Thermal Sensations PDF

Thermal Sensations Asghar Ghasemi  Temperature Temperature is a measure of the motions and vibrations of the particles of the object Almost all cells in our body are temperature-sensitive but some cells are extremely sensitive to temperature and are called...

Thermal Sensations Asghar Ghasemi  Temperature Temperature is a measure of the motions and vibrations of the particles of the object Almost all cells in our body are temperature-sensitive but some cells are extremely sensitive to temperature and are called thermoreceptors (thermosensors)  Temperature coefficient ( 10 coefficient) 10 coefficient is typically used to measure the reaction rate change as a function of temperature increase by 10° C. Most cells: Q10 values are between 1-3 (average: 2) Thermoreceptors: Q10 value >3 We can perceive changes in our average skin temperature of as little as 0.01° C if this change affects the entire surface of the body simultaneously Skin T > 45° Skin T < 10° C C Thermoreceptors: Nociceptors Warmth receptors Cold receptors  Cold and warmth spots The cold and warmth receptors are located immediately under the skin at discrete separated with no overlapping between their receptive fields Most areas of the body have 3 to 10 times as many cold spots as warmth spots. Number (spots/cm2) of cold spots in different areas of the body: Lips: 15 to 25 Finger: 3 to 5 Trunk: less than 1 Filingeri, D. Compr Physiol, 2016. 6(3): p. 1429.  The locations at which a thermal stimulus is detected are known as warm and cold spots and are assumed to mark the receptive fields of underlying thermoreceptors Insensitive areas  Paradoxical cold At very high skin temperatures (>45° C), there is a sensation of paradoxical cold, caused by activation of a part of the cold receptor population. Warmth and cold transductions 1. Chemical stimulation of the endings Cold and warmth receptors are stimulated by changes in their metabolic rates 2. Direct physical effects of temperature TRP (transient receptor potential) channels, which are non-selective cation channels Skin temperature (°C)  Static and dynamic responses of thermoreceptors Static response: response to steady-state temperature (tonic response) Dynamic response: response to changes in temperature (phasic response)  Thermoreceptor adaptation NO adaptation for skin temperature 40°C Slow adaptation (to a great extent but never 100%) for skin temperature 20-40 °C How we perceive different gradation of thermal sensation? By the relative degrees of stimulation of the different types of receptors Spatial summation of thermal sensations It is difficult to judge gradations of temperature when small skin areas are stimulated because the number of cold or warmth endings in any one surface area of the body is slight. However, when a large skin area is stimulated all at once, the thermal signals from the entire area are cumulative. For example, rapid changes in temperature as little as 0.01° C can be detected if this change affects the entire surface of the body simultaneously. Conversely, temperature changes 100 times as great often will not be detected when the affected skin area is only 1 square centimeter in size.  Thermal sense pathways Thermal signals are transmitted in pathways parallel to those for pain signals (Lateral AL) On entering the spinal cord, the signals travel for a few segments upward or downward in the and then terminate mainly in laminae I, II, and III of the dorsal horns— the same as for pain. Second order neurons cross to the opposite anterolateral sensory tract and terminate in both: (1) the reticular areas of the brain stem and (2) the ventrobasal complex of the thalamus (3) A few thermal signals are also relayed to the cerebral somatic sensory cortex from the ventrobasal complex. Removal of the entire cortical postcentral gyrus in a person reduces but does not abolish the ability to distinguish gradations of temperature. Thermoregulatio n Thank you