Measurement of contractile and electrical properties of single human thenar motor units in response to intraneural motor-axon stimulation

G. Westling, R. S. Johansson, C. K. Thomas, B. Bigland-Ritchie

Research output: Contribution to journalArticle

67 Scopus citations

Abstract

1. A method is described for measuring contractile properties of single human motor units. Conventional human microneurographic techniques were adapted to stimulate individual motor axons in the median nerve, with the use of negative current pulses and a tungsten microelectrode, while recording motor-unit electromyographic activity (EMG) and isometric force responses from the thenar muscles. 2. EMG signals were recorded from both proximal and distal thenar muscle surfaces. Force was recorded in two directions (thumb flexion and abduction). This allowed calculation of the direction and magnitude of resultant force exerted by each unit. 3. Data accepted as originating from a single unit satisfied all the traditional 'all-or-none' criteria. Additional criteria also required the following: 1) a wide safety margin between the threshold for unit activation and the current intensity needed to elicit responses from other units; 2) that the characteristic direction in which each unit generated force did not change during the recording period; and 3) whenever F-responses were encountered, the second EMG waveform was identical to the first - a highly improbable event if more than one unit had been excited. 4. Respiration and blood pressure waves introduced baseline fluctuations that distorted the force measurements. These fluctuations were minimized by synchronizing stimuli to the pulse pressure cycle and resetting the baseline electronically just before stimulus onset. 5. Combining motor-axon stimulation at a site remote from the muscle with electronic resetting of the force baseline and delivery of stimuli at fixed intervals after the pulse pressure waves allowed the full time course of human motor-unit twitch and tetanic force and EMG signals to be recorded accurately without signal averaging.

Original languageEnglish (US)
Pages (from-to)1331-1338
Number of pages8
JournalJournal of neurophysiology
Volume64
Issue number4
DOIs
StatePublished - Jan 1 1990

ASJC Scopus subject areas

  • Neuroscience(all)
  • Physiology

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