Question

the pathway of trigeminal nerves, facial nerves, and vestibulcochlear nerves in more detail

Answer 1

PATHWAY OF TRIGEMINAL NERVE -

The trigeminal nerve arises by two roots(a smaller medial motor root and a larger sensory root) from the ventrolateral aspect of the pons.

The sensory root passes forward and laterally over the apex of the petrous temporal bone to enter the middle cranial fossa. Here it exhibits a rounded enlargement, the trigeminal (gasserian) ganglion.

The subarachnoid expansion over this portion of trigeminal nerve is called meckel's cave.

The convex distal surface of ganglion gives origin to three large divisions of the trigeminal nerve - ophthalmic,maxillary and mandibular.

1.The ophthalmic nerve (purely sensory) arises from the anteriolateral aspect of the ganglion and enters the lateral wall of the cavernous sinus where it lies below the trochlear nerve.

In the cavernous sinus, it divides into three branches :nasociliary, lacrimal and frontal. All these branches enter the orbit through the superior orbital fissure.

This nerve gives sensory supply to upper third of the face.

2. The maxillary nerve (purely sensory) arises from the ganglion and enter the lateral wall of the cavernous sinus where it occupies the lowest position and leaves the cavernous sinus to enter the pterygopalatine fossa through the foramen rotundum.

This nerve gives sensory supply to the middle third of the face.

3. The mandibular nerve is the largest division and mixed nerve ( sensory and motor ) arises from the trigeminal ganglion and immediately enters the infratemporal fossa through the foramen ovale.Here, just below the basal skull it is joined by motor root of the trigeminal nerve. Motor root accompanies the sensory root,, these passes deep at the side of trigeminal ganglion and leaves the skull through mandibular foramen to join the mandibular division of trigeminal nerve to form the trunk of mandibular nerve.

The following branch arise from mandibular nerve are -

Meningeal branch, nerve to medial pterygoid, masseteric nerve, deep temporal nerve, lateral pterygoid, buccal nerve, auriculotemporal nerve, lingual nerve, inferior alveolar nerve.

This nerve gives sensory supply to the lower third of the face and motor supply to muscles of mastication, mylohyoid, anterior belly of digastric, tensor palati and tensor tympani.

PATHWAY OF FACIAL NERVE -

This nerve consists of two distinct roots :a) a large medial motor root (the facial nerve proper) and b) a small lateral sensory root (the nervous intermedius). These two roots arise from the pontomedullary junction.

The roots of the facial nerve passes laterally and forward in the cerebello-pontine angle. The main trunk i.e motor root enters the external auditory meatus accompanied by the small sensory root (nervous intermedius of wrisberg), vestibule cochlear nerve and labyrinthine vessels. At the lateral end of meatus two roots unites to form the trunk of facial canal where it runs above the bony labyrinth of the internal ear and then bends posteriorly in the medial wall of the middle ear, forming genu of facial nerve.Then the nerve turns 90 degrees and runs in the posterior wall of the middle ear till it reaches the stylomastoid foramen at the base,through which it leaves the cranial cavity. Finally, it runs anteriolateral to enter the parotid gland, where it divides into five terminal branches.

The facial nerve gives off following branches -

-greater petrosal nerve

-nerve to stapedius

-chorda tympani nerve

-posterior auricular nerve

-nerve to posterior belly of digastric and stylohyoid muscles

-Terminal branches (temporal, zygomatic, buccal, mandibular and cervical) to the muscles of facial expression.

PATHWAY OF VESTIBULOCOCHLEAR NERVE -

The vestibular nerve arises from the lateral aspect of the pontomedullary junction,passestthrough the pontocerebellor angle and enters the internal acoustic meatus along with facial nerve and labyrinthine vessels.

The vestibular ganglion (also called scarpa's ganglion) is located on the vestibular nerve in the lateral part of the internal acoustic meatus. In the lateral part of the internal acoustic meatus, the nerve divides into three distinct branches : superior and inferior divisions and singular nerve. These branches innervation the sensory receptors for equilibrium in the membranous labyrinth of the internal ear.

The cochlear nerve also arises from the lateral aspect of the pontomedullary junction.At the medial end of the internal acoustic meatus, it enters into bony labyrinth of the middle ear and reaches the modiolus of the internal ear. In the modiolus,the cochlear nerve possesses a sensory ganglion called the spiral ganglion made of bipolar neurons. The peripheral processes of these neurons innervates the sensory receptor of hearing (the organ of corti).