Physiology of Visual Processing by Retina

Seeing something starts with eye, two important matter is associated with eye: 1. the reflected light from object and 2. the physiology of eye. First we would focus one the physiology of eye and its inner core, retina, then we will concentrate on how the reflected light from object hits retina.

Physiology of eye:

The outermost part of our eye is consist of a protective covering called sclera, wchic is almost transparent for light in the front side of eye; we call it cornea. Just behind the cornea, we have an anterior chamber or aqueous humor: a transparent fluid similar to plasma, it is secreted from the Ciliary body. Ciliary body is a structure which works as a support for lens. Just in-front of the lens, there is a layer of tissue called iris. Iris controls the opening of pupil diameter in order to control the amount of light entering into the eye. So, practically pupil is that only small circle through which light can hit lens and then refracted through the lens and vitreous body and reach to the retina. The function of the front portion of eye is only allowing the light to enter into the eye and refract it through the lens to retina. Retina is the light sensitive layer of tissue where photo-transduction takes place. The energy from photons converts into the electrical signal in the photo receptor cells, known as  Rod and Cone cell resides in retina.

The structure of eye and retina allows the light from objects traverse through the lens and the vitreous body and project the image on retina with minimal distortion. Looking at the above image of light projection on retina, it is observable, human retina has a center for projecting lights, named Fovea.  Just behind the retina, there is a layer of black melanin pigment epithelium absorbs the rest of the light not captured by the retina. In order to the make the projected image less distorted, the retinal cells shift away from the center  fovea. Moreover, the proximal layer of retinal neurons in-front of the photoreceptor  are also unmyelineted so that this layer is relatively transparent to light.

Organization of Retina:
The schematic view of retina shows several layers of retinal cells. 1. Ganglion cells, 2. Bipolar cell, 3. Photoreceptor cells (Rods and cone)  and 4. Melanin epithelium tissue. Besides these, we have Horizontal and Amacrine cells which grows transverse to the above cells and create horizontal connectivity. the physical interaction of light and neural activity of retina starts from the layer of photoreceptors, whenever light hits the photosensitive rods and cone, the action potential generates in these cells. Unlike other peripheral receptors, rods and cone cells are considered as a part of central nervous system. The action potential or spikes generated in the photoreceptor transmitted to the bipolar cell though the synaptic connectivity. Bipolar cell send the electric signal to the ganglion cell where the signals from the rods and cones are integrated. Within these three layer of cells , a great deal of visual processing takes place. The axons of the ganglion cell forms the optic nerve and leave the retina to send the visual signal towards visual cortex. If we follow the neural signal flow for creating vision, that goes following, Light --> Photoreceptor --> Bipolar cell --> Ganglion Cell --> Lateral geniculate nucleus in Thalamus --> Visual cortex. Vision processing take place in each layer of signal pathway.