Neurophysiology of Skin Thermal Sensations

Abstract

Undoubtedly, adjusting our thermoregulatory behavior represents the most effective mechanism to maintain thermal homeostasis and ensure survival in the diverse thermal environments that we face on this planet. Remarkably, our thermal behavior is entirely dependent on the ability to detect variations in our internal (i.e. body) and external environment, via sensing changes in skin temperature and wetness. In the past 30 years, we have seen a significant expansion of our understanding of the molecular, neuroanatomical and neurophysiological mechanisms that allow humans to sense temperature and humidity. The discovery of temperature-activated ion channels which gate the generation of action potentials in thermosensitive neurons, along with the characterization of the spino-thalamo-cortical thermosensory pathway, and the development of neural models for the perception of skin wetness, are only some of the recent advances which have provided incredible insights on how biophysical changes in skin temperature and wetness are transduced into those neural signals which constitute the physiological substrate of skin thermal and wetness sensations. Understanding how afferent thermal inputs are integrated and how these contribute to behavioral and autonomic thermoregulatory responses under normal brain function is critical to determine how these mechanisms are disrupted in those neurological conditions which see the concurrent presence of afferent thermosensory abnormalities and efferent thermoregulatory dysfunctions. Furthermore, advancing the knowledge on skin thermal and wetness sensations is crucial to support the development of neuro-prosthetics. In light of the above, this review will focus on the peripheral and central neurophysiological mechanisms underpinning skin thermal and wetness sensations in humans.

Warm hands, cold heart: progressive whole‐body cooling increases warm thermosensitivity of human hands and feet in a dose‐dependent fashion

“…In humans and primates, fast‐conducting myelinated Aδ‐fibres innervate the skin with varying density depending on the skin site (Filingeri, ), selectively respond to skin cooling (range 30–14°C; maximal impulse frequency within 27–22°C range; conduction velocity 3.8–4.4 m s −1 ; receptive field <1 mm) and mediate innocuous cold sensations (Darian‐Smith et al . ).…”

“…). In contrast, unmyelinated C fibres respond to skin warming, fire optimally within a temperature range of 30–45°C (maximal impulse frequency within 36–42°C range; conduction velocity 0.4–2 m s −1 receptive field <1 mm) and mediate innocuous warm sensations (Konietzny & Hensel, ; Filingeri, ; but note that a subclass of C fibre, i.e. C2, responds to noxious skin cooling <15°C and mediates noxious cold sensations; Campero et al .…”

Afferent thermosensory function in relapsing–remitting multiple sclerosis following exercise‐induced increases in body temperature

“…In humans, magnitude estimation of skin thermal sensations is determined by afferent impulses produced by peripheral skin thermoreceptors (Filingeri et al, 2017b) and by their integration operated by central (sub-cortical/cortical) neural structures (Filingeri, 2016). Due to the central, and not peripheral, nature of MS lesions within the nervous system (Noseworthy et al, 2000), it could be therefore suggested that the pronounced reduction in cold sensitivity observed in our relapsingremitting MS group could be dependent on heat-induced alterations in the processing of afferent somatosensory inputs within central neural centers.…”

“…Indeed, the neuro-anatomical and -physiological differences between the human peripheral and central pathways for cold (served by myelinated nerve fibers) and warm (served by non-myelinated nerve fibers) skin thermosensitivity (Dostrovsky & Craig, 1996;Iannetti et al, 2003), allow for the independent assessment of myelinated and nonmyelinated afferent neural pathways (Filingeri, 2016). The opportunity to concurrently and non-invasively evaluate both myelinated and non-myelinated afferent pathways is particularly relevant in the context of a demyelinating disease such as MS (Noseworthy et al, 2000).…”

“…represents a unique index of afferent function (Filingeri, 2016), and its assessment could prove advantageous to non-invasively evaluate somatosensory function within normally functioning non-myelinated pathways and within demyelinated afferent pathways in MS.…”