Figure 2. Examples of EMG responses evoked from the MLF.

A, records from the surface of the spinal cord at C1–C2 level illustrating potentials evoked by double 20 μA stimuli. Averages of 20 traces. B, simultaneously obtained records from neck muscles.The three dotted vertical lines indicate the most likely onset of the direct volleys after the first stimulus, the indirect volleys evoked by the 2nd stimulus and EMG potentials evoked by these stimuli, respectively. Boxes indicate the time windows within which the areas of the early components of the EMG responses were measured. C, diagram of the most direct connections between fibres stimulated in the MLF and motoneurons (MN) innervating neck muscles. It takes into account <0.5 ms latencies of direct volleys in axons of reticulospinal (RS) neurons, <1 ms additional delays of indirect volleys and <1 ms latencies of EMG responses with respect to the indirect volleys evoked by the second stimulus (none appearing after the first stimulus) which would not leave time for additional relay neurons. D, similar diagram of the most direct connections between neurons stimulated in the red nucleus (RN) and motoneurons innervating neck muscles. The ∼1.5 ms longer latencies of EMG responses would require one or two additional synaptic delays, related to transsynaptic and not direct activation of rubrospinal neurons and their trans-interneuronal, and not direct, actions on neck motoneurons.