AssessmentsAnatomy and physiology of the eye
Anatomy and physiology of the eye
USMLE® Step 1 style questions USMLE
A 75-year-old man presents with drooping of the right eyelid and diplopia. Physical examination shows the right eye is pointing downwards and outwards. Which of the following nerves is most likely affected in this patient?
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 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 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.
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.
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.
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.
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.
- "Medical Physiology" Elsevier (2016)
- "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)