Mechanism of Vision: Structure, Different Parts, Functioning

The sense of vision is vital to humans and animals in perceiving the surroundings through the detection and processing of light. The mechanism of vision should be understood for clear interaction between an individual and the surrounding environment, and to diagnose and treat some complications that concern vision.

Anatomy Of The Eye

The eye is a unique, complex organ with major structures that function in harmony with the capture and processing of light.

Cornea

• This is the clear, curved front that focuses light.

• It gives the maximum optical power to the eye and acts protectively.

Pupil

• This is the aperture-like opening in the centre of the iris that controls the amount of light falling on the retina. Its diameter will vary with the intensity of light.

• It will be small in bright light and big in dim light.

Iris

• That part of the eye that surrounds the pupil; controls the size of the pupil.

• Composed of smooth muscle cells that contract and relax to alter the diameter of the pupil.

Lens

• The transparent, elastic structure behind the pupil focuses light onto the retina.

• Its shape changes to focus on objects at different distances– accommodation.

Retina

• It is the light-sensitive layer lining the back of the eye, comprising photoreceptors—rods and cones.

• It captures the light images and translates them into electrical signals.

Optic nerve

• It is the nerve transmitting the visual information from the retina to the brain.

• The nerve takes the signals to the visual cortex for interpretation.

Pathway Of Light Through The Eye

• The light passes through several structures before reaching the retina.

• The cornea bends most of the light that enters and sends it to the lens for focusing.

• It provides approximately 65-75% of the total focusing power of the eye.

• The Iris acts in changing the size of the pupil to control the amount of light entering inside.

• In bright conditions, the pupil constricts, allowing limited light to enter; under poor illumination, it dilates to let more light fall on the retina.

• The lens focuses light further and concentrates it on the retina.

• It changes shape for close and far vision, in a process called accommodation.

The Retina And Photoreceptors

The retina has a significant role in the transduction of light into neural signals.

There are two types of photoreceptors in the retina:

Rods:

• They are responsible for low light conditions that prevail at night and for peripheral vision.

• They are very sensitive to light but incapable of registering colour.

Cones:

• Responsible for colour vision and central vision with high acuity

• Three types of cones are sensitive to red, green, and blue light.

Rods are more numerous and are distributed throughout the peripheral retina, while cones are concentrated in the central retina, particularly in the fovea, the area responsible for sharp central vision.

Conversion Of Light To Electrical Signals

• The process by which light is converted into electrical signals is called phototransduction.

• Rhodopsin is a light-sensitive pigment of rods that is changed chemically by the absorption of light, thus initiating phototransduction.

• Cones contain photopsins (iodopsins) sensitive to different wavelengths of light, that is, red, green, and blue, thereby enabling colour discrimination.

• Light absorption leads to a series of chemical reactions which in turn change the shape of the photopigments.

• This alters the membrane potential of the photoreceptor cell and generates electrical signals.

Transmission Of Signals To The Brain

• The visual information travelling from the retina to the brain is now in the form of electrical signals.

• Photoreceptors synapse onto bipolar cells, which in turn synapse onto ganglion cells. The axons of ganglion cells make up the optic nerve, which sends the signals to the brain.

• The optic nerve transmits the signal to the visual cortex, which is the part of the brain that processes visual information.

• It is located in the occipital lobe in the back of the brain.

Image Processing In The Brain

• The visual cortex is responsible for interpreting visual information.

• It analyses edges, shapes, colours, and distances in the visual scene, allowing visual recognition of objects and their arrangement in space.

• Brain integration of visual information with other sensory modalities (e.g., sound, touch) creates a fully perceived environment, guiding our navigation and interactions with our surroundings.

Common Vision Disorders

Several common disorders of vision alter the way we see.

Myopia (nearsightedness)

• Difficulty seeing distant objects.

• It is caused by an eye that's too long or a too-curved cornea,

• This creates the condition whereby light focuses in front of the retina.

Hyperopia

• This means farsightedness or difficulty seeing close objects.

• It may be due to the eye being too short or the cornea too flat, which will cause light to focus behind the retina.

Astigmatism

• When there is an abnormal shape in the curve of the cornea or the lens, it may result in blurry vision and the condition where light will focus on more than one point in the retina.

Presbyopia

• A loss in the flexibility of the lens with ageing eventually leads to an inability to focus on close objects.

• Commonly occurs in people over the age of 40.

• These disorders can be genetic or develop over time.

• Some of the symptoms are blurry vision, tired eyes, and headaches.

• Treatments range from corrective lenses, such as glasses or contacts, to surgical ones like LASIK, and PRK.

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