The contribution of sensory nerves to the onset threshold for cutaneous vasodilatation during gradual local skin heating of the forearm and leg

During local skin heating, the temporal onset of vasodilatation is delayed in the leg compared to the forearm, and sensory nerve blockade abolishes these differences. However, previous work using rapid skin heating did not allow for determination of sensory nerve influences on temperature thresholds for vasodilatation. Two sites were examined on both the forearm and leg, one control (CTRL), and one treated for sensory nerve blockade (EMLA). Skin blood flux was monitored using laser-Doppler probes, with heaters controlling local skin temperature (Tloc). Tloc was increased from 32-44 °C (+ 1 °C·10 min− 1). Stimulus-response curves were constructed by fitting a four-parameter logistic function. EMLA significantly increased Tloc onset in the forearm (CTRL = 35.3 ± 0.4 °C; EMLA = 36.8 ± 0.7 °C) and leg (CTRL = 36.5 ± 0.4 °C; EMLA = 38.4 ± 0.5 °C; both P < 0.05). At both CTRL and EMLA, Tloc onset was higher in the leg compared to the forearm (both P < 0.05). In the forearm, median effective temperature to elicit 50% vasodilatation (ET50) was similar between sites (CTRL = 39.7 ± 0.3 °C; EMLA = 40.2 ± 0.4 °C; P = 0.09); however, in the leg, EMLA significantly increased ET50 (CTRL = 40.2 ± 0.3 °C; EMLA = 41.0 ± 0.3 °C)(P < 0.05). At CTRL sites, no limb difference was observed for ET50 (P = 0.06); however, with EMLA, ET50 was significantly higher in the leg (P < 0.05). EMLA significantly increased the gain of the slope at the forearm, (CTRL = 0.31 ± 0.01%CVCmax·°C− 1; EMLA = 0.45 ± 0.07%CVCmax·°C− 1), and leg (CTRL = 0.37 ± 0.05%CVCmax·°C− 1; EMLA = 0.54 ± 0.04%CVCmax·°C− 1)(both P < 0.05). At CTRL sites, the gain was significantly higher in the leg (P < 0.05); however, for EMLA, no significant limb difference existed (P = 0.10). These data indicate that the onset of vasodilatation occurs at a lower temperature in the forearm than the legs, and sensory nerves play an important role in both limbs.