Lab 9 - Descending Pathways to the Spinal Cord

The Tectospinal Pathway - Midbrain

Figure 1 is a transverse cut through the rostral midbrain. The fibers of the tectospinal tract arise from the deep layers of the superior colliculus, sweep anteromedially around the periaqueductal gray and oculomotor nuclei, decussate and descend as the tectospinal tract. The tract has not formed as yet at this level. The superior colliculus receives input from the retina (optic tract), diffuse areas of the cerebral cortex, basal ganglia, inferior colliculus, spinal cord and reticular formation. A major component of the tectospinal tract is involved in postural reflexes to visual and acoustic stimuli. The direct tectospinal tract is relatively small in humans.

Figure 2 is a transverse cut through the midbrain/pons junction. The fibers (crossed) of the tectospinal tract are located along the midline, just anterior to the medial longitudinal fasciculus. They remain in close association with the medial longitudinal fasciculus through the brain stem. Recall that the medial longitudinal fasciculus contains two ascending fiber groups that affect the extraocular motor nuclei. These fiber groups originate in the vestibular and abducens nuclei.