What Are Retinal Ganglion Cells?
Retinal ganglion cells are photosensitive cells in the retina that send light information to the brain. Retinal ganglion cells, along with photoreceptors and bipolar cells, make up the visual pathway to relay visual information from the eye to the brain. Eye diseases such as glaucoma can lead to damage of the retinal ganglion cells, causing irreversible vision loss because these cells cannot regenerate.
- Retinal ganglion cells are output neurons of the retina that send light information to the brain for visual image interpretation.
- Retinal ganglion cells receive signals from bipolar cells, and send signals to the lateral geniculate nucleus (LGN) in the thalamus.
- Glaucoma is an eye disease that causes death of retinal ganglion cells and irreversible loss of vision.
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Understanding Retinal Ganglion Cells
Retinal ganglion cells are one of the many types of neurons in the retina. Neurons are cells of the nervous system that receive and send electrical signals within the body. The retina has five different types of neurons:
These cells sense light coming into the eye and convert light into electrical signals.
- Bipolar cells
These cells transmit information from photoreceptors to retinal ganglion cells.
- Retinal ganglion cells
These cells transmit information from bipolar cells to the brain, through the optic nerve.
- Horizontal cells
These cells inhibit bipolar cells.
- Amacrine cells
These cells inhibit retinal ganglion cells.
Retinal ganglion cells transmit visual data from the retina to the brain. They are the only cell type to send information out of the retina. There are approximately 1.2 million retinal ganglion cells in each eye. Their axons are located in the nerve fiber layer of the retina. These axons send information to the brain through the optic nerve, which is the bundle of nerve fibers that connects the eye to the brain. Signals from retinal ganglion cells go through the optic nerve to the lateral geniculate nucleus (LGN) in the thalamus of the brain, where the input is processed into visual perception. The receptive field of a retinal ganglion cell needs approximately 10 photons to generate a visual response.
- The pathway of visual information processing is photoreceptor > bipolar cell > retinal ganglion cell > optic nerve > lateral geniculate nucleus (LGN) in the thalamus > visual cortex of the occipital lobe.
- Intrinsically photosensitive retinal ganglion cells (ipRGCs): important for the circadian rhythm and pupillary response. ipRGCs help to synchronize the circadian clock with sunlight detection. Loss of these cells may contribute to sleep disturbances.
- Midget ganglion cells: synapse with amacrine cells and midget bipolar cells
- Diffuse (parasol) ganglion cells: synapse with all bipolar cells, except midget bipolar cells
Damage to Retinal Ganglion Cells
Retinal ganglion cell degeneration has considerable visual impact and is a factor contributing to irreversible blindness. Although many eye diseases can damage retinal ganglion cells, the most common one is glaucoma.
Glaucoma is a neurodegenerative disease that causes slow, progressive death of retinal ganglion cells. Because retinal ganglion cells make up the optic nerve, glaucoma damages the optic nerve located inside of the eye. For most forms of glaucoma, elevated intraocular pressure and aging are important risk factors that cause pathological changes to the retinal ganglion cells. High eye pressure can compress and damage the retinal ganglion cells, thus impairing the light information processing capability. However, there are also types of glaucoma where the eye pressure is normal.
Once glaucoma causes retinal ganglion cell death, the change is irreversible because retinal ganglion cells cannot regenerate. Glaucoma initially causes peripheral vision loss. Over time, glaucoma progresses and damages a larger extent of the optic nerve, until barely any healthy tissue is left. People do not notice glaucoma affecting their vision until it becomes severe and causes ‘tunnel vision’. Currently the only treatment for glaucoma is preventing further optic nerve damage by lowering intraocular pressure, either through eye drops or eye surgery.
Loss of Retinal Ganglion Cells With Age
As we age, the optic nerve naturally loses retinal ganglion cells. However, the amount of retinal ganglion cells that naturally die with aging is minimal, compared with the excessive loss of retinal ganglion cells that characterizes glaucoma. One way that ganglion cell loss is quantified is using an instrument called an optical coherence tomography (OCT), that scans the microscopic layers of the eye and measures its thickness in microns. The normal rate of retinal nerve fiber layer thickness loss is 1 to 2 microns per year. In glaucoma, the rate of loss is 3 to 5 microns per year, or more.
Aging puts the eyes more at risk to develop glaucoma. Adults over 60 years old are most likely to get glaucoma, but glaucoma can occur at any age. Although older age increases the risk of glaucoma, not all elderly individuals develop glaucoma. The incidence of glaucoma is approximately 2% in Americans aged 40 to 65, and approximately 5% in Americans over 65 years old.
Treatment of Retinal Ganglion Cells Damage
Ganglion cells cannot regenerate, so damaged ganglion cells cannot be revived or ‘cured’. The aim of glaucoma treatment is to prevent further death of retinal ganglion cells, and to halt the progression of optic nerve damage. Currently, the only treatment available to prevent retinal ganglion cell damage is lowering intraocular pressure. Intraocular pressure can be lowered by eye drops or eye surgery. Eye drops are usually the first line treatment for glaucoma, and eye surgery is performed for glaucoma that is difficult to control or in a severe stage.