Proposed cortical and sub-cortical contributions to the long-latency stretch reflex in the forearm

The short- and long-latency (SLSR, LLSR) components of the stretch reflex response were investigated in the forearm using a paired transcranial magnetic stimulation (TMS)-stretch reflex protocol. Responses to TMS were recorded in the flexor and extensor carpi radialis muscles (FCR, ECR) after conditioning with a rapid wrist extension movement. The cortical stimuli were timed to elicit a motor-evoked potential (MEP) at either the SLSR or LLSR onset in the FCR muscle. Responses were also collected in TMS-alone and stretch reflex-alone conditions. Six intensities of magnetic stimulation were applied in all conditions. In the FCR muscle, MEP amplitude when timed to arrive at the LLSR onset was significantly greater than the sum of the MEP and stretch reflex responses when given separately. MEP amplitudes at the SLSR onset in the FCR muscle and in the ECR muscle at both SLSR and LLSR onset were not significantly different from that expected from the sum of the two stimuli given separately. This indicates heightened corticospinal excitability at a time corresponding to the passage of an afferent volley induced by the stretch, and raises the possibility of a transcortical loop of the LLSR in the forearm. The extent of MEP facilitation was generally consistent across all stimulus intensities tested. A reduced component of the LLSR was evident when the stretch response was timed to arrive during the silent period following the cortical stimulus, suggesting both cortical and sub-cortical components to the reflex response.