Guyton and Hall Physiology Chapter 51 - The Eye II. (Receptor and Neural Function of the Retina)

Structure of the Retina

• The retina is composed of several layers, including the pigmented layer, photoreceptor layer (rods and cones), outer limiting membrane, outer nuclear layer, outer plexiform layer, inner nuclear layer, inner plexiform layer, ganglion cell layer, stratum opticum, and inner limiting membrane.
• Light travels through these layers in reverse order, reaching the photoreceptor layer first.

Rhodopsin and the Visual Cycle

• Rods contain rhodopsin, a light-sensitive pigment.
• Rhodopsin consists of scotopsin (a protein) and retinal (a derivative of vitamin A).
• When light hits rhodopsin, retinal changes shape from cis to trans, causing rhodopsin to dissociate into scotopsin and all-trans retinal.
• This process initiates a cascade of events leading to hyperpolarization of the rod cell.
• The cycle is completed as all-trans retinal is converted back to 11-cis retinal, allowing it to bind to scotopsin and regenerate rhodopsin.

Light and Dark Adaptation

• When exposed to bright light, the amount of rhodopsin and retinal in both rods and cones decreases.
• Retinal is converted into vitamin A.
• This process reduces sensitivity to light.
• In dark conditions, the cycle reverses, restoring sensitivity.

Cone Function

• Cones are responsible for color vision.
• There are three types of cones, each sensitive to a specific wavelength of light: blue (445 nm), green (535 nm), and red (570 nm).

Lateral Inhibition

• Lateral inhibition enhances contrast detection and sharpness of visual perception.
• It involves the interaction between neighboring neurons in the retina, where stimulated neurons inhibit the activity of nearby neurons.

Ganglion Cells

• There are two main types of ganglion cells: M cells and P cells.
• M cells are sensitive to movement and low contrast and are insensitive to color.
• P cells are sensitive to detail, color, and light intensity.
• A third type, photosensitive retinal ganglion cells (pRGCs), contain melanopsin and send signals to non-visual areas, such as the suprachiasmatic nucleus.

Color Contrast Mechanisms

• Color contrast mechanisms involve pathways in the retina that either excite or inhibit ganglion cells depending on the color of light.
• This provides a basis for color differentiation in the brain.

Layers of the Retina

• The retina has multiple layers including a pigmented layer, photoreceptor layer, outer limiting membrane, outer nuclear layer, outer plexiform layer, inner nuclear layer, inner plexiform layer, ganglion cell layer, stratum opticum, and inner limiting membrane.
• Distal parts of the retina contain photoreceptor cells (rods and cones), bipolar, horizontal, and amacrine cells.
• Proximal parts of the retina contain ganglion cells that send signals to the optic nerve.
• Light travels through these layers, starting from the ganglion cell layer to the pigmented layer, and then back to the ganglion cell layer.

The Pigment layer

• The pigment layer contains melanin, which prevents light from reflecting within the eyeball.
• Melanin absorbs light to prevent it from scattering and ensure clear vision.

Photoreceptor cells

• Rods and cones are photoreceptor cells.
• Rods contain rhodopsin, which is a light-sensitive pigment.
• Cones contain three different color pigments: a blue-sensitive pigment, a green-sensitive pigment, and a red-sensitive pigment.

Rhodopsin and Color Perception

• Rhodopsin decomposes on exposure to light, initiating a signal.
• The breakdown of rhodopsin excites nerve fibers leading from the eye.
• Different types of cones have different wavelengths of peak light sensitivity, allowing for color perception.
• The peak absorbencies of cone pigments are at 445 nanometers (blue), 535 nanometers (green), and 570 nanometers (red).
• Rhodopsin has a peak absorption at 505 nanometers.

Retinal Sensitivity Regulation

• The eye adapts to light and dark conditions by adjusting retinal sensitivity.
• Light Adaptation is a process that decreases retinal sensitivity to prevent overstimulation in bright light.
• Dark Adaptation increases retinal sensitivity to allow for better vision in low light.

Mechanisms of Retinal Adaptation

• Changes in the concentrations of rhodopsin and color pigments contribute to light and dark adaptation.
• Pupillary size changes rapidly, adjusting light input to the eye.
• Neural adaptation, involving neurons in the visual pathway, also contributes.

Detachment of the Retina

• Detachment of the retina is a serious condition that can occur due to injury, fluid, or blood collection between the retina and the pigmented epithelium.
• It can also be caused by contraction of collagenous fibrils in the vitreous humor, pulling on the retina.
• Detached retinas can remain functional for several days but can become permanently damaged if not repaired.

Retina Structure

• The retina is the light-sensitive layer of the eye, containing rods and cones.
• Rods are responsible for dim light vision, while cones are responsible for color vision.
• The retina is composed of 10 layers with the rods and cones located on the outer edge.
• Light enters the retina from the inside, passing through several layers before reaching the rods and cones.

Vitamin A Deficiency and Night Blindness

• Vitamin A is crucial for the production of retinal and rhodopsin, which are essential for vision.
• Severe vitamin A deficiency can lead to night blindness because the amount of light available at night is insufficient for adequate vision.
• Night blindness can be reversed in less than an hour by intravenous injection of vitamin A.

Rod Excitation and Hyperpolarization

• Exposure to light causes the rod receptor to hyperpolarize, meaning an increase in negativity of the membrane potential.
• Hyperpolarization is caused by the decomposition of rhodopsin, which decreases the rod cell membrane's sodium conductance.
• This sodium conductance decrease leads to a decrease in the inward flow of sodium ions, resulting in hyperpolarization.

Dark Adaptation

• The eye has a dynamic range of light sensitivity of 500,000 to 1 million times, adapting to changes in illumination.
• This adaptation ensures that the retina can detect both dark and light areas in the image.
• Pupillary size changes contribute to rapid adaptation, while neural adaptation involves the neurons in the visual chain adjusting their signals.

Color Vision

• The three types of cones in humans are sensitive to different wavelengths of light, corresponding to red, green, and blue.
• These cones contain pigments that absorb specific wavelengths, leading to the perception of color.
• Mixing appropriate combinations of red, green, and blue light can recreate almost all gradations of color.
• The spectral sensitivities of the cones explain most of the phenomena of color vision.

Explore the intricate structure of the retina, including its various layers and the role of rhodopsin in the visual cycle. Understand how light interacts with these components and the processes involved in light and dark adaptation. This quiz provides a comprehensive overview of retinal anatomy and function.