The long-latency reflex is composed of at least two functionally independent processes

Abstract

Pruszynski JA, Kurtzer I, Scott SH. The long-latency reflex is composed of at least two functionally independent processes. J Neurophysiol 106: 449 -459, 2011. First published May 4, 2011 doi:10.1152/jn.01052.2010.-The nervous system counters mechanical perturbations applied to the arm with a stereotypical sequence of muscle activity, starting with the short-latency stretch reflex and ending with a voluntary response. Occurring between these two events is the enigmatic long-latency reflex. Although researchers have been fascinated by the long-latency reflex for over 60 years, some of the most basic questions about this response remain unresolved and often debated. In the present study we help resolve one such question by providing clear evidence that the human long-latency reflex during a naturalistic motor task is not a single functional response; rather, it appears to reflect the output of (at least) two functionally independent processes that overlap in time and sum linearly. One of these functional components shares an important attribute of the short-latency reflex (i.e., automatic gain scaling, sensitivity to background load), and the other shares a defining feature of voluntary control (i.e., task dependency, sensitivity to goal target position). We further show that the task-dependent component of long-latency activity reflects a feedback control process rather than the simplest triggered reaction to a mechanical stimulus. upper limb; feedback; background load; intent; spatial target

Goal-Dependent Modulation of Fast Feedback Responses in Primary Motor Cortex

“…The most common finding is that the short-latency response is the same for both instructions, whereas the long-latency response is greater when the subject is instructed to resist versus let go. This pattern is strikingly robust and has been reported for muscles at the elbow, wrist, finger, and ankle (Pruszynski and Scott, 2012).…”

“…Many studies have established that rapid motor responses (i.e., stretch reflexes) to mechanical perturbations are goal-dependent (Shemmell et al, 2010;Pruszynski and Scott, 2012). These studies typically issue verbal instructions, such as "resist" and "let go," that indicate how the participant should respond to a mechanical perturbation (Hammond, 1956).…”

“…Primary motor cortex (M1) is a logical candidate to mediate goal-dependent modulation of the long-latency stretch response (Scott, 2004(Scott, , 2012. M1 is an important component of the transcortical feedback pathway (Shemmell et al, 2010;Hatsopoulos and Suminski, 2011;Pruszynski and Scott, 2012) and a key node for generating voluntary motor commands (Porter and Lemon, 1993).…”

Early compensatory and anticipatory postural adjustments following anterior cruciate ligament reconstruction

“…Our results, however, are in contrast with those of Madhavan and Shields (2011), who reported higher amplitude of long-latency muscle responses to unexpected perturbations in patients with ACLR compared to healthy individuals during a dynamic single-leg weight-bearing task. The differences between Madhavan and Shields (2011) and the results of the present study might be attributed to the task being performed, since long-latency responses may be modulated in a manner appropriate to meet the motor demands and give protection to forthcoming perturbations (Dietz et al 1994;Shemmell et al 2010;Pruszynski et al 2011). The weight-bearing task being performed in Madhavan and Shields (2011) is a closed kinetic chain movement, in which activation of the knee extensor muscles causes lower anterior shear 15 forces on the tibia and lower strain on the ACL than during open kinetic chain movements (Lutz et al 1993) that are similar to the semi-reclined task in this study.…”

“…Since long-loop circuits are involved, this process can take place, thus allowing the sensory signal to be specifically associated to the task and the motor response to be adapted instantaneously to the actual circumstances (Lewis et al 2006;Kurtzer et al 2008;Pruszynski and Scott 2012;Pruszynski et al 2011;Casabona et al 2012;Valle et al 2013).…”