David S. Campion, M.D.

EMG (Electromyogram)

This shows diagrammatically two "MOTOR UNITS". A motor unit is a single nerve fiber and the hundreds of individual muscle fibers that it supplies or innervates. The nerve fiber is the trigger that tells the muscle to contract. The sequence is this: a signal from the brain – the upper motor neuron – travels down the spinal cord and transmits the signal to a motor cell in the spinal cord – the lower motor neuron (LMN). These LMNs are spread the length of the spinal cord, the ones in the neck – the cervical region – have their nerve fibers connecting to muscles in the shoulder, arm and hand, while the LMNs in the lower back, the lumbar-sacral region, connect to the hip girdle, thigh, leg and foot muscles.

For simplicity represented here are two LMNs sending their nerve signals down to two and three muscle fibers respectively. When the nerve signal reaches the nerve – the neuro-muscular junction – a chemical acetylcholine is released and a new electrical signal travels along the muscle fiber and causes the muscle fiber to contract. A large muscle like quadriceps in the thigh may have a thousand nerve fibers (bundled together in the femoral nerve) each nerve fiber supplying a thousand muscle fibers! The more signals the stronger the muscle contraction.

In EDX we study these electrical signals traveling along the muscle, and as these signals cause the contraction we call them motor unit potentials. Most people know that an electrocardiogram (EKG) is measured by putting a series of electrodes on the chest wall and the EKG gives information as to the heart function. Similarly an Electromygram (EMG) gives very valuable information as to the muscle we are recording from, but more importantly the EMG also gives information about the function of the nerves going to the muscle, including the effects of damage to the nerve. By studying many muscles along the arm or leg for example, we can tell if a disc is pressing on the nerve root (radiculopathy) and which nerve root or roots are damaged.

The only problem, from the patient’s standpoint, is that to accurately pick-up these signals we need to put a very small pin through the skin into the muscle! A reference pin is slipped under the skin and then the EMG pin, which is smaller than an acupuncture needle, is placed in a muscle and the electrical signals are observed. As detailed in the section NORMAL MUSCLE, the motor unit potentials picked up tell the EDX physician about the nerves supplying that muscle and in the section DENERVATION, REINNERVATION and MYOPATHY you will find explanations of the electrical events accompanying these conditions.

FEATURED TOPICS

• Electromyogram (EMG)
• Denervation
• Reinnervation
• Myopathy

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