It has been claimed that a central oscillator may produce a detectable rhythmicity in the EMG or mechanical waveforms associated with tremor (for example, McAuley et al. 1997). Underlying this idea is the concept that a high-frequency modulation of the motor neurones can produce a ripple of the same frequency in their output, presumably by a mechanism which biases their likelihood of firing in a cyclical manner. Thus the resultant rhythm is a feature of the behaviour of a population of motoneurones rather than a single cell. Although this seems a reasonable concept, the low-pass filtering properties of the neuromuscular system might heavily attenuate the oscillation. We have used a simple method of producing motor neuronal entrainment using repetitive cutaneous stimulation to study the extent of this attenuation.

Small cutaneous stimuli (3 times detection threshold, 50 µs width) were applied using ring electrodes to the proximal interphalangeal joint of the index finger of six subjects with ethical permission and their consent. During this procedure the subject used the first dorsal interosseus muscle of the same hand to generate an approximately constant force (15 N, less than 50 % maximal voluntary contraction for all subjects) against a load cell. A damped visual display of the force was provided to assist the subject. With stimulation at low frequency, complex large force fluctuations were easily recorded using an averaging technique. Their profile, which was triphasic, matched those described for EMG changes by Stephens and co-workers (for example, Jenner & Stephens, 1982). With higher frequencies, the size of the response decreased, but a time-locked mechanical fluctuation was still confidently detectable at ~40 Hz, either by averaging or by FFT techniques. A representative example is shown in Fig. 1.We conclude that a synchronising influence such as repetitive mild ipsilateral cutaneous stimulation can produce an adequate input to produce easily detectable rhythmicity at high frequency without undue attenuation. By implication, an oscillator higher in the CNS should be able to do the same.

Figure 1. Cutaneous stimulation at 1.0 Hz (left) and 36 Hz (right). There is a phase-locked mechanical response that is much smaller at the higher frequency.

Jenner, J.R. & Stephens, J.A. (1982). J. Physiol. 333, 405-419. abstract

McAuley, J.H., Rothwell, J.C. & Marsden, C.D. (1997). Exp. Brain Res. 114, 525-541.