Effect of caffeine on self-sustained firing in human motor units

Abstract

This study examined the effect of caffeine on self-sustained firing (SSF) of human motor units. At physiological doses, caffeine acts as a competitive antagonist to the inhibitory effects of adenosine. This antagonism has many possible effects on the central nervous system. One of these effects is to increase the release of the excitatory neurotransmitters serotonin and noradrenaline. In addition, caffeine increases serotonin concentration in brainstem regions that have excitatory projections to spinal motor neurons. Since plateau potentials, which are responsible for SSF, are facilitated by these neurotransmitters, we hypothesized that caffeine would increase the frequency at which SSF occurs. A double-blind, repeated-measures design using either drug (6 mg kg(-1) caffeine) or placebo (flour) was carried out on seven male subjects who reported ingesting less than 200 mg week(-1) caffeine. We investigated the occurrence of SSF in tibialis anterior motor units (214 trials) and found a significant (P < 0.05) increase in the occurrence of SSF in the caffeine trial (87.0 +/- 5.8 %) compared to the placebo (64.6 +/- 9.7 %). These data further verify the presence of SSF in the tibialis anterior motor units of young men and provide indirect evidence of the facilitation of plateau potentials by monoamines in the human neuromuscular system.

Figure 1. Example of self-sustained firing

A, representative recording from one subject. The accelerometer channel indicates the application of the vibrator to the tendon. The frequency channel shows the instantaneous (dots) and average (line) firing rates of the control unit. The intramuscular channel reveals the recruitment of a motor unit (test unit) during vibration followed by prolonged firing. This recruitment coincides with an increase in the tibialis anterior EMG (TA EMG) and torque, since additional motor units are likely to be recruited elsewhere in the muscle. B, 13 s of recording on an expanded time base illustrating the period before and after vibration. The control unit fired steadily at 9.93 Hz before vibration and 9.56 Hz after vibration, indicating that descending drive to the motor neuron pool did not increase. The test unit (larger unit) recruited during vibration maintained a steady firing frequency of 9.84 Hz for 34 s after vibration was removed, and only terminated when the subject was instructed to relax. The amplitude of the test unit was attenuated during the vibration due to the depression of the skin for the 1.5 s the vibrator was applied (verified by a sham trial).

Figure 2. Isometric ramp contraction

This was performed immediately after each SSF trial to test the sensitivity of the control unit. As torque increased, the firing frequency of the control unit also increased, indicating the sensitivity of the control unit to increased descending drive. The arrow denotes the firing frequency of the control unit (9.93 Hz) during the SSF trial. Since the control unit's firing frequency increases well beyond this point with further voluntary effort, we conclude that the control unit was not firing at a saturated frequency during the SSF trial.

Figure 3

A, the occurrence of self-sustained firing (SSF) across all trials. The frequency of occurrence of SSF in the caffeine trial is significantly different from all other trials, but there were no differences between the two pre-test trials, or the placebo trials. Each fraction denotes the number of trials displaying SSF over the total number of trials recruited during vibration. Caffeine pre-test (CF Pre), 46.5 ± 9.8 %; caffeine post-test (CF Post), 87.0 ± 5.8 %; placebo pre-test (PL Pre), 61.7 ± 10.0 %; placebo post-test (PL Post), 64.6 ± 9.1 %. B, the effect of caffeine on the occurrence of self-sustained firing expressed as a percentage change from the pre-test for each condition. Caffeine caused a significant increase in the occurrence of SSF compared to placebo (*P < 0.05, **P < 0.01).