Surface EMG recordings
In this section you will model using the surface EMG recording technique of Compound Muscle Action Potential (CMAP), which records the summed electrical activity from many motor unit action potentials. Disturbances of motor function occur commonly in many diseases of the nervous system and can often be detected clinically by study of electromyographic activity.
You will use the surface EMG technique to record the Compound Muscle Action Potential (CMAP) of the hypothenar muscle using skin surface electrodes, and electrically stimulate, with low current levels, the Ulnar Nerve at the elbow near the medial epicondyle (your not-so-funny "funny" bone!) or at the wrist.
Since you can measure the distance between the stimulating electrodes and the middle of your recording electrodes, you know the distance between stimulation and recording sites. On the recording screen, you can measure the time taken between applying the electrical stimulus and the start of the CMAP response (the "latency" of the response), and from the equation Velocity = Distance / Time, you can calculate the Conduction Velocity of the Ulnar Nerve.
Changes in Ulnar Nerve CV can be found in disturbances of nerve function (as in diabetic neuropathy, entrapment syndromes, spinal cord injury) and therefore are of diagnostic value.
In the subsequent tabs, you can measure the impact of varying skin temperature on the Ulnar Nerve CV measured with electrical stimulation applied at the wrist, and the effect of age from newborn to old. The changes seen with varying temperature were obtained from similar real-life class experiments, while the effects of development were obtained from a reading of the literature.
Surface EMG setup and electrode placement
Connect the snap cables to the correct electrode positions on the hand.
Once you have connected the snap cables to their correct positions, continue to the recording tab to start collecting values.
Recording a CMAP to stimulation at the elbow
Place the stimulating bar (the black bar with the red-and-black lead) on the ulnar nerve at the medial epicondyle (the groove at the elbow), within the shaded area.
Please attach the snap cables in the Setup tab before attempting this experiment.
Adjust the parameters below, then press "Stimulate" to record a CMAP response of the hypothenar muscle to stimulation at the elbow.
You can always move the stimulating bar around if you're not getting the expected CMAP response.
Identify the lowest stimulus intensity at which you can get a response. Using the stimulus strength two levels above this, perform 6 trials.
| Trial no. | Measured latency (ms)1 | Distance (mm)2 | Conduction velocity (m/s) |
|---|---|---|---|
| 1 | 300 | ||
| 2 | 300 | ||
| 3 | 300 | ||
| 4 | 300 | ||
| 5 | 300 | ||
| 6 | 300 | ||
| Average | 300 |
1 Time from applying stimulus to the first negative trough.
2 Distance between middle of stimulating electrode and middle of two recording electrodes (set to 300mm for convenience).
Recording a CMAP to stimulation at the wrist
We will now repeat the experiment for a second stimulating position at the wrist. Place the stimulating bar on the ulnar nerve in the wrist, within the shaded area.
Please attach the snap cables in the Setup tab before attempting this experiment.
Adjust the parameters below, then press "Stimulate" to record record the CMAP response of the hypothenar muscle to stimulation at the wrist.
Identify the lowest stimulus intensity at which you can get a response. Using the stimulus strength two levels above this, perform 6 trials.
| Trial no. | Measured latency (ms)1 | Distance (mm)2 | Conduction velocity (m/s) |
|---|---|---|---|
| 1 | 70 | ||
| 2 | 70 | ||
| 3 | 70 | ||
| 4 | 70 | ||
| 5 | 70 | ||
| 6 | 70 | ||
| Average | 70 |
1 Time from applying stimulus to the first negative trough.
2 Distance between middle of stimulating electrode and middle of two recording electrodes (set to 70mm for convenience).