Electrodes on skin, stimulate nerves w/electrical impulses. NCS record large myelinated fibers. Small myelinated & unmyelinated fibers (autonomic & somatic pain nerves) not recorded by this test.

Most common NCS: Motor & sensory conduction studies & late responses measuring compound muscle action potentials (CMAPs), sensory nerve action potentials (SNAPs), F-waves, & H-reflexes.

Definitions: Conduction velocity: Speed of the fastest conducting nerve fibers. Latency: Time from stimulation to initial deflection of the CMAP or SNAP. Amplitude of CMAP or SNAP: Distance from baseline to negative peak (upward); Duration: Time from initial deflection from baseline to return. Antidromic: Stimulating toward the sensory receptor; Orthodromic: Stimulating away from the sensory receptor. Conduction block: Focal demyelination so severe that action potential cannot propagate along that area. CMAP amplitude is ↓ by >50% from distal to proximal stimulation sites. Temporal dispersion: An increase in the duration & decrease in the amplitude of the action potential due to variation in the firing times & conduction of individual nerve fibers.

Motor NCS: CMAP: Summated record of synchronously activated muscle action potentials.

Sensory NCS: Represents sum of individual SNAPs.

Late responses: F-wave & H-reflex may be used to assess proximal nerve segments.

F-wave: Pathway: Antidromic activation of motor neurons → anterior horn cells → orthodromic impulses traveling back along motor axons; response involving motor fibers only.

H-reflex: Pathway: Afferent sensory fiber → synapse at spinal cord → efferent α motor neuron; electrical equivalent of the ankle reflex.

Repetitive nerve stimulation: Nerve stimulated at different frequencies, assessing for decremental/incremental response.

Needle inserted into muscle & electrical activity recorded. No electrical stimulation is given. Allows evaluation of spontaneous activity & motor unit action potentials (MUAPs).