Clonus in man: a rhythmic oscillation maintained by a reflex mechanism

Area of Science:

• Neuroscience
• Human Physiology
• Motor Control

Background:

• Clonus is an involuntary, rhythmic muscle contraction often associated with spasticity due to upper motor neuron lesions.
• The precise mechanisms generating clonus, particularly its dependence on reflex loops versus central pattern generators, remain debated.

Purpose of the Study:

• To investigate the underlying mechanisms of clonus in patients with lower limb spasticity.
• To assess the role of reflex pathways and mechanical factors in clonus generation.
• To evaluate the impact of afferent fiber blockade on clonus persistence.

Main Methods:

• Studied 7 patients with spasticity from upper motor neuron lesions.
• Evoked soleus H reflexes at varying intervals between clonic beats to assess resetting.
• Calculated the resetting index based on H reflex timing relative to clonic beats.
• Applied calf compression to block group I afferent fibers and observed effects on ankle clonus.

Main Results:

• A high resetting index (near 1.0) indicated significant reflex sensitivity.
• Calf compression completely abolished ankle clonus within 10 minutes.
• Clonus diminished progressively in duration and frequency before disappearance.
• Clonus duration was found to be dependent on joint angle and muscle stretch.

Conclusions:

• Clonus generation is significantly influenced by reflex and mechanical feedback loops.
• Findings do not support the hypothesis of a central spinal generator for clonus.
• The study highlights the importance of afferent input and muscle stretch in maintaining clonus.