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


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.

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