Tag: human brain and sense organs

The retina contains the cells, that respond to light. These specialized cells are called as photoreceptors. There are 2 types of photoreceptors in the retina: rods and cones. 

In order to reduce the amount of light entering into the eye, pupils constrict, and this constriction helps to decrease the strong light, which can be harmful to cells in the retina. The retina is the innermost layer of the eye which contains photoreceptor cells, that are useful for the formation of the image and perceiving an object. Another advantage of constriction of the pupil is to improve the clarity of image by blocking some of the rays of light, which can optically cause distortion of the image. The size of the pupil is controlled by the action of the pupillary sphincter muscle and dilator muscle.

Rhodopsin is a light-sensitive receptor protein. It is a biological pigment in photoreceptor cells of the retina. It is the primary pigment found in rod photoreceptors. They are extremely sensitive to light, enabling vision in low-light conditions. 

Refraction is the phenomenon which makes image formation possible by the eye as well as by cameras and other systems of lenses. Most of that refraction in the eye takes place at the first surface since the transition from the air into the cornea is the largest change in index of refraction which the light experiences. About 80% of the refraction occurs in the cornea and about 20% in the inner crystalline lens.

The retina is a layered structure with several layers of neurons interconnected by synapses. The only neurons that are directly sensitive to light are the photoreceptor cells. These are mainly of two types: the rods and cones. Rods function mainly in dim light and provide black-and-white vision, while cones support the daytime vision and the perception of colour. The third type of photoreceptor, the intrinsically photosensitive ganglion cell, is important for reflexive responses to bright daylight. Neural signals from the rods and cones undergo processing by other neurons of the retina. The output takes the form of action potentials in retinal ganglion cells, whose axons form the optic nerve. 

The abducens nerve innervates the lateral rectus eye muscle that turns the eye outward away from the midline. 

All photoreceptors share a common visual pigment: a mixture of a vitamin A-based chromophore (retinal) and a seven-transmembrane-helix apoprotein (opsin). Together, they form rhodopsin. The pigments belong to a special class of receptors called as G protein-coupled receptors, characterised by signaling through heterotrimeric G proteins. 

The plica semilunaris in humans is a small fold of tissue on the inside corner of the eye. It is the vestigial remnant of the nictitating membrane, an organ that is fully functional in some other species of mammals.

At extremely low intensities of stimuli, when only rods are stimulated, the retina shows a variable sensitivity to light according to its wavelength, being most sensitive at about 5000 angstroms, the absorption maximum of the rod visual pigment, rhodopsin. There is a characteristic shift in the maximum sensitivity from 5000 angstroms for scotopic (night) vision to 5550 angstroms for photopic (day) vision, called Purkinje shift.