Train of Four

Look up ulnar nerve motor innervation, specifically in the hand.

I understand that placing the machine on the ulnar nerve stimulates the adductor pollis which in turn adducts the thumb. But in this particular nerve muscle response, why are we only counting twitches of the thumb and not other finger movements? Thanks.

You can test TOF on the hand or on the obicularis oculi muscle around the eye from stimulation of the facial nerve. The reason you should use the ulnar nerve to check TOF is because twitches come back later than they do if you stimulate the eye muscle. This means that the muscles for respiration (diaphragm, abdominal muscles, etc) return before twitches show up on the hand and make it a little more safe to know if there is residual paralysis.

I understand that placing the machine on the ulnar nerve stimulates the adductor pollis which in turn adducts the thumb. But in this particular nerve muscle response, why are we only counting twitches of the thumb and not other finger movements? Thanks.

Traditionally the thinking has been to monitor a muscle group as far away from the electrodes as possible to rule out direct muscle stimulation by the stimulator device.

The fingers are ignored, the reasoning goes, because there is no way to really know if movement is due to lack of adequate blockade of the neuromuscular junction (measured by neural stimulation) or just because directly applied electricity is causing the muscle fibers themselves to depolarize and contract.

The adductor pollicis is far enough away from the electrodes to say with confidence that its movement is not due to direct electrical stimulation.