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Compound action potential is a measure of the combined electrical potential of a group of cells or fibers in a single nerve. Each fiber that makes up a nerve can transmit a distinct electrical signal. This electrical potential can be graphically analyzed, and often occurs at different intensities, depending on the stimulus. The trailing axon of a nerve cell or a neuron, in contrast, typically fires the same type of signal no matter how it is stimulated. Nerve signals that travel through muscle cells during complex motion often trigger compound action potential, while simple movement and normal sensations generally don’t.
Although not always present in the peripheral nervous system, this potential is generally more regular in the central nervous system. Impulses that travel through the spinal cord and brain, and control muscle movement and organ function, are typically numerous and continuous. Electrical potential is also influenced by different diameter fibers that make up most nerves. Large diameter fibers generally need a less intense stimulus to respond, while narrower fibers often begin to respond when the stimulus is stronger.
There are many different types of nerve fibers, and a single nerve can have fibers with various diameters. The compound action potential of the nerve is a combination of the action potential of all the fibers. Generally, the stronger the stimulus, the more nerve fibers that are activated, meaning the potential is higher. Surgeons can use these data for gauging how much anesthesia to give a patient. Certain fibers are affected more than others depending on the type of anesthetic used.
Compound action potential is also measured to detect nerve damage and determine its extent. Electrodes attached to the skin on either side of an arm, for example, can be used to time the conduction of nerve fibers. By comparing the results to a normal value, a medical professional can see if the electrical activity of a nerve is being disrupted. In medicine, peripheral nerves are classified either with letters, which are generally used for skin nerves, or Roman numerals often used to categorize muscle nerves.
To measure this combined electrical potential, analysts use computers and software that display test readings on a graph. The membrane potential can be identified before a vertical line shows the spike in neural activity. Conditions such as carpal tunnel syndrome are often diagnosed this way, since an affected nerve generally has a slower conduction speed than a normal one.