Spinal stretch reflexes support efficient control of reaching

Abstract

Efficiently controlling the movement of our hand requires coordinating the motion of multiple joints of the arm. Although it is widely assumed that this type of efficient control is implemented by processing that occurs in the cerebral cortex and brain stem, recent work has shown that spinal circuits can generate efficient motor output that supports keeping the hand in a static location. Here, we show that a spinal pathway can also efficiently control the hand during reaching. In our first experiment we applied multi-joint mechanical perturbations to participant's elbow and wrist as they began reaching towards a target. We found that spinal stretch reflexes evoked in elbow muscles were not proportional to how much the elbow muscles were stretched but instead were efficiently scaled to the hand's distance from the target. In our second experiment we applied the same elbow and wrist perturbations but had participants change how they grasped the manipulandum, diametrically altering how the same wrist perturbation moved the hand relative to the reach target. We found that changing the arm's orientation diametrically altered how spinal reflexes in the elbow muscles were evoked, and in such a way that were again efficiently scaled to the hand's distance from the target. These findings demonstrate that spinal circuits can help efficiently control the hand during dynamic reaching actions, and show that efficient and flexible motor control is not exclusively dependent on processing that occurs within supraspinal regions of the nervous system.

Temporal dynamics of the sensorimotor convergence underlying voluntary limb movement

“…S5). Our results regarding facilitative effects on muscle activity are consistent with previous evidence that the Ia monosynaptic stretch reflex system is engaged in reaching movements (21). On the other hand, somatosensory feedback signals through afferents with other modalities, such as Golgi tendon organs and cutaneous receptors, are also known to act on spinal motor neurons and are involved in the modulation of muscle activity (9,11,(28)(29)(30).…”

“…They also send inhibitory inputs to motor neurons of the antagonist muscles through a process, known as reciprocal inhibition (20). A previous study using a sudden stretching of the limb showed that the stretch reflex system is engaged in reaching movements (21). Therefore, we examined the relationship between the afferent component and changes in joint angle that accompany changes in muscle length.…”

Express visuomotor responses reflect knowledge of both target location and conscious intent during reaches of different amplitudes

“…Initiation and online control of real-world visuospatial actions rely on visuomotor circuits that must integrate multisensory information about the body and target positions, which is inherently variable and noisy. The current data is consistent with previous evidence suggesting that the putative subcortical express circuits can be primed to generate flexible context-dependent motor outputs that support the accomplishment of visuospatial tasks from both static (Kurtzer, 2015; Weiler et al, 2019; Contemori et al, 2022) and dynamic postures (Cross et al, 2019; Kozak et al, 2019; Weiler et al, 2021).…”

Fast reoptimization of human motor patterns in non-Earth gravity fields locally induced by a robotic exoskeleton

“…These results are consistent with previous studies performed with different experimental paradigms, which suggested that somatosensory feedback, in particular haptic, allows a fast and significant adaptation ( 11, 54–56 ). This is probably due to the very quick adaptation of somatosensory loops and to the predominant role of the somatosensory system in the early learning of new dynamics ( 43, 44, 57, 58 ). Such a quick adaptation could therefore be a feature of the CNS to easily handle object manipulation, even when deprived of specific visual or vestibular information ( 59 ), which is a common type of action in daily life.…”