Image formation on the retina

In exactly the same manner that a glass lens can focus an image on a sheet of paper, the lens system of an eye can focus an image on the retina. The image is inverted and reversed with respect to the object. However, the mind perceives objects in the upright position despite the upside-down orientation on the retina because the brain is trained to consider an inverted image as the normal.

 

Accomodation – is eye lens adjustment for various distances.

Parasympathetic control of accomodation – the ciliary muscle is controlled almost entirely by the parasympathetic nervous system. Stimulation of the PNS contracts the ciliary muscle which relaxes the lens ligaments and increases the refractive power. With an increased refractive power the eye is capable for focusing an object nearer at hand than when the eye has less refractive power. Consequently, as a distant object moves toward the eye, the number of parasympathetic impulses on the ciliary muscle must be progressively increased for the eye to keep the object constanly in focus.

Sympathetic control. Sympathetic stimulation has a weak effect in relaxing the ciliary muscle, but this plays almost the role in the normal accomodation mechanism.

 

Visual acuity – is ability of human eye to discriminate between point sources of light. Normal clinical value: 20/20, that is, test chart is at 20 feet and person can see those letters clearly which he should normally be able to see at 20 feet.

 

Retina – is the light-sensitive portion of the eye, containing the cones, which are responsible for colour vision, and the rods, which are mainly responsible for vision in the dark. When the rods and cones are excited, signals are transmitted through successive neurons in the retina itself and finally into the optic nerve fibers and cerebral cortex.

Functions:

1. Vision – due to rods and cones presence, retina is responsible for photopic and scotopic vision.

2. Reflexes – concerned with light and accomodation reflexes.

3. Tone, posture and equillibrium.

Retinal impulses help to maintain tone, posture and equillibrium.

Rods and cones are photoreceptors.

Functions:

1. Rods are responsible for dark vision.

2. Cones are responsible for colour vision or daytime vision.

Photochemicals – these are light-sensitive chemicals that decompose on exposure to light and excite nerve fibers leading from eye.

1. Rhodopsin - photochemical present in rods. Composed of scotopsin plus 11-cis-retinal.

2. Iodopsin – photochemical present in cones. Composed of photopsin plus 11-cis-retinal.

Photopic vision – colour-vision or day light vision is called photopic vision.

Scotopic vision – night vision (or black and white vision).

When a person is in dark, scotopsin combines with 11-cis-retinal (derived from vitamine A) and forms rhodopsin that causes depolarization of sensory receptors which in turn excites optic nerve so the person is able to see in dark.

As soon as person comes in the light the rhodopsin splits into scotopsin and all-transretinal causing hyperpolarization which causes decreased excitation of optic nerve.

Protanopia (red blindness) – is observed due to missing of red cones.

Deuteranopia (green blindness)- due to green cones missing.

Tritanopia (due to blue cones missing).

 

The visual pathways

Are the pathways from the retinae to the visual cortex. After nerve impulses leave the retinae thay pass backward through the optic nerves. At the optic chiasma all the firbers of the retinae cross to the opposite side, where they join the fibers from the opposite temporal retinae to form the optic tracts. The fibers of each optic tract synapse in the dorsal lateral geniculate nucleus, and from here the geniculo calcarine fibers pass by way of the optic radiation, or geniculo-calcarine tract, to the primary visual cortex in the calcarine area of occipital lobe.