Hey everyone! Here's the answer to this weeks round of Neuroscience trivia!
The key was to establish which cranial nerves were controlling the symptoms presented. Another hint was that the surgery was to repair his internal jugular vein. the left shoulder weakness and inability to turn his head left against resistance suggests damage to the sternocleidomastiod muscle, which is innervated by Spinal Accessory nerve (CN XI). A flat soft palatal arch leads one to think there may be damage to the vagus nerve (CN X) which supplies most of the pharynx, larynx, and palate. The lack of gag reflex really gives it away, since the gag reflex receives efferent (motor) input from CN X and afferent (sensory) input from Glossopharyngeal nerve (CN IX). Once you have established that CN IX is involved, it is easy to distinguish the correct answer of dry mouth, since it is known that CN IX innervates the visceral efferent branches of the parotid gland which supplies saliva to the oral vestibule. Why does this matter you may ask? Because cranial nerves IX, X, and XI all exit the jugular foramen alongside the internal jugular vein. These nerves were probably severed when the original trauma occurred. Or else his surgeon should start looking for a new job...
Why are the other choices wrong?
A) Ptosis (sagging eyelid) on the left side that disappears on upward gaze would suggest a problem with sympathetic innervation, which is not present with his other symptoms.
B) Profound ptosis on the left side would be produced by an Oculomotor nerve palsy (CN III) which exits out of the superior orbital fissure (within the eye socket), which would not satisfy the other symptoms presented.
C) Dry eye on the left side would be produced by severing the lacrimal nerve which goes to the lacrimal gland. This wouldn't have happened during this surgery because the fibers from the Facial nerve (CN VII) which add into the fibers of the Opthalmic division of the Trigeminal nerve (V1) at the ciliary ganglion, do so within the orbit.
D). Gustatory sweating over the left parotid gland is actually a condition called Frey's syndrome. This is most often caused when one undergoes surgical resection of the parotid gland (found anterior to the ear within the infratemporal fossa). The surgeon may accidentally sever some parasympathetic fibers within the gland, which then heal and fuse with sympathetic fibers. Thus, when the person thinks about food and salivates, the sympathetic innervation that once told the gland to supply saliva to the mouth when food was about to enter, now tells the skin of the cheek to release sweat. Weird right?
Whew. That's a long one! I wanted everyone to see a typical test question, and then see the typical explanations of the wrong answers that can be found in most books I am studying from. Hopefully you stuck with me this far!
I love you all and I can feel the encouragement flowing every day. I am so blessed to be here doing exactly what I've always wanted to do. It's hard work, but I wouldn't have it any other way.
Love and blessings,
Melissa
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