Experimental introduction
In this experiment you will set up skin electrodes (surface EMG) and record the Compound Muscle Action Potential (CMAP), which represents the summed electrical activity from many motor unit action potentials, and use this to measure nerve conduction velocities for two stimulation sites along the length of the nerve.
The easiest places to stimulate the ulnar nerve are in the groove of the elbow next to the medial epicondyle (funny bone) and on the wrist. These places are optimal because there is minimal superficial fat and the nerve runs close to the skin.
NOTE: It takes patience and trial-and-error to successfully find the nerve under the skin for stimulation. This will be helped by using areas and subjects with lower levels of sub-cutaneous fat, as this tissue can interfere with being able to get sufficient current to the nerve to stimulate it.
Surface EMG Recording (Compound Muscle Action Potential, CMAP)
What is a CMAP response?
The two panels below show what the hypothenar muscle CMAP response should or may look like. Panel A is a recording obtained from a subject with very little sub-cutaneous fat and with the stimulating electrode in the ideal place on top of the skin at the elbow, to stimulate the ulnar nerve running under the skin. There is only a single early response. Panel B shows an early and a late response, likely from some late conducting or more distant nerves.
You are aiming to get a recording like that in Panel A:
What isn't a CMAP response?
Note that there should be a distinct delay (a latency) between the stimulus and the response, like shown in Panel A below. You may see responses like those in panels B and C that look similar in shape to panel A but do not have a delay between the stimulus and response.
These responses are most likely to be due to current spread from the stimulating electrodes to your recording electrodes, along the skin or other conductive pathways. There always has to be a consistent delay between stimulating a nerve and APs reaching a more distant point - nerve conduction is not instantaneous.
Note on measured conduction velocities
Please note that the Conduction Velocities (CVs) generated in these virtual experiments for the ulnar nerve (in the Nerve Recordings and the Varying Temperature experiments) are derived from real-life student experiments using the same procedures as shown in the simulation.
Trained neurologists will tell you that these CVs are considerably lower than the ulnar nerve CV they can record, which can be about 50-60 m/s or thereabouts.
The values we implemented in this simulation reflect the student (in)experience in real-life lab classes and are meant solely as a training exercise.