Lab 5 - Somatosensory, Viscerosensory and Spinocerebellar Pathways

Spinal Trigeminal Pathway - Brainstem to Diencephalon

The images move from the brainstem to the diencephalon. View and identify the structures in the illustration.

Mid Pons

The fascicles of the trigeminal roots can be seen near the posterolateral margin of the pons tegmentum flanked by the main sensory trigeminal nucleus laterally and the trigeminal motor nucleus medially. Recall that at this level of the brain stem, the main sensory trigeminal nucleus replaces the spinal trigeminal nucleus. The axons of the main sensory trigeminal nucleus decussate and join the 2° spinal trigeminal afferents in the ventral trigeminothalamic tract. The axons of the trigeminal motor neurons exit the brain stem to innervate the muscle of the jaw.

Rostral Pons

At this level, the mesencephalic trigeminal tract and nucleus are found along the anterolateral margin of the periaqueductal gray. Recall that the mesencephalic trigeminal nucleus contains the cell bodies of the 1° afferentsinnervating jaw proprioceptors. Most of their central processes descend to terminate in the motor nucleus of the trigeminal nerve. The ventral trigeminothalamic tract, which contains the crossed 2° afferents of the spinal and main sensory trigeminal systems, remains in close association with the medial lemniscus.

Midbrain

The mesencephalic trigeminal tract and nucleus continue to be located within the anterolateral aspect of the periaqueductal gray. The ventral trigeminothalamic tract is located along the posterior aspect of the medial lemniscus.

Midbrain-Diencephalon

At this level, the ventral trigeminothalamic tract is still located along the posterior margin of the medial lemniscus. The fibers of both somatosensory tracts are located lateral to the red nucleus and are preparing to enter the thalamus.

Diencephalon

The ventral trigeminothalamic tract fibers mediating fine tactile discrimination and proprioception for the face (main sensory trigeminal nucleus fibers) terminate in the ventral posteromedial (VPM) nucleus of the thalamus. The axons of these 3° trigeminal afferents travel from the VPM through the posterior limb of the internal capsule to terminate in the ventrolateral aspect of the postcentral gyrus.