The Imaging and Vision System of Our Eyes
Various Structures Function to Control Light as it Traverses the Eye
Nov 26, 2008
For us to form an image of an object, light has to first enter the eye through the cornea and then the pupil via the aqueous humour. From here it passes through the iris and then through the crystalline lens. The light then traverses the vitreous humour before reaching the retina. It is here that the light transfers its energy into an electrical impulse that is carried by the optic nerve through the visual pathway and then through the occipital cortex to be finally interpreted by the brain.
Controlling Light
The human eye is dedicated to focusing light onto the retina. The media for transmission within the eye is significantly transparent to minimize any loss of light while the cornea and lens act to converge the light onto the retina. Most of the light from the external environment is refracted at the cornea and this is the same area of the eye that is reshaped in many laser vision correction procedures like LASIK. Of the light that reaches the internal regions of the eye, the intensity is maintained at relatively constant levels by the changing size of the pupil. This is adjusted by the relative contractions of the circular and radial muscles of the iris so that the retina is not damaged by excessive light nor is sight hampered by insufficient light entry.
The lens behind the iris functions to focus the light and this is done via contractions of the ciliary muscles. When these muscles contract, they pull the lens to flatten it resulting in a sharp image of a distant object. When released, the lens thickens allowing a nearby object to be focused.
The wall of the eyeball is made up of three layers. The outermost is the sclera, giving the eyeball the characteristic white colour. The middle layer is the choroid, containing blood vessels that nourish and dispose of waste from the retina cells. It is dark to minimize reflections within the eye. The innermost layer is the retina where the light sensitive cells and neurons are located.
From Light to Vision
Two unique areas exist on the retina, the fovea and the optic disk. The human fovea is an indentation located directly opposite the lens. It allows for increased colour vision and detail in observation. The optic disk, also known as the ‘blind spot’ has a notable absence of light sensitive cells where the optic nerve fuses with the retina. The rest of the retina contains rods and cones. These cells differ in their response to light. Rods function better in darker conditions through a greater sensitivity to light. Cones conversely respond better to higher intensities and different wavelengths of light, providing colour vision and better perception of detail.
Rods and cones contain variants of the opsin family of light sensitive proteins. Primary opsins break down into secondary opsins upon contact with light of specific intensity and wavelength. These open ion channels and generate electrical impulses in the cell membranes of the rods and cones. Neurons then convey these impulses to the visual cortex in the brain via the optic nerve. Here the projected image begins to be interpreted.
