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Physiological Recordings of High and Low Output NMJs on the Crayfish Leg Extensor Muscle
Published on: November 17, 2010
The pain-induced decrease in low-threshold motor unit discharge rate is not associated with the amount of increase in
Dario Farina1, Lars Arendt-Nielsen, Silvestro Roatta
1Center for Sensory-Motor Interaction, Department of Health Science and Technology, Aalborg University, Fredrik Bajers Vej 7 D-3, DK-9220 Aalborg, Denmark. df@hst.aau.dk
Summary
Experimental muscle pain reduces motor unit discharge rates. However, the increase in motor unit twitch torque during pain is not associated with this discharge rate decrease, suggesting other mechanisms maintain force.
Area of Science:
- Neuroscience
- Human Physiology
- Pain Research
Background:
- Nociceptive afferent activation typically reduces motor unit discharge rates during static contractions.
- Experimental muscle pain may increase motor unit twitch force, potentially compensating for reduced discharge rates to maintain constant muscle force.
Purpose of the Study:
- To investigate the association between changes in motor unit discharge rate and the increase in spike-triggered average torque during experimental muscle pain.
- To determine if altered motor unit twitch torque during pain correlates with reduced discharge rates.
Main Methods:
- Sixteen subjects performed isometric ankle dorsiflexion at 10% maximal voluntary contraction (MVC).
- Motor unit discharge rate and spike-triggered average torque were measured before and after injecting hypertonic (painful) or isotonic (control) saline into the tibialis anterior muscle.
Main Results:
- Hypertonic saline injection significantly decreased motor unit discharge rate (from 9.9±1.3 to 8.9±1.0 pulses per second).
- The peak spike-triggered average torque increased significantly during pain (from 0.56±0.55 to 0.95±1.02 mNm).
- No significant correlation was found between the decrease in discharge rate and the increase in spike-triggered average torque (R=0.08).
Conclusions:
- Pain-induced increases in estimated motor unit twitch torque are not due to changes in muscle fiber action potential.
- These torque modifications during pain are independent of the observed decrease in motor unit discharge rate.
- The maintenance of constant force during static painful contractions is not explained by a direct matching of contractile and control motor unit properties.
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