Instruction-dependent modulation of the long-latency stretch reflex is associated with indicators of startle

• Published in: Experimental brain research Vol. 230; no. 1; pp. 59 - 69
• Main Authors: Ravichandran, Vengateswaran J., Honeycutt, Claire F., Shemmell, Jonathan, Perreault, Eric J.
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg Springer Berlin Heidelberg 01.09.2013; Springer; Springer Nature B.V
• Subjects: Acoustic Stimulation; Acoustics; Adult; Arm - innervation; Arm - physiology; Biological and medical sciences; Biomedical and Life Sciences; Biomedicine; Brain research; Data Interpretation, Statistical; Elbow; Elbow Joint - physiology; Electromyography; Female; Fundamental and applied biological sciences. Psychology; Humans; Linear Models; Male; Motor ability; Movement - physiology; Muscle strength; Muscle, Skeletal - innervation; Muscle, Skeletal - physiology; Neurology; Neurosciences; Photic Stimulation; Physiological aspects; Posture; Psychomotor Performance - physiology; Reflex, Startle - physiology; Reflex, Stretch - physiology; Research Article; Vertebrates: nervous system and sense organs; Young Adult; New Zealand; United States; United States > US; Chicago Illinois; Startle; Triggered reaction; Stretch reflex; Long-latency reflex
• ISSN: 0014-4819; 1432-1106; 1432-1106
• DOI: 10.1007/s00221-013-3630-1

Long-latency responses elicited by postural perturbation are modulated by how a subject is instructed to respond to the perturbation, yet the neural pathways responsible for this modulation remain unclear. The goal of this study was to determine whether instruction-dependent modulation is associated with activity in brainstem pathways contributing to startle. Our hypothesis was that elbow perturbations can evoked startle, indicated by activity in the sternocleidomastoid muscle (SCM). Perturbation responses were compared to those elicited by a loud acoustic stimulus, known to elicit startle. Postural perturbations and startling acoustic stimuli both evoked SCM activity, but only when a ballistic elbow extension movement was planned. Both stimuli triggered SCM activity with the same probability. When SCM activity was present, there was an associated early onset of triceps electromyographic (EMG), as required for the planned movement. This early EMG onset occurred at a time often attributed to long-latency stretch reflexes (75–100 ms). The nature of the perturbation-triggered EMG (excitatory or inhibitory) was independent of the perturbation direction (flexion or extension) indicating that it was not a feedback response appropriate for returning the limb to its original position. The net EMG response to perturbations delivered after a movement had been planned could be explained as the sum of a stretch reflex opposing the perturbation and a startle-evoked response associated with the prepared movement. These results demonstrate that rapid perturbations can trigger early release of a planned ballistic movement, and that this release is associated with activity in the brainstem pathways contributing to startle reflexes.