DESIGN OF THE MR-STRETCHFINGERS DEVICE FOR THE STUDY OF NEUROMECHANICS OF STRETCH REFLEXES IN THE FINGER MUSCLES
Date
2023-05
Authors
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Journal ISSN
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Publisher
University of Delaware
Abstract
Human motor control consists of feedforward and feedback mechanisms, both
aiming to reach a desired muscle state. Feedforward control does not involve sensory
feedback and instead generates a movement based on the desired state. In humans,
feedforward control is involved in voluntary reactions to a stimulus [1]. By contrast,
feedback control considers movement errors and sensory feedback, with the aim of
correcting movements. However, feedback control occurs on several timescales, and
the speed of these responses is constrained by the neuromuscular pathways required
for producing them. One method of feedback control in humans is stretch reflexes.
Long latency responses (LLRs) specifically are believed to occur due to a combination
of inputs from the corticospinal tract (CST) and reticulospinal tract (RST). LLRs are
important to study biomedically, as they are involved in many neurological diseases
that affect voluntary motor control. The CST can be damaged due to stroke [3]. For
instance, 50% of people who experienced a stroke have a residual motor disability which
can be due to reduced corticospinal drive resulting in a loss of movement accuracy and
muscle flaccidity [4]. The RST can assume some of the lost CST function; however, the
RST’s role in regaining motor function is not well understood. Understanding the role
these pathways play in finger control at the metacarpophalangeal (MCP) joint has not
been extensively studied in the past. The goal of this project was to develop a device
which would elicit long latency responses in participants, validate that these responses
are elicited, and utilize fMRI imaging to map the neural pathways activated during
perturbations.
In Aim 1 we present the development of the MR-StretchFingers device, which
aims to apply perturbations at the MCP joint. We then present results which show that
the device specificationsare met and it is capable of applying perturbations at the MCP joint and limiting other movement within those specifications. In Aim 2, we examine
the signal-to-noise ratio (SNR) measured using surface electromyography (sEMG) in
two muscles: first dorsal interosseus muscle which aids with flexion of the index finger,
and the flexor digitorum superficialis muscle, which aids with middle/ring/pinky finger
flexion. Results show that SNR was maximal during flexion movements corresponding
to either index or middle/ring/pinky flexion, matching the physiological understanding
of the muscles. Further, the results were of similar magnitude to SNR values for other
muscles in literature. We then present an LLR study, which aimed to determine if LLRs
are being elicited. We found that LLRs were being elicited in all cases at the group
level, in both FDI and FDS with background torque applied by either the index finger
only or middle/ring/pinky fingers only. Finally, we present an MRI conditions pilot
which aims to find a control and experimental condition suitable for fMRI imaging.
The results indicate two conditions which were suitable. In Aim 3, we present an
MRI pilot study, which involved simultaneous mechanical perturbations by the MR SF with fMRI. The results indicate that there is some activation in the motor cortex
and brainstem due to LLR modulation.
Overall, this study provides preliminary evidence that there is subcortical and
cortical activity that modulates the LLRs elicited by finger muscles. Future studies
can expand on these results by including more participants and fine tuning the pro tocol to provide a clearer understanding of the neural correlates of LLRs and how
the LLR in finger muscles are modulated by factors such as the perturbation veloc ity. Further, other factors affecting LLRs, such as task instruction or varying levels of
background torque can be examined both outside and inside the MRI to understand
LLR modulation. These findings can increase our understanding of human secondary
motor pathways and lead to new rehabilitation strategies and therapies that can aid
individuals with various motor impairments.