Notes
Cranial Nerve Overview
Sections
Brainstem Longitudinal View (Gross Anatomy)
Overview
- Here, we'll learn the anatomy of the cranial nerves as they exit the brainstem and also the location of the cranial nerve nuclei.
- Start a table, so we can set up our understanding of the cranial nerve nuclei.
- Denote that 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.
Key Related Anatomy
- Let's start with an anterior view of the brainstem, which is how we commonly study the brainstem in anatomy lab.
- First, label the superior-inferior orientational plane.
- To begin, draw the cervical spinal cord.
- Next, draw the medulla as a narrow column – think of it as an extension of the spinal cord.
- Specify its medullary pyramids, which contain descending motor fibers.
- Draw the ovoid-shaped pons and indicate its large basis, which comes from its large motor nuclei – like a bag of marbles.
- Next, draw the cerebral peduncles of the midbrain.
- Now, let's indicate the major fibers that create striations in the surface of the brainstem, so we can understand the source of these striations.
- Within the cerebral peduncle, show that vertical striations come from the corticospinal tracts, which carry descending motor fibers.
- Within the pontine basis, show that horizontal striations come from ponto-cerebellar crossing fibers.
- Within the medullary pyramid, show that 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.
- Now, include the surrounding thalamic structures for anatomical context.
- Draw the thalami, the major sensory integration center.
- Then, between the cerebral peduncles, draw the hypothalamus.
- Specify the pituitary stalk (in midline)
- And then the mammillary bodies.
Cranial Nerves
- Now, let's show the cranial nerves as they exit the brainstem.
- We'll start with the midline cranial nerves: CNs 3, 4, 6, 12, and 11; they all innervate midline musculature (elsewhere we refer to them as the somatomotor group).
- Show that CN 3, the oculomotor nerve, exits the midbrain near midline.
- Then, show that CN 6, the abducens nerve, exits near midline from the pontomedullary junction.
- Now, show that 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.
- Show that cranial nerve 11, the spinal accessory nerve, emerges laterally from the cervical spinal cord – we include it here because its nuclei lie centrally.
- Now, show that CN 4, the trochlear nerve, wraps around the outside of the pons (even though it originates from midline of the lower midbrain).
- Write that 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
- Next, let's draw the solely special sensory set: CNs 1, 2, 8.
- We skip CN 1, which lies superior to the brainstem.
- Draw the optic chiasm of CN 2, which is the bundle of crossing fibers of the optic nerve as it transitions to optic tract. We see that if the pituitary enlarges, it can grow into and injure the optic chiasm.
- Next, at the pontomedullary junction, draw CN 8, the vestibulocochlear nerve.
- Then, next to it, start the last CN set: the pharyngeal arch set, with CN 7, the facial nerve.
- Now, exiting laterally from the medulla, draw CN 9, the glossopharyngeal nerve, and below it, draw CN 10, the vagus nerve.
- Then, draw CN 5, the trigeminal nerve, exiting the mid/lower pons.
Brainstem Sections (Radiographic Anatomy)
Overview
- Next, let's draw axial sections of the brainstem and their cranial nerve nuclei in radiographic view.
- For each section, show that anterior will be the front and posterior will be the back.
Midbrain
- First, draw the midbrain.
- Posteriorly, draw the colliculi, which relay visual and auditory information.
- Draw the periaqueductal gray area, which surrounds the cerebral aqueduct (of Sylvius).
- Then, anteriorly, draw 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, draw the:
- Bilateral, thinly-shaped, substantia nigra, which are essential for motor activation.
- Then draw the bilateral, circular-shaped, red nuclei.
Now, let's include the cranial nerve nuclei.
Medial
Medially, draw the motor nuclei:
- Cranial Nerve 3
- The oculomotor complex of CN 3 in midline.
- Draw the Edinger-Westphal nucleus of CN 3, which is a key autonomic part of this complex.
- Oculomotor complex anatomy is quite "complex.
- 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.
- Cranial Nerve 4
- Next, draw the trochlear nucleus of CN 4 adjacent to CN 3.
Lateral
Laterally, draw the sensory nucleus:
- Cranial Nerve 5
- The mesencephalic trigeminal nucleus of CN 5 (which is a sensory nucleus).
Pons
- Next, drop down a level and draw the pons: the mid-brainstem level.
- Demarcate the pontine basis, anteriorly, 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
Medially, draw the motor nuclei:
- Cranial Nerve 6:
- In midline, draw the abducens nucleus of CN 6.
- Cranial Nerve 5:
- And lateral to it, draw the motor trigeminal nucleus of CN 5.
- Cranial Nerve 7:
- Then, the facial nucleus of CN 7.
- Next, the superior salivatory nucleus of CN 7.
Lateral
Laterally, draw the sensory nuclei:
- Cranial Nerve 5:
- Now, the principal sensory nucleus of CN 5.
- Cranial Nerve 8
- Then, the vestibulocochlear nucleus of CN 8.
Medulla
- Now, draw the medulla.
- Cap off its most anterior region with the medullary pyramids: the medullary correlate of the cerebral peduncles, which carry the descending corticospinal tract fibers – they decussate at the cervico-medullary junction.
- Now, draw the inferior olive 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
Medially, draw the motor nuclei:
- Cranial Nerve 12:
- Start with the hypoglossal nucleus, CN 12
- Cranial Nerve 10:
- Then, the dorsal motor nucleus of the vagus, CN 10
- Cranial Nerve 9:
- Then, the inferior salivatory nucleus, CN 9
- Lastly, the nucleus ambiguus, CNs 9 and 10.
Lateral
Then, draw the sensory nuclei:
- Start with the most medial-lying of the sensory nuclei, the solitary tract nucleus of CNs 7, 9, and 10.
- Then, the medullary extension of the vestibulocochlear nucleus, CN 8
- Finally the spinal trigeminal nucleus, CN 5.