Absence of Local Sign Withdrawal in Chronic Human Spinal Cord Injury

Abstract

Local sign withdrawal, a reflex to direct the limb away from noxious cutaneous stimuli, is thought to be indicative of a modular organization of the spinal cord. To assess the integrity of such an organization of the spinal cord in chronic human spinal cord injury (SCI), we tested the electromyogram (EMG) and joint torque responses to cutaneous stimuli applied to 6 locations of the leg in 10 SCI volunteers and 3 spinal-intact controls. The 6 locations included the medial arch of the foot, the second metatarsal, the dorsum, the region over the sural nerve at the lateral malleolus, and the anterior and posterior aspects of the lower leg. Although spinal-intact subjects demonstrated local sign withdrawal, the data from SCI subjects indicated that an invariant flexion response pattern was produced regardless of stimulus location. Ankle dorsiflexion and hip flexion were produced in all subjects at all locations and no difference in the ratio of hip:ankle torques could be detected for the 6 test locations. A windup-crossover test, employing a sequence of 6 stimuli at 1-s intervals was used to assess whether common neuronal pathways were responsible for the loss of modular organization. An additional 10 SCI volunteers were tested using stimuli in which the stimulus location was switched between the 2nd and 3rd stimulus of the test sequence. The response to the crossover stimulus more closely resembled the response to the 3rd stimulus of a windup sequence than a response without conditioning stimuli. These results indicate that increased excitability produced by windup at one stimulus site is maintained at the 2nd site. This observation suggests that deep dorsal horn neurons, typically associated with musculotopic mapping, may be reorganized in chronic spinal cord injury.

Ankle Load Modulates Hip Kinetics and EMG During Human Locomotion

“…In incomplete SCI subjects, using the Lokomat to resist swing phase movements has been shown to enhance flexor muscle activity (Lam et al 2008). As well, seated SCI subjects demonstrate an invariant flexion response to cutaneous stimulation regardless of stimulus location on the lower limb (Schmit et al 2003). If the cutaneous stimulation of the thigh and shank during walking do indeed create a flexion response in spinal cord injury subjects, this may have potentially decreased the magnitude of the major finding of the current study, that SCI subjects increase hip extension torque when the limb is loaded during stance phase.…”

Modulation of flexor reflexes by static and dynamic hip proprioceptors in chronic human spinal cord injury

“…The current results more closely resemble the enhanced excitation of the reflex response during late stance and early swing that is observed in response to tactile levels of stimuli in spinal intact subjects; 41 however, this modulation of the electrocutaneous reflex is further altered in incomplete SCI. 42 One possible explanation for this difference is that the spinal organization of the flexor reflex is fundamentally changed in chronic SCI, 43 altering the reflex modulation of the TA during locomotion. Alternatively, modulation of the flexor reflex response may be dominated by a central drive involving the locomotor pattern generator, rather than modulation from proprioceptive afferent feedback.…”

Flexor reflex decreases during sympathetic stimulation in chronic human spinal cord injury

“…While all responses were attenuated during conditioning, the second and third windup responses showed a lesser degree of depression. This pattern could be indicative of inhibition of the flexor reflex afferents early in the flexor reflex pathways rather than in the deep dorsal horn interneurons or motoneurons where windup of the flexion reflex is thought to occur (Hornby et al 2003, Schmit et al 2003). …”

“…The stimulus amplitude was set to 50mA, a stimulus that typically generated a vigorous response (ankle torque > 5Nm). A total of 3 stimuli were applied at a 1s interval to obtain a measure of windup of the flexor reflex response (Hornby, et al, 2003, Schmit, et al, 2003). The rationale for using 3 stimuli was that windup stabilizes by the 3 rd stimulus (at interstim intervals less than 5s.)…”