What muscle is primarily controlled by the trochlear nerve?

The trochlear nerve, also known as cranial nerve IV, primarily innervates the superior oblique muscle, which plays a crucial role in eye movement. This muscle is responsible for downward and outward movement of the eye and is unique among the extraocular muscles; it is the only muscle innervated by the trochlear nerve.

Understanding the function of the superior oblique muscle reveals the significance of this nerve in coordinating complex eye movements, particularly in actions such as looking down when crossing the eyes. Furthermore, the trochlear nerve can be tested clinically, where a patient may present with difficulty in eye movements, specifically looking down, if this nerve is compromised.

In contrast, the other muscles listed in the choices are controlled by different cranial nerves. The inferior rectus muscle is innervated by the oculomotor nerve (cranial nerve III), while both the lateral rectus muscle, responsible for lateral eye movement, and the medial rectus muscle, which controls movement towards the nose, are innervated by the abducens nerve (cranial nerve VI) and the oculomotor nerve (cranial nerve III), respectively. This distinction underscores the specialized role of the trochlear nerve in eye muscle control.