Somatosensation refers to a general sense rather than a special sense. It encompasses proprioception and interoception, as well as modalities like pressure, vibration, light touch, tickle, itch, temperature, pain, proprioception, and kinesthesia.
Rather than being located in a single specialized organ, somatosensory receptors are dispersed throughout the body, including the skin, muscles, tendons, ligaments, joint capsules, and even within visceral organs.
Two key somatosensory signals—pain and temperature—are transduced by free nerve endings. These rely on thermoreceptors (responsive to temperature differences from body temperature) and nociceptors (detecting potentially damaging mechanical, chemical, or thermal stimuli).
When tissues are stressed or injured, they release chemicals that activate proteins in nociceptors. An example is capsaicin, the heat-inducing molecule in spicy foods, which binds to ion channels sensitive to temperatures above 37°C. Capsaicin remains attached for an extended period, reducing pain responses from the activated nociceptor; this property makes it useful as a topical analgesic.
When a textured surface causes the skin on your finger to vibrate, these low-frequency vibrations are detected by Merkel cells (also called type I cutaneous mechanoreceptors), located in the stratum basale of the epidermis. In contrast, deep pressure and higher-frequency vibration are registered by lamellated (Pacinian) corpuscles, found in deeper layers of the dermis or subcutaneous tissue.
Light touch is sensed by tactile (Meissner) corpuscles, while hairs on the skin are wrapped by the hair follicle plexus to detect hair movement (such as an insect crawling on the surface). Skin stretch is transduced by bulbous corpuscles (also known as Ruffini corpuscles or type II cutaneous mechanoreceptors).
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