I remember in my first year of medical school how much time my roomate and I spent trying to figure out the eye muscles. I honestly dont think I truly understood it until I started studying for the Step 1 exam. This was the case for most other subjects as well.
I want to say at the beginning that the exact details of eye muscle functioning are probably not the highest yield material for Step 1 studying. Also, explaining this in words is definitely not the most effective way of explaining the eye. Animation would be ideal here, alas. With that said lets get into this crazyness.
The first thing I want to point here is a foundational point. The eye muscles and the eye socket itself point outward at about a 20 degree angle away from the midline. Let me be clear: NOT the eyeball but rather the orbit and the majority of the muscles come out of the skull at a 20 degree angle away from the midline. You can see this from the picture. The eyeball itself however is pointed forward as you can see.
THE SUPERIOR RECTUS EXAMPLE
Lets look at the superior rectus (the central muscle in the image here) to get an idea of how this particular non-alignment plays out. If you look carefully at the insertion point of the superior rectus here we can see that if it were to contract not only would the eye turn upwards it would also intort. To visualize intorsion lets first imagine a point on the very top of the eyeball – the exact center of eye in the image (since it is a superior view). Intorsion would mean this point moves medially towards the midline causing the eye to essentially rotate about its own axis. Now we can better imagine the action of the superior rectus: eye elevation and slight intorsion. Intorsion, however, would give us a tilted view of the world and with each eye intorting if we wanted to look up, things would appear very confusing.
The question then arises: How can we look up without causing intorsion of the eye?
To prevent this intorsion nature ingeniously created another muscle (the inferior orbital in this case) to essentially cancel out the intorsion by causing extorsion (outward rotation of the eye about its axis). In addition to extorsion, the inferior orbital elevates the eye as well. So to recap we have two muscles working in concert with each other: one causing elevation and intorsion (superior rectus) and the other causing elevation and extorsion (the inferior orbital). The intorsion and extorsion cancel each other out and the eye can look up without tilting.
Look at the following image to get a better understanding of how the inferior orbital cause elevation and extorsion. To really see if you understand this concept try to work out for yourself using these images how the eye might handle looking down. To get you started, realize that in order to look down the eye would need to use both the inferior rectus as well as the superior orbital. Which one would cause the extorsion? Which one causes the intorsion? Do you see how this cancels out to produce leveled depression of the eye?
TESTING THE SUPERIOR AND INFERIOR ORBITAL MUSCLES
One last point I want to make has to do with how we test the superior and inferior orbital muscles. Lets use the superior orbital muscle as an example. In order to test whether it is working generally we ask the patient to turn the eye medially (adduct) and then look down. The reason we do this is to remove the depressing ability of the inferior rectus. If, after removing the inferior rectus’ depressing ability, the eye can look down then the superior orbital muscle is functioning.
To understand how turning the eye medially “removes” the inferior rectus’ depressing ability we need to revisit the idea that the major eye muscles are coming out of the skull at roughly 20 degrees from the midline (see image above). If, for example, the right eye turns medially then its central axis line would be roughly perpendicular to the line of the inferior rectus. At this point if it were the only muscle to contract it would cause the eye to extort or rotate outwardly about its own axis. In reality the other muscles would prevent this from happening.
Now that the depressing ability of the inferior rectus is removed by turning the eye medially would can test the superior orbital muscle ability to depress the eye and therefore its functioning. The Inferior Orbital muscle is essentially the same idea. We test it by turning the eye medially and then looking up instead.
As always I am more than open to comments and discussion. Good luck with the studying!