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Lab 9 (ƒ 10) - Cranial Nerve Nuclei and Brain Stem Circulation

Cranial Nerve V-Review

Figure 1(A) is a section taken at mid-pontine level. The spinal trigeminal nucleus and tract (somatosensory afferents of Cranial Nerve V, VII, IX, X) continue to be located anterior to the vestibular nuclei. Notice that the descending root fibers of the facial nerve are located medial to the spinal trigeminal nucleus and tract. Recall the location of the medial longitudinal fasciculus; we will use it as a landmark at more rostral sections.

Figure 1(B) is a section through the rostral half of the pons at the level of entry of the trigeminal nerve root (somatosensory and motor axons of Cranial Nerve V) into the pons tegmentum. Three nuclear groups associated with trigeminal nerve can be seen at this level.

• The main sensory trigeminal nucleus is a short column of cells that extends from the rostral pole of the spinal trigeminal nucleus up to the rostral part of the trigeminal root entry zone. These neurons convey discriminative touch and proprioception from the face area.
• The mesencephalic trigeminal nucleus is a slender column of cells found in the lateral margin of the gray forming the floor of the fourth ventricle. It extends from the level of the rostral border of the trigeminal root entry zone to the rostral midbrain. The mesencephalic trigeminal nucleus contains the cell bodies of 1° afferents forming proprioceptors in the jaw joints and jaw muscles.
• The third trigeminal nucleus, the trigeminal motor nucleus, appears in the middle third of the pons, medial to the main sensory trigeminal nucleus. It is located in the anterolateral region of the tegmentum, medial to the fibers of the entering trigeminal root and posterior to the superior olivary complex.  Some corticopontine fibers pass into the pons tegmentum to end bilaterally - with a slight contralateral dominance - in the trigeminal motor nucleus: Additional indirect cortical input is provided by the pons reticular formation.

Figure 1(c) is is a section through the lower midbrain near its junction with the pons. Only the mesencephalic trigeminal nucleus (1° proprioceptive afferents of Cranial Nerve V) is present near the anterolateral margin of the periaqueductal gray. The trochlear nucleus (axons exit brain as Cranial Nerve IV) can be seen in the periaqueductal gray immediately above the medial longitudinal fasciculus. This nucleus is located in the caudal midbrain at the level of the inferior colliculus. The cortical input to the trochlear nucleus is indirect and mediated by midbrain (superior colliculus and vertical gaze centers for saccades) and pontine (the superior vestibular nuclei for smooth pursuit) structures.