Descending Brainstem Systems Contribute to Ankle Clonus in Humans with Spinal Cord Injury

Area of Science:

• Neuroscience
• Motor Control
• Spinal Cord Injury Research

Background:

• Ankle clonus is a common, involuntary muscle contraction after spinal cord injury (SCI).
• The exact mechanisms driving clonus are unclear, but brainstem systems are implicated in regulating spinal motoneuron excitability.
• Neuromodulatory brainstem inputs may compensate for lost descending pathways post-SCI, contributing to involuntary contractions.

Purpose of the Study:

• To investigate the role of descending brainstem pathways in the generation and maintenance of ankle clonus following chronic SCI.
• To test the hypothesis that enhanced brainstem drive increases motoneuron excitability in individuals with SCI-related clonus.

Main Methods:

• Quantified motoneuron excitability using soleus H-reflex and maximal motor response (H/M ratio).
• Assessed brainstem involvement via long-latency cutaneous reflex (LLR) and StartReact response (reaction time to startling stimuli).
• Studied participants with chronic SCI, with and without ankle clonus, measuring responses in tibialis anterior and soleus muscles.

Main Results:

• Participants with clonus exhibited significantly higher H/M ratios, indicating increased motoneuron excitability compared to those without clonus.
• Individuals with clonus showed prolonged and augmented LLRs and reduced reaction times to startling stimuli.
• LLR duration positively correlated with StartReact response and H/M ratio, linking brainstem activity to motoneuron excitability.

Conclusions:

• Findings suggest that descending brainstem pathways contribute to the maintenance of ankle clonus after chronic SCI.
• Enhanced monoaminergic and reticulospinal tract activity may underlie the increased motoneuron excitability observed in clonus.
• These results highlight the potential involvement of brainstem neuromodulation in SCI-induced spasticity and involuntary movements.