MUNIX: Motor Unit Number Index

Sanjeev D. Nandedkar, Ph.D

Introduction

Muscle fibers are organized in functional entities called motor units. A motor unit (MU) consists of all muscles fibers innervated by one motor neuron (Figure 1A). The motor neuron is located in the spinal cord. When it discharges, an action potential (AP) propagates along the motor axon (or the nerve fiber) to reach the muscle. The muscle fibers respond by producing an action potential and contraction. The contraction is recognized as a muscle twitch while the electrical activity can be recorded using an electromyograph. The summated electrical activity of all muscle fibers in the MU is called the ‘motor unit potential (MUP’). The MUPs are usually recorded and analyzed in the needle EMG examination. However, it is possible to record the MUP using surface electrodes also. Due to large distance from the muscle fibers, the surface recorded motor unit potential (SMUP) waveforms tend to have lower amplitude compared to the needle recordings. The main spike of the needle recorded MUP is also sharper and has a high pitch sound.