The trochlear nerve [IV] is a cranial nerve that carries GSE fibers to innervate the superior oblique muscle, an extra-ocular muscle in the orbit. It arises in the midbrain and is the only cranial nerve to exit from the posterior surface of the brainstem (Fig. 8.54). After curving around the midbrain, it enters the inferior surface of the free edge of the tentorium cerebelli, continues in an anterior direction in the lateral wall of the cavernous sinus (Figs. 8.53 and 8.54; see Fig. 8.45), and enters the orbit through the superior orbital fissure.
The trigeminal nerve [V] is the major general sensory nerve of the head and also innervates muscles that move the lower jaw. It carries general somatic afferent (GSA) and branchial efferent (BE) fibers:
The GSA fibers provide sensory input from the face, anterior one-half of the scalp, mucous membranes of the oral and nasal cavities and the paranasal sinuses, the nasopharynx, part of the ear and external acoustic meatus, part of the tympanic membrane, the orbital contents and conjunctiva, and the dura mater in the anterior and middle cranial fossae.
The BE fibers innervate the muscles of mastication; the tensor tympani, tensor veli palatini, and mylohyoid muscles; and the anterior belly of the digastric muscle.
The trigeminal nerve exits from the anterolateral surface of the pons as a large sensory root and a small motor root (Fig. 8.54). These roots continue forward out of the posterior cranial fossa and into the middle cranial fossa by passing over the medial tip of the petrous part of the temporal bone (Fig. 8.53).
In the middle cranial fossa the sensory root expands into the trigeminal ganglion (Fig. 8.53), which contains cell bodies for the sensory neurons in the trigeminal nerve and is comparable to a spinal ganglion. The ganglion is in a depression (the trigeminal depression) on the anterior surface of the petrous part of the temporal bone, in a dural cave (the trigeminal cave). The motor root is below and completely separate from the sensory root at this point.
Arising from the anterior border of the trigeminal ganglion are the three terminal divisions of the trigeminal nerve, which in descending order are: the ophthalmic nerve (ophthalmic division [V1]).
the maxillary nerve (maxillary division [V2]), and the mandibular nerve (mandibular division [V3]).
The ophthalmic nerve [V1] passes forward in the dura of the lateral wall of the cavernous sinus (see Fig. 8.45), leaves the cranial cavity, and enters the orbit through the superior orbital fissure (Fig. 8.53).
The ophthalmic nerve [V1] carries sensory branches from the eyes, conjunctiva, and orbital contents, including the lacrimal gland. It also receives sensory branches from the nasal cavity, frontal sinus, ethmoidal cells, falx cerebri, dura in the anterior cranial fossa and superior parts of the tentorium cerebelli, upper eyelid, dorsum of the nose, and the anterior part of the scalp.
The maxillary nerve [V2] passes forward in the dura mater of the lateral wall of the cavernous sinus just inferior to the ophthalmic nerve [V1] (see Fig. 8.45), leaves the cranial cavity through the foramen rotundum (Fig. 8.53), and enters the pterygopalatine fossa.