Lab 6 - Auditory, Vestibular, Gustatory and Olfaction Systems

Central Auditory Pathway - Pons (continued)

The images in this section are all from the pons. Starting at the figure 1, scroll up toward the midbrain. The text that follows provides explanations for each section.

Figure 1, Layer A was taken at the pontomedullary junction where rootlets of both the facial (VII) nerve and the abducens (VI) nerve can be seen in the pons tegmentum. Notice that the pyramidal tracts (corticofugal fibers) are buried within the pons base and surrounded by pontine nuclei and the middle cerebellar peduncle. Recall that the middle cerebellar peduncle is formed by crossed axons from the pontine nuclei. In the floor of the fourth ventricle, the sulcus limitans is a shallow groove between the facial colliculus and the vestibular (sensory) area. Observe that parts of the genu (knee) of the facial (VII) root are located on the medial and lateral aspects of the abducens nucleus. Notice also the abducens (VI) root coursing anterior to the abducens nucleus. Which extraocular muscle does the abducens nerve innervate? (6-3). Locate the following somatosensory structures: the medial lemniscus, spinothalamic tract, ventral trigeminal lemniscus, the spinal trigeminal nucleus and tract.

The 2° auditory afferents, which originated in the cochlear nuclear complex, either terminate in the ipsilateral superior olivary complex, or cross the midline to terminate in the contralateral superior olivary complex, or bypass the superior olivary complex. The crossing 2° afferents form the trapezoid body which is a conspicuous bundle of transverse fibers lining the anterior floor of the pontine tegmentum. The tertiary auditory fibers and the secondary auditory fibers, which bypass the superior olivary complex, collect to form the lateral lemniscus, lateral to the superior olive. It appears lightly stained because it contains the cell soma of the nucleus of the trapezoid body (a nucleus which is part of the superior olivary complex) and of the pontine reticular formation.

Figure 1, Layer B is at the mid pons at the level of the superior cerebellar peduncle, which makes its appearance and forms the lateral wall of the fourth ventricle. In the floor of the fourth ventricle, the median eminence replaces the facial colliculus. Lateral to it, the sulcus limitans separates it from the vestibular (sensory) area that is much reduced in size. The corticofugal fibers continue to be surrounded by the pontine nuclei whose decussated axons form the middle cerebellar peduncle in the pons base. The crossed sensory tracts are located in a cluster in the pons tegmentum. These include the medial lemniscus, spinothalamic tract, ventral trigeminal lemniscus and lateral lemniscus, which all appear in the anterolateral "corner" of the pons tegmentum. Notice also the main trigeminal sensory nucleus which sends most of its axons across the midline. The mesencephalic trigeminal nucleus and tract are located in the angle formed by the lateral wall and floor of the fourth ventricle.

The entry of the trigeminal nerve into the pons tegmentum marks the rostral extent of the superior olivary complex. The 2° auditory afferents terminate ipsilaterally within the superior olivary complex, or cross the midline in the trapezoid body to terminate in the contralateral superior olivary complex or bypass the superior olivary complex. Many neurons in the superior olivary complex receive bilateral input from the two cochlear nuclear complexes and are responsive to stimulation of either ear. The axons of the 3° (superior olivary complex) afferents ascend in the lateral lemniscus. Some of the superior olivary neurons send their axons back to the cochlea and cochlear nuclear complex and form the olivocochlear bundle. Recall that olivocochlear bundle provides neural efferent feedback to the ear.

Figure 1, Layer C is a section through the pons-midbrain junction. Notice that the superior cerebellar peduncles have shifted into the core of the tegmentum. The fourth ventricle is narrowing to form the cerebral aqueduct. The corticofugal fibers (corticopontine, corticobulbar and corticospinal) continue to be surrounded by the pontine nuclei. The medial lemniscus, spinothalamic tract, and ventral trigeminal lemniscus line part of the floor of the tegmentum. Locate the mesencephalic trigeminal tract and nucleus. Observe where the rootlets of the trochlear (IV) nerve exit the brain stem on its posterior surface.

The lateral lemniscus is shifted laterally and posteriorly and with the inward shift of the superior cerebellar peduncles now lies near the posterolateral margin of the brain stem. The light areas within the core of the lateral lemniscus are the cell soma of the nuclei of the lateral lemniscus. Some of the fibers in the lateral lemniscus (2° and 3° afferents) terminate within these nuclei. Their cells (3° and 4° afferents) send their axons in the lateral lemniscus to the inferior colliculus.