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.