Visual Anatomy

Can you describe what a beautiful sunset looks like? How about the face of your mother? Do you remember what it was like seeing your favorite film in the theatre?

You can probably describe all of these visuals in great detail. How you were able to see all of these items, however, may prove to be a bit more scientifically complex. This explanation will provide you with the knowledge surrounding vision, visual acuity, pathways, and the anatomy and physiology of it all.

• How do we see?
• What parts of our anatomy and physiology contribute to vision?
• What is the function of the photoreceptors?
• What is the vision process?

Visual Anatomy and Physiology

To understand the visual system, one must first know the basic physiology of vision itself. To put it plainly, vision is comprised of the refraction of light absorbed by the eye. This then follows a visual pathway that forms into a neural impulse, which is followed by the process of perception.

Before we discuss the pathways themselves, we will first go over the actual anatomy of the eye, and how to identify each region.

Fg. 1 Diagram of the eye, Wikimedia commons

Iris: The colored circular structure of the eye that is referred to when you identify your ‘eye color’.

Lens: Located directly behind the iris, the lens is a curved disk that is extremely important in vision. It bends and flexes its shape to help focus light refractions on the retina.

Cornea: The cornea lays just on top of the iris and lens. It acts as a clear protective barrier to the eye.

Sclera: This is the outer white layer of the eye.

Ciliary body: A circular-shaped muscle behind the iris. It is attached to the lens to help flex and bend its shape.

Vitreous body: A transparent and gelatinous substance that makes up the inner space of the eye

Choroid: Middle vascular layer of the eye.

Retina: One of the most essential visual structures. Located in the very back of the eye, it is home to the photoreceptors.

Fovea: Resting in the center of the retina and packed with cone photoreceptors, the fovea contributes to the most detailed vision, known as central vision.

Optic nerve: An abundance of nerve fibers attached to the back of the eye, that sends visual information to the brain.

The Photoreceptors

Photoreceptors are extremely essential to vision. The amount of these receptors is unique to humans. Located in the retina of the eye, these visual receptors are cells that contain photopigments, which convert light into neural impulses. There are two different types of these photoreceptors, with separate responsibilities in vision.

Rods

This type of photoreceptor is used for vision at night and in low lighting. In humans, these photoreceptors are very abundant but do not contribute to color vision.

Cones

Cones are light-sensitive cells that work best during the day. They are responsible for detailed vision and allow us to see color.

The Vision Process

Each anatomical component of the eye plays a crucial role in the visual pathway and contributes to how we see images of people, places, and objects. Now that the location and explanation of the anatomical features of the eye have been discussed, let's explore the step-by-step vision process. This is the very first process that takes place before we humans have any perception of the actual image we are seeing.

1. Light enters the eye and is refracted (bent) by the cornea.

2. The iris adjusts the pupil opening to the amount of light passing through.

3. The muscle called the ciliary body helps flex and shape the lens to focus the light on the retina.

4. The light is refracted onto the retina where the fovea lies.

5. Photoreceptors then convert the light into neural impulses that are carried through the optic nerve.

The Visual Pathway

The optic nerve then transports the neural impulse to the brain, where the visual pathway continues in the lateral geniculate nucleus (LGN).

The LGN is located at the mid-brain, in the thalamus. The neural impulse travels through the optic chiasm where they are crossed between the two brain hemispheres of the LGN.

Once the impulse reaches the LGN, all of the visual information is organized between the two visual fields. As you can see from the diagram, the information from the right visual field corresponds to the left hemisphere of the LGN, while the left visual field corresponds to the right hemisphere. From here, the information travels to the primary visual cortex near the back of the brain.

Fg. 2 Visual pathway in the LGN, Wikimedia commons

This displays the general overview of the clinical anatomy and physiology of the visual system before the brain can process the neural impulse into what we call perception.

Special Senses: Anatomy of the Visual System

With all of the features mentioned above surrounding the anatomy and physiology of the visual field, it is no wonder human eyesight includes helpful senses. These allow you to see images of the moving world in the safest and optimally possible way.

Color Vision

The ability to see color can prove useful in many aspects of life, such as seeing traffic lights or other means of identification. This ability is all thanks to the photoreceptors in the retina called cones. Each cone receptor corresponds to the three primary colors; red, blue, and green, which ultimately leads to color vision.

Accommodation

This is when the ring-shaped muscles attached to the lens of the eye, known as the ciliary body, contract and adjust the lens. The adjustment of the lens is what allows us to focus between either near or farther objects.

Visual Acuity

Have you heard of the phrase “20/20 vision”? This is the measurement of visual acuity. Essentially, 20 out of 20 represents the outcome of an eye test when a person can see an average amount of detail on a letter test from 20 feet away. Visual acuity is defined as the amount of detail and sharpness of one's visual perception. Once again, the cone photoreceptors contribute to this ability, as they are responsible for detailed vision.