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Lab 3 (ƒ5) - Somatosensory, Viscerosensory and Spinocerebellar Pathways

Spinal Trigeminal Pathway - Brainstem to Diencephalon (continued)

The images move from the brainstem to the diencephalon. Moving to the next section (moving to the right through the images) is moving rostrally. At each level read the corresponding section of text and identify the bold structures.

Mid Medulla

The 1° somatosensory afferents of the vagus nerve innervate the dura in the posterior cranial fossa and part of the skin of the ear. These afferents join the spinal trigeminal tract and some end in the spinal trigeminal nucleus. Small fascicles of the 2° afferents cross the midline and collect in the ventral trigeminothalamic tract. The ventral trigeminothalamic tract is located lateral to the medial lemniscus. The spinal trigeminal pathway is considered to be the cranial homologue of the spinothalamic pathway: That is, it carries information about crude touch, temperature and pain from the head and face. Lesions of the spinal trigeminal tract result chiefly in loss or diminution of pain and thermal sense in the face area, with little effect on tactile sensibility of the face. In the medulla, the spinothalamic tract lies near the lateral surface just posterior to the inferior olive. The spinoreticular fibers, which ascend with the spinothalamic tract, and 2° spinal trigeminal afferent fibers give off branches that terminate in the medullary reticular formation.

Upper (Open) Medulla

The glossopharyngeal (IX) nerve carries, in addition to visceral and gustatory afferents, cranial somatosensory afferents that innervate the mucous membranes of the posterior oral cavity, the middle ear and Eustachian tube. These 1° somatosensory afferents join the spinal trigeminal tract and some terminate in the spinal trigeminal nucleus. At all levels of the spinal trigeminal nucleus, the 2° axons cross the midline to collect in the ventral trigeminothalamic tract. This tract remains in close proximity to the medial lemniscus.

Caudal Pons

The descending limb of the facial root passes lateral to the abducens nucleus and sweeps anteriorly on its course out of the pons. The facial nerve contains cranial somatosensory afferents that innervate the skin of the external ear. These fibers enter the spinal trigeminal tract and some descend to terminate in the spinal trigeminal nucleus. The spinal trigeminal nucleus, the cranial homologue of the substantia gelatinosa, sends its axons across the midline to travel in the ventral trigeminothalamic tract and terminate in the ventral posteriomedial (VPM) thalamus. The spinothalamic tract is located at the lateral tip of the medial lemniscus. Some of the spinoreticular fibers traveling with the spinothalamic fibers and some branches of 2° spinal trigeminal afferents are leaving to terminate in the pontine reticular formation.

Mid Pons

The trigeminal nerve is the largest cranial nerve and enters the brain stem at mid pontine levels. The ventral trigeminothalamic tract, which contains the crossed 2° afferents of the trigeminal system, remains in close association with the medial lemniscus and spinothalamic tract. The spinothalamic tract is located near the lateral tip of the medial lemniscus. The spinoreticular fibers and 2° spinal trigeminal afferents continue to give off branches to the pontine reticular formation.

Rostral Pons

The ventral trigeminothalamic tract remains in close association with the medial lemniscus and spinothalamic tract. The spinothalamic tract is located at the posterolateral tip of the medial lemniscus. Spinoreticular fibers and branches of 2° spinal trigeminal afferents continue to end in the pontine reticular formation and in the periaqueductal gray surrounding the cerebral aqueduct.

Lower Midbrain

The ventral trigeminothalamic tract is located along the posterior aspect of the medial lemniscus. The crossed (predominantly) and uncrossed 2° and 3° spinothalamic tract afferents continue to be located at the posterolateral tip of the medial lemniscus. The spinoreticular fibers and 2° spinal trigeminal afferents continue to send branches to the periaqueductal gray.

Upper Midbrain

At this level, the ventral trigeminothalamic tract is located along the posterior margin of the medial lemniscus. The spinothalamic tract is located at the posterolateral tip of the medial lemniscus. It contains 2° neospinothalamic afferents and 3° paleospinothalamic afferents that are predominantly crossed.

Midbrain (Coronal)

The component of the ventral trigeminothalamic tract carrying fibers from the spinal trigeminal nucleus is terminating in the ventral posteromedial (VPM) nucleus (fast pain sensation) and in the intralaminar nuclei (slow pain, temperature) of the thalamus. The VPM can be identified as the pale finger shaped area located along the superior margin of the medial lemniscus just below the centromedian nucleus. The 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. The intralaminar nuclei project diffusely to several cortical areas. Locate the centromedian nucleus and nucleus parafascicularis, which form the intralaminar thalamic nuclei. The axons of the VPL travel in the posterior limb of the internal capsule to end in the postcentral gyrus of the parietal lobe.