Point 13
Intro
Now
we can do some problem solving. Lesions involving the
vestibular nerve, nuclei, and descending pathways will
result in problems such as stumbling or falling TOWARDS
THE SIDE OF THE LESION. Think of the NORMAL side
as being in control and pushing against the weak side. For
example, if you have a patient with a lesion that has
destroyed the LEFT vestibular nerve, the LEFT
vestibular nuclei and the LEFT lateral
vestibulospinal tract are "tuned" down. Meanwhile, the
normal RIGHT nerve is fine and firing away and thus
the RIGHT lateral vestibulospinal tract is also in
good shape. The two lateral vestibulospinal tracts usually
counteract each other functionally, but now the RIGHT
side takes over. The end result? STUMBLING AND FALLING TO
THE WEAK SIDE, IN THIS CASE TO THE LEFT.
Imbalance in the vestibulospinal tracts can be demonstrated in healthy medical students. Put a penny on the floor and stand directly over it so that it lies between your feet. Bend your head forward to stare at the penny. While staring at the penny turn to the RIGHT five complete turns. At the end of the five turns stop, try to stand erect and hold your arms straight ahead. If you go back to the rubber tire -cupula experiment, you can see that upon stopping, the LEFT vestibular nerve is dominating, so which way do you stumble? Following this spinning exercise, you might also experience some nausea, autonomic disturbances and vertigo (you are spinning, or the room is spinning). In addition, there will be an involuntary to and fro oscillation of the eyes. This is called nystagmus and demonstrates the CONNECTIONS BETWEEN THE VESTIBULAR APPARATUS AND NUCLEI IN THE BRAIN STEM THAT INNERVATE MUSCLES THAT MOVE THE EYES.
We first need to look at the ABDUCENS nucleus. This nucleus lies in the dorsal pons. The nucleus and the looping fibers of C.N. VII (to be discussed soon) comprise a bump on the floor of the fourth ventricle called the facial colliculus. C.N. VI fibers pass ventrally from the abducens nucleus, exit at the pontomedullary junction and eventually reach the lateral rectus (LR6). We will evaluate the effects of lesions involving this nucleus and nerve later in this point. First we need to talk about how the vestibular system influences eye movements.