Which action of extra-ocular muscle is spared in involvement of Occulomotor nerve :
## **Core Concept**
The oculomotor nerve (cranial nerve III) is responsible for controlling several extraocular muscles that move the eye. These include the medial rectus, superior rectus, inferior rectus, and inferior oblique muscles, which are involved in movements such as adduction, elevation, depression, and intorsion (rotating the top of the eye toward the nose). The oculomotor nerve also carries parasympathetic fibers that control pupil constriction.
## **Why the Correct Answer is Right**
The correct answer, **D. Superior oblique**, refers to the action of the superior oblique muscle. This muscle is responsible for intorsion (rotating the eye inward), with secondary actions of depression (especially when the eye is adducted) and abduction. The superior oblique muscle is innervated by the trochlear nerve (cranial nerve IV), not the oculomotor nerve. Therefore, in the case of oculomotor nerve involvement, the action of the superior oblique muscle is spared.
## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because the medial rectus muscle, responsible for adduction of the eye, is innervated by the oculomotor nerve. Therefore, its action is not spared in oculomotor nerve involvement.
- **Option B:** This option is incorrect because the inferior rectus muscle, which depresses the eye, is also innervated by the oculomotor nerve. Hence, its action is affected, not spared.
- **Option C:** This option is incorrect as it likely refers to another muscle innervated by the oculomotor nerve, given the context of the question.
## **Clinical Pearl / High-Yield Fact**
A key clinical point to remember is that in a third nerve palsy (oculomotor nerve palsy), the eye is typically deviated downward and outward (down and out) due to the unopposed action of the lateral rectus (innervated by the abducens nerve, CN VI) and the superior oblique muscles. The sparing of the superior oblique muscle's action explains this characteristic deviation.
## **Correct Answer:** D. Superior oblique