Inner Segment

The part of a photoreceptor cell that is closest to the front of the eye and is thought to channel light to the outer segment, where the visual cycle takes place.

The inner segment is one of the regions of a photoreceptor. In all vertebrates (animals with a spine), the inner segment serves not only as the site of energy production and oxidation, but also as a region of high refractive index (for light). The latter property causes the inner segment to act as a "waveguide" that channels light to the outer segment (where it is transformed into an electrical signal that travels down the optic nerve).