of visual information, adding more detail to what we see.

The central area where light strikes is activated, while the surrounding area is inhibited by lateral inhibition, enhancing contrast and creating a more defined image.

Lateral inhibition is crucial for enhancing visual contrast, allowing us to perceive edges, patterns and details more sharply. This is similar to how sharpening enhances image clarity.

Amacrine cells are a type of interneuron in the retina that analyze visual signals before they are sent to the brain. They play a role in various functions, including detecting movement, responding to changes in light, and contributing to color vision.

They are involved in the initial processing of visual information, aiding in the detection of movement, changes in illumination, and contributing to the perception of color.

X cells are a type of retinal ganglion cell with small dendritic fields. They transmit precise details of visual images, providing information about the specific location of visual input.

Y cells are large retinal ganglion cells with broad dendritic fields, enabling them to receive input from a wider area of the retina. They transmit signals quickly, alerting the brain to new visual events.

These cells respond to rapid changes in visual images, signaling the brain almost instantaneously when a new visual event occurs. They provide information about the general location of the event to guide eye movement.

The optic nerve is formed by the axons of ganglion cells, transmitting visual information from the retina to the brain. It's a bundle of nerve fibers carrying signals from the eye.

Each ganglion cell receives input from an average of 60 rods and 2 cones. This convergence means that information from a large number of photoreceptor cells is combined in one ganglion cell.

Each retina contains 100 million rods, 3 million cones, and 1.6 million ganglion cells. Many photoreceptor cells converge on a single ganglion cell, effectively summarizing the input