In Vivo Measurement of the Compound Muscle Action Potential in a Rat

Place an anesthetized long-Evans rat on a heating mat.

Insert a reference electrode into the fourth finger and a recording electrode into the hypothenar muscles.

These are a group of muscle fibers in the palm that are innervated by the ulnar nerve.

Next, position a ground electrode on the upper forearm, a stimulating cathode near the elbow, and an anode near the armpit.

Apply a constant electrical stimulus through the cathode to the ulnar nerve.

This stimulation induces an action potential across the axon, leading to neurotransmitter release at the neuromuscular junction.

The neurotransmitter release causes the muscle fiber to depolarize and contract, generating electrical activity.

Record the combined electrical activity from all muscle fibers, called the compound muscle action potential, or CMAP.

Adjust the cathode's position and stabilize the cathode with an electrode holder to acquire optimum recording.

Gradually increase the electrical stimulation until a maximum response is obtained from all muscle fibers.

Before beginning the procedure, confirm the appropriate level of sedation by pedal withdrawal and corneal reflex in a 12-week-old female Long-Evans rat. Place the rat on a feedback-controlled heat mat to record the compound muscle action potential. Insert the reference electrode through the dorsal aspect of the fourth digit in the recording electrode through the hypothenar muscle. Place the ground electrode through the skin on the superior aspect of the forearm between the stimulating and recording electrodes.

Then, insert the percutaneous stimulating needle cathode approximately four millimeters distal to the cubital tunnel at the elbow, and insert the anode approximately one centimeter proximally through the skin of the axillary region.

Next, use a semi-automated computer-controlled axonal excitability program linked to a constant current stimulator in an amplifier to apply a one-millisecond square-wave pulse to the ulnar nerve with the cathode needle electrode, and to record the compound muscle action potential from the hypothenar muscle. Carefully adjust the angle and/or position of the cathode until an optimal biphasic response curve with a consistent amplitude is achieved, and stabilize the cathode with a repositionable electrode holder.

For threshold tracking, it is critically important that there is a proportionate response to the stimulus. A successful response is indicated by a sigmoid shape in the stimulus-response curve.

To record a stimulus-response curve, incrementally increase the stimulus intensity of a one-millisecond impulse until a maximum response is obtained.