Trochlear nerve is the fourth of the 12 paired cranial nerves (i.e. cranial nerve IV). It is a motor nerve that innervates a single muscle; the superior oblique muscle of the eye.
The tendon of superior oblique muscle passes through a fibrous loop located anteriorly on the medial side of the orbital cavity. This fibrous loop is called “trochlea” (Latin for Pulley). The trochlear nerve is named after this structure.
Trochlear nerve emerges from the posterior aspect of midbrain just below the inferior colliculus. It then curves around the lateral side of the lateral side of the cerebral peduncle and runs forward towards the eye in the subarachnoid space.
The nerve passes between the superior- and posterior- cerebral arteries. It pierces the dura just under the free margin of the tentorium cerebelli.
It runs on the lateral wall of the cavernous sinus and finally enters the orbit through the superior orbital fissure. It then ends by supplying the superior oblique muscle.
Unique features of trochlear nerve:
There are certain features that are unique to the trochlear nerve as compared to other cranial nerves. These are:
- It is most slender of the cranial nerves and is the smallest in terms of the number of axons it contains.
- It has the greatest intracranial length.
- It is the only cranial nerve that crosses to the other side (decussates) before innervating the target.
- It is the only cranial nerve that exits from the posterior aspect of the brainstem.
The trochlear nerve is a motor nerve that supplies only one muscle, the superior oblique muscle. Thus it controls the movements of the eye brought about by this muscle.