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Generally, the somatosensory sense is referred to as, “the sense of touch.” However, it accounts for various other sensations beyond that. For instance, it allows an organism to feel how hot or cold their surroundings are. It allows organisms to feel pain when their tissues sustain damage. Yet, this myriad of somatosensations wouldn’t be possible without somatosensory receptors. Somatosensory receptors are the dendrites of sensory neurons that detect somatosensory stimuli and transduce them into electrical signals that can be interpreted by the brain. There are a plethora of somatosensory receptors in the human body, but they are classified into three main types: nociceptors, thermoreceptors, and mechanoreceptors. Nociceptors are free nerve endings that transmit pain signals resulting from tissue damage. Thermoreceptors are also free nerve endings, and they pick up the temperature of the environment, whether it is warm or cold. More information on mechanoreceptors can be found in Somatosensory Receptors Part I: Mechanoreceptors.
Thermoreceptors are located in the skeletal muscles, skin, liver, and hypothalamus. They are categorized into high-threshold receptors which respond to temperatures above 45o C or below 15o C, and low-threshold receptors, which respond to temperatures between 45o C and 15o C. The nerve fibers that make up thermoreceptors are Aδ (A delta) fibers, thin myelinated fibers which conduct signals quickly, and C fibers, unmyelinated fibers which conduct signals much more slowly. Therefore, in thermoreceptors, Aδ fibers are responsible for conducting cold sensations, while warm sensations are conducted through C fibers. Since pain signals are also conducted through Aδ and C fibers, extreme temperatures from high-threshold thermoreceptors can also induce pain. Temperatures below 0o C and above 50o C typically cause tissue damage in humans, which means that pain signals can also be conducted through thermoreceptors if the external temperature falls within these ranges.
Somatosensory Science Facts, Page 130, Question 134
One important function of thermoreceptors is to regulate body temperature. Body temperature is determined by two factors: external temperature and core temperature. Thermoreceptors located in the skin are responsible for detecting outside temperature and external stimuli. They then relay that information to the brain. Meanwhile thermoreceptors located in internal organs such as the brain, spinal cord, and viscera detect changes in core body temperature. Temperature information travels through the preoptic area of the hypothalamus, which is responsible for thermoregulation (the maintenance of body temperature). If any shift in body temperature is detected, the brain triggers physiological and behavioral responses that oppose the temperature shifts. Physiological responses include shivering and sweating, while behavioral responses require conscious thought, such as putting on or taking off a jacket.
Meanwhile, nociceptors can be categorized into three different types: mechanonociceptors, polymodal nociceptors, and silent nociceptors. Mechanonociceptors detect the initial pain of a high intensity mechanical force (such as a cut, a pinch, or a stretch in the very beginning, when it is at its highest intensity). These nociceptors are composed mainly of myelinated Aδ fibers, as these high intensity signals must be conveyed quickly. Polymodal nociceptors respond to extreme chemical conditions as well as extreme thermal conditions (such as extreme heat or extreme coldness). They consist mainly of C fibers, as temperature signals are conveyed a little more slowly. Finally, silent nociceptors are activated by mechanical stimuli on injured areas after tissue damage. They can be made up of Aδ fibers or C fibers, depending on the type of pain they convey.
Somatosensory Science Facts, Page 106, Question 101
Nociceptors are also categorized based on where they are located in the body. First, there are cutaneous nociceptors, which are located in the skin and innervate the epidermis and dermis layers of the skin. There are also muscle nociceptors, which innervate the tendon and muscle belly. Joint nociceptors activate the ligaments, adipose tissue, fibrous capsule, menisci, and synovial layer of joints, while visceral nociceptors are located in the gut. Each of these locations contain all three types of nociceptors.
These thermoreceptors and nociceptors allow organisms to detect the temperature of our environment as well as tissue damage. However, their impact goes far beyond simply detecting stimuli. For instance, if extreme heat is detected by a thermoreceptor, this signal will be propagated from neuron to neuron until it reaches the postcentral gyrus (where the somatosensory cortex is located), and the insula (a brain region which plays a major role in homeostatic function). The insula will allow the stimulus to be perceived, and the brain will then send a signal to motor neurons, which will allow an organism to move away from the stimulus. Thermoreceptors and nociceptors allow us to move away from noxious stimuli, or protect ourselves from harsh conditions. Essentially, they shield us from danger, allowing us to protect ourselves from the hostilities of the world.
This information and more on Somatosensory can be found in our book, “Somatosensory Science Facts” (in press) and the lead Authors are Sehej Bindra and Charles Pidgeon, Ph.D. Several other students from the Neuroscience Sensory Unit are co-authors.
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