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Date of Graduation
5-15-2025
Semester of Graduation
Spring
Degree Name
Master of Science (MS)
Department
Department of Biology
First Advisor
Corey Cleland
Second Advisor
Mark Gabriele
Third Advisor
Roshna Wunderlich
Abstract
The nociceptive withdrawal response (NWR) is a protective response to noxious stimuli that causes the movement of a limb or appendage away from the stimulus. Previous behavioral studies from our laboratory demonstrated that the heat-evoked NWR of the rat tail consists of four components: local bend, caudal bend progression, tail-base rotation, and tail stiffening. We hypothesize that tail stiffening through muscular co-contraction is an evolved mechanism to lessen the neural computational burden the CNS faces during the NWR. Co-contraction could arise from either sensory input from the noxious stimulus or sensory input from the resulting tail movement. The specific aim of our study was to determine if tail movement-evoked responses contribute to co-contraction during the NWR by comparing electromyographic (EMG) responses to noxious heat stimuli during tail movement and isometric (no movement) conditions.
In adult rats (n = 10), fine-wire EMG electrodes were inserted bilaterally into 8 tail muscles (6 intrinsic, dorsal lateral; 2 extrinsic, M. sacrocaudalis dorsalis). Placement of electrodes in the extrinsic muscles was verified by electrical stimulation and postmortem dissection. The NWR was evoked using a heat stimulus (980 nm laser diode) applied bilaterally to 6 locations distributed rostral-caudally along the tail. The tail was secured to a rigid rod to confine tail movement to rotation at the base of the tail. EMG and tail movement were recorded in two conditions: isometric (no tail-base rotation, accomplished by fixing the rigid rod during the trial) and movement (tail-base rotation).
Our results revealed the presence of extensive movement-evoked co-contraction, especially for intrinsic muscles, during the tail NWR. Our results also demonstrated differences between the intrinsic and extrinsic muscles: intrinsic muscles showed substantially more movement-related co-contraction than extrinsic muscles, and extrinsic muscle responses, but not intrinsic muscle responses, depended on stimulus location and degree of angular movement.
Our results suggest that extrinsic and intrinsic tail muscles may have different functions during the heat-evoked tail NWR. Specifically, we believe that the intrinsic muscles are responsible for stiffening the tail during the NWR through primarily movement-evoked co-contraction, while extrinsic muscles are primarily responsible for creating movement of the tail away from the heat stimulus. Our results provide insight on the simplification of movement by the CNS as limb stiffening can lessen the mechanical complexity of the tail, while also establishing the rat tail as a model system to further study co-contraction. A better understanding of the mechanisms underlying co-contraction may be critical to understanding the excess levels of co-contraction that are present in a variety of neurological disorders such as Parkison’s disease and Down syndrome.
