Cranial Nerve Nuclei & Functions
Cranial Nerve Nuclei & Functions
Overview
- Here, we will create an overview of the organization and functions of the cranial nerve nuclei.
- We will use the embryological cell columns to categorize the anatomy and physiology of the cranial nerves as a helpful mnemonic – don't be alarmed by the embryologic vernacular, ultimately it will serve as helpful memory device.
Coronal Brainstem
First, draw a coronal brainstem:
Label the midline of our diagram.
- Next, use the sulcus limitans to split the left side of the brainstem.
- Show that it divides each half of the brainstem into the medial-lying Motor/Efferent division, derived from the basal plate, and lateral-lying Sensory/Afferent division, derived from the alar plate.
General Anatomy: Bedside Approach
Overview
- First, let's create an overview of the positions of the cranial nerve nuclei that we can easily recreate at the bedside.
Medial
- Along the medial half of the motor division, we'll draw the General Somatic Efferent cell column – they supply striated muscle with volitional actions.
General Somatic Efferent (GSE)
Include:
- CN 3 in the superior midbrain
- CN 4 in the inferior midbrain
- CN 6 in the pons
- CN 12 spanning the height of the medulla
- CN 11 in the cervical spinal cord.
- Soon we'll see that this set of CNs activates midline musculature, which helps us remember that they all lie in midline.
General Visceral Efferent (GVE)
- Next, to them, we'll draw the parasympathetic motor nuclei, the General Visceral Efferent cell column, which supply visceral musculature – remember "REST and DIGEST" as hint for their functions.
Include:
- Small subnuclei of CN 3 in the superior midbrain;
- CN 7 in the inferior pons;
- CN 9 in the superior medulla.
- And a large nucleus of CN 10 in the medulla.
Lateral
- In the lateral motor area, we'll draw the Special Visceral Efferent cell column, which is analogous to the General Visceral Efferent column in that it supplies striated muscles with volitional control but is derived from special embryological origins: the pharyngeal arches.
Special Visceral Efferent (SVE)
Draw:
- CN 5 in the mid pons,
- CN 7 in the inferior pons,
- CNs 9 and 10 spanning the height of the medulla.
Special Visceral Afferent/General Visceral Afferent (SVA/GVA)
- Next, in the medial sensory region, draw, the sole nucleus of the Special Visceral Afferent/General Visceral Afferent cell column, which combines CNs 7, 9, and 10 and spans the height of the medulla.
- As it's cell column suggests, this provides numerous functions, which are, again, special in that they are derived from the embryological pharyngeal arches.
Special Sensory Afferent (SSA)
- Lateral to this, draw the sole nucleus of the Special Sensory Afferent cell column.
- Draw CN 8 from the pons into the medulla.
- We'll see that this nucleus supplies one of the special senses.
General Somatic Afferent (GSA)
- Next, at the lateral edge of the brainstem, draw the General Somatic Afferent cell column, which is really just one nucleus CN 5, spanning the brainstem into the upper cervical spinal cord, but this nucleus has important subnuclei, so draw it specifically as having:
- A large midbrain/pontine component,
- A small pontine component
- A long component from the pons into the cervical spinal cord.
Simple Rostral/Caudal Organization: Primordial Fish
- One simplistic way to recall this rostral–caudal organization is to imagine that you are a primordial fish swimming through the great sea.
- First, you smell food (cranial nerve 1, the olfactory nerve);
- then, you visualize it (cranial nerve 2, the optic nerve);
- next, you fix your eyes on it, for which you use your extraocular eye muscles — innervated by cranial nerve 3 (the oculomotor nerve),
- cranial nerve 4 (the trochlear nerve), and cranial nerve 6 (the abducens nerve);
- you chew the food using cranial nerve 5 (the trigeminal nerve) and then taste it and smile using cranial nerve 7 (the facial nerve).
- Then, you listen for predators with cranial nerve 8 (the vestibulocochlear nerve)
- while you swallow the meal with cranial nerves 9 (the glossopharyngeal nerve) and 10 (the vagus nerve);
- you lick your lips with cranial nerve 12 (the hypoglossal nerve);
- and toss your head from side to side with cranial nerve 11 (the accessory nerve).
Cranial Nerve Functions
CNs 3, 4, 6, 12, 11, 8
Now, let's introduce the key functions of each CN.
- First, we'll address CNs 3, 4, 6, 12, 11, and 8 — they are the most straightforward.
- Cranial Nerve 3
- Indicate the oculomotor complex supplies CN 3, which innervates several extraocular muscles: the medial, superior, and inferior recti, and levator palpebrae, which raises the eyelid when looking up, and inferior oblique.
- Show that the Edinger-Westphal CN 3 subnucleus produces pupillary constriction.
- Cranial Nerve 4
- Now, indicate that trochlear nucleus of CN 4 which innervates the superior oblique.
- When the long, thin trochlear nerve is injured, the affected eye is elevated (aka hypertropic).
- Patients with a fourth nerve palsy tilt their head away from the affected eye (see the end of the chapter for further discussion).
- Cranial Nerve 6
- Indicate that the abducens nucleus of CN 6 innervates the lateral rectus.
- Cranial Nerve 12
- Indicate that the hypoglossal nucleus of CN 12 provides tongue protrusion.
- Cranial Nerve 11
- Indicate that the spinal accessory nucleus of CN 11 provides head turn.
- Cranial Nerve 8
- Now, indicate that the vestibulocochlear nucleus of CN 8 provides hearing and balance; hearing is one of the special senses.
- CNs 1 and 2 are part of this group of special sensory afferent nuclei, but lie superior to the brainstem, so we exclude them here.
CNs 5, 7, 9, 10
- Next let's address all of subnuclei of cranial nerves 5, 7, 9, and 10 — they are numerous and complex.
- Cranial Nerve 7
- Indicate that the superior salivatory nucleus of CN 7 provides lacrimation and salivation, amongst other facial secretions.
- Cranial Nerves 9 & 10
- Then, label the inferior salivatory nucleus of CN 9, which innervates the otic ganglion, which supplies the parotid gland, and the dorsal motor nucleus of the vagus nerve, CN 10.
- We will bundle the actions of CN 10 along with additional actions of CN 9 later, as they are most easily remembered together.
- Cranial Nerve 5
- Next, indicate the motor trigeminal nucleus of CN 5, which provides mastication.
- Then label the sensory division subnuclei:
- The mesencephalic trigeminal nucleus, which spans from the midbrain to the mid pons,
- The principal sensory trigeminal nucleus, which is restricted to the mid pons,
- The spinal trigeminal nucleus, which spans from the mid pons to the upper cervical spinal cord.
- Indicate that they all play a role in facial sensation.
- Cranial Nerve 7 (cont)
Next, let's complete CN 7.
- As already shown, it forms the superior salivatory nucleus.
- So now, indicate it also forms the facial nucleus, which provides facial strength.
- And that it comprises part of the solitary tract nucleus, along with CNs 9 and 10; specify that the CN 7 component receives taste sensation from the anterior two-thirds of the tongue.
- Cranial Nerves 9 & 10 (cont)
- Now, show that CNs 9 and 10 comprise nucleus ambiguus.
- Finally, we can elaborate on the functions of CNs 9 and 10.
- Indicate that both are involved in cardio-pulmonary function, swallowing, CN 9 helps in salivation, and CN 10 is instrumental to gut motility.
- Cranial Nerve 5 (cont)
- For completeness, also show that the spinal trigeminal nucleus also receives CNs 7, 9, and 10, which provide clinically insignificant sensory coverage from the external ear.
Somatomotor & Pharyngeal Arch Terminology
- Before we move on, let's take a moment to consolidate our understanding of the somatomotor set and pharyngeal groups to help our understanding and recall of these cranial nerve functions.
Somatomotor
- The somatomotor set is considered the brainstem extension of the spinal neurons because it innervates somite tissue derivatives. However, whereas the spinal neurons contain general somatic efferent and afferent and general visceral efferent and afferent cells, the somatomotor cranial nerve set comprises only general somatic efferent cells and a single general visceral efferent nucleus — the Edinger–Westphal nucleus.
Pharyngeal Arch
- The pharyngeal arch nerves were best categorized in Charles Judson Herrick's early 1900s observations about the role of these cranial nerves in the gill arches of fish provides insight into their purpose in humans.
- Whereas fish use these nerves to coordinate jaw movements that pump water across their gills for oxygen transfer; as we'll see humans use them for chewing (CN 5), facial expression (CN 7), speaking and swallowing (CNs 9 and 10).
- In regards to the embryology of the pharyngeal arches (according to the simplest, most common definition), the first pharyngeal arch derives cranial nerve 5, the second pharyngeal arch derives cranial nerve 7, the third pharyngeal arch derives cranial nerve 9, and the fourth and sixth pharyngeal arches derive cranial nerve 10.