Action potentials recorded with evoked potential techniques: modes and sites of generation

Short latency evoked potentials (auditory and somatosensory) include compound action potentials which have been thought to be generated as a result of synchronous nerve impulses (volley) in the sensory pathways. The results of studies in experimental animals with intracranial electrodes or lesions and in patients with clearly localized and identified brain lesions have been interpreted as evidence that these waves are initiated at specific sites such as the neural outflows from the relevant nuclei of the respective pathways. However recordings of the auditory nerve-brain stem evoked responses (ABR) intracranially during neurosurgical procedures and considerations of the latency of the very early somatosensory evoked potentials (SEP) have cast doubt as to this suggested mode of generation of these far-field potentials and, therefore, their sites of initiation. Careful studies of SEP in humans, in vitro experiments with nerve bundles and computer simulations have shown that far-field action potentials can be initiated at sites where the propagated action potential volley passes through boundaries between volume conductor regions differing in geometry and in conductivity. These studies have provided insight into the modes of generation of SEP and ABR and their possible sites, showing that they may be more sensitive to the passive properties of the surroundings of nerve tracts rather than their functional state per se.