Motor Point Stimulation in Spinal Paired Associative Stimulation can Facilitate Spinal Cord Excitability

Abstract

Paired associative stimulation at the spinal cord (spinal PAS) has been shown to increase muscle force and dexterity by strengthening the corticomuscular connection, through spike timing dependent plasticity. Typically, transcranial magnetic stimulation (TMS) and transcutaneous peripheral nerve electrical stimulation (PNS) are often used in spinal PAS. PNS targets superficial nerve branches, by which the number of applicable muscles is limited. Alternatively, a muscle can be activated by positioning the stimulation electrode on the “motor point” (MPS), which is the most sensitive location of a muscle to electrical stimulation. Although this can increase the number of applicable muscles for spinal PAS, nobody has tested whether MPS can be used for the spinal PAS to date. Here we investigated the feasibility of using MPS instead of PNS for spinal PAS. Ten healthy male individuals (26.0 ± 3.5 yrs) received spinal PAS on two separate days with different stimulation timings expected to induce (1) facilitation of corticospinal excitability (REAL) or (2) no effect (CONTROL) on the soleus. The motor evoked potentials (MEP) response curve in the soleus was measured prior to the spinal PAS, immediately after (0 min) and at 10, 20, 30 min post-intervention as a measure of corticospinal excitability. The post-intervention MEP response curve areas were larger in the REAL condition than the CONTROL conditions. Further, the post-intervention MEP response curve areas were significantly larger than pre-intervention in the REAL condition but not in the CONTROL condition. We conclude that MPS can facilitate corticospinal excitability through spinal PAS.

Motor point stimulation induces more robust F‐waves than peripheral nerve stimulation

“…A constant-current stimulation with 1-ms pulse duration was used to deliver MPS and PNS using a constant-current electrical stimulator (model DS7AH, Digitimer, Welwyn Garden City, UK). The shorter pulse duration (200-400 μs) is commonly used for MPS in clinical settings, while we (Fok et al, 2020;Nakagawa et al, 2020) as well as other groups (Charlton, 2003;Ridding & Taylor, 2001) used MPS with 1-ms pulse duration. In the present study, we used MPS with a longer F I G U R E 1 The locations of electrodes for motor point stimulation (MPS), peripheral nerve stimulation (PNS) and EMG recording from the soleus (SOL) muscle.…”

“…At the same time, MPS induces an antidromic firing along the alpha motoneurons, same as PNS, which could cause the activation of a motor neuron inducing an F‐wave. For example, we demonstrated that MPS is usable to induce spinal plasticity through a paired associative stimulation protocol, suggesting that MPS induces the antidromic firing activating the motor neuron cell body (Fok et al, 2020). In our previous studies, we found motor responses at the latency of F‐wave when we applied electrical stimulation with supramaximum intensities in the soleus muscle (SOL).…”

Neuromuscular recruitment pattern in motor point stimulation

“…If this is the case, orthodromic drive coming from the Ia‐nerve via spinal cord would not be produced much. Thus, for MPS, antidromic drive from the motor nerve would not collide with the orthodromic drive unlike in PNS, which may be useful for intervention methods using antidromic impulse to spinal motor neuron such as paired associative stimulation 24 . On the other hand, MPS would not be useful for interventions through the Ia‐nerve such as inducing plasticity of reciprocal inhibition by repetitive Ia‐sensory stimulation 25 .…”

Utility of transcranial magnetic stimulation in the assessment of spinal cord injury: Current status and future directions

“…Another study in 22 individuals with SCI (C3–L5; AIS C–D) reported loss of MEP facilitation in the tibialis anterior muscle by prior (∼50–60 ms) conditioning stimulation of the tibial nerve, but intact facilitation with conditioning stimulation of the common fibular nerve ( 54 ). In addition, precise calculations of the CMCT have been used to design targeted paired-associative stimulation neuromodulation approaches to facilitate functional recovery after SCI ( Figure 2 ) ( 60 – 64 ). The paired-associative stimulation approaches are based on the Hebbian principle of associative plasticity, i.e., “neurons that fire together, wire together” ( 65 , 66 ).…”

“…Use of upper conduction time (central motor conduction time) and lower conduction time (peripheral conduction time) for targeting the stimulation to lower limb motoneuron cell bodies. Use of latencies of MEP, F-wave and M-wave for precise calculations of the conduction times [Reprinted from Fok et al, 2020 ( 64 ); reprint permitted under Creative Commons Attribution License; CC BY].…”