Day 282: Review – Transcutaneous spinal cord stimulation of the cervical cord modulates lumbar networks
It’s that time again! My biweekly critical review paper is due for my PI. He gets a copy and so do all of you. This is a particularly interesting study that falls in line with a lot of research that I am doing, so it’s interesting to see how other groups are progressing. Overall I think this is a great study and while it is behind a paywall, I think I summed it up very well. The drawing they did (above) is awesome, especially for a scientific journal where we normally use simple line figures. Anyway, let’s get to it.
Communication between the cervical and lumbar enlargements of the spinal cord is necessary to facilitate bipedal locomotion. Studies have already shown that cervical activation through arm cycling can reduce the Hoffman reflex (H-reflex) response of the soleus muscle. While we know that transcutaneous spinal cord stimulation (TSS) can activate spinal networks, we don’t understand what networks are being activated and the differences between volitional activation and TSS activation. For example, we do not know if activation of the cervical spinal cord using TSS influences the lumbar circuitry. This study examines the effect of several different conditions on the H-reflex amplitude including TSS of the cervical and lumbar enlargements.
The researchers recruited thirteen neurologically intact subjects (4 female; 9 male). The experiment consisted of eliciting the soleus H-reflex in six different randomized conditions: 1.) Arms static without TSS, 2.) Arms static with TSS over lumbar enlargement, 3.) Arms static with TSS over cervical enlargement, 4.) Arm cycling without TSS, 5.) Arm cycling with TSS over lumbar enlargement, and 6.) Arm cycling with TSS over the cervical enlargement. Each condition consisted of ten H-reflex stimuli with a three second minimum between pulses. TSS used 10kHz 1ms long stimulus trains at a frequency of 30Hz and the stimulation intensity was found by determining when the subject felt a sustained sensation at the anode electrodes.
Using a 1 x 3 factor ANOVA (static arms, static w/ lumbar TSS, static w/ cervical TSS) they found that cervical TSS caused significant conditioning effects and reduced the amplitude of the H-reflex when compared to the static arms and no TSS condition. Interestingly, they found that lumbar TSS had no significant effect on H-reflex amplitude when compared to the static arms and no TSS condition. Applying a 1 x 4 factor ANOVA (static arms, cycling w/out TSS, cycling w/ lumbar TSS, cycling w/ cervical TSS) they found a significant reduction in H-reflex amplitude across conditions when compared to the static no stimulation condition. However, the did not find any significant difference between the cycling conditions.
Overall the researchers wanted to investigate cervico-lumbar connectivity during TSS and the differences in interlimb coupling when TSS was applied to the cervical or lumbar enlargements. While tonic TSS of the cervical enlargement significantly modulated activity of the lumbar networks, the combination of arm cycling with cervical or lumbar TSS did not result in significant additional suppression of the soleus H-reflex beyond arm cycling or cervical TSS alone.
This is consistent previous studies and is likely mediated by reinforced presynaptic inhibition in 1a lumbar afferent terminals. While these connections are weakened or eliminated after stroke, repeated activation of these circuits through arm and leg cycling reengages these propriospinal connections and enhances interlimb coordination. The authors conclude that it is possible that cervical TSS recruits similar networks within the spinal cord influencing propriospinal connections between the cervical and lumbar enlargements of the spinal cord.
What is fascinating about this experiment is that these findings are in line with the results of other research that has been conducted, but they use TSS in place of volitional movement. This means we can also determine things like length of inhibition and using this information we could time the response so that when the cervical enlargement is in its refractory period. This could potentially increase the response of the H-reflex instead of suppressing it and could reduce the stimulus amplitude needed for effectiveness, thus increased subject comfort and lower power consumption for mobile battery powered neuromodulatory devices.
Barss TS, Parhizi B, Mushahwar VK. Transcutaneous spinal cord stimulation of the cervical cord modulates lumbar networks. J Neurophysiol. 2020;123(1):158‐166. doi:10.1152/jn.00433.2019