Review of Medical Physiology - William F. Ganong 2002

Physiology of Nerve and Muscle Cells
Excitable Tissue: Nerve
Properties of Mixed Nerves

Mammalian peripheral nerves are formed by A large number of axons surrounded by a fibrous sheath known as the epineurium. Consequently, extracellularly recorded potential changes represent the algebraic sum of potential changes across numerous axons. The threshold current intensity and the distance from the stimulating electrodes vary for individual nerve axons. Subthreshold stimuli are insufficient to trigger excitation. Threshold-strength stimuli elicit excitation in axons with a low excitation threshold, resulting in minor potential changes. As the intensity of the stimulating current increases, excitation also occurs in axons with a high excitation threshold. The Amplification of electrical phenomena remains proportional to the stimulating current strength until the process encompasses all nerve axons. A stimulus that causes the excitation of all nerve axons is termed maximal; any further increase in its intensity (supramaximal stimulus) no longer leads to an increase in the recorded potential.

Compound Action Potential

Another property of mixed nerves that distinguishes them from an isolated axon is the presence of multiple peaks in the action potential curve. This multi-peaked action potential is referred to as a compound action potential (Fig. 2-13). Its distinct appearance is due to the fact that mixed nerves are composed of fiber groups with varying conduction velocities. Therefore, when all nerve fibers are stimulated, The activity of fast-conducting fibers reaches the recording electrodes earlier than that of slow-conducting fibers. Furthermore, the greater the distance between the stimulating and recording electrodes, the more pronounced the Separation between the peaks corresponding to fast- and slow-conducting fibers. The number and size of the peaks depend on the types of fibers that make up the specific nerve under investigation. When using a stimulus weaker than maximal, the shape of the compound action potential also depends on the number and type of fibers responding to the stimulus.