The retina, a light-sensitive tissue located at the back of the eye, plays a fundamental role in vision. It converts light into electrical signals that the brain interprets as images. This process is made possible by the retina’s complex, layered structure, which houses specialized cells. Understanding the arrangement of these cells, from the outermost layer near the choroid to the innermost layer facing the vitreous humor, shows how visual information is captured and processed.
The Light-Sensing Cells
The journey of light through the retina begins at its back, with the photoreceptor cells: rods and cones. These neurons are positioned closest to the retinal pigment epithelium (RPE) and the choroid to capture incoming light. Photoreceptors are responsible for converting light energy into electrical signals, a process known as phototransduction.
Rod photoreceptors are sensitive to low light levels, making them responsible for vision in dim conditions and peripheral vision. They provide black-and-white vision and detect motion, with 120 million rods found in the human retina, predominantly in the periphery. Conversely, cone photoreceptors operate in brighter light and are responsible for color vision and high visual acuity. The human retina contains 6 million cones, with the highest concentration in the fovea, the central region of the macula that provides sharp, detailed vision.
Intermediate Processing Layers
Following light detection by photoreceptors, the electrical signals are transmitted to intermediate layers of cells for processing and modulation. Bipolar cells transmit signals, receiving them from photoreceptors and transmitting them to retinal ganglion cells. There are distinct types of bipolar cells, some responding to increases in light (ON-bipolar cells) and others to decreases in light (OFF-bipolar cells), contributing to the initial encoding of visual contrast.
Horizontal cells, located in the same layer as bipolar cells, modulate signals laterally across the retina. They receive input from multiple photoreceptors and provide inhibitory feedback, which enhances visual contrast and helps the retina adapt to varying light conditions. Amacrine cells, interneurons, refine signals within the inner plexiform layer, influencing ganglion cell output. These cells contribute to visual functions, including motion detection and processing of transient visual events.
The Signal Transmitters
The final stage of retinal signal processing before information leaves the eye occurs at the retinal ganglion cells (RGCs). These neurons form the “front-most” layer of the retina, situated closest to the vitreous humor. RGCs receive processed signals from bipolar and amacrine cells, integrating this information.
The axons of these ganglion cells converge at the optic disc to form the optic nerve. This nerve transmits the visual information as electrical impulses from the retina to various areas of the brain for further interpretation. There are 0.7 to 1.5 million retinal ganglion cells in the human retina, and each receives input from numerous photoreceptors, with the exact number varying depending on the retinal location.
Support Cells and Retinal Environment
Beyond visual processing neurons, other cells and structures provide support and maintain the retinal environment. Müller glial cells are a common type of glial cell in the retina, extending radially across its thickness. They offer structural support, regulate the extracellular environment by maintaining ion balance and clearing neurotransmitters, and contribute to the metabolic health of retinal neurons.
Behind the photoreceptors, the retinal pigment epithelium (RPE) forms a layer of pigmented cells. The RPE is important for photoreceptor health, performing functions such as absorbing stray light to enhance visual acuity, transporting nutrients to the photoreceptors, and removing waste products. It also plays a role in the visual cycle by processing vitamin A metabolites, which are necessary for phototransduction.