Characterization of the tendon vibration reflex response in hemi-spastic stroke individuals

The objective of our study was to assess the role of persistent inward currents, or PICs, on the excitability of motoneurons innervating spastic muscle in hemi-spastic stroke individuals. This was accomplished by examining the effects of tonic vibration applied to the tendon of the biceps brachii muscle. The elicited TVR (tonic vibration reflex) provides a useful way to assess the degree of excitability of spinal neurons in spastic syndromes, and it has additional features that may signify the presence of PICs in spastic motoneurons. We applied sinusoidal stretches of varied duration to the biceps tendon of two hemi-spastic stroke individuals and one neurologically intact individual. We recorded the resulting TVR response from electromyographic(EMG) signals obtained from the biceps as well as force recorded at the wrist. The results of our preliminary study show that the initial rise of the TVR force response as well as the force magnitude are generally greater in spastic muscle, perhaps a marker of motoneuron excitability. Additionally, a shorter vibration duration was sufficient to evoke a response on the spastic side of our tested stroke subjects. However, the key marker of PICs - the decay of the force response as well as sustained after-discharge did not exhibit clear differences. Our present data suggests that motoneurons innervating spastic muscle are more readily activated, and thus exhibit increased excitability, which could possibly be a function of greater depolarization, without a change in PIC magnitude. Our data does not rule out the possibility of subthreshold activation of the PIC resulting in enhanced motoneuron depolarization.

I. Introduction

Spasticity is characterized by exaggerated reflex responses to muscle stretch or tendon tap, perhaps as the result of increased motoneuron excitability, although this has not been definitively established. The objective of our study was to assess the role of persistent inward currents, or PICs, on motoneurons innervating spastic muscle in hemiparetic stroke individuals, by examining the effects of tonic vibration applied to the biceps tendon. The TVR (tonic vibration reflex) provides a useful way to assess the degree of excitability of spinal neurons in spastic syndromes, and it has additional features that may signify the presence of PIC's in spastic motoneurons. The TVR relies on the fact that small amplitude high frequency vibration applied to the muscle tendon generates a phase locked train of Ia afferent impulses at the frequency of the vibration signal (provided the frequency is not too high i.e., cycles/second)[1]. These high frequency Ia afferent trains induce progressive excitation of the homonymous motoneurons, and appear to excite persistent inward currents in these motoneurons, as evidenced by the slow increase in isometric force during the vibration sequence, and, even more visibly, by the sustained force that persists after the vibration is turned off [1] [2]. This sustained force is termed “hang up”, and it is an excellent marker for the presence of monoamine dependent PIC's. If PIC's are increased in amplitude in motoneurons that innervate spastic muscle, we would expect to see enhanced TVR induced EMG and torque responses in such muscles, with prolongation of force and EMG well after vibration is terminated. In a similar study, McPherson et.al., reported significantly greater increases in elbow flexion torque and EMG activity in the spastic as compared with the non-spastic limbs of chronic hemi -spastic stroke survivors, both during biceps vibration and up to 5 s following vibration cessation. The vibration was applied on the belly of the biceps muscle for a duration of 8_seconds with a vibration frequency of 112 Hz.