PHYSIOLOGY OF SEEING

 PHYSIOLOGY OF SEEING

Vision is the special sense of sight that is based on the transduction of light stimuli received through the eyes. The eyes are located within either orbit in the skull. The bony orbits surround the eyeballs, protecting them and anchoring the soft tissues of the eye.

Physiological events of vision consists of following;

* Refraction of light entering the eye

* Focusing of image on the retina by accommodation of lens

* Convergence of image

* Photo-chemical activity in retina and conversion into neural impulse

* Processing in brain and perception

During the first event, light wave travels parallel to each other but they bend when passes from one medium to another. This phenomenon is called refraction.

Before light reach the retina, it passes through the clear cornea, aqueous humor, lens, vitrous humor, so refraction takes place in every medium before it falls on retina.

In normal eye, light wave is focused on the retina.

Accommodation of lens to focus image:

Accommodation is a reflex process to bring light rays from object into perfect focus on retina by adjusting the lens. For accommodation to view closer object, ciliary muscle contract and lens become thick which causes focus on closer object. Similarly, when distant object is viewed, ciliary muscles relaxes, so the tension of ligament become greater which pull lens and lens become thinner, due to which image forms on retina.

Convergence of image: Human eye have binocular vision, it means although we have two eye, we perceive single image. In binocular vision, two eye ball turns slightly inward to focus a close object so that both image falls on corresponding points on retina at the same time. This phenomenon is called convergence.

Photochemical activity in rods: Each eye contains 125 million rods and 7 million cone cells.

There are three different types of cone cells and each cone cell contains different photo-pigment and are sensitive to red, green and blue. The perception of color depends upon which cone are stimulated.

Processing of image in brain and perception: All visual information originates in retina due to stimulation of rods and cones which are then conveyed to the brain. Photoreceptor cells, bipolar cells and ganglion cells transmit impulses directly from the retina to the brain to perceived the image.