Resource Centre - Accommodation & Visual Phototransduction

Accommodation & Visual Phototransduction

Accommodation of the Eye

What is accommodation?
Accommodation is the ability of the eye to adjust its focus so that a clear image of an object, over a range of distances, can be formed on the retina.

When and why is accommodation needed?
Accommodation is needed when our eyes change from focusing on an object that is far away to focusing on a nearer object and vice versa.

When light rays enter our eyes, the rays should converge onto the retina. Accommodation is needed to make sure the point of convergence is always on the retina so that the image can be sent, as electrical impulses, to be read in the brain. For example, when our eyes are focused on a distant object, the cornea and the lens cause the light to converge onto the focal point of the retina. If, however, the object was moved nearer and the lens shape stayed the same then the light rays would converge behind the retina, which would give an indistinct, blurry image. Therefore, the eye uses accommodation to alter the shape of the lens so that light rays from objects from different distances will always be focused upon the focal point of the retina to give clear, sharp images.

How does accommodation work?
For accommodation to work, the lens needs to be able to adjust its shape. This is controlled by the brain, suspensory ligaments and the ciliary muscles.

When the lens is at rest, it is focused for distant objects as the light rays are almost parallel and, therefore, do not need much refraction to focus the image upon the retina. At rest, or when viewing distant objects, the ciliary muscles are relaxed, making the suspensory ligaments taut which pull on the lens capsule, making it less convex, or flatter.

When the eye is focusing upon a near object the light rays from this are more diverging and, hence, need more refraction for the light rays to converge upon the focal point of the retina. When focusing on a near object, the brain automatically sends signals to the ciliary muscles, causing them to contract, this makes the passive suspensory ligaments slack which allows the lens to revert to its natural, more rounded shape allowing the eye to focus on nearer objects.

Presbyopia is the inability of the lens to focus on objects as they move nearer the eye. It results from the loss of elasticity of the lens, which occurs with increasing age. To find out more about presbyopia, click here.

Contact lenses are used to correct problems with accommodation.

Visual Phototransduction

What is Visual Phototransduction?
Visual phototransduction is the process of transformation of light into electrical signals by photoreceptors e.g. rods and cones, in the retina of the eye.

Photoreceptors
There are two types of photoreceptors in the retina - rods and cones. These are specialised cells which, when activated by light, send electrical signals to the optic nerve. The fibres in the optic nerve then carry the electrical signals to the brain where they are converted into an image.

Rod cells are very sensitive to light but cannot differentiate between colours whereas cone cells are less sensitive but can detect different colours.

There are many more rods than cones in the retina, rods outnumber cones around 20:1.

There is only one type of rod but three types of cones, each one sensitive to red, green or blue, the combinations of which enable us to see a range of different colours.

The Process of Visual Phototransduction
The photoreceptor cells contain molecules called photopigments, which absorb specific wavelengths of light. The photopigments consist of a protein called opsin and a chromophore called 11-cis-retinal. When the 11-cis-retinal molecule absorbs light, it changes conformation to the all-trans configuration and the new shape can no longer fit to the binding site of the opsin protein. This activates opsin and sets of a process of reactions, which, effectively leads to the closing of sodium channels, which alters the electrical state of the photoreceptor cell membrane. This causes signals to be sent to the intermediary cells e.g. bipolar cells and ganglion cells which pass the signals to the optic nerve and, ultimately, to the brain.

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