Flexion-Withdrawal Reflexes in the Upper-Limb Adapt to the Position of the Limb

INTRODUCTION Noxious electrical stimulation of the fingers produces a coordinated reflex response in upper limb muscles that results in the withdrawal of the limb, analogous to removing the hand from touching a hot stove [1, 2]. In general, the upper limb withdrawal is accomplished by a proximaldistal progression of shoulder extension, elbow flexion, and wrist extension [1]. However, exceptions to coordinated withdrawal movement patterns have been observed. For example, some stimulation sites in the lower limb make the leg extend rather than flex [3]. The response of the upper limb has not been studied in the same detail as the lower limb; however, it has been shown that the reflex response can modulate with the phase of movement, as occurs with reaching tasks in the upper limb [4]. The cutaneous afferent feedback (Aδ, C-fiber) responsible for the withdrawal reflex does not make direct connections onto motor neurons in the spinal cord. This suggests that these complex neural circuits would be mutable under different task conditions, operating in the best manner to remove the limb from the painful environment. The purpose of the present experiment was to test the flexibility of the flexion-withdrawal reflex and torque responses by observing the behavior of the limb when it is in different positions. We hypothesized that the flexionwithdrawal response would coordinate to match the limitations placed on the arm due to positioning.