RESUMO
Pyramidal motor evoked potential (PMEP) was recored in rat using the specially designed stimulating electrode for preferential activation of a pyramidal cell layer of the motor cortex. The PMEPs, recorded in the upper levels of a neural axis (medulla and C7), were composed of short latency complex waves and a long latency positive wave with a large amplitude and prolonged duration. However, the PMEP recorded at the lower level of a neural axis (T8) only showed the long latency a large positive wave. Conduction velocity of the short latency wave was approximately 11.1m/sec. Judging by the conduction velocities of these waves, it appeared that the short latency-wave complex originated from direct activation of pyramidal cells in the motor cortex (D-wave). The long latency-wave seemed to be evoked by indirect activation of the pyramidal cells (I-wave). This view was further supported by the serial depth recordings of PMEPs in the medulla as well as by the field mapping of PMEPs in the T4 spinal cord. PMEPs completely disappeared following the acute lesion of an internal capsule, indicating that these waves originated from the motor cortex rather than the brain stem nuclei such as the reticular nuclei.