Electrical stimulation of peripheral nerves activates a large number of large diameter fibres, predominantly cutaneous afferents of Group I and II.  These afferents are derived from cutaneous receptors, muscle spindles (primary and secondary endings), Golgi tendon organs and joint capsule receptors.  The afferents for the SEP are conducted along the dorsal columns, medial lemnisci and the caudal division of the ventral posterior lateral nucleus of the thalamus.

Untitled-1 copy

Recording of the SEP. A shows position of the electrodes over the scalp (oblique view).  B shows the SEPs recorded from the electrodes in A (note the polarities are reversed compared with usual conventions).  C shows the cortical surface of the hemisphere, where CS is the central sulcus.  D shows SEPs recorded from the cortical surface.

It is likely that the typical waveforms seen in human cortex are generated in somatosensory cortex in area 3b and 1:


Figure: A significant enlargement of the P25-N30 deflection (shown in red) of greater than 10 mV, defined as a giant SEP, corresponds to an enhanced excitability of the somatosensory cortex and supports a cortical origin for the myoclonus.




A central tenet of the current classification of myoclonus into cortical and subcortical types is that in cortical myoclonus the SEPs are enlarged.  Although giant evoked potentials are typically recorded from sites corresponding to somatosensory cortex, their origin may be more widespread, or they may arise from other cortical areas or represent abnormal input from subcortical structures.  However, since the N1/N20 response classically represents the thalamic afferent volley it is assumed that the abnormal enlarged responses of PME and similar conditions are not related to thalamic or other forms of abnormal input, but that the cortical area from which the evoked potentials are recorded is itself abnormal. 

With respect to the definition of a giant SEP, this can vary greatly, although a figure of >10 mV is hard to dispute. In the laboratory at Tygerberg Hospital, mean values for normal individuals (average age 40 years) was N20: 1.8 mV; N20-P25: 3.7 mV; P25-N30: 2.6 mV.