The C-reflex is an abnormal long latency response, elicited at rest in a muscle by electrical stimulation of a peripheral nerve, typically the median nerve. An afferent signal is postulated to pass up to somatosensory cortex, which activates the primary motor cortex and in turn the corticospinal pathway, resulting in a myoclonic jerk. The C-reflex can be recorded using a conventional nerve conduction study program on a standard EMG machine, or using the F-wave application.

The usual value is 40 ms, which is equivalent to twice the latency from stimulation of motor cortex to the onset of an induced motor response.

Talmakkies C wave2

Late response in patient with PME.(Series of stimuli at increasing current strength, compound muscle action potential at left of figure; horizontal time base of 10 msec/division)