Clinical Demonstration:
Nerve Conduction Studies and Electromyography

C. Chalk
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The electrical properties of the nervous system have been recognized at least since the time of Galvani's studies of animal electricity in the 18th century.

From Luigi Galvani's "Commentary on the Effects of Electricity on Muscular Motion", 1791.
However, it has only been since the 1940s that developments in electronics have allowed physicians to record and measure the electrical function of the nervous system in patients. The purpose of today's session is to demonstrate some techniques for measuring the electrical function of the peripheral nervous system. These techniques, known as nerve conduction studies and electromyography, are used by neurologists in the evaluation of patients with diseases of the peripheral nerves and muscles.


Nerve Conduction Studies

If a brief electrical shock is applied to a peripheral nerve, the subject will experience two things. First, of course, an electrical sensation will be felt. Second, if the shock is large enough, the muscles innervated by the nerve will make a brief, forceful contraction (a twitch). The effect of the shock is to depolarize some or all of the immediately subjacent axons, causing them to generate action potentials. Unlike action potentials generated normally in the nervous system, which occur rather randomly and which propagate in one direction only, those resulting from the electrical shock travel as a synchronous wave, both up the nerve (towards the brain, experienced as the sensation of a shock) and distally (towards the muscle, causing the twitch).



A Note on Extracellular and  Intracellular Recordings
The intracellular technique records a transmembrane potential by inserting a micropipette into one cell and recording the potential changes with respect to an extracellular reference electrode. intracell.gif (1947 bytes)
The extracellular technique records potential changes at the membrane surface rather than across the membrane.

Note that the techniques we will be demonstrating here are all extracellular recordings.

Adapted from "Anatomy", 3rd Edition; Gardner, Gray, O'Reilly
Motor and Sensory Function of the Median Nerve

Clinically it is helpful to study the motor and sensory functions of peripheral nerves separately. However, most peripheral nerves are mixed nerves, with motor and sensory axons randomly intermingled, and depolarizing the nerve with an electric shock generates action potentials in both motor and sensory axons. Fortunately, at their distal ends, all mixed nerves form discrete motor and sensory branches, which can be studied separately. In the median nerve, for example, motor and sensory axons are completely intermingled proximal to the wrist. In the hand, however, the motor axons are gathered into a motor branch to muscles in the thumb, while sensory axons coalesce to form sensory branches innervating the lateral three digits.  Thus, any action potentials recorded from nerve branches in, say, the index finger reflect the activity of median nerve sensory axons exclusively. Median motor axon activity could be recorded from the motor branch innervating the thumb muscles, but in fact it is easier to record muscle action potentials directly from the thumb muscle (muscle action potentials are generated when the action potentials travelling along the motor axons reach the thumb muscle). 

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