Thermal Sensations and Thermoreceptors

Thermal Sensations

• Temperature is a measure of the motion and vibrations of the particles in an object.
• Most body cells are temperature-sensitive, but some are highly sensitive to temperature; these are called thermoreceptors (also known as thermosensors).
• Thermoreceptors have a Q10 value greater than 3.
• The average Q10 value for most cells is between 1–3.
• Q10 coefficient measures the reaction rate changes due to a 10°C increase in temperature.
• Thermoreceptors can detect skin temperature changes as small as 0.01°C if the change affects the entire body simultaneously.
• Thermoreceptors are categorized into nociceptive and non-nociceptive types.
• Nociceptors are further divided into heat and cold nociceptors, defined by their respective thermal thresholds.
  • Heat nociceptors (skin temp > 45°C)
  • Cold nociceptors (skin temp < 10°C)
• Non-nociceptors can be further divided into location and stimulation. – Central (hypothalamus, viscera) – Peripheral (skin)
• Warm receptors and cold receptors have different numbers in the skin.
  • Cold receptors are more abundant (3-10 times more than warm receptors).
  • Specific locations have varying concentrations of these receptors (e.g., lips have more than the trunk).
• The locations where temperature is detected are called warm and cold spots. They mark the receptive fields of the thermoreceptors and are discrete.
• The different types of receptors (warmth, cold) have distinct activity levels related to specific temperature ranges (activation thresholds).
• The sensation of paradoxical cold occurs at very high skin temperatures (> 45°C) due to activation of a part of the cold receptor population
• Thermal transduction involves both chemical stimulations of the receptors, related to metabolic changes and the direct physical effects of temperature on temperature-gated ion channels (such as TRP).
• Different TRP channels (TRPA1, TRPM8, TRPV1, TRPV2, TRPV3 and TRPV4) have specific temperature sensitivities and play different roles in thermoreception
• Thermoreceptor adaptation is not complete at all skin temperatures; it is substantial, but not fully adapted from stimulus at skin temps 20-40 degrees Celsius
• Thermal signals travel in parallel with pain signals through distinct spinal cord pathways, primarily in laminae I, II, and III of the dorsal horn.
• Sensory information from thermoreceptors is relayed through specialized neurons to the brain's thermoregulatory centers in the hypothalamus and other brain regions.
• Spatial summation is important for thermal sensation because localized stimulation of small areas often results in weaker perception; larger areas of stimulation result in greater perception.