Abstract
The loss or reduction of supraspinal inputs after spinal cord injury provides a unique opportunity to examine the plasticity of neural pathways within the spinal cord. In a series of nine experiments on a patient, quadriplegic due to spinal cord injury, we investigated interlimb reflexes and self-sustained activity in completely paralyzed and paretic muscles due to a disinhibited propriospinal pathway. Electrical stimuli were delivered over the left common peroneal nerve at the fibular head as single stimuli or in trains at 2–100 Hz lasting 1 s. Single stimuli produced a robust interlimb reflex twitch in the contralateral thumb at a mean latency 69 ms, but no activity in other muscles. With stimulus trains the thumb twitch occurred at variable subharmonics of the stimulus rate, and strong self-sustained activity developed in the contralateral wrist extensors, outlasting both the stimuli and the thumb reflex by up to 20 s. Similar behavior was recorded in the ipsilateral wrist extensors and quadriceps femoris of both legs, but not in the contralateral thenar or peroneal muscles. The patient could not terminate the self-sustained activity voluntarily, but it was abolished on the left by attempted contractions of the paralyzed thumb muscles of the right hand. These responses depend on the functional integrity of an ascending propriospinal pathway, and highlight the plasticity of spinal circuitry following spinal cord injury. They emphasize the potential for pathways below the level of injury to generate movement, and the role of self-sustained reflex activity in the sequelae of spinal cord injury.
Highlights
In human bipedal locomotion the arm swing is thought to be a vestigial remnant of quadrupedal gait patterns [1]
A tightly coupled interlimb reflex twitch contraction with a reproducible latency was produced by stimulation of only the left leg and appeared only in the right thumb
The duration of the silent period produced during a voluntary contraction ranged from 197 to 227 ms with a mean of 217 ms. These findings indicate that the EMG activity triggered by the interlimb reflex was not modulated as is a voluntary contraction, even though mechanisms sufficient to produce a silent period remained intact
Summary
In human bipedal locomotion the arm swing is thought to be a vestigial remnant of quadrupedal gait patterns [1] This limb co-ordination occurs through the linking via propriospinal pathways of caudal central pattern generators that control hindlimb movement with central pattern generators in the rostral spinal cord controlling forelimb movement [2]. Long propriospinal connections mediate interlimb cutaneomuscular reflexes in healthy human subjects [3,4], these have been demonstrable only during voluntary contractions of the target muscle. They may provide interlimb co-ordination for locomotion [4] or protection against limb trauma [3]. Studies reporting bidirectional propriospinal activation show that the descending pathway is easier to demonstrate than the ascending pathway [4,8,9], and that the ascending pathway is stronger ipsilaterally than contralaterally [8,9]
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