Cranial Nerve Nuclei

Cranial Nerve Overview

Overview

Cranial nerves exit the brainstem and also the location of the cranial nerve nuclei.
9 (of the 12) cranial nerves originate from within the brainstem.
Those 9 cranial nerves originate from approximately 16 major cranial nerve nuclei.
We refer to these as "major" because we could parse these 16 nuclei even more finely into at least 31 cranial nerve nuclei, if we chose to.

Brainstem Longitudinal View (Gross Anatomy)

Key Related Anatomy

Medullary pyramids contain descending motor fibers.

The ovoid-shaped pons has a large basis, which comes from its large motor nuclei – like a bag of marbles.
Cerebral peduncles of the midbrain.
Major fibers create striations in the surface of the brainstem:
Within the cerebral peduncle, vertical striations come from the corticospinal tracts, which carry descending motor fibers.
Within the pontine basis, horizontal striations come from ponto-cerebellar crossing fibers.
Within the medullary pyramid, rostrally there are vertical striations from the corticospinal tracts, and caudally there are horizontal striations from the decussation of the pyramids at the cervico-medullary junction.
The thalami: the major sensory integration center.
Hypothalamus is between the cerebral peduncles.
Pituitary stalk (in midline)
Mammillary bodies.

Cranial Nerves

Midline cranial nerves: CNs 3, 4, 6, 12, and 11; they all innervate midline musculature (elsewhere we refer to them as the somatomotor group).
CN 3, the oculomotor nerve, exits the midbrain near midline.
CN 6, the abducens nerve, exits near midline from the pontomedullary junction.
Cranial nerve 12, the hypoglossal nerve, emerges paracentrally from the mid-medullary level. Just like CNs 3 and 6, CN 12 has a central (midline) course.
Cranial nerve 11, the spinal accessory nerve, emerges laterally from the cervical spinal cord – we include it here because its nuclei lie centrally.
CN 4, the trochlear nerve, wraps around the outside of the pons (even though it originates from midline of the lower midbrain).
CN 4 decussates (crosses midline) as it exits the brainstem – the trochlear nerve that emerges on the right comes from the left CN 4 nucleus – it's the only CN to make this decussation (with the exception of a subnucleus of CN 3).

solely special sensory set: CNs 1, 2, 8

Skip CN 1, which lies superior to the brainstem.
CN 2 is the bundle of crossing fibers of the optic nerve as it transitions to optic tract. If the pituitary enlarges, it can grow into and injure the optic chiasm.
CN 8, the vestibulocochlear nerve is at the pontomedullary junction.
The pharyngeal arch set, with CN 7, the facial nerve.
CN 9, the glossopharyngeal nerve, exits laterally from the medulla.
CN 10, the vagus nerve, is below CN 9.
CN 5, the trigeminal nerve, exits the mid/lower pons.

Brainstem Sections (Radiographic Anatomy)

Axial Sections

Midbrain

Review midbrain.
Posteriorly, are the colliculi, which relay visual and auditory information.
The periaqueductal gray area surrounds the cerebral aqueduct (of Sylvius).
Anteriorly are the cerebral peduncles (aka crus cerebri), which look like Mickey Mouse ears.
The anterior localization of the cerebral peduncles helps us remember that throughout the brainstem, motor tracts localize anteriorly (whereas sensory pathways mostly localize posteriorly).

Then, from anterior to posterior:

Cranial nerve nuclei:

Medial

In the 1950s, R. Warwick created a classic model for the oculomotor complex, and although details of this model have been updated over time, its basic construct remains unchanged.
The model shows that the superior rectus subnuclei project contralaterally, the single levator palpebrae subnucleus projects bilaterally, and the remaining subnuclei project ipsilaterally.
As an important clinical corollary, levator palpebrae subnucleus lesions naturally cause bilateral eye muscle palsies and superior rectus subnucleus lesions also produce bilateral palsies because when the superior rectus fibers exit their subnucleus, they immediately pass through the contralateral subnucleus.

Lateral Sensory Nucleus

Pons

Review the pons: the mid-brainstem level.
Anteriorly is the pontine basis, which comprises representative pontine nuclei and pontocerebellar fibers, which cross into the middle cerebellar peduncle as an important step in the corticopontocerebellar pathway, which modulates movement.

Medial Motor Nuclei

Lateral Sensory Nuclei:

Medulla

Review medulla.
Anterior region: the medullary pyramids, which are the medullary correlate of the cerebral peduncles, which carry the descending corticospinal tract fibers – they decussate at the cervico-medullary junction.
Inferior olive lies just behind the medullary pyramids, which contains climbing fibers.
The inferior olives project to the contralateral cerebellum as part of the clinically important triangle of Guillain-Mollaret.

Medial Motor Nuclei

Lateral Sensory Nuclei

The most medial-lying of the sensory nuclei is the solitary tract nucleus of CNs 7, 9, and 10.
The medullary extension of the vestibulocochlear nucleus, CN 8
The spinal trigeminal nucleus, CN 5.