The Retina
Light rays from an image are focused onto the retina by glasses or contacts and the anterior structures of the eye. Light rays are converted from a physical wave into a chemical and electrical message to be interpreted by the brain. This process is called phototransduction. In the sensory retina, there five main cell types and nine layers involved in this process.
The sensory retina is an optically clear structure. For phototransduction to begin, an image must pass through the anterior blood supply and every layer of the sensory retina until the image finally reaches the photoreceptor layer. The photoreceptor layer is composed of rods and cones. There are roughly 120 million rods in the adult retina. These cells allow us to see the difference between light and dark. Rods are utilized for night vision and low lighting conditions.
There are 6 to 7 million cones in the adult eye. Cones are only utilized for day or brighter conditions. These cells are specialize for color detection. There are three types of cones present in the retina: blue sensitive cones, which are sensitive to shorter wavelengths of light (420 to 440nm), green sensitive cones, which are stimulated by more intermediate wavelengths of light (534 to 545nm) and red sensitive cones, which are sensitive to longer wavelengths of light (564 to 580nm). The brain analyzes the number and intensity of cone cell responses to identify the colors of an image.
Rods and cones differ slightly in the phototransduction process. To read about these differences check out the biochemistry of the visual process.
When light strikes a photoreceptor cell, the process of phototransduction begins. The light ray changes the opsin of the rod or cone from 11-cis retinal to all-trans retinal. This structural change closes the cation channels in the photoreceptor cell. The result is hyperpolarization (creates a negative charge) of the photoreceptor. Hyperpolarized photoreceptor cells send less glutamate to bipolar cells.
The bipolar cells respond to glutamate in two different ways. The ionotropic glutamate receptors depolarize the bipolar cells, so less glutamate hyperpolarizes these cells. Metabrotropic receptors do the opposite, hyperpolarizing the bipolar cell with higher glutamate concentrations. These two receptors affect bipolar cells differently to allow more refinement in stimulation, which is used in color, contrast and edge detection.
Photoreceptor cells also affect the horizontal cells. A grouping of photoreceptors provide input to a single horizontal cell. When the photoreceptors hyperpolarize the horizontal cell also hyperpolarizes. This causes the horizontal cell to release less GABA. A negative feedback loop is initiated as less GABA depolarizes the entire group of photoreceptors. The phototeceptors firing from a light stimulus are unaffected by GABA depolarization.
This utlizes a center surround system to allow our visual system to sharpen edges. The photoreceptors stimulated by light continue firing (hyperpolarized) to the horizontal and bipolar cells, while the surrounding photoreceptors that aren't stimulated are actually further depolarized by the inhibitory effect of GABA from the horizontal cells.
Amacrine cells are the next step in propagating the visual message along the visual pathway. There are 40 types of amacrine cells. These are very specialized cells that provide a link from horizontal and bipolar cells to the ganglion cells. Most amacrine cells use the neurotransmitters GABA and glycine and have an inhibitory (depolarizing) effect on the horizontal and bipolar cells.
The ganglion cells are the last stop for the visual message in the eye, as the ganglion cell nerve fibers converge to form the optic nerve. Ganglion cells significantly condense visual information. The visual process starts with over 120 million photoreceptors and ends with just over one million ganglion cells. The two major types of ganglion cells are midget and parasol ganglion cells.
The midget ganglion cells detect color and detail. These cells project to the parvocellular system portion of the lateral geniculate nucleus (LGN). The parasol ganglion cells detect light and movement and project to the magnocellular portion of the LGN. Besides carrying visual information, there are photosensitive ganglion cells that provide input for circadian rhythms and pupil size.
With all of this complicated processing taking place it's no surprise the retina is the most metabolically demanding tissue in the body by weight. A lack of adequate blood supply is the link between microvascular conditions (hypertension and diabetes) to eye problems. To meet the demands of the retina, the ophthalmic artery provides an anterior blood supply via the central retinal artery and a posterior blood supply via the choroid.
The central retinal artery has three divisions. The first, is the radial peripapillary capillaries that supply the nerve fiber layer. Next, the inner capillaries supply the ganglion cell layer. Outer capillaries are the third division, which supply the inner plexiform layer. The central retinal artery supplies 20 to 30% of the retina.
For the other 70 to 80% of the retina (along with iris and ciliary body), the choroid supplies vascular support. The choroid is a mixture of blood, blood vessels and connective tissue. Next to the choroid lies the choriocapillaris. The choriocapillaris is a vascular extension of the choroid, but possesses a fine meshwork of connective tissue that allows easy transfer of metabolites in and out of the retina.
Bruch's membrane acts as a filter between the blood supply of the choriocapillaris and the retinal pigmented epithelium (RPE) of the retina. The RPE is a supportive layer to the photoreceptors and posterior half of the sensory retina.
The RPE works to prevent sun damage to the retina, maintain the structure of the choroid, works as a blood barrier, is a source for immune response in the retina, maintains pH balance of the retina, conducts phagocytosis to remove metabolic debris from the retina. With macular degeneration, Bruch's membrane thickens as it accumulates calcium and lipids (waste products of metabolism). Also, the RPE supplies the enzymes and vitamin A necessary to convert all-trans retinol back into 11-cis retinal to recharge the photoreceptors and restart the process of phototransduction.
Summary
Photoreceptors consists of cones (daylight, color, detail) and rods (night vision, movement).
Bipolar, horizontal and amacrine cells refine visual information (edges, colors, contrast).
Ganglion cell condense visual info (midget from cones, parasol from rods).
Most of the retina's blood supply comes from the choroid.
The RPE does it all: recharges photoreceptors, removes waste, immune response, pH balance, sun protection and structural support.
And if you're really nerdy and want a more detailed look into phototransduction and the visual vitamin A cycle. Check out biochemistry of the visual process.
The sensory retina is an optically clear structure. For phototransduction to begin, an image must pass through the anterior blood supply and every layer of the sensory retina until the image finally reaches the photoreceptor layer. The photoreceptor layer is composed of rods and cones. There are roughly 120 million rods in the adult retina. These cells allow us to see the difference between light and dark. Rods are utilized for night vision and low lighting conditions.
There are 6 to 7 million cones in the adult eye. Cones are only utilized for day or brighter conditions. These cells are specialize for color detection. There are three types of cones present in the retina: blue sensitive cones, which are sensitive to shorter wavelengths of light (420 to 440nm), green sensitive cones, which are stimulated by more intermediate wavelengths of light (534 to 545nm) and red sensitive cones, which are sensitive to longer wavelengths of light (564 to 580nm). The brain analyzes the number and intensity of cone cell responses to identify the colors of an image.
Rods and cones differ slightly in the phototransduction process. To read about these differences check out the biochemistry of the visual process.
When light strikes a photoreceptor cell, the process of phototransduction begins. The light ray changes the opsin of the rod or cone from 11-cis retinal to all-trans retinal. This structural change closes the cation channels in the photoreceptor cell. The result is hyperpolarization (creates a negative charge) of the photoreceptor. Hyperpolarized photoreceptor cells send less glutamate to bipolar cells.
The bipolar cells respond to glutamate in two different ways. The ionotropic glutamate receptors depolarize the bipolar cells, so less glutamate hyperpolarizes these cells. Metabrotropic receptors do the opposite, hyperpolarizing the bipolar cell with higher glutamate concentrations. These two receptors affect bipolar cells differently to allow more refinement in stimulation, which is used in color, contrast and edge detection.
Photoreceptor cells also affect the horizontal cells. A grouping of photoreceptors provide input to a single horizontal cell. When the photoreceptors hyperpolarize the horizontal cell also hyperpolarizes. This causes the horizontal cell to release less GABA. A negative feedback loop is initiated as less GABA depolarizes the entire group of photoreceptors. The phototeceptors firing from a light stimulus are unaffected by GABA depolarization.
This utlizes a center surround system to allow our visual system to sharpen edges. The photoreceptors stimulated by light continue firing (hyperpolarized) to the horizontal and bipolar cells, while the surrounding photoreceptors that aren't stimulated are actually further depolarized by the inhibitory effect of GABA from the horizontal cells.
Amacrine cells are the next step in propagating the visual message along the visual pathway. There are 40 types of amacrine cells. These are very specialized cells that provide a link from horizontal and bipolar cells to the ganglion cells. Most amacrine cells use the neurotransmitters GABA and glycine and have an inhibitory (depolarizing) effect on the horizontal and bipolar cells.
The ganglion cells are the last stop for the visual message in the eye, as the ganglion cell nerve fibers converge to form the optic nerve. Ganglion cells significantly condense visual information. The visual process starts with over 120 million photoreceptors and ends with just over one million ganglion cells. The two major types of ganglion cells are midget and parasol ganglion cells.
The midget ganglion cells detect color and detail. These cells project to the parvocellular system portion of the lateral geniculate nucleus (LGN). The parasol ganglion cells detect light and movement and project to the magnocellular portion of the LGN. Besides carrying visual information, there are photosensitive ganglion cells that provide input for circadian rhythms and pupil size.
With all of this complicated processing taking place it's no surprise the retina is the most metabolically demanding tissue in the body by weight. A lack of adequate blood supply is the link between microvascular conditions (hypertension and diabetes) to eye problems. To meet the demands of the retina, the ophthalmic artery provides an anterior blood supply via the central retinal artery and a posterior blood supply via the choroid.
The central retinal artery has three divisions. The first, is the radial peripapillary capillaries that supply the nerve fiber layer. Next, the inner capillaries supply the ganglion cell layer. Outer capillaries are the third division, which supply the inner plexiform layer. The central retinal artery supplies 20 to 30% of the retina.
For the other 70 to 80% of the retina (along with iris and ciliary body), the choroid supplies vascular support. The choroid is a mixture of blood, blood vessels and connective tissue. Next to the choroid lies the choriocapillaris. The choriocapillaris is a vascular extension of the choroid, but possesses a fine meshwork of connective tissue that allows easy transfer of metabolites in and out of the retina.
Bruch's membrane acts as a filter between the blood supply of the choriocapillaris and the retinal pigmented epithelium (RPE) of the retina. The RPE is a supportive layer to the photoreceptors and posterior half of the sensory retina.
The RPE works to prevent sun damage to the retina, maintain the structure of the choroid, works as a blood barrier, is a source for immune response in the retina, maintains pH balance of the retina, conducts phagocytosis to remove metabolic debris from the retina. With macular degeneration, Bruch's membrane thickens as it accumulates calcium and lipids (waste products of metabolism). Also, the RPE supplies the enzymes and vitamin A necessary to convert all-trans retinol back into 11-cis retinal to recharge the photoreceptors and restart the process of phototransduction.
Summary
Photoreceptors consists of cones (daylight, color, detail) and rods (night vision, movement).
Bipolar, horizontal and amacrine cells refine visual information (edges, colors, contrast).
Ganglion cell condense visual info (midget from cones, parasol from rods).
Most of the retina's blood supply comes from the choroid.
The RPE does it all: recharges photoreceptors, removes waste, immune response, pH balance, sun protection and structural support.
And if you're really nerdy and want a more detailed look into phototransduction and the visual vitamin A cycle. Check out biochemistry of the visual process.
Comments
Post a Comment