The human retina contains approximately 6 to 7 million cones in total, comprising only 5% of the total number of retinal photoreceptors, but our visual acuity relies on as little as 100000 cones. Rod cells use glutamate as their neurotransmitter and synapse onto second-order bipolar cells at the outer plexiform layer. The configuration of rods into the retinal system allows them to use their unique sensitivity to photons and integrate the photon signal for longer by converging multiple rods onto a single RGC and thus reducing background noise. Rods have a higher sensitivity to single photons of light than cones, whereas cones are more sensitive to specific wavelengths (colors) of light. More rods converge onto a single retinal ganglion cell (RGC), whereas, cones keep a 1 to 1 ratio allowing for this difference in spatial acuity to emerge. It is essential to understand that rods poor spatial acuity is not a result of any inherent inferiority in the rod cell, but a result of wiring. This phenomenon can be appreciated when objects are more visible when one looks just above or just below the intended object in a dark setting. Although the rods differ from the cones in ways that seem to make them inferior, the off-axis visual quality using rods is remarkably good. Also, rods cannot function during the daytime as they are "photo-bleached" and need 20 minutes to recover, and only after 40 minutes after sunset or immersion into darkness can all the rods come online and help with creating scotopic vision. Rods have a slow speed of response, and their spatial acuity and contrast sensitivity are also very low, directly contrasting the rapid, high spatial acuity, and high contrast sensitivity of cones. Rods differ from their color-sensing cone counterparts in a variety of ways. Rods are concentrated in the outer retina and their density increases as one moves outward towards the periphery of the retina, with there being zero rods in the central fovea. In humans, approximately 95% of the photoreceptors in our retina are rods, and they specialize in registering low-light levels, thus helping to create a black and white vision- known as scotopic vision. It is presumed this evolutionary change allowed for organisms to have better survival in low-light settings. Rod cells can be traced back to approximately 500 million years ago when a piscine ancestor evolved rods to supplement the already pre-existing cones. The retina itself consists of six different cell lines divided into ten different layers, each playing a specific role in creating and transmitting vision. The different cell types perform a particular role and form functional circuits that specialize in detecting specific variations and movements of light. The retina is fed oxygen from a unique dual blood supply that divides the retina into outer and inner layers for more efficient oxygenation. Retinal tissue develops to become the most metabolically expensive tissue in the human body, consuming oxygen more rapidly than any other tissue. This long and complex embryonic development makes the retina vulnerable to genetic and environmental insults that can negatively affect retinal development. Development of the retina is a long and complex process that begins during the fourth week of embryogenesis and continues into the first year of life. Located in the posterior portion of the eyeball, the retina is the only extension of the brain that can be viewed from the outside world and gives ophthalmologists a rare window into real-time pathology affecting the retina. The retina is the innermost layer in the eye that is responsible for the visual processing that turns light energy from photons into three-dimensional images.
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