USMLE-1 Thermal Sensation & Receptor Physiology Quiz-4 скачать в хорошем качестве

USMLE-1 Thermal Sensation & Receptor Physiology Quiz-4

Understanding Thermal Sensations Our skin helps us feel temperature changes, from freezing cold to burning hot. This happens through specialized sensors called thermal receptors, which detect cold, warmth, and extreme temperatures that cause pain. There are three main types: Cold receptors: These spot cooling sensations. They're small, myelinated nerve endings (type Aδ) just under the skin, transmitting signals quickly at about 20 m/sec. They activate most around 24°C and fade above 40°C. Warmth receptors: These detect heating. They're free nerve endings using slower, unmyelinated type C fibers (0.4–2 m/sec). They start firing around 30°C and stop near 49°C. Pain receptors: These kick in for extremes, like below 10–15°C (freezing cold) or above 45°C (burning hot). They work with the others to signal danger. Receptors are scattered in "spots" under the skin—3–10 times more cold spots than warm ones. Lips have the most cold spots (15–25 per cm²), while the trunk has fewest (less than 1 per cm²). How do they work? Temperature alters the receptors' metabolic rates, speeding or slowing chemical reactions inside cells (doubling every 10°C rise). This chemical change excites the nerves. Sensations build from relative activation: cool feels different from warm based on which receptors fire more. Extremes feel similar (both painful) because pain dominates. Receptors adapt: A sudden drop feels intensely cold at first but fades over seconds to minutes (never fully). They respond strongly to changes, not just steady temps—explaining why entering hot water feels scalding initially. Signals travel like pain pathways: into the spinal cord (laminae I–III), then up to the brainstem, thalamus, and slightly to the cortex for fine discrimination. Spatial summation helps: Tiny changes (0.01°C) are detectable over large areas, but not small ones. Overall, this system protects us by sensing environmental shifts, blending detection, adaptation, and pain for survival