Physiology of the human eye

Dr. Mona Koaik, MD, MSc. Ophthalmologist

Duration:

The human eye is a complex organ that works through a series of intricate processes involving light, lenses, and neural signaling. Here’s a breakdown of its physiology:

Light refraction is the key to how the eye creates vision. Light enters the eye through the cornea, which acts as a clear, protective outer covering. The cornea bends or ‘refracts’ the light towards the pupil, which is the black circular opening in the center of the iris.

The iris is the colored part of the eye, which adjusts the size of the pupil to regulate the amount of light entering the eye. In bright conditions, it contracts to reduce the pupil size which restricts too much light from entering the eye, and in dim conditions, it dilates to increase the pupil size which allows more light in, giving you better vision when it’s dark out.

Beyond the pupil lies the lens which is a flexible, transparent structure that further refracts light to focus it precisely onto the retina. Muscles attached to the lens can change its shape, allowing the eye to focus on objects at different distances. This process is known as accommodation.

Light focused by the cornea and lens forms an inverted image on the retina. Once the brain receives the image, it automatically inverts it again so that we see things correctly. The retina lines the inner surface at the back of the eye and contains millions of photoreceptor cells called rods and cones that convert light into electrical signals.

This stage of the process is called phototransduction. Rods and cones contain light-sensitive pigments (like rhodopsin in rods and opsins in cones) that change shape when exposed to light. This change triggers a cascade of chemical and electrical signals, ultimately generating nerve impulses that travel to the brain for interpretation.

These nerve impulses are called signal processing. The nerve impulses travel through the retina to the optic nerve, which carries them to the vision centre in the brain. The brain is able to process and combine both signals coming from both eyes in order to create a unified visual perception.

The part of the brain that receives and processes these signals from the eye is called the visual cortex, which is located in the occipital lobe of the brain. It’s here that the brain interprets the electrical signals as images and enables us to perceive color, shape, depth, and movement.

Furthermore, how the human eyes are placed in the skull allows for binocular vision. This is where each eye sees a slightly different image. This disparity helps with depth perception and spatial awareness.

In summary, the human eye functions like a complex optical instrument, converting light into electrical signals that the brain then interprets as clear images that have color, shape and depth perception. Each part of the eye plays a crucial role in this process, from focusing light onto the retina to transmitting visual signals to the brain which then forms images in our brain known as vision.

Presenter: Dr. Mona Koaik, Ophthalmologist, Ottawa, ON

Local Practitioners: Ophthalmologist