Central nervous system disorders

VII/1.3: Cranial nerves

VII/1.3.1: General remarks

Practically all of the twelve pairs of cranial nerves can be seen from the cerebral basis. The first two pairs are not considered as actual cranial nerves. When describing the cranial nerves, we are going to mention only a few characteristics which can be observed from the cerebral basis, but the introduction of their microscopic features and branch system will be set aside.

VII/1.3.2: Cranial nerve I; Olfactory nerve

The first pair of (not actual) cranial nerves are the olfactory nerves (CN I). Their widened ends are called olfactory bulbs, and lay above the ethmoid bone’s cribriform plate. The bulbs meet the olfactory filaments that come from the nasal cavity. From here, they continue as olfactory tracts in the sulcus with similar name. The tracts widen and turn into the olfactory trigone behind the sulcus. The trigone’s lateral side is the lateral olfactory stria, and the medial side is constituted by the medial olfactory stria. There is the anterior perforated area or substance between the sides. The oblique diagonal band of Broca runs behind it.

VII/1.3.3: Cranial nerve II; Optic nerve

Optic nerves (CN II) are also not considered as actual cranial nerves. Their fibers partly cross in the optic chiasm and run in the optic tract to the CGL as described above.

The following actual cranial nerves are all directly connected with the brainstem.

VII/1.3.4: Cranial nerve III; Oculomotor nerve 

The oculomotor nerve (CN III) exits the brainstem at the bottom of the interpeduncular fossa, at the oculomotor sulcus which is located medially from the cerebral crura.

VII/1.3.5: Cranial nerve IV; Trochlear nerve

The trochlear nerve (CN IV) is the only cranial nerve that doesn’t come out on the ventral surface of the brainstem. Instead, it starts dorsally from the midbrain tectum below the two inferior colliculi. It appears at the basal surface only after going round the brainstem in the ambient cistern and reaches the lateral side of the cerebral crus.

VII/1.3.6: Cranial nerve V; Trigeminal nerve 

The trigeminual nerve (CN V) exits the brainstem where the pons meets the brachium pontis. The finer band at the upper part of the nerve corresponds to the motor root, while the larger, more bulky part corresponds to the sensory root.

VII/1.3.7: Cranial nerve VI; Abducens nerve 

The abducent nerve (CN VI) originates at the border of the pons and the medulla oblongata, where the bulbopontine sulcus crosses the preolivary sulcus.

VII/1.3.8: Cranial nerve VII; Facial nerve

The facial nerve (CN VII) comes out from the lower part of the pons where the pons and the brachium pontis meet.

VII/1.3.9: Cranial nerve VIII; Acoustic or vestibulocochlear nerve

The acoustic or vestibulocochlear nerve (CN VIII) exits the brainstem right beside the facial nerve, and sends oblique fibers toward it, which can only be seen if we carefully pull the two nerves apart. For this reason the nerve is also called intermediate or intermediofacial nerve.

VII/1.3.10: Cranial nerve IX; Glossopharyngeal nerve;

Cranial nerve X; Vagus nerve;

Cranial nerve X; Accessory nerve

The glossopharyngeal (CN IX), vagus (CN X) and accessory (CN XI) nerves

leave the postolivary sulcus in this cranio-caudal order, and all of them have more fibers than the one before. Additional fibers join them from the cervical section of the spinal cord - these are called spinal root, while the fibers above are called cranial root.

VII/1.3.11: Cranial nerve XII; Hypoglossal nerve

The hypoglossal nerve (CN XII) comes out from the preolivary sulcus with usually more than ten fibers that usually gather into two stronger bundles.