Essentially, the technique employed by neurophysiologists to recorded motor unit activity has remained unchanged since the 1920s.
A fine wire is placed in a hypodermic needle, which is placed inside the skeletal muscle of the volunteer (Fig. 13 from Adrian & Bronk, 1929, J Physiol
). The small size of the fine wire offers a small recording volume, which is necessary to isolate the muscle fibers of an active single motor unit.
In order to improve stability, a modification to the electrode can be made by moving the recording sites from the distal tip of the wires to a region along the length of the wire (Fig. 1 from Enoka et al., 1988, Exp Neurol
). This allows the electrode to be held in place by placing it under the skin and above the muscle fascia.
I have used this branched bipolar configuration to measure motor unit recruitment in the biceps brachii and I always thought it would be sweet to show an actual image of the wires as opposed to a cartoon representation.
That's when I noticed Aspex
, a company that makes scanning electron microscopes, had a promotion on called "send us your sample". Basically, you find something around the house/lab, pack it in an envelope, and send it to them for a free analysis. I have always been interested in microscopy (my early work
) and though I would send in a branched bipolar electrode.
Aspex did a fine job. In this image, you can really appreciate how the insulation is removed to reveal the stainless-steel core of the wire. The wire is 50 micrometers in diameter and the total length of the exposed region is ~0.7 mm.
In addition to magnifying the wire itself, I asked Aspex to zoom in on the wire where it emerges from the hypodermic needle. The results again look very nice and really make you think twice about what is piercing the skin!
All-in-all a pretty sweet result, taking approximately 3 weeks from the time I dropped the electrode in the mail to the time I was email my PDF of images. Here is a link to the "scan your sample" promotion. Act fast, who knows how long they will be doing this!