Cranial Nerve Embryology
Cranial Nerve Embryology
Brainstem
- Here, we will learn the embryology of the cranial nerves.
First, draw half of brainstem in coronal view, from superior to inferior:
- Midbrain.
- Pons
- Medulla
- Then, include the cervical spinal cord.
- Define the midline of the brainstem.
- Next, use the sulcus limitans to further divide this half of the brainstem in two parts.
Neural Tube
- Let's see how the sulcus limitans will help us divide the motor cranial nerve nuclei from the sensory subnuclei, and help us predict the localization of each subnucleus based on its motor or sensory function.
- To start, draw the developing neural tube in cross-section – the spinal cord maintains this appearance.
- Label the central canal.
- Show that the sulcus limitans divides the neural tube into dorsal and ventral divisions.
- Then, include the dorsal-lying alar plates, which house sensory neurons.
- Next, include the ventral-lying basal plates, which house motor neurons.
- Now, show that during development, the dorsal brainstem opens (imagine the opening of an orange: the dorsal slices move out laterally, the ventral slices remain medial).
- Next, redraw the neural tube as internal organization of the brainstem: with the CSF canal along the dorsum of the brainstem, and add the sulcus limitans, which separates the dorsum of the brainstem into the: bilateral basal plates, medially, and the alar plates, laterally.
Table
- So, now, set up a table and show that the sulcus limitans divides the brainstem into a medial-lying Motor/Efferent division, derived from the basal plate, and a lateral-lying Sensory/Afferent division, derived from the alar plate.
- And distinguish the different rostro-caudal levels of the brainstem, as well.
- Indicate that we'll first learn the somatomotor set, which comprises CNs 3, 4, 6, 12, and 11.
- It is considered the brainstem extension of the spinal neurons because it innervates somite tissue derivatives.
- Indicate, however, that it is purely from basal plate derivatives (shown in our diagram, as well) thus, 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, which has neural crest cell elements.
Cranial Nerve Diagram
General Somatic Efferent (GSE)
- Now, in our table, indicate that we'll draw the General Somatic Efferent cell column along the medial half of the motor division – they supply striated muscle with volitional actions.
Draw the following:
- CN 3 in the superior midbrain, the oculomotor complex, which innervates several extraocular muscles: the medial, superior, and inferior recti, and levator palpebrae, which raises the eyelid when looking up, and inferior oblique.
- CN 4 in the inferior midbrain, the trochlear nucleus, which innervates the superior oblique.
- When the long, thin trochlear nerve is injured, the affected eye is elevated (aka hypertropic).
- CN 6 in the pons, innervates the lateral rectus.
- CN 12 spanning the height of the medulla, hypoglossal nucleus, which provides tongue protrusion.
- CN 11 in the cervical spinal cord, the spinal accessory nucleus of CN 11, which provides head turn.
- This set of CNs activates midline musculature, which helps us remember that they all lie in midline.
- 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:
- The small subnuclei of CN 3 in the superior midbrain, the Edinger-Westphal subnucleus, which produces pupillary constriction.
Soley Sensory Set
- Next, let's learn the Soley Sensory Set, which comprises CNs 1, 2, and 8, and are placode derivatives (derived from ectodermal thickenings) and the pharyngeal arch group, which comprises CNs 5, 7, 9, and 10, and are neural crest cell derivatives (with some placode elements, as well).
Secondary Brain Vesicles
Draw the secondary brain vesicles from cranial to caudal:
- Telencephalon
- Diencephalon
- Mesencephalon
- Metencephalon
- Myelencephalon
- And then the remainder of the caudal neural tube.
- Next draw an outline of the embryo, along the ventral surface, draw 5 pharyngeal arches.
Pharyngeal Arches
Now, let's show how the neural crest cells migrate into the pharyngeal arches and to form the pharyngeal arch cranial nerves.
- Show that CN 5 migrates into arch 1
- Then, that CN 7 migrates into arch 2
- Then, that CN 9 migrates into arch 3
- Then, that CN 10 migrates into arches 4 and 6 – (for reference, the superior laryngeal branch lies within the 4th pharyngeal arch and the recurrent laryngeal branch lies within the 6th pharyngeal arch).
Soley Sensory Set (cont.)
- Now, let's specify key placodes (which are areas of thickened surface ectoderm), which form CNs 1, 2, and 8 (the solely sensory set of CNs), from cranial to caudal.
- First, at the nasal prominences, draw the olfactory placode, which derives the olfactory epithelium and olfactory nerve (CN 1).
- Then, show that the optic placode, which forms the optic nerve (CN 2), originates from the diencephalon.
- And show that the otic placode, which forms the vestibulocochlear nerve (CN 8), originates from the hindbrain.
Background of Pharyngeal Arches
- 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).
Pharyngeal Arches (cont.)
Now, pick back up with our coronal brainstem diagram:
- Draw CN 7 in the inferior pons, the superior salivatory nucleus, which provides lacrimation and salivation, amongst other facial secretions.
- CN 9 in the superior medulla, the inferior salivatory nucleus, which innervates the otic ganglion, which supplies the parotid gland.
- And a large nucleus of CN 10 in the medulla, the dorsal motor nucleus of the vagus nerve; we'll learn its actions in a moment.
- 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.
Draw:
- CN 5 in the mid pons, the motor trigeminal nucleus, which provides mastication.
- CN 7 in the inferior pons, which forms the facial nucleus; it provides facial strength.
- CNs 9 and 10 spanning the height of the medulla, they comprise nucleus ambiguous, which are both involved in cardio-pulmonary function, swallowing – CN 9 helps in salivation, and CN 10 is instrumental to gut motility.
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, the solitary tract nucleus, 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.
- Specify that the CN 7 component receives taste sensation from the anterior two-thirds of the tongue. The CN 9 and 10 components, as mentioned, are involved in cardio-pulmonary function, swallowing, and gut motility – CN 9 provides the afferent loop to the gag reflex.
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, the vestibulocochlear nucleus, which 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.
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, the mesencephalic trigeminal nucleus, which spans from the midbrain to the mid pons.
- A small pontine component, the principal sensory trigeminal nucleus, which is restricted to the mid-pons.
- A long component from the pons into the cervical spinal cord, 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.
- 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.
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).