AssessmentsAnatomy and physiology of the eye
USMLE® Step 1 style questions USMLE
A 25-year-old woman comes to the office because of blurred vision that has been worsening over the past year. The blurred vision occurs both at far and near distances and without associated eye pain, photophobia, erythema, or discharge during this time. Her past medical history is significant for myopia. Upon manifest refraction, you find that the patient's best visual improvement is only achieved with the addition of cylindrical power to the lens on the right eye. Which structure is most likely responsible for this condition?
Our eyes allow us to visualize the world around us. They do this by converting light waves into neural signals so that our brains can process them.
The eye itself is shaped like a sphere that is elongated horizontally, as opposed to being perfectly round, and only the anterior one-sixth of the eye is visible. The rest of the eye is contained within the orbit, or eye socket, of the skull.
Now, the eye consists of three layers: the outermost fibrous layer, the middle vascular layer, and the inner neural layer.
The outer fibrous layer contains two main structures: the sclera and the cornea.
The sclera makes up the majority of the outer layer and is the white portion of the eye. It’s like a tough, fibrous covering that protects the more delicate structures within the eye and it also acts as an anchoring point for the extrinsic eye muscles to attach to.
The sclera is like a wall that’s built around the eye, that only has a tiny opening at the back to let the optic nerve through.
As the sclera approaches the anterior portion of the eye it reaches a transition point known as the corneal limbus where it becomes the cornea.
The cornea itself is a transparent, dome shaped clear layer that covers the iris and the pupil. It allows light to enter the eye, and its curved shape helps focus light on the retina in the back of the eye.
At the periphery of the cornea, there are stratified squamous epithelial cells which continually divide and regenerate the cornea, and they help to heal after a corneal injury or abrasion.
The cornea doesn’t contain blood vessels and therefore immune cells can't access the cornea. As a result, it’s one of the few parts of the body that is considered "immune privileged" since it can be transplanted without the fear of an immune response and organ rejection.
Moving inwardly from the fibrous layer, the next layer of the eye is the middle vascular layer, which is also called the uvea.
Structures within this layer include the iris, pupil, choroid, and ciliary body.
The word "iris" derives from a Greek word meaning "rainbow", and that makes sense since the iris is what we think of as the colorful part of the eye.
Eye color is determined by the amount of melanin in the iris.
People with a high concentration of melanin have dark brown eyes, those with medium amounts have green eyes, and people with low concentrations of melanin have blue eyes.
The iris sits behind the cornea and it is composed of two distinct groups of muscle: the sphincter pupillae muscle, sometimes referred to as circular muscle, and the dilator pupillae muscle, otherwise known as radial muscle. These muscles help control the the size of the pupil, which is the central opening at the center of the iris.
The sphincter pupillae muscle surround the iris like a tiny circle, and in bright light this muscle tightens around the pupillary opening, reducing the size of the pupil. When it’s dark, the dilator pupillae muscle pulls the iris radially or outwardly from the pupil which helps increase the diameter of pupillary opening, allowing more light to enter the eye.
After light passes through the cornea and pupillary opening of the iris, it reaches a biconvex transparent structure called the lens which is located in a space called the posterior chamber.
Biconvex means that the lens is curved on both sides.
The lens itself can bend, allowing it to become flatter or rounder, and this in turn bends the light entering the eye.
A structure called the ciliary body, controls the degree to which the lens becomes flatter or rounder.
The ciliary body includes the ciliary muscle and tiny projections from the ciliary muscle called ciliary processes.
The ciliary processes connect to suspensory ligaments which attach directly to the lens and hold the lens in place behind the iris as well as help it change shape.
When the ciliary muscles relax the ciliary processes pull on the suspensory ligaments like a taut rope, and that makes the lens flatter.
Finally, the last structure of the vascular layer is a membrane called the choroid which is full of blood vessels that provide nutrients to most of the eye.
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- "Physiology" Elsevier (2017)
- "Human Anatomy & Physiology" Pearson (2018)
- "Principles of Anatomy and Physiology" Wiley (2014)
- "Pupil shape as viewed along the horizontal visual field" Journal of Vision (2013)