2-Minute Neuroscience: Trochlear Nerve (Cranial Nerve IV)

The trochlear nerve (cranial nerve IV) supplies one of the extraocular muscles: the superior oblique muscle. In this video, I discuss the anatomy and function of the trochlear nerve, as well as describe what can happen when the nerve is damaged.

TRANSCRIPT:

Welcome to 2-minute neuroscience, where I explain neuroscience topics in 2 minutes or less. In this installment I will discuss the trochlear nerve.

The trochlear nerve, also known as cranial nerve IV, is responsible for supplying one of the extraocular muscles of the eye: the superior oblique muscle. The superior oblique helps the eye to move down and out. To create this type of movement, the muscle passes through a pulley-like structure called the trochlea of the superior oblique, which is where the nerve gets its name.

The trochlear nerve originates in a small nucleus in the midbrain. The nerve fibers decussate, or cross over to the other side, of the brainstem before leaving the brainstem near the junction of the midbrain and pons. The trochlear nerve is the only cranial nerve that leaves the brainstem from the back, or posterior surface, of the brainstem. It’s also the only cranial nerve to completely originate from a nucleus contralateral to the structure it supplies.

The trochlear nerve is a very delicate nerve that is relatively easily damaged. Damage can be congenital or occur due to other causes like trauma. The symptoms of trochlear nerve palsy, however, are typically not as noticeable as those that result from damage to the oculomotor or abducens nerve. Because the superior oblique helps to move the eye downwards, when the nerve is damaged the eye tends to deviate upwards since there is no opposing force coming from the superior oblique. This can result in diplopia, or double-vision. Some patients will adopt a head tilt as a compensatory mechanism to better align the eyes and reduce the diplopia. If the palsy does not resolve on its own or through less invasive treatments, patients may undergo surgery to weaken an opposing muscle (usually the inferior oblique) to minimize the deviation.

References:

Vilensky JA, Robertson WM, Suarez-Quian, CA. The Clinical Anatomy of the Cranial Nerves: The Nerves of “On Old Olympus Towering Top.” 1st ed. John Wiley & Sons, Inc.; 2015.

Vanderah TW, Gould DJ. Nolte's The Human Brain. 7th ed. Philadelphia, PA: Elsevier; 2016.

Caption author (Arabic): Shwan Hameed