An index of spindle efficacy obtained by measuring electroneurographic activity and passive tension in the rat soleus muscle.
While muscle spindle afferent discharges are known to change with altered muscle use, the way in which the changes in spindle discharge are affected by modifications to the elastic properties of the muscle-tendon unit remains to analyze. This paper describes a methodology to define, in the rat, a spindle efficacy index. This index relates the spindle afferent discharges recorded from electroneurograms (ENG) due to muscle stretch to the passive elastic properties of the muscle-tendon unit quantified during the stretch imposed for the ENGs recordings. The stretches were applied to the rat soleus muscle after the Achilles tendon was severed. The spindle afferent discharges were characterized from the root mean square (RMS) values of electroneurograms (ENGs) recorded from the soleus nerve. The first step of the study was to validate the definition of dynamic and static indices (DI and SI) of spindle discharges from RMS-ENG as classically done when isolated afferents are studied. The slopes of the DI-stretch velocity or SI-stretch amplitude relationships gave the indices of spindle sensitivity under dynamic and static conditions, respectively. Incremental stiffness was calculated to describe the passive elastic properties during the dynamic and static phases of ramp and hold stretches applied at different amplitudes and velocities. The spindle efficacy index (SEI) is the ratio between the indices of spindle sensitivity and incremental stiffness values. Both spindle discharges and incremental stiffness increased with stretch amplitude under dynamic and static conditions. The corresponding SEI values were constant whatever the stretch amplitude. This result validates the relationship between spindle discharges and passive incremental stiffness. This method can be proposed to study, in the rat, the spindle function when the muscles are suspected to present changes in their neuromechanical properties.
Rosant
Cédric
C
Laboratoire de Biomécanique et Génie Biomédical, UMR-CNRS 6600, Université de Technologie de Compiègne, BP 20529, F-60205 Compiègne, France.
Pérot
Chantal
C
D000818:Animals; D004594:Electrophysiology / Q000379:methods*; D008297:Male; D009470:Muscle Spindles / Q000502:physiology*; D018482:Muscle, Skeletal / Q000502:physiology*; D051381:Rats; D017208:Rats, Wistar; D012026:Reflex, Stretch / Q000502:physiology*