Epidural and transcutaneous spinal cord stimulation facilitates descending inputs to upper-limb motoneurons in monkeys

Journal of Neural Engineering, 2021 · DOI: https://doi.org/10.1088/1741-2552/abe358 · Published: March 22, 2021

Simple Explanation

This study investigates how spinal cord stimulation (SCS) can improve upper-limb movement after spinal cord injury. The researchers tested different types of SCS on monkeys to see how they affect muscle responses and the signals from the brain that control movement. The study found that stimulating the spinal cord from the front (ventral) directly activated muscles. Stimulating from the back (dorsal) enhanced brain signals to muscles, especially at certain frequencies. The findings suggest that a combination of stimulation techniques could be beneficial for restoring upper-limb function in people with spinal cord injuries, by both directly activating muscles and improving the brain's control over them.

Study Duration

Not specified

Participants

5 neurologically intact monkeys

Evidence Level

Not specified

Key Findings

1
Ventral epidural SCS effectively recruits upper-limb muscles, requiring lower current intensities and reliably following high-frequency trains.
2
Dorsal epidural SCS and transcutaneous SCS facilitate supraspinal-evoked responses, particularly at intermediate stimulation frequencies.
3
A computational model suggests that the frequency-dependence of SCS effects can be explained by synaptic excitation gated by upstream inhibition, possibly via presynaptic mechanisms.

Research Summary

The study examined the effects of epidural and transcutaneous spinal cord stimulation (SCS) on upper-limb muscle responses and modulation of supraspinal-evoked movements in monkeys. Ventral epidural SCS elicited robust movements at lower current intensities, while dorsal epidural SCS and transcutaneous SCS facilitated supraspinal-evoked responses, especially at intermediate stimulation frequencies. The findings suggest that multicontact electrodes allowing access to both dorsal and ventral epidural sites may be beneficial for combined therapeutic purposes, and that the interaction of direct, synaptic, and presynaptic effects should be considered when optimizing SCS-assisted rehabilitation.

Practical Implications

Combined Therapeutic Approach

Multicontact electrodes accessing both dorsal and ventral epidural sites could be beneficial for combined therapeutic purposes, allowing for both direct muscle activation and facilitation of supraspinal inputs.

Optimizing SCS Parameters

The interaction of direct, synaptic, and presynaptic effects should be considered when optimizing SCS-assisted rehabilitation protocols to maximize effectiveness.

Frequency-Specific Stimulation

Intermediate stimulation frequencies (e.g., 50 Hz) may be more effective for facilitating supraspinal inputs compared to lower or higher frequencies.

Study Limitations

1
Experiments were performed on neurologically intact animals under anesthesia, limiting generalizability to awake, spinal cord-injured subjects.
2
The study acknowledges that reafference or nociceptive effects could have contributed to the facilitatory effects of transcutaneous stimulation.
3
The computational model is a simplified representation and does not capture all the biophysical complexities of spinal circuits.

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