Motor root of trigeminal nerve

At a glance

English

Motor root of trigeminal nerve

Latin (TA2)

Radix motoria nervi trigemini

System

Nerve

Region

Head

Side

Paired, left and right

About the motor root of trigeminal nerve

The trigeminal nerve leaves the brainstem as two roots on the lateral surface of the pons, at the junction with the middle cerebellar peduncle. The large one is the sensory root; the motor root is much smaller and lies just above and medial to it. Sometimes it is called the portio minor, against the portio major of the sensory root. It carries all the trigeminal nerve's motor fibres, and they end up entirely in the mandibular division.

Its fibres come from the trigeminal motor nucleus in the middle of the pons, an oval cluster of neurons in the upper lateral tegmentum, medial to the principal sensory nucleus of the trigeminal. The root passes forwards across the posterior cranial fossa with the sensory root and crosses the petrous ridge into the middle cranial fossa. There it runs beneath the trigeminal ganglion, without entering it, and passes through the foramen ovale, where it merges with the sensory fibres of the third division to make the mixed mandibular nerve.

The muscles it serves all derive from the first pharyngeal arch. They are the four muscles of mastication (masseter, temporalis and the medial and lateral pterygoids), and also mylohyoid, the anterior belly of digastric, and the two tensors, tensor veli palatini and tensor tympani. The motor nucleus forms the outgoing side of the jaw jerk: tapping the chin with the mouth slightly open stretches the closing muscles, sensory signals pass via the mesencephalic trigeminal nucleus, and the motor nucleus responds with a small jaw closure. Because the motor nucleus receives input from both cerebral hemispheres, a stroke on one side rarely weakens chewing much, whereas a brisk jaw jerk points to upper motor neuron disease on both sides.

Damage to the root or nucleus weakens the jaw muscles on the same side. The masseter and temporalis feel soft on clenching, the muscles waste with time, and the jaw swings towards the weak side on opening because the healthy lateral pterygoid pushes it across. Tumours of the posterior fossa or the cavernous region and lesions of the pons are typical causes.

Course and supply

Arises from

The trigeminal motor nucleus in the mid-pons; the root emerges from the lateral pons, above and medial to the sensory root.

Ends in

Passes beneath the trigeminal ganglion and joins the mandibular nerve at the foramen ovale, distributing through its motor branches.

Supplies

Motor to masseter, temporalis, the medial and lateral pterygoids, mylohyoid, the front belly of digastric, tensor veli palatini and tensor tympani.