Interaction of Stretch Reflex Loop with Descending B-Oscillations and the Generation of Tremor in Parkinson’s Disease: A Study of Motor Unit Firing Synchrony and Patterns (P5.369)

OBJECTIVE: Though the neural mechanisms underlying rest and postural parkinsonian tremors have been studied extensively, their understanding remains limited. In this study, we investigated tremor-related motor unit (MU) firing patterns and synchrony, as MU behaviors determine the neurogenic components of limb tremors. Their study may also yield information on the tremor-related synaptic input to motoneurons. BACKGROUND: Previous measurements in parkinsonian patients exerting force with the index finger have shown enhanced MU synchrony and tremor-related spike doublets and triplets [1]. Their incidence directly reflected the difference between the mean MU firing rate and tremor frequency while the mean interspike interval within them was approximately 50ms. A further step was undertaken now by including postural and rest tremor measurements. METHODS: We performed spectral, coherence, cross-correlation and interspike interval analyses on simultaneous records of MU activities and rest and postural tremors in 21 patients. Sixteen normal volunteers participated as control subjects. RESULTS: Interchanging normal and abnormal activity epochs were recorded displaying common features among the different tremor types. Specifically, 6-10-Hz, normal-sized, and 4-8-Hz, abnormally large, tremor components, ca. 1.5-Hz apart, were derived, each having invariant frequency in rest and postural tremor records for a given patient. The former component was related to normal-like MU behaviors while the latter with tremor-related spike-doublets and triplets and enhanced MU synchrony. These doublets/triplets exhibited fixed mean interspike intervals (30-50-ms i.e. beta-range) which co-vary with the period-differences between the two components. CONCLUSIONS: Our findings are compatible with a pair of principally neural, frequency-linked oscillators, one of them being physiological. Accordingly, we propose a two-state oscillatory action in the spinal stretch reflex loop dictated by intermittent, descending beta-oscillations, as a suitable model of parkinsonian tremor genesis. [1]Christakos CN, Erimaki S, Anagnostou E, Anastasopoulos D (2009) Tremor-related motor unit firing in PD: implications for tremor genesis. J Physiol (Lond) 587:4811-4827 Disclosure: Dr. Agapaki has nothing to disclose. Dr. Anastasopoulos has nothing to disclose. Dr. Erimaki has nothing to disclose. Dr. Christakos has nothing to disclose.