The retina contains two kinds of light cells: the rod cells and the cone cells.
The rod cells are active in low light.
The cone cells are active in normal daylight.
There are three kinds of cone cells, each containing a different pigment.
When come cells are activated their pigments absorbs light.
The spectral sensitivities differ as to wavelength: the most sensitive to shortwave lengths, one most sensitive to medium wavelengths and the third senitive to medium-to-long wavelengths, all within the visible spectrum.
These receptors are known as short (S), medium (M), and long (L) wavelength cones, but are also often referred to as blue, green, and red cones.
The peak sensitivities of differing cones result in blue, green, and yellow-green regions of the spectrum, respectively.
The absorption spectra of the three systems overlap, covering the visible spectrum.
The innermost, light-sensitive layer of tissue of the eye.
The focused image on the retina, translates that image into electrical neural impulses to the brain to create visual perception.
The retina consists of several layers of neurons interconnected by synapses.
There are 18 layers that can be identified in the retina.
Each layer contains specific cell types or cellular compartments that have different nutritional requirements.
The retina is supported by an outer layer of pigmented epithelial cells.
The light sensing cells in the retina are the photoreceptor cells, which are of two types: rods and cones.
Rods function mainly in dim light and provide black-and-white vision.
Cones function in well-lit conditions and are responsible for the perception of color, as well as high-acuity vision used for tasks such as reading.
There is a third type of light-sensing cell, the photosensitive ganglion cell.
The photosensitive ganglion cell is involved in circadian rhythms and reflexive responses such as the pupillary light reflex.
When light strikes the retina it starts a cascade of chemical and electrical events the trigger nerve impulses that are sent to visual centers in the brain through the optic nerve fibers.
The neural signals emanating from the rods and cones undergo processing by other neurons.
These neurons output in the form of action potentials in retinal ganglion cells whose axons form the optic nerve.
The retina is considered part of the central nervous system (CNS) and the only part of the CNS that can be visualized non-invasively.
The light sensing cells are in back of the retina, so that light has to pass through layers of neurons and capillaries before it reaches the rods and cones.
The ganglion cells, whose axons form the optic nerve, are at the front of the retina
The optic nerve must cross through the retina en route to the brain.
Where the optic nerve crosses through the retina there are no photoreceptors, and is referred to then blind spot.
An area of the central retina is adapted for high-acuity vision, the fovea centralis, is avascular, and has minimal neural tissue in front of the photoreceptors, minimizing light scattering.
Light passes through several transparent nerve layers to reach the rods and cones, and then a chemical change in the rods and cones send a signal back to the nerves.
The rod and cones signals are processed in these layers.
First, the signals start as raw outputs of points in the rod and cone cells, to the bipolar and horizontal cells, then to the amacrine cells and ganglion cells then to the optic nerve fibers.
Subsequently, the nerve layers identify shapes, bright points surrounded by dark points, edges, and movement.
The center of the fovea holds very few blue-sensitive cones.
The retina has ten distinct layers.
From closest to farthest from the vitreous body:
Inner limiting membrane, a basement membrane elaborated by Müller cells.
Nerve fiber layer with axons of the ganglion cell bodies
Ganglion cell layer which contains nuclei of ganglion cells, the axons of which become the optic nerve fibers, and some displaced amacrine cells.
Inner plexiform layer which contains the synapse between the bipolar cell axons and the dendrites of the ganglion and amacrine cells.
Inner nuclear layer which contains the nuclei and surrounding cell bodies of the amacrine cells, bipolar cells, and horizontal cells.
Outer plexiform layer with projections of rods and cones ending in the rod spherule and cone pedicle, respectively.
These make synapses with dendrites of bipolar cells and horizontal cells.
Outer nuclear layer with cell bodies of rods and cones.
External limiting membrane that separates the inner segment portions of the photoreceptors from their cell nuclei.
Inner segment / outer segment layer of rods and cones.
The outer segments contain a highly-specialized light-sensing apparatus.
Retinal pigment epithelium is a single layer of cuboidal epithelial cells.
It is closest to the choroid, and provides nourishment and supportive functions to the neural retina.
Retinal layers can be grouped into four processing stages:
transmission to bipolar cells
transmission to ganglion cells, which also contain photoreceptors, the photosensitive ganglion cells
transmission along the optic nerve.
The optic nerve projects to the superior colliculus, the suprachiasmatic nucleus, and the nucleus of the optic tract.
It passes through the other layers, creating the optic disc.
The entire retina is approximately 72% of a sphere about 22 mm in diameter.
The entire retina contains about 7 million cones and 75 to 150 million rods.
The optic disc, a part of the retina sometimes called the blind spot because it lacks photoreceptors.
The optic disc is located at the optic papilla, where the optic-nerve fibers leave the eye.
The optic disc appears as an oval white area of 3 mm².
Temporal to this optic disc is the macula, at whose center is the fovea, a pit that is responsible for our sharp central vision but is actually less sensitive to light because of its lack of rods.
Human’s possess one fovea.
Around the fovea extends the central retina for about 6 mm and then the peripheral retina.
The retina is no more than 0.5 mm thick.
The retina has three layers of nerve cells and two of synapses.
The optic nerve carries the ganglion cell axons to the brain, and the blood vessels that supply the retina.
The ganglion cells lie innermost in the eye.
Photoreceptive cells lie beyond the eye.
Light first passes through and around the ganglion cells and through the thickness of the retina, before reaching the rods and cones.
Light is absorbed by the retinal pigment epithelium or the choroid.
White blood cells in capillaries in front of the photoreceptors can be seen as
bright moving dots when looking into blue light: blue field entoptic phenomenon.
Between the ganglion cell layer and the rods and cones there are two layers of neuropils.
Synaptic contacts are made at the neuropils layer.
The central retina predominantly contains cones.
The peripheral retina predominantly contains rods.
There are about seven million cones and a hundred million rods in the retina.
The macula has a yellow pigmentation, and is known as the macula lutea.
The fovea allows for the sharpest vision the eye can attain.
There are about 150 million receptors and only 1 million optic nerve fibers, indicating convergence and mixing of signals.
One area of the retina can control another, inhibiting the sum of messages sent to the higher regions of the brain.
The ophthalmic artery supplies the retina via two distinct vascular networks: the choroidal network, and the retinal network.
The choroidal network supplies the choroid and the outer retina.
The retinal network supplies the retina’s inner layer.
The photoreceptor amplification process uses large quantities energy for vision and, thus, requires a large supply of nutrients supplied by the blood vessels in the choroid, which lies beyond the retinal pigment epithelium.
The nutrients required include glucose, fatty acids, and retinal.
The choroid network supplies about 75% of these nutrients to the retina and the retinal vasculature only 25%.
When light strikes the disks in the rods and cones, it’s 11-cis-retinal changes to all-trans-retinal.
The retinal is pumped out to the surrounding retinal pigment epithelium (RPE) where it is regenerated and transported back into the outer segments of the photoreceptors.
The retinal pigment epithelium recycling function protects the photoreceptors against photo-oxidative damage.
This recycling function allows the photoreceptor cells to have decades-long useful lives.
Retinal blood circulation changes are seen with aging, exposure to air pollution, and may indicate cardiovascular diseases such as hypertension and atherosclerosis.
Determining the width of arterioles and venules near the optic disc is used technique to identify cardiovascular risks.
The retina translates an optical image into neural impulses.
The optimal image starts with the patterned excitation of the color-sensitive pigments of its rods and cones, the retina’s photoreceptor cells.
The neural system and various parts of the brain work in parallel to form a representation of the external environment in the brain.
Cones respond to bright light and mediate high-resolution color vision during daylight illumination.
Rods respond to dim light and mediate lower-resolution, monochromatic vision under very low levels of illumination.
At mesopic light levels, both the rods and cones are actively contributing to pattern information.
The spectral sensitivity refers to the
response of cones to various wavelengths of light.
Spectral sensitivity of a cone falls into one of three subtypes: blue, green, and red, known as short, medium, and long wavelength-sensitive cone subtypes.
Lack of one or more of the cone subtypes that causes individuals to have deficiencies in color vision/color blindness.
Photoreceptors expose to light hyperpolarizes its membrane.
Photoreceptor outer cell segment contains a photopigment, and inside the cell cyclic guanosine monophosphate (cGMP) keeps the Na+ channel open, and in the depolarized resting state.
Photopigment is bleached away in bright light and only replaced as a chemical process, so the transition from bright light to darkness the eye can take up to thirty minutes to reach full sensitivity.
In the transfer of visual signals to the brain,
The visual pathway transfers visual signals to the brain by the retina and is vertically divided in two, a temporal half and a nasal half.
The axons from the nasal half cross the brain at the optic chiasma to join with axons from the temporal half of the other eye before passing into the lateral geniculate body.
There are more than 130 million retinal receptors.
There are approximately 1.2 million axon fibers in the optic nerve.
The fovea of the retina produces the most accurate information.
The fovea occupies about 0.01% of the visual field, less than 2° of visual angle, about 10% of axons in the optic nerve are devoted to the fovea.
The retina sends neural impulses representing an image to the brain, and it compresses those impulses to fit the capacity of the optic nerve.
Compression is necessary because there are 100 times more photoreceptor cells than ganglion cells.
Retinal signal flow: Photoreceptors → Bipolar → Ganglion → Chiasm → LGN → V1 cortex
Inherited and acquired diseases or disorders that affects the retina:
Retinopathy-hypertension and diabetes
A number of different instruments are available for the diagnosis of diseases and disorders affecting the retina include ophthalmoscopy and fundus photography.
Adaptive optics has been used to image individual rods and cones.
The electroretinogram is used to non-invasively measure the retina’s electrical activity, which is affected by certain diseases.
Optical coherence tomography is a non-invasive technique that can obtain a 3D volumetric or high resolution cross-sectional tomogram of the fine structures of the retina, with histologic quality.
Retinal vessel analysis non-invasively allows for the examination of the small arteries and veins in the retina which allows for conclusions about the function of small vessels elsewhere.
Retinal vessel analysis is a predictor of cardiovascular disease and may be helpful in the early detection of Alzheimer’s disease.
The retinal microvascular signs are associated with cardiovascular diseases.
Retinal imaging could be used as a screening, diagnostic, and prognostic tool for cardiovascular diseases.
Retinal vascular diameter correlates with acute coronary syndrome incidence, especially in women and can predict its occurrence.
Retinal vascular diameter is more strongly related to acute coronary syndrome in mid life rather than in the elderly population.
Treatment modalities of management for retinal disease include:
Intravitreal medication, such as anti-VEGF or corticosteroid agents
Use of nutritional supplements
Modification of systemic risk factors for retinal disease
Retinal gene therapy