•••quote:•••Originally posted by Chris_P:
•As far as the accessory nerve is concerned, if the lesion is above the point where it enters sternocleidomastoid, that muscle as well as trapezius will be paralysed and the unopposed action of the other scm will cause the head to be turned away from the side of the lesion. This won't happen if the lesion is below sternocleidomastoid (in the posterior triangle - most common area of injury). In this case the patient will be weak on the side of injury when asked to shrug shoulders.
I'll look up about the CNV injuries - more often they are sensory than motor (ie trigeminal neuralgia).
Chris•••••Here is what I am having a problem with. First of all, when I say "turn" I mean to rotate the head like you were looking to your left. Also, when I say "tilt" (ie, tilt head to left) I mean to basically put your left ear on your left shoulder. As far as I understand, the action of the SCM contracting unilaterally is to turn the head to the opposite direction (the LEFT SCM turns the head to the RIGHT). Contracting SCM bilaterally cuases flexion of the neck. So, losing your LEFT SCM due to LEFT CNXI lesion would cause the unopposed action of the RIGHT SCM to turn the head to the LEFT (towards the side of the lesion). I have also read in Moore that the SCM also tilts the head when contracted unilaterally. I am assuming that if the deviation is really TOWARDS the side of the lesion that it is because the tilting action of the unopposed SCM is more powerful than the turning action. Is this correct? Are we thoroughly confued yet?
As for the jaw deviation, I understand now. I had forgotten the origin is the sphenoid bone and the insertion (movable end) is the mandible. So, contracting the right pterygoids would cuase the jaw to move to the left. So a lesion of the right would cuase the unopposed action of the left to cause a deviation to the right (same side as lesion). Right?