The distribution of cutaneous sudomotor and alliesthesial thermosensitivity in mildly heat-stressed humans: an open-loop approach

Abstract

The distribution of cutaneous thermosensitivity has not been determined in humans for the control of autonomic or behavioural thermoregulation under open-loop conditions. We therefore examined local cutaneous warm and cool sensitivities for sweating and whole-body thermal discomfort (as a measure of alliesthesia). Thirteen males rested supine during warming (+4 degrees C), and mild (-4 degrees C) and moderate (-11 degrees C) cooling of ten skin sites (274 cm2), whilst the core and remaining skin temperatures were clamped above the sweat threshold using a water-perfusion suit and climate chamber. Local thermosensitivities were calculated from changes in sweat rates (pooled from sweat capsules on all limbs) and thermal discomfort, relative to the changes in local skin temperature. Thermosensitivities were examined across local sites and body segments (e.g. torso, limbs). The face displayed stronger cold (-11 degrees C) sensitivity than the forearm, thigh, leg and foot (P = 0.01), and was 2-5 times more thermosensitive than any other segment for both sudomotor and discomfort responses (P = 0.01). The face also showed greater warmth sensitivity than the limbs for sudomotor control and discomfort (P = 0.01). The limb extremities ranked as the least thermosensitive segment for both responses during warming, and for discomfort responses during moderate cooling (-11 degrees C). Approximately 70% of the local variance in sudomotor sensitivity was common to the alliesthesial sensitivity. We believe these open-loop methods have provided the first clear evidence for a greater facial thermosensitivity for sweating and whole-body thermal discomfort.

Figure 1. Limb sweat rates during warming and cooling of six discrete skin sites

Data are means (± s.e.m.), averaged across all limbs, during mild warming (4°C; ▵), mild cooling (−4°C; ▿) or moderate cooling treatments (−11°C; ▾) applied to local skin sites. Moderate cooling was applied immediately after mild warming, so the baseline was elevated relative to the other treatments. Sweat rates are percentages of the value observed when the thermal stimulus was first perceived (time = 0).

Figure 2. Local cutaneous thermosensitivities for sudomotor control

Data are mean (± s.e.m.) sweat rate changes, averaged across all limbs, relative to local skin temperature change, during mild warming (4°C; hatched bars), mild cooling (−4°C; grey bars) and moderate cooling (−11°C; black bars) of 10 discrete skin sites. Significant differences (P < 0.05) between local sites were only apparent with respect to the face (†).

Figure 3. Segmental cutaneous thermosensitivities for sudomotor control

Segments are groups of locally treated skin sites: face; torso (chest, abdomen and lower back); upper limb (arm, forearm and hand); lower limb (thigh, leg and foot); proximal limb (arm, forearm, thigh and leg); and limb extremities (hand and foot). Data are mean (± s.e.m.) sweat rate changes, averaged across all limbs, relative to local skin temperature changes, during mild warming (4°C; hatched bars), mild cooling (−4°C; grey bars) and moderate cooling (−11°C; black bars) of 10 discrete skin sites. Significant differences (P < 0.05) between body segments were only apparent with respect to the face (†) and torso (‡).

Figure 4. Segmental cutaneous thermosensitivities for alliesthesia

Segments are groups of locally treated skin sites. Data are mean (± s.e.m.) whole-body thermal discomfort changes, relative to local skin temperature changes, during mild warming (4°C; hatched bars), mild cooling (−4°C; grey bars) and moderate cooling (−11°C; black bars) of 10 discrete skin sites. Significant differences (P < 0.05) between sites were only apparent with respect to the face (†).

Figure 5. Local cutaneous thermosensitivities for sudomotor control and whole-body thermal discomfort (alliesthesia)

A, data are site-specific, mean sudomotor (grey bars) and alliesthesial (hatched bars) thermosensitivities (± s.e.m.) derived across three localized thermal treatments (n = 12): mild warming (4°C), mild cooling (−4°C) and moderate cooling (−11°C). Significant differences (P < 0.05) between local sites were only apparent with respect to the face (†), abdomen (‡) and back (§). The inset (B) displays the relation between changes in whole-body thermal discomfort and local skin temperature (T_skl), and represents the alliesthesial thermosensitivity for these treatments at the face.