Can neck muscle spindle afferents activate fusimotor neurons of the lower limb?

INTRODUCTION

We investigated whether vibratory stimulation of the dorsal neck muscles activates fusimotor neurons of lower limb muscles in relaxed human subjects.

METHODS

The triceps surae (TS) muscles of seated subjects (n = 15) were conditioned to leave their muscle spindles in either an insensitive (hold-long) or sensitive (hold-short) state. A vibrator (80 HZ) was applied to the dorsal neck muscles for 10 seconds. The tendon jerk was evoked from the right TS immediately before (during) or 5 seconds after (interposed) the offset of vibration.

RESULTS

The size of the reflex after hold-long muscle conditioning and after neck vibration was significantly smaller than the control hold-short reflex (P < 0.001). However, after hold-short conditioning, neck vibration significantly increased tendon jerk amplitude, both during (P = 0.001) and interposed (P = 0.026).

CONCLUSION

Dorsal neck vibration increases spinal reflex excitability of the TS in relaxed and seated subjects, but not through fusimotor excitation.