Novel human models for elucidating mechanisms of rate-sensitive H-reﬂex depression

Biomedical Journal, 2020 · DOI: 10.1016/j.bj.2019.07.007 · Published: February 26, 2020

Spinal Cord Injury Physiology Rehabilitation

Simple Explanation

This study investigates whether the depression of the H-reflex during repetitive stimulation is due to mechanisms acting before or after the synapse between nerve and muscle. The study uses novel human models to differentiate between pre-synaptic and post-synaptic inhibitory mechanisms non-invasively. The findings could lead to new rehabilitation strategies to improve neuromodulation and understand neuroplasticity in humans.

Study Duration

Not specified

Participants

23 healthy adults, 2 with acute sensory-impaired SCI, 5 healthy adults without rate-sensitive depression

Evidence Level

Not specified

Key Findings

1
Testing MEPs were not diminished in healthy subjects with or without typical rate-sensitive H-reﬂex depression, or in subjects with sensory-impaired SCI.
2
MEP responses were similar in healthy subjects with versus without rate-sensitive H-reﬂex depression.
3
Results support a pre-synaptic locus of rate-sensitive H-reﬂex depression for the ﬁrst time in humans.

Research Summary

The study used novel human neurophysiologic models to investigate whether the mechanism of rate-sensitive H-reﬂex depression lies in the pre-synaptic or post-synaptic locus in humans. Results from these novel in vivo human models support a pre-synaptic locus of rate-sensitive H-reﬂex depression for the ﬁrst time in humans. Spinal reﬂex excitability can be modulated separately from descending corticospinal inﬂuence, offering potential targets for neuromodulatory intervention.

Practical Implications

Neuromodulation Targets

Both spinal segmental circuits and corticospinal circuits may be suitable targets for neuromodulation, and potential benefits may be additive.

Rehabilitation Strategies

The ability to differentiate between pre-synaptic and post-synaptic inhibitory mechanisms raises the possibility to develop novel rehabilitation strategies.

Understanding Neuroplasticity

Findings contribute to understanding the neuroplasticity of these pathways in humans, aiding in the development of more effective interventions.

Study Limitations

1
Small sample size in the SCI group (Group II).
2
The neurophysiologic basis for the absence of rate-sensitive depression in Group III participants is not known and is beyond the scope of the present study.
3
Future studies are needed to conﬁrm that H-reﬂex rate-sensitive depression is likewise lost in patients with AIS C or D SCI.

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