Sensory root of trigeminal nerve

At a glance

English

Sensory root of trigeminal nerve

Latin (TA2)

Radix sensoria nervi trigemini

System

Nerve

Region

Head

Side

Paired, left and right

About the sensory root of trigeminal nerve

The sensory root, or portio major, is the bulkier of the two roots of the trigeminal nerve. It is formed by the central processes of the first-order neurons whose cell bodies sit in the trigeminal ganglion, in Meckel's cave near the apex of the petrous temporal bone. From the back of the ganglion the root passes backwards over the petrous apex and enters the lateral part of the pons at its middle, alongside the much thinner motor root, which lies on its medial side.

Inside the brainstem the incoming fibres sort themselves by the kind of sensation they carry. Fibres for fine, discriminative touch and pressure from the face end in the principal (chief) sensory nucleus, a cluster of second-order cells in the caudal pons close to where the root enters. The same nucleus takes conscious position sense from the jaw. Fibres for pain and temperature turn downwards in the spinal trigeminal tract to reach the spinal trigeminal nucleus, which stretches into the upper cervical cord. Proprioceptive fibres from the muscles of chewing are the exception: their cell bodies lie in the mesencephalic nucleus rather than in the ganglion.

From the principal nucleus most second-order axons cross the midline and climb in the ventral (anterior) trigeminothalamic tract to the ventral posteromedial nucleus of the thalamus on the opposite side. Information from the mouth and teeth takes a different route: a smaller dorsal trigeminothalamic tract carries it to the thalamus of the same side. From the thalamus the signal is relayed to the face area of the postcentral gyrus.

Clinically the root lies in the cerebellopontine angle, where a looping artery, most often the superior cerebellar artery, can press on it and is thought to cause many cases of trigeminal neuralgia. Surgeons relieve the pressure by microvascular decompression, and procedures that damage the root or ganglion on purpose are also used to control this pain. Tumours of the angle, such as vestibular schwannoma, may first show as reduced corneal sensation.

Course and supply

Arises from

The central processes of pseudounipolar neurons in the trigeminal ganglion, leaving its posterior concave border.

Ends in

Enters the lateral pons and ends in the principal sensory nucleus, the spinal trigeminal tract and nucleus, and, for some fibres, the mesencephalic nucleus.

Supplies

Carries all general sensation from the face, scalp front, eye, nasal and oral cavities, teeth and much of the dura into the brainstem.