Figure 2: The triple stimulation test (TST) principle.

On the left, a schematic diagram of the motor tract is simplified to four corticospinal axons with monosynaptic connections to four peripheral axons (a simplification which does not account for the complexity of corticospinal connections); horizontal lines represent the muscle fibres of the four motor units. Recordings are shown on the right: (A) TST test, (B) TST control, (C) response to a single stimulus at wrist and (D) superimposition of recordings A, B and C. In this example a submaximal transcranial stimulus excites 75% of the axons (three axons out of four). Desynchronization of the three action potentials is assumed to occur within the corticospinal tract (or possibly at the spinal cell level). (A, 1) Transcranial stimulation excites three out of four axons. (A, 2) After a delay, a maximal stimulus applied to the wrist evokes the first negative (upward) deflection in the TST test trace; this response is followed by that of the multiple-discharge volleys (not figured on the left scheme). (A, 3) After a delay, a maximal stimulus is applied to Erb’s point; (A, 4) a synchronized response from the three axons excited initially by the transcranial stimulus is recorded as the second large deflection of TST test trace. (B, 1) A maximal stimulus is applied to Erb’s point; (B, 2) after a delay, a maximal stimulus applied to the wrist evokes the first deflection of TST control trace; (B, 3) after a delay, a maximal stimulus is applied to Erb’s point; (B, 4) a synchronized response from the four axons is recorded as the second deflection of TST control trace. (C) The response evoked by stimulating the wrist serves as a baseline for measurement of the amplitude and area of the second deflection of the TST curves. (D) On the superimposed traces, the smaller size of the second deflection of the TST test trace, compared with that of the TST control trace, demonstrates that not all spinal axons of the target muscle were excited by transcranial stimulation (in this example both amplitude and area ratios should be 75% if the four individual MUPs have identical sizes). Calibrations: 2 mV and 5 ms. (Figure from Magistris, M. R., K. M. Rosler, A. Truffert and J. P. Myers (1998). “Transcranial stimulation excites virtually all motor neurons supplying the target muscle. A demonstration and a method improving the study of motor evoked potentials.” Brain 121: 437–450 (with kind permission of the authors and Oxford University Press (Magistris et al., 1998a).)