Recording Forelimb Muscle Activity in Head-Fixed Mice with Chronically Implanted EMG Electrodes

I'm interested in how parts of mouse motor cortex, called primary and secondary motor cortex, interact with each other to influence muscle activity during different kinds of movements. So we know that both primary and secondary motor cortex project to the spinal cord, but it's unclear how they differentially influence movement. So I'm using this protocol to compare the effects of inactivating primary and secondary motor cortex on forelimb muscle activity throughout movement to understand how they differentially control movement.

The use of forelimb muscle EMG has enabled us to measure the impact of optogenetic inactivation, of different motor system regions on motor output. For example, we've been able to show that direct motor cortical influence on muscles is specific to certain motor behaviors and particular muscle activity states during those behaviors. New large scale multi electrode arrays like neuro-pixels now enable us to record from large neuron populations across multiple motor system regions simultaneously.

Performing EMG recordings during these neural recordings will enable us to characterize how the interactions between these motor system regions depend on muscle activities state. To begin the fabrication of the electrode set first tie two pieces of PFA coated braided stainless steel wires together with a single knot. Insert an 18 gauge needle into a piece of corrugated cardboard.

Position the knot six centimeters from the needle's insertion end. Then pull the electrode strands against the cardboard to tighten it around the needle. Carefully remove the needle to maintain the knot's integrity and pull the knot tight two times to tighten it further.

Next, tape the electrode taut against a piece of flat cardboard. Ensure that the two connector end wires are together and the insertion end wires are spread apart. With a scalpel, make precise nicks in the insulation, marking the ends where the insulation will be removed.

Make a series of approximately six cuts at each marked location, with two cuts each on top, at the sides, and underneath the wire. Angle the scalpel blade to cut along the loosened insulation lengthwise and use a pair of forceps to carefully remove it from the wire. Strip one millimeter insulation off the end of each wire at the connector end, and five millimeters off at the insertion end.

Twist the wire segments at the insertion end together and crimp the exposed ends in the shaft of a 0.5 inch 27 gauge needle. Next, use a pair of straight forceps to carefully reinsert each soldered brass fitting back into the connector. Make sure that the wires of each electrode pair are adjacent and untangled.

Coat the pin slots with epoxy to insulate all metal or wire near the connector from the tissue. To begin, extend the incision on the back of the neck of an anesthetized mouse with a head plate implantation. With a blunt bone scraper separate the skin under the neck incision from the underlying tissue.

Work the scraper under the skin and behind the neck incision to clear a path for the electrodes. Insert the tip of a large needle driver through the triceps incision and out of the neck incision. Clamp the needle driver around the electrode needle lengthwise.

Then pull it through to the triceps incision. Next, bend a 27 gauge needle to give it a slight curve. Hold the needle with the needle driver, then press it against the handle of a pair of forceps to result in an additional bend of 5 to 10 degrees.

Add three total bends at different positions along the length of the needle. Next, identify the entry and exit points on the muscle. With a pair of fine forceps, remove any fat in fascia obscuring the entry and exit site.

After submerging three to five millimeters of wire parallel to the muscle fibers, use the needle driver to insert the needle into the proximal end of the muscle. Now push the needle through the muscle to the exit site. Once the needle exits the muscle, grab the tip with blunt forceps and pull the needle through.

With a pair of forceps, tie a loose distal knot at the exit site. Tighten the knot down to a one centimeter loop. Push the loop with the forceps and position it over the exit site.

After identifying the location over which the distal knot is to be closed gently grasp the loop with a pair of fine bent forceps and pull the loop tight over the forceps. Remove the fine forceps from the knot. Push the knot toward the exit site with the fine bent forceps and pull the needle end with fingers to finish tightening it.

Use the straight fine forceps to grasp the exit knot. Then tightly curl the distal wire around the forceps to bend the wire around the knot and toward the muscle. Finally, cut the wire off about 0.5 millimeters distal to the distal knot, leaving a small nub curled around the knot.

All the implanted electrodes produced a clean electromyographic signal 36 days after implantation. However, after 72 days, the wrist extensor electrode failed.