![]() Photoreceptors are tightly packed together, thus allowing a large number of photons to be absorbed across a small area of the retina. Structurally, photoreceptors are neuroepithelial cells that can absorb light and convert it into an electrical signal - phototransduction. ![]() Figure 2 - Colour sensitivity at different wavelengths Structure of Photoreceptors Hence, they are solely responsible for vision in the daylight. In contrast, cones are concentrated in the fovea - the central part of this contains no rods. This is also the part of the retina with the highest acuity of vision. In contrast to rods, cones are much less sensitive to light. Figure 1 - The layers of the retina Types of Photoreceptor Finally, we will discuss the clinical consequences of retinal disease.įor the anatomy of the visual pathway, please see our sister article on TeachMeAnatomy. In this article, we will focus on exploring the structure of photoreceptors and their function. These electrical signals are passed via the optic nerve to the visual cortex allowing us to visualise our surroundings. It consists of photoreceptor cells that convert light energy into nerve impulses. The retina is the innermost layer of the eye. Synaptic region – allows the communication between the photoreceptor cell and the bipolar cell.Cell body – contains the nucleus of the cell.It also provides the energy needed for phototransduction. Inner segment – houses metabolic organelles such as lysosomes, mitochondria, and endoplasmic reticulum.Cilium – connects the outer and inner segments together.It consists of numerous tightly stacked membrane discs that contain proteins required for phototransduction. Outer segment – captures light and converts it to an electrical stimulus.Overall, rods and cones are structurally compartmentalised into five distinct regions: The structure of photoreceptors is well-adapted to their function. Structurally, photoreceptors are neuroepithelial cells that can absorb light and convert it into an electrical signal – phototransduction. And they all act together to distinguish coloured visual input. Although the different cones have a specific colour name, each cone is sensitive to a variety of colours/wavelengths. ![]() Red and green cones are much more abundant and concentrated mainly in the fovea, whilst blue cones are few and found outside the fovea. The other main function of cones is colour vision. It is mediated by three different types of cones, which are sensitive to different ranges of light wavelengths. ![]() In contrast, cones are concentrated in the fovea – the central part of this contains no rods. This is also the part of the retina with the highest acuity of vision. In contrast to rods, cones are much less sensitive to light. Thus, patients with degenerative changes of rod cells, such as retinitis pigmentosa, may present with a symptom of night-blindness known as nyctalopia and peripheral vision loss. In terms of their distribution, rods are found on the outside of the fovea and contribute to peripheral vision. However, as the light levels increase their phototransduction cascades become saturated and are unable to reflect changes in light intensity. They can signal the absorption of a single photon! Hence, they are mainly responsible for scotopic vision (in low-light levels). Rods are much more sensitive to light than cones. The number of rods and cones in the human retina is around 120 million and 6 million cells, respectively. In this article, we will only focus mainly on the rod and cones. There are three main types of photoreceptors in the human eyes called rods, cones, and intrinsically photosensitive retinal ganglion cells. The axons of the ganglion cells then form the nerve fibre layer of the retina, which exits the eye as an optic nerve. The impulse is then relayed by the bipolar cell to the ganglion cell. The photoreceptors are involved in phototransduction, a process of converting light photons to an electrical impulse. The three main cells in the neural retina are (from the outermost to innermost): The neural retina consists of multiple layers. The RPE is also involved in a visual cycle as it regenerates photosensitive pigments. The tight junctions between the RPE cells form part of the blood-retinal barrier, which helps to prevent molecules passing from the choroid into the retina. It is responsible for the nourishment and support of the neural retina. The RPE is a single layer of cuboidal epithelial cells and located in the outermost layer of the retina. The retina consists of layers, which can be subcategorized into retinal pigmented epithelium (RPE) and neural retina.
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