Neuronal networks in the human spinal cord ineteract with peripheral signals to generate and sustain clonus during stepping, standing and following manual induced stretch.
Fig. 1
Fig. 1 EMG activity (µV) of the SOL, MG and TA during (A, B) stretch-induced ankle clonus, (C, D) non-weight bearing standing, and (E, F) one step during stepping on a treadmill using body weight support. (B, D, and F) show 0.6 seconds selected from between the dotted lines in (A, C, and E, respectively) plotted on an expanded time scale.
Fig. 2
Fig. 2 (A and B) Demonstrates method of calculations of muscle-tendon length (MTL) 40 milliseconds prior to a clonic EMG burst from (A) TA and (B) SOL muscles from a SCI subject. The MTL (percent shange length, %SL; secondary axis) is depicted by the bold curve and is plotted with EMG (µV, primary axis) versus the change in MTL. The veritcal dashed line represents the apporximate division between MTL shortening (left of the line) and the MTL stretch (right of the line). Thin solid line - SOL clonic EMG; Thin dashed line - MG clonice EMG; Heavy dashed line - TA clonic EMG; Heavy solid line - grouped muscle clonic EMG.
Clonus presents behaviorally as rhythmic distal joint oscillation and is a common pathology that occurs secondary to SCI and other neurological disabilities. There are two primary theories as to the underlying mechanism of clonus. The prevailing one is that clonus is generated and sustained by recurrent activation of stretch reflexes. We compared EMG of the SOL, MG and TA from SCI subjects during stretch-induced ankle clonus, non-weight bearing standing and stepping. The most common ankle muscle activation pattern following a quick stretch of the plantarflexors (stretch-induced ankle clonus) was one of temporally synchronized EMG activity of the SOL, MG and TA. Most subjects displayed the same EMG coordination pattern among SOL, MG and TA during clonus occurring during non-weight bearing standing and stepping as during stretch-induced ankle clonus. In all conditions the occurrence of SOL, MG and TA clonic EMG activity was not restricted to periods following muscle-tendon stretch. Further, clonic EMG in all paradigms had similar burst frequency, burst duration and silent period duration. The similarities in burst characteristics and coordination patterns across conditions indicated that clonic EMG patterns occurred over a wide range of kinematic and kinetic conditions and thus proprioceptive inputs. These results suggest that the repetitive clonic bursts could not be attributable solely to immediate afferent feedback such as recurrent muscle stretch but rather clonus is the representation of the oscillatory properties of the inter- and moto-neuronal pools interacting with peripheral signals. Clonus represents the interneuronal functional connections between extensors and flexors with oscillatory properties that can be activated by many different peripheral stimuli after neurologic injury.
Related Publications:
Beres-Jones JA, Johnson TD, Harkema SJ. Clonus after human spinal cord injury cannot be attributed solely to recurrent muscle-tendon stretch. Exp Brain Res 2003.
Davis BC, Beres JA, Johnson TD, Dobkin BH, Harkema SJ. Quantitative assessment of load and speed effects on lower limb muscle coordination during locomotion. Soc Neurosci 2000;26(1):156.