Single Unit Spindle Responses to Muscle Vibration in Man

This chapter discusses the responses of muscle spindle afferents to muscle vibration in human subjects, using the microneurographic technique originally described by Vallbo and Hagbarth. With a vibrator of the type commonly used to elicit tonic vibration reflexes in humans, the spindle primary endings can be driven up to firing rates greatly exceeding those resulting from sustained passive stretch alone. Muscle stretch increases the spindle response to vibration, the spindle primaries following higher vibration frequencies than the secondaries. During muscle shortening, on the other hand, the vibration-induced spindle excitation is minimal, the response being better preserved in the secondaries than in the primaries. Primary endings strongly driven by the vibrator are often unable to respond to a tendon tap applied during vibration, but the tap may temporarily unload the spindles on its falling phase, causing a pause in the vibration-induced excitation. Fusimotor coactivation occurring during a voluntary contraction sensitizes spindle endings to the vibratory stimulus. The tonic vibration reflex resulting from the vibratory stimulus is a contraction of the α-type, which unloads spindle endings and reduces their sensitivity to vibration.