Corticospinal Tract Jitter

Corticospinal influences on distal and proximal muscles of the upper limb were stud­ied in 14 normal volunteers. The techniques of electrical and magnetic corticospinal tract acti­vation through intact scalp were used. Recordings were made of the discharges of single motor units from the abductor digiti minimi and biceps brachii muscles. Poststimulus time histograms (PSTHs) of the motor unit discharges were constructed. The latency variation of consecutive single motor unit responses (the corticospinal tract jitter) was determined for each period of increased firing probability (peak). The rise time of excitatory postsynaptic potential (EPSP) was estimated from the duration of the period of increased firing proba­bility, and its amplitude from the size of the PSTH peak. The threshold stimuli produced one or more narrow periods of increased firing prob­ability in the PSTHs of all the motor units stud­ied. A relatively small corticospinal tract jitter (289–428 µs in abductor digiti minimi and 358–431 µs in biceps brachii muscle) with no sta­tistically significant differences between the mus­cles was found in each period of increased fir­ing probability. The rise time of EPSP in both muscles ranged between 1.1 and 1.5 ms. The size of the first peaks after the anodal stimula­tion indicated that of the underlying EPSP ampli­tude was approx. 5.0 mV for the abductor digiti minimi and approx. 4.7 mV for the biceps brachii motor units. The finding of relatively small corti­cospinal tract jitter and short EPSPs rise times seem to support the existence of a monosy­naptic projection from the brain to the motoneurones innervating both muscles. The estimated sizes of EPSPs also suggest similar effectiveness of synaptic transmission in both cases. Our results do not support the anticipated greater effectiveness of corticospinal tract connexions to the motoneurones supplying distal muscles of the upper limb.