EMG/NCS

• Neurons have an intracellular resting membrane potential of -70 mV. When nerves conduct electrical impulses along their axon, the membrane is depolarized by an influx of sodium (Na+) ions via voltage-gated sodium channels (Figure 1).

• This influx causes a voltage change to adjacent regions and changes the resting potential further down the axon thereby propagating the potential down the axon (Figure 2). The axon membrane potential will then stabilize with the inactivation of sodium channels and delayed opening of voltage-gated potassium channels.

Figure 2: A traversing action potential is conducted down the axon as the axon membrane depolarizes, then repolarizes.

• When unmyelinated, fibers can conduct potentials in the range of 1-5 m/sec. However, myelinated motor nerve axons and many sensory nerve axons have conduction velocities up to 150 m/sec. This is due to a process called saltatory conduction.
  • Produced by Schwann cells, myelin sheaths are concentrically wrapped around an axon.
  • In between myelin sheaths, there are gaps of exposed axon called nodes of Ranvier. It is at these nodes where action potentials occur.
  • When an action potential occurs at a node, that current flows passively within the myelinated internode of the axon until the next node is reached.
  • Saltatory Conduction: Current flows across the neuronal membrane and jumps from node to node (Figure 3).

Figure 3: Myelinated axon showing saltatory conduction