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

Abstract

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.

Fractionation of the visuomotor feedback response to directions of movement and perturbation

“…Such a possibility is similar in some sense to the rapid motor responses to muscle stretch: different functional responses at different delay times ( Crago et al 1976 ; Pruszynski et al 2011b ). For stretch responses, although the initial short-latency response is not modifiable [except on a long timescale ( Wolpaw et al 1983 )] and only exhibits gain scaling ( Marsden et al 1976 ; Pruszynski et al 2009 ), the longer-latency responses can be modulated according to the task demands ( Dimitriou et al 2012 ; Kurtzer et al 2008 ; Nashed et al 2012 , 2014 ; Shemmell et al 2009 ) and may involve cortical circuits similar to those involved in voluntary motor control ( Pruszynski et al 2011a ; Zuur et al 2009 , 2010 ) as well as subcortical components ( Grey et al 2001 ; Lewis et al 2004 ).…”

Primary motor cortex and fast feedback responses to mechanical perturbations: a primer on what we know now and some suggestions on what we should find out next

“…Notably, we also established a causal role for M1 by applying TMS over human M1 and showing that the long-latency stretch response in shoulder muscles was potentiated even when the shoulder joint was not displaced by the mechanical perturbation. As described above, such potentiation (Day et al, 1991; Palmer and Ashby, 1992; Lewis et al, 2004) must reflect the impact of elbow afferent information onto a cortical circuit controlling shoulder muscles since local shoulder afferents are not physically affected by the perturbation.…”

Ipsilateral Motor Pathways after Stroke: Implications for Non-Invasive Brain Stimulation

“…Up-regulation of contralesional motor cortex excitability and the CRPP pathway may be important for paretic arm function after stroke (Turton et al, 1996; Netz et al, 1997; Alagona et al, 2001; Lewis et al, 2004; Misawa et al, 2008), particularly in poorly recovered patients (Turton et al, 1996; Netz et al, 1997; Gerloff et al, 1998; Caramia et al, 2000; Trompetto et al, 2000; Lewis and Perreault, 2007; Misawa et al, 2008). The degree of reorganization toward contralesional hemisphere control may depend on the residual integrity of white matter tracts from the ipsilesional hemisphere (Ward et al, 2006, 2007; Stinear et al, 2008; Grefkes and Fink, 2011).…”