For example, the epidermal layers are thicker in the heel region and could act as insulators to the electrical stimulus. These differences in NWR threshold and sensation could be related to differences in skin thickness or innervation density across the sole of the foot. The perceived stimulus quality also varies across the sole of the foot. The NWR is more difficult to evoke from some areas of the foot sole than others, and the heel region is especially difficult requiring higher stimulation intensities. The purpose of the present study was to investigate potential mechanisms underlying differences in NWR threshold and sensation across different sites in the sole of the foot. Research has shown that the RRF expands during chronic pain conditions caused by central sensitization, and thus the NWR can be used to probe central sensitization. In humans, the RRF is typically assessed via electrical stimulation of several sites at the sole of the foot. Consequently, the reflex receptive field (RRF) of a muscle is defined as the skin area from where nociceptive stimulation elicits a NWR in that muscle. Since the NWR has a modular organization, stimulation of a certain skin area will activate reflex activity in a group of synergistic muscles leading to withdrawal of the affected skin area. The NWR can be evoked using cutaneous electrical stimulation and assessed by surface electromyography from selected muscles in the lower limb. This reflex has been the subject of substantial research both in humans and animals as the NWR excitability reflects spinal nociceptive processes. The nociceptive withdrawal reflex (NWR) protects the body by withdrawing the limb from a potential damaging stimulus. The nerve staining and modeling results do not explain differences in NWR thresholds across the sole of the foot which may suggest that central mechanisms contribute to variation in NWR excitability across the sole of the foot. Instead a lack of IENFs at the heel decreased the electrical activation thresholds compared to models including IENFs. The computer simulation of the effects of skin thicknesses and innervation densities on thresholds of modeled Aδ and Aβ fibers did not reveal differences in pain and perception thresholds across the foot sole as have been observed experimentally. For each of the 9 combinations of site and electrode size, a total of 3000 Aβ fibers and 300 Aδ fibers was modeled. The model included three different sites in the sole of the foot (forefoot, arch and heel) and three different electrode sizes (diameters: 9.1, 12.9, and 18.3 mm). The modeling comprised finite element analysis of the volume conduction combined with a passive model of the activation of branching cutaneous nerve fibers. Secondly, mathematical modeling was used to investigate to what degree differences in skin thicknesses affect the activation thresholds of Aδ and Aβ fibers in the sole of the foot. However, the results indicate that there are no nociceptive intraepidermal nerve fibers (IENFs) innervating the heel. No differences in innervation densities were found across the sole of the foot using the two staining techniques: Na v1.7 immunochemistry (small nociceptive fibers (1-way ANOVA, NS)) and the Sihler’s method (myelinated nerve fibers (1-way ANOVA, NS)). 1) Staining for the Na v1.7 antigen (small nociceptive fibers) and 2) the Sihler whole nerve technique (myelinated part of the nerve). The first part of the study investigated the neural innervation in different sites of the sole of the foot using two different staining techniques. The aim of the present study was to investigate potential peripheral mechanisms for any site dependent differences in reflex thresholds. However, elicitation of NWRs is highly site dependent, and NWRs are especially difficult to elicit at the heel. Human nociceptive withdrawal reflexes (NWR) can be evoked by electrical stimulation applied to the sole of the foot.
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