Cranial nerve pathways Flashcards
Where is the cell body of an upper motor neuron (UMN) located?
In the cortex (precentral gyrus)
Where does the axon of an UMN project?
It projects all the way into the brainstem, where it decussates (crosses over) in the distal medulla. It will then ultimately synapse on the cell body of a lower motor neuron (LMN).
Where do UMN for spinal nerves project?
In spinal nerves, UMN only project from the contralateral primary cortex to cell bodies of LMN ipsilateral to the innervated muscle.
Where do the UMN for most cranial nerves project?
For most cranial nerves, UMN project bilaterally to cell bodies of LMN ipsilateral to the innervated muscle.
What is the significance of the difference between spinal nerve UMN and most cranial nerve UMN?
Since most cranial nerve UMN project bilaterally, it is very hard to disrupt function through a UMN lesion since there is redundancy/two of them to the ipsilateral muscle.
Where are the cell bodies for lower motor neurons (LMN) located?
In either the ventral/anterior horn of the spinal cord (spinal nerves) or in a cranial nerve nuclei in the brain stem (cranial nerves).
Where does the axon of a LMN project to?
Projects out of the spinal cord/brainstem to the ipsilateral innervated muscle(s).
What is the general UMN-LMN pathway?
An UMN synapses with a LMN cell body in a ventral horn (spinal nerves) or cranial nerve nuclei in the brainstem (cranial nerve). The LMN then acts upon the ipsilateral muscle(s).
What will happen if there is a UMN issue for a spinal nerve?
The muscle contralateral to the side of the lesion will be affected (ex. issue with cell bodies/axons in left cortex will result in loss of function of the right side).
What will happen if there is a LMN issue? (both spinal and most cranial nerves).
The muscle ipsilateral to the lesion will be affected.
What cranial nerves are exceptions to the rule?
The facial nerve (CN VII) and the hypoglossal nerve (CN XII)
What is special about the hypoglossal nerve?
It behaves exactly like a spinal nerve (no UMN redundancy). A lesion in the UMN will affect the contralateral tongue muscles, while a LMN lesion will affect the ipsilateral tongue muscles. Weakness of the tongue muscle will cause deviation to that side (unopposed contralateral contraction).
What is special about the facial nerve?
The innervation of the upper face is like most cranial nerves (bilateral UMN), while the lower face is like a spinal nerve (contralateral UMN control).
What will happen if there is a LMN lesion to the facial nerve?
The entire ipsilateral face will be affected. (ex a left LMN facial nerve lesion will paralyze the entire left side of the face).
What will happen if there is a UMN lesion to the facial nerve?
The contralateral lower face will be affected, while the upper face will be spared (due to UMN redundancy). (ex. a left UMN facial nerve lesion will paralyze the right lower face while the right upper face is unaffected).
What is special about the trigeminal nerve (CN V) is regards to parasympathetic innervation?
While CN V has no pre-ganglionic parasympathetic cell bodies, many ganglia and post-synaptic fibers travel with CN V (in fact, all four parasympathetic ganglia in the region associate with CN V).
How many nuclei does CN V have?
Three sensory and one motor.
Where do the sensory nuclei of CN V extend?
They extend from the midbrain all the way through the medulla and into the spinal cord.
