Biology subjects

Torell, F.

Publications and source records attributed to Torell, F..

5 recordsLinked to original sources

bioRxiv2025

Preparation duration shapes the goal-directed tuning of stretch reflex responses

Stretch reflex responses are important for counteracting perturbations, and the modulation of stretch reflex gains can assist the voluntary movement quality and performance. Recent findings suggest that movement preparation includes goal-directed tuning of muscle spindles, facilitating a concurring modulation of reflex gains. In the context of delayed-reach, goal-directed modulation of stretch reflex gains is present when the homonymous muscle is unloaded (i.e., antagonist load) and the preparatory delay is sufficiently long (>250 ms and <750 ms). This study examined how multiple preparatory delays impact the goal-directed modulation of short- and long-latency stretch reflex (SLR and LLR) gains of the pectoralis, anterior and posterior deltoid muscles in a delayed-reach task. We used electromyographic signals to quantify stretch reflex responses of healthy subjects that experienced haptic perturbations induced by a robotic manipulandum in a delayed-reach task. We found that a preparatory delay of 300 ms is sufficient for goal-directed tuning of SLR responses in the pectoralis and posterior deltoid muscles, and 400 ms allows for even stronger goal-directed tuning of stretch reflex responses. In contrast, there was no goal-directed modulation in the SLR of the anterior deltoid. Tuning of LLR gains was robustly goal-dependent throughout, with negligible modulation across preparatory delays. Our results clarify the minimum preparatory time required for goal-directed tuning of stretch reflex gains and characterise the relationship between reflex gains and preparation duration. This relationship is not strictly linear, likely reflecting the interplay of multiple feedback mechanisms functioning at different time frames.

neuroscience↗

bioRxiv2024

Temporal shift in task factor influence across the stretch reflex

Mechanical perturbations applied to the arm can elicit reflexive actions. These rapid corrective responses include the stretch reflex, which consists of different components: the short-latency reflex (SLR) as well as the early and late long-latency reflex (LLR). In this study, we explore how different task factors dynamically influence these reflex components in the context of a specific delayed-reach paradigm. Using multiple linear regression (MLR), we analysed reflex responses from seven muscles actuating the right arm to examine the effects of mechanical load, preparatory delay, perturbation and target direction, as well as their two-factor interactions. The MLR analysis shows that our delayed-reach tasks engaged shoulder girdle muscles, whereas the biceps and triceps primarily acted as stabilizing muscles, with rapid responses triggered regardless of perturbation direction. Specifically, our analyses show that the earliest corrective response, the SLR, exhibited some task/target-dependent modulation particularly in muscles of the shoulder girdle, although background (pre-)loading decreased this modulation. The SLR was primarily influenced by the main factors Load and Perturbation, along with the interaction Load x Perturbation. Perturbations aligned with the load direction were associated with increased electromyographic (EMG) activity across all examined muscles. While there was a small but significant effect of load during the early LLR, this effect diminished by the late LLR epoch. Task-dependent modulation was most pronounced at the late LLR epoch, suggesting greater top-down modulation of this reflex component. In particular, the late LLR was shaped by the factors Perturbation and Target, as well as the interaction Perturbation x Target. Targets and perturbations in opposing directions resulted in heightened EMG activity, and shoulder muscles exhibited stronger LLR responses for targets located farther along the muscle shortening direction. Across all reflex epochs, a longer preparatory delay (750 ms) was associated with higher EMG activity.

neuroscience↗

bioRxiv2024

Local muscle pressure stimulates the principal receptors for proprioception

Proprioception plays a crucial role in motor coordination and self-perception. Muscle spindles are the principal receptors for proprioception. They are believed to encode muscle stretch and signal limb position and velocity. Here, we applied percutaneous pressure to a small area of extensor muscles at the forearm while recording spindle afferent responses, skeletal muscle activity and hand kinematics. Three levels of sustained pressure were applied on the spindle-bearing muscle when the hand was relaxed and immobile ( isometric condition) and when the participants hand moved rhythmically at the wrist. As hypothesized to occur due to compression of the spindle capsule, we show that muscle pressure is an adequate stimulus for human spindles in isometric conditions, and that pressure enhances spindle responses during stretch. Interestingly, release of sustained pressure in isometric conditions lowered spindle firing below baseline rates. Our findings urge a re-evaluation of muscle proprioception in sensorimotor function and various neuromuscular pathologies. HIGHLIGHTSO_LILocal muscle pressure is an adequate stimulus for spindles in isometric conditions C_LIO_LIRelease of pressure in isometric conditions lowers spindle firing below baseline rates C_LIO_LILocal muscle pressure enhances spindle afferent responses during muscle stretch C_LI

neuroscience↗

bioRxiv2022

Goal-directed modulation of stretch reflex gains is reduced in the non-dominant upper limb

Most individuals experience their dominant arm as being more dexterous than the non-dominant arm, but the neural mechanisms underlying this asymmetry in motor behaviour are unclear. Using a delayed reach task, we have recently demonstrated strong goal-directed tuning of stretch reflex gains in the dominant upper limb of human participants. Here, we used an equivalent experimental paradigm to address the neural mechanisms that underlie the preparation for reaching movements with the non-dominant upper limb. We found only minor goal-directed differences in the short latency stretch reflex of the non-dominant limb. There were consistent effects of load, preparatory delay duration and target direction on the long latency stretch reflex. However, by comparing stretch reflex responses in the non-dominant arm with those previously documented in the dominant arm, we demonstrate that goal-directed tuning of short and long latency stretch reflexes is markedly weaker in the non-dominant limb. The results indicate that the motor performance asymmetries across the two upper limbs is partly due to the more sophisticated control of reflexive stiffness in the dominant limb, likely facilitated by the superior goal-directed control of muscle spindle receptors. Our findings therefore suggest that independent fusimotor control plays a role in determining performance of complex motor behaviours and support existing proposals that the dominant arm is better supplied for executing more complex tasks, such as trajectory control. Key pointsO_LIMost of us routinely rely on the dominant arm to perform more complex and demanding motor tasks, but the mechanisms enabling the superior motor performance of the dominant limb are unclear. C_LIO_LIA better understanding of the motor asymmetry across the two arms might provide key insight into core sensorimotor principles. C_LIO_LIThis study shows that goal-directed tuning of short and long latency stretch reflexes in the non-dominant arm is markedly weaker than in the dominant arm. C_LIO_LIOur results suggest that the more sophisticated control of reflexive stiffness in the dominant limb, likely facilitated by superior fusimotor control, partly underpins the laterality of motor performance. C_LI

neuroscience↗

bioRxiv2022

Assistive loading promotes goal-directed tuning of stretch reflex gains

Voluntary movements are prepared before they are executed. Preparatory activity has been observed across the CNS and recently documented in first order neurons of the human PNS i.e., in muscle spindles. The changes seen in these sensory organs suggest that the independent modulation of stretch reflex gains may represent an important component of movement preparation. The aim of the current study was to further investigate the preparatory modulation of short- and long-latency stretch reflex responses ( SLR and LLR) of the dominant upper limb. Specifically, we investigated how different target parameters (target distance and direction) affect the preparatory tuning of stretch reflex gains in the context of goal-directed reaching, and whether any such tuning depends on preparation duration and the direction of background loads. We found that target distance produced only small variations in reflex gains. In contrast, both SLR and LLR gains were strongly modulated as a function of target direction, in a manner that facilitated the upcoming voluntary movement. This goal-directed tuning of SLR and LLR gains was present or enhanced when the preparatory delay was sufficiently long (>250 ms) and the homonymous muscle was unloaded i.e., when a background load was first applied in the direction of homonymous muscle action (assistive loading). The results extend further support for a relatively slow-evolving process in reach preparation that functions to modulate reflexive muscle stiffness, likely via the independent control of fusimotor neurons. Such control can augment voluntary goal-directed movement and is triggered or enhanced when the homonymous muscle is unloaded. Significance StatementIt is well-known that movement preparation improves motor performance. That is, briefly delaying the onset of a goal-directed movement can significantly benefit the overall quality of movement. However, the mechanisms underlying movement preparation remain unclear. In this study we examined the preparatory modulation of short- and long-latency stretch reflex responses in the dominant upper limb. We found that goal-directed tuning of stretch reflex gains is consistently triggered or enhanced in cases where preparation is sufficiently long (>250 ms) and a background - assistive- load is first applied in the direction of homonymous muscle action. A better understanding of movement preparation will likely also benefit the development of rehabilitation regimes and movement augmentation devices.

neuroscience↗