Opsis Therapeutics
Cellular Structure

Photoreceptors

Photoreceptors are neurons found only in the retina. They are adapted to receive light and convert it into electrochemical impulses. Those impulses then traverse a network of neurons leading to the brain. Photoreceptors are an essential component of the visual system and are responsible for starting the cascade of events that leads to what we subjectively call “vision”. If photoreceptors die as a result of disease or injury, the retina cannot replace them, and the resulting vision loss is permanent.

Cellular Structure

Cellular Structure

Photoreceptors are polarized neurons that contain specialized, antenna-like structures for receiving light signals and transmitting those signals into electrochemical impulses. The light-receiving elements of the cell are contained in the outer segment, while the synaptic terminal connects with bipolar cells, which in turn connect to retinal ganglion cells en route to the brain. There are two basic types of photoreceptors, rods and cones. Depicted here is a rod photoreceptor.

Phototransduction

Phototransduction

The outer segments of photoreceptors contain proteins called opsins, which in turn house chromophores. Through a process known as phototransduction, chromophores undergo isomerization when they absorb light, forcing a structural change in the opsin. This event triggers a cascade of chemical amplifications that lead the photoreceptor to transmit a signal. The system must then be restored in order to participate in another light detection cycle.

Color Sensitivity

Color Sensitivity

In humans, rod-type photoreceptors in the retina detect low-level, gray-scale light, such as is required for peripheral or night vision. Cone-type photoreceptors detect more intense light necessary for visual acuity during the day. Cones are also responsible for color vision and they accomplish this via the specialization of three cone sub-types ('Long' (red), 'Medium' (green), and 'Short' (blue)), each containing distinct opsins. Signals from the three cone sub-types are ultimately merged and interpreted upstream to convey a final color perception.

Subtype Distribution

Subtype Distribution

The human retina has approximately six million cones and one-hundred twenty million rods. The distribution of these cells is uneven. With the exception of the center, where they are less prominent, rods are spread evenly across the retina. Conversely, cones are concentrated in the center in the macula, and particularly within a sub-structure of the macula known as the fovea. These regions convey the highest degree of visual acuity and thus are most important for our visual function.