Human spinal locomotor control is based on flexibly organized burst generators

Area of Science:

• Neuroscience
• Motor Control
• Spinal Cord Injury Research

Background:

• Epidural electrical stimulation (EES) of the human lumbar spinal cord can elicit rhythmic motor outputs in leg muscles of individuals with paralysis.
• EES allows investigation of spinal rhythm and pattern generating circuits independent of volitional control or sensory feedback.

Purpose of the Study:

• To explore the range of rhythmic motor patterns generated by the functionally isolated human lumbar spinal cord.
• To identify fundamental temporal components underlying these motor patterns.

Main Methods:

• Ten individuals with chronic, motor-complete spinal cord injury received EES.
• Surface electromyographic (EMG) activity was recorded from lower limb muscles (quadriceps, hamstrings, tibialis anterior, triceps surae).
• Rhythmic EMG activity segments were analyzed for temporal components and activation patterns.

Main Results:

• Rhythmic activity was detected in seven participants, exhibiting physiologically appropriate cycle durations and phase timings for locomotion.
• Multi-muscle activation patterns showed diverse configurations, including locomotor-like synergies.
• Statistical analysis identified three common temporal activation components, two related to extension/flexion phases and a third independent component.

Conclusions:

• The human lumbar spinal cord possesses flexible neural circuits capable of generating diverse locomotor outputs from constant EES input.
• These circuits utilize basic, combinable temporal activation patterns, interpreted as outputs from spinal burst generators.
• Findings suggest potential for harnessing spinal circuit flexibility to enhance locomotor control and adaptation post-spinal cord injury.