THE RETINA

Ophthalmologist Carolina Valdivia, MD describes the retina: its structure and functional role in enabling us to see.

The optics of the eye create an image of the visual world on the retina, which serves a similar function to the image processor or film in a camera. Light striking it initiates a sequence of chemical and electrical events that ultimately trigger nerve impulses. These are sent to various visual centers of the brain through fibers of the optic nerve.

Retinal tissue is supplied abundantly with blood and normally has a reddish hue. It represents the only part of the central nervous system that can be visualized by direct observation. I examine it using a diagnostic instrument called an ophthalmoscope, which illuminates the structure by shining a concentrated light beam through through pupil and lens. Changes in its color or appearance may indicate disease.

If you experience changes in sharpness or color perception, flashes of lights, floaters, or distortion in vision, you should visit an ophthalmologist for an examination.

BASIC ORGANIZATION AND STRUCTURE

Four layers enclose this light-sensitive tissue. From outside to inside, these are:

• Sclera (white part of the eye)
• Large blood vessels of the choroid
• Choriocapilaris
• Bruch’s Membrane (separates the retinal pigmented epithelium from the choroid).

The retina consists of 10 tissue layers. From the outside (closest to Bruch's Membrane) to the inside (closest to the gelatinous vitreous humor), these are:

1. Pigmented epithelium.
2. Photoreceptor outer/inner segment (rods and cones).
3. Outer limiting membrane.
4. Outer nuclear layer (cell bodies of rods and cones).
5. Outer plexiform layer (axons of rods and cones, horizontal cell dendrites, bipolar dendrites).
6. Inner nuclear layer (nuclei of horizontal cells, bipolar cells, amacrine cells, and Müller cells).
7. Inner plexiform layer (axons of bipolar cells and amacrine cells, dendrites of ganglion cells).
8. Ganglion cells (nuclei of ganglion cells and displaced amacrine cells).
9. Nerve fiber layer (axons from ganglion cells traversing the tissue to leave the eye at the optic disc).
10. Inner limiting membrane (separates the retina from the vitreous humor).

I have included three excellent videos on this page to assist you in understanding the complexity of this unique photosensitive structure.

Light entering the eye is focused first by the cornea and then by the crystalline lens. This system is so powerful that light rays intersect at a point just behind the lens (inside the vitreous humor) and diverge from that point onto the retinal surface.

The retina contains four basic classes of neurons in addition to the light-sensitive photoreceptor cells (rods and cones). A neuron is an electrically excitable cell that processes and transmits information by electrical and chemical signaling.

• Horizontal neurons.
• Bipolar neurons.
• Amacrine cells.
• Ganglion cells.

There also is one major type of glial cell called a Müller cell. Glial cells comprise the support structure for the central nervous system.

Nearly all synapses occur in the plexiform layers. All visual information passes across at least two synapses, one in the outer plexiform layer and another in the inner plexiform layer, before leaving the eye.

Image analysis occurs in both plexiform layers. The outer plexiform layer isolates visual information into channels that either are “on” or “off”. It also conducts a spatial analysis on the visual input. The inner plexiform layer is focused more on processing transient visual stimuli and moving stimuli. The output neurons of the inner plexiform layer – the ganglion cells – transmit the visual message to the rest of the brain.

PHOTORECEPTORS (RODS AND CONES)

Rods and cones are positioned away from incoming light, which passes by these photoreceptors before being reflected back onto them. Light initiates a chemical reaction with the violet-colored protein pigment iodopsin in cones (activated in situations of ample light known as photopic conditions) and with the reddish-purple protein pigment rhodopsin in rods (activated in low-light or scotopic conditions).

Activated photoreceptors stimulate bipolar cells, which in turn stimulate ganglion cells. The impulses continue into the axons of the ganglion cells, through the optic nerve, and to the visual cortex located at the back of the brain, where the image is perceived as right-side up. The brain actually has the ability to detect a single photon of light (the smallest packet of energy) being absorbed by a photoreceptor.

Approximately 7 million cones are present in the retina of each eye. They are responsible for vision in average to bright light conditions and for color vision. The highest concentration of cones is found in the macula, the area that is responsible for central vision. The fovea, located at the center of the macula, contains only cones and no rods. Once a cone pigment is bleached by light, it takes about six minutes for it to regenerate.

There are about 120 to 130 million rods in the retina of each eye. They process images in dim light and are sensitive to movement, and to shapes. The highest concentration of rods is in the periphery. They successively decrease in density moving in toward the macula.

Rods do not detect color. For this reason, it is difficult to distinguish the color of an object at night, in other low-light conditions, or in our peripheral vision. Once a rod pigment is bleached by light, it takes about 30 minutes for it to regenerate.

Defective or damaged cones result in color deficiency; defective or damaged rods result in problems seeing in the dark and at night.

FOVEA

RETINAL NEURONS

Despite there being only four basic classes of neurons, along with photoreceptors, a number of neuronal types are present in each of these classes.

• Rod photoreceptor (1 cell type)
• Cone photoreceptor (3-4 cell types that are responsive to light of different wavelengths – blue, green, red).
• Horizontal neurons (3 cell types)
• Bipolar neurons (9-11 cell types)
• Amacrine cells (about 40 cell types)
• Ganglion cells (5 main classes of cell types)
1. Midget cell
2. Parasol cell
3. Bistratified cell
4. Photosensitive ganglion cell
5. Other ganglion cells projecting to the superior colliculus for eye movements (saccades)

Two main kinds of chemical synapses occur in the retina – ribbon synapses made by photoreceptor and bipolar terminal endings, and conventional synapses made by horizontal and amacrine cells.

• Ribbon synapses are characterized by a dense ribbon-like structure found just before the synapse that functions as a conveyor belt to bring synaptic vesicles – small sacs storing neurotransmitters that are released at the synapse – to the presynaptic membrane where they bind and release their contents.
• Conventional synapses are characterized by bunches of synaptic vesicles that are positioned against the membrane to which they bind.

Additionally, cone photoreceptors make an unusual chemical synapse, called a flat or basal junction, mainly onto OFF-type bipolar cells.

RESPONSE TO LIGHT

Distal neurons – those closest to the pigmented epithelium (photoreceptor, horizontal and bipolar cells) – respond to light with sustained stratified membrane potential changes. Membrane potential is the difference in voltage between the interior and exterior of a cell. Most of these cells also respond by hyperpolarizing in response to light stimuli. Hyperpolarization is a change in a cell's membrane potential that makes it more negative.

All photoreceptors hyperpolarize in response to light. Through synapses, they cause horizontal and OFF-bipolar cells to hyperpolarize and the ON-bipolar cells to depolarize. Horizontal cells act as lateral inhibitory neurons. Two types of amacrine cell responses – transient and sustained – have been identified. Transient amacrine cells respond by depolarizing. Sustained amacrine cells may either depolarize or hyperpolarize to light. Action potentials are often observed on the depolarizing responses of amacrine cells. All ganglion cells generate action potentials. Three basic types of responses are observed: ON, OFF, and ON-OFF cells.

RETINAL DISEASES

Several problems can afflict this light-sensitive tissue. A brief list is presented below.

• Retinal Detachment typically occurs when vitreous humor leaks through a tear or break, causing retinal tissue to peel away from its underlying layer.
• Retinitis Pigmentosa (RP) is an inherited condition causing degeneration of the rod photoreceptors.
• Diabetic Retinopathy is a condition in which excess glucose in the bloodstream causes tiny capillaries in the back of the eye to swell and leak fluid, leading to blurry vision.
• Age-Related Macular Degeneration (AMD) is a chronic eye condition that affects central vision in people age 50 and older.
• Epiretinal Membrane (macular pucker) is a scar tissue-like membrane that forms over the macula.
• Retinal Tear occurs when the vitreous humor that fills the center of the eye sags and pulls away from the retina, forming small, jagged flaps on the surface.
• Macular Hole is a small break in the macula.
• Cone Dystrophy is an inherited ocular disorder characterized by the loss of cone cells, the photoreceptors responsible for both central and color vision.
• Central Serous Chorioretinopathy is characterized by the accumulation of fluid under the macula.

REFERENCE:

Dowling JE. The Retina: An Approachable Part of the Brain, Revised Edition. Boston, MA: Belknap Press of Harvard University Press, 2012.

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