Lab 7 - Visual System

Central Visual Pathways - The Cortex

Locate the optic nerve on the inferior surface of the whole brain. Each optic nerve contains uncrossed, myelinated axons of the retinal ganglion cells. Fibers in the optic nerve are thus axons of third-order (3°) afferents in the visual pathway. The second-order afferents are the bipolar cells. Identify the optic chiasm on the inferior surface of the brain. Fibers arising from the nasal half of each retina (representing the temporal visual hemifields) cross in the optic chiasm to the opposite optic tract. Notice the proximity of optic chiasm to the infundibulum (the pituitary stalk). Recall that fibers arising from the temporal areas of the retina (and representing the nasal visual hemifields) do not cross.

Locate the oculomotor nerves emerging from the midbrain within the interpeduncular fossa. Recall that the oculomotor nerve is positioned between the posterior cerebral artery and superior cerebellar artery and is vulnerable to vascular abnormalities. The abducens nerves emerge more caudally at the pontomedullary junction. Recall that the trochlear nerves emerge from the posterior surface of the midbrain and can only be visualized if the cerebellum is removed. These three cranial nerves (the abducens, trochlear and oculomotor) provide the motor innervation to the extraocular muscles.

In the anterior view of the brainstem (fig.1) follow the optic tract from the chiasm posteriorly to the lateral geniculate body of the thalamus. The optic chiasm is located at approximately the same rostral-caudal level as the anterior commissure.

Look for the components of the central visual pathway in fig.3. The fibers of the optic tract enter the superior colliculus via the brachium of the superior colliculus. The fibers of the optic tract may also synapse in the lateral geniculate body. The axons of the lateral geniculate body