A simulation study of reflex instability in spasticity: origins of clonus

Clonus is defined as an involuntary rhythmic muscle contraction that generally occurs in people who have sustained lesions involving descending motor pathways in the neuraxis, and is usually accompanied by other signs of reflex hyperexcitability such as spasticity. This paper hypothesizes that clonus arises when two conditions occur simultaneously: 1) the reflex pathway contains long delay times (implying innervation of distal limb muscles, exacerbated when these muscles display slow twitch properties) and 2) the excitability of the motoneurons is enhanced. This paper tested this dual hypothesis by developing a computer model representing the ankle reflex pathway. This model included the ankle muscles, afferent and efferent pathways, and a monosynaptic spinal link between spindle afferents and motoneurons. Simulations show that as the motoneuron current threshold was reduced (reflecting increased excitability of spinal motoneurons), normal reflex responses became unstable and oscillations developed similar to those observed in spastic patients. In parallel, when we choose reflex delay times typical for distal leg muscles in man, system stability is poor, and oscillations occur readily with increasing motoneuron excitability. As simulated pathway delays are reduced, oscillatory behavior is also reduced, and usually damps out. Conversely, as simulated reflex delays are increased, oscillations increase in amplitude and do not decay. Finally, these two phenomena interact, so that increasing motoneuron excitability will induce reflex oscillations for intermediate loop delays. These findings support the hypothesis that unstable oscillatory behavior, such as the oscillations observed in clonus, will occur when the motoneuron excitability increases in a reflex pathway containing long delays. This change in excitability is mediated by a reduction in motoneuron firing threshold, rather than by an increase in feedback gain. Furthermore, we demonstrate that sustained oscillations occur readily through self reexcitation, which reduces the need to propose that a "central oscillator" must be involved in generating clonus.