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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.