Tempo-spatial integration of nociceptive stimuli assessed via the nociceptive withdrawal reflex in healthy humans

Abstract

Spatial information of nociceptive stimuli applied in the skin of healthy humans is integrated in the spinal cord to determine the appropriate withdrawal reflex response. Double-simultaneous stimulus applied in different skin sites are integrated, eliciting a larger reflex response. The temporal characteristics of the stimuli also modulate the reflex, e.g., by temporal summation. The primary aim of this study was to investigate how the combined tempo-spatial aspects of two stimuli are integrated in the nociceptive system. This was investigated by delivering single- and double-simultaneous stimulation and sequential stimulation with different interstimulus intervals (ISIs ranging 30-500 ms) to the sole of the foot of 15 healthy subjects. The primary outcome measure was the size of the nociceptive withdrawal reflex (NWR) recorded from the tibialis anterior (TA) and biceps femoris (BF) muscles. Pain intensity was measured using a numerical rating scale (NRS) scale. Results showed spatial summation in both TA and BF when delivering simultaneous stimulation. Simultaneous stimulation provoked larger reflexes than sequential stimulation in TA, but not in BF. Larger ISIs elicited significantly larger reflexes in TA, whereas the opposite pattern occurred in BF. This differential modulation between proximal and distal muscles suggests the presence of spinal circuits eliciting a functional reflex response based on the specific tempo-spatial characteristics of a noxious stimulus. No modulation was observed in pain intensity ratings across ISIs. Absence of modulation in the pain intensity ratings argues for an integrative mechanism located within the spinal cord governed by a need for efficient withdrawal from a potentially harmful stimulus.NEW & NOTEWORTHY Tempo-spatial integration of electrical noxious stimuli was studied using the nociceptive withdrawal reflex and a perceived intensity. Tibialis anterior and biceps femoris muscles were differentially modulated by the temporal characteristics of the stimuli and stimulated sites. These findings suggest that spinal neurons are playing an important role in the tempo-spatial integration of nociceptive information, leading to a reflex response that is distributed across multiple spinal cord segments and governed by an efficient defensive withdrawal strategy.

Keywords: nociception; nociceptive withdrawal reflex; spatial summation; temporal summation.

Graphical abstract

Figure 1.

Diagram indicating the stimulated sites, recorded muscles, and representative raw EMG traces obtained with single and simultaneous stimulation. A: figure showing the location of the stimulating electrodes in the sole of the foot: M and L, medially and laterally located, respectively. B: distal (tibialis anterior) and proximal (biceps femoris) muscles from which sEMG recordings (C) were obtained. Raw EMG traces for both TA and BF illustrate the response of a representative subject to single and simultaneous stimuli in M and L sites. BF, biceps femoris; EMG, electromyography; NWR, nociceptive withdrawal reflex; sEMG, surface electromyography; TA, tibialis anterior.

Figure 2.

Stimulus types used in the experiment: single, simultaneous, and sequential. Black rectangles indicate stimulus artifact. The NWR was quantified in a 70-ms window (blue box) starting 80 ms after the trigger of second stimulation. Sequential stimulation was delivered with varying interstimulus intervals ranging from 30 to 500 ms. ISI, interstimulus interval; NWR, nociceptive withdrawal reflex.

Figure 3.

Box and whiskers plot showing nNWR for simultaneous and sequential (averaged across ISIs) stimulation in both TA (left) and BF (right) muscles. For TA muscle, there was a significant difference between simultaneous and sequential stimulation (*P < 0.05, Wilcoxon signed-rank test). When compared with single stimulation (not shown), simultaneous stimulation showed increased reflex magnitude for both TA and BF muscles (øP < 0.01, Friedman’s test). Sequential stimulation, however, only provoked larger reflexes than single stimulation in BF (øP < 0.01, Friedman’s test). BF, biceps femoris; ISIs, interstimulus intervals; nNWR, magnitude of the nociceptive withdrawal reflex; TA, tibialis anterior.

Figure 4.

Box and whiskers plot illustrating the magnitude of the NWR when using sequential stimulation with different ISIs (30–500 ms). The top row shows the results obtained in the tibialis anterior muscle, whereas the bottom row displays the results of the biceps femoris muscle across the ISIs. Values are shown as nNWR, normalized to the NWR due to single stimulation (see methods). Opposite tendencies in the proximal (BF, bottom row) compared with the distal (TA, top row) muscles were observed. For larger ISIs, TA-NWR was facilitated, whereas BF-NWR was reduced (*P < 0.05, Wilcoxon signed-rank test). BF, biceps femoris; ISI, interstimulus interval; nNWR, magnitude of the nociceptive withdrawal reflex; NWR, nociceptive withdrawal reflex; TA, tibialis anterior.

Figure 5.

Box and whiskers plot showing pain intensity ratings due to single, simultaneous, and sequential stimulation of M and L. Simultaneous and sequential stimulation were perceived as more painful than single stimulus (*P < 0.05, Wilcoxon signed-rank test). No significant difference was found between simultaneous and sequential stimulus. Pain threshold (NRS = 5): red horizontal line. L, lateral; M, medial; NRS, numerical rating scale.

Figure 6.

Box and whiskers plot of pain intensity ratings reported for sequential stimulation in the medial electrode (M), in the lateral electrode (L), and as a combination of both (M-L). Different interstimulus intervals (ISIs) are shown. Perception was above pain threshold (NRS = 5: red horizontal line) in all cases, and there was no significant effect of ISI on the perception of pain intensity (P > 0.05, Friedman’s test). NRS, numerical rating scale.