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Motor unit number estimation (MUNE)

Authors
Clifton L Gooch, MD
Robert Henderson, MD
Section Editor
Jeremy M Shefner, MD, PhD
Deputy Editor
John F Dashe, MD, PhD

INTRODUCTION

Electrophysiologic testing of nerve and muscle function has played a critical role in the diagnosis of neuromuscular disease for over half a century. Advances in computer technology and technical refinements have enabled the development of more sophisticated electrodiagnostic methods, which are providing information on the motor nerve and its function in health and disease. Motor unit number estimation (MUNE) is a technique that can be used to determine the approximate number of motor neurons in a muscle or group of muscles. In addition, MUNE methods provide a means of measuring motor unit size, enabling tracking of both loss of motor units and the compensatory phenomenon of collateral reinnervation. MUNE is used most often in neuromuscular disorders such as amyotrophic lateral sclerosis and spinal muscular atrophy.

This topic will review the methodology and utility of MUNE. Other electrodiagnostic methods are discussed separately. (See "Overview of nerve conduction studies" and "Overview of electromyography".)

BASIC PRINCIPLES OF MOTOR NERVE FUNCTION

The peripheral nerve is a collection of individual fibers, or axons, bundled together in much the same fashion as individual telephone lines within a telephone cable. Most peripheral nerves are mixed nerves, containing both sensory axons (carrying information from peripheral sensory receptors to the sensory ganglions) and motor axons (carrying commands for muscle contraction from the spinal cord to the muscle). Routine nerve conduction studies (NCS) can selectively test both the sensory axons (sensory NCS) and motor axons (motor NCS). Sensory NCS enable relatively selective stimulation and recording of sensory action potentials in a peripheral nerve, but, although they are a critical part of routine electrodiagnosis, they have little relevance to the assessment of motor function.

Each spinal motor neuron and the axon it supplies, with all of its branches and their innervated muscle fibers, is known as a motor unit. When the healthy motor axon is depolarized, all of its branches and muscle fibers are activated in an all-or-none fashion. Most peripheral nerves contain hundreds of motor axons, each of which supplies a small percentage of the total number of muscle fibers within a muscle. For full contraction of that muscle and full strength, all of the motor units in that muscle must be activated together. When minimal force is needed for a delicate task, only one or a few motor units are activated.

PHYSIOLOGIC MEASURES OF MOTOR NERVE FUNCTION

During standard motor nerve conduction studies (NCS), bipolar electrodes at the skin surface stimulate the nerve strongly enough to activate all of the motor axons within it, resulting in full depolarization and contraction of the muscle, which corresponds with activation of all of its motor units and component muscle fibers. The electrical impulse generated by this muscle activity is recorded by electrodes placed over the muscle on the skin surface. In a healthy muscle, all the motor units and all their muscle fibers are activated simultaneously during this test, generating the compound motor action potential (CMAP), which is the maximum motor response. The amplitude of the CMAP corresponds to the total number of motor units and muscle fibers activated. The time required for transmission from the stimulation site to the recording site and generation of a waveform is measured, and the conduction velocity can be calculated.

             

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Literature review current through: Nov 2016. | This topic last updated: Mon Mar 02 00:00:00 GMT 2015.
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