Characterization of interlimb interaction via transcutaneous spinal stimulation of cervical and lumbar spinal enlargements

Abstract

The use of transcutaneous electrical spinal stimulation (TSS) to modulate sensorimotor networks after neurological insult has garnered much attention from both researchers and clinicians in recent years. Although many different stimulation paradigms have been reported, the interlimb effects of these neuromodulation techniques have been little studied. The effects of multisite TSS on interlimb sensorimotor function are of particular interest in the context of neurorehabilitation, as these networks have been shown to be important for functional recovery after neurological insult. The present study utilized a condition-test paradigm to investigate the effects of interenlargement TSS on spinal motor excitability in both cervical and lumbosacral motor pools. Additionally, comparison was made between the conditioning effects of lumbosacral and cervical TSS and peripheral stimulation of the fibular nerve and ulnar nerve, respectively. In 16/16 supine, relaxed participants, facilitation of spinally evoked motor responses (sEMRs) in arm muscles was seen in response to lumbosacral TSS or fibular nerve stimulation, whereas facilitation of sEMRs in leg muscles was seen in response to cervical TSS or ulnar nerve stimulation. The decreased latency between TSS- and peripheral nerve-evoked conditioning implicates interlimb networks in the observed facilitation of motor output. The results demonstrate the ability of multisite TSS to engage interlimb networks, resulting in the bidirectional influence of cervical and lumbosacral motor output. The engagement of interlimb networks via TSS of the cervical and lumbosacral enlargements represents a feasible method for engaging spinal sensorimotor networks in clinical populations with compromised motor function.NEW & NOTEWORTHY Bidirectional interlimb modulation of spinal motor excitability can be evoked by transcutaneous spinal stimulation over the cervical and lumbosacral enlargements. Multisite transcutaneous spinal stimulation engages spinal sensorimotor networks thought to be important in the recovery of function after spinal cord injury.

Keywords: human neurophysiology; interlimb reflexes; propriospinal; spinal cord; transcutaneous spinal stimulation.

Graphical abstract

Figure 1.

Interlimb conditioning of arm and leg muscle spinally evoked motor responses (sEMRs). Top: ascending interlimb conditioning of arm muscle sEMRs [70 ms condition-test interval (CTI)] in a representative subject. A and B: the unconditioned (blue and green) and conditioned (red and purple) sEMRs recorded from nondominant (A) and dominant (B) arm muscles in response to lumbosacral transcutaneous spinal stimulation (TSS) or fibular nerve conditioning stimulation, respectively. Bottom: descending interlimb conditioning of leg muscle sEMRs (70 ms CTI) in the same subject. C and D: the unconditioned (blue and green) and conditioned (red and purple) sEMRs recorded from left (C) and right (D) leg muscles in response to cervical TSS or ulnar nerve conditioning stimulation, respectively. BIC, biceps brachii; ECR, extensor carpi radialis; FCR, flexor carpi ulnaris; MH, medial hamstrings; SOL, soleus; TA, tibialis anterior; TRIC, triceps brachii; VL, vastus lateralis.

Figure 2.

Facilitation of spinally evoked motor response (sEMR) amplitudes in leg muscles: grouped data (n = 16; 8 females, 8 males) detailing the magnitude of interlimb conditioning by muscle and condition-test interval (CTI; given in milliseconds) in response to cervical transcutaneous spinal stimulation (TSS) (red) or ulnar nerve conditioning stimulation (purple). The amplitude of the average peak-to-peak (PTP) amplitude for each control (CTRL) as well as each CTI was normalized to the maximum PTP amplitude (as defined in the text) obtained in the same muscle during collection of recruitment curves. Significant differences between sEMR amplitude at each CTI compared with control responses are indicated at top of bar graphs: *0.005 < P ≤ 0.010, **P ≤ 0.005. L, left; MH, medial hamstrings; R, right; SOL, soleus; TA, tibialis anterior; VL, vastus lateralis.

Figure 3.

Facilitation of spinally evoked motor response (sEMR) amplitudes in arm muscles: grouped data (n = 16; 8 females, 8 males) detailing the magnitude of interlimb conditioning by muscle and condition-test interval (CTI; given in milliseconds) in response to lumbosacral transcutaneous spinal stimulation (TSS) (red) or fibular nerve conditioning stimulation (purple). The amplitude of the average peak-to-peak (PTP) amplitude of control sEMRs (CTRL) as well as sEMRs recorded during each CTI was normalized to the maximum PTP amplitude (as defined in the text) obtained in the same muscle during collection of recruitment curves. Significant differences between the sEMR amplitude at each CTI compared with control responses are indicated at top of bar graphs: *0.005 < P ≤ 0.010, **P ≤ 0.005. BIC, biceps brachii; D, dominant; ECR, extensor carpi radialis; FCR, flexor carpi radialis; ND, nondominant; TRI, triceps brachii.