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Is the long-latency stretch reflex in human masseter transcortical?
Sophie L Pearce1, Timothy S Miles, Philip D Thompson
1Department of Physiology, The University of Adelaide, 5005, Adelaide, South Australia, Australia.
Experimental Brain Research
|April 25, 2003
Summary
The long-latency stretch reflex in the human masseter muscle does not involve the motor cortex. This jaw reflex is not influenced by voluntary commands or motor set, indicating a subcortical pathway.
Area of Science:
- Neuroscience
- Human motor control
Background:
- The neural pathway for the long-latency stretch reflex (LLSR) in the human masseter muscle is not fully understood.
- Investigating the motor cortex's involvement is crucial for elucidating this reflex pathway.
Purpose of the Study:
- To determine if the human masseter LLSR involves corticomotoneuronal (CM) cells.
- To assess the influence of motor cortex excitability on the LLSR.
Main Methods:
- Ten subjects underwent transcranial magnetic stimulation (TMS) while performing a jaw-holding task.
- Short- and long-latency stretch reflexes were evoked by jaw displacements.
- TMS was delivered at intervals timed to coincide with the short-latency stretch reflex and LLSR to assess corticomotoneuronal excitability.
Main Results:
- Conditioning muscle stretch did not significantly alter motor-evoked potential (MEP) areas in the masseter muscle across various conditioning-testing intervals.
- No significant modulation of MEPs was observed when TMS coincided with either the short-latency stretch reflex or the LLSR.
- Subjects could not voluntarily modify the short- or long-latency stretch reflexes.
Conclusions:
- The long-latency stretch reflex in the human masseter muscle does not appear to involve the motor cortex.
- The LLSR pathway is not influenced by motor set, suggesting it operates at a subcortical level.