Somatosensory Receptors PDF

Somatosensory Receptors PH5208 Dr. Moira Jenkins Sensation • general senses: • somatic sensory • visceral sensory • Sensation from a structure: • Visceral (primarily composed of smooth muscle, cardiac muscle, or gland) • Somatic (everything else) Pain, temperature, vibration, joint position, light...

Somatosensory Receptors PH5208 Dr. Moira Jenkins Sensation • general senses: • somatic sensory • visceral sensory • Sensation from a structure: • Visceral (primarily composed of smooth muscle, cardiac muscle, or gland) • Somatic (everything else) Pain, temperature, vibration, joint position, light and deep touch, • “special” senses: • • • • • sight (vision) taste (gustation) smell (olfaction) hearing equilibrium The somatosensory system reports the body’s interactions with its immediate surroundings via “touch and feel” sensing of contact pressure against the skin, and via thermal sensations 3 classes of receptors: • mechanoreceptors • thermoreceptors • nociceptors → these receptors are “tuned” to discriminate among different qualities of the various stimuli to which they are responsive, e.g. texture and shape, hot vs. cold, innocuous vs. noxious the nervous signaling transduced by these receptors is conveyed by a primary afferent fiber that enters the spinal cord via a dorsal root Kandel & Schwartz Fig. 22-1 Skin- loaded with receptors Epidermis (stratified squamous) and dermis-connective tissue Thin Skin/Hairy • Found everywhere on the body except palms and soles • Lucidum absent or thin, corneum thinner • Hair root plexus-free nerve ending at root, detect movement Thick Skin/Glabrous/Smooth/Nonhairy • Found on soles, palms, fingertips • • • • Fascia System of connective tissue that encases organs and binds them together Initially thought to be passive **Now know a sensory organ and even actin/myosin More receptors than skin? Especially interstitial, type III and IV Dysfunction after injury, nonuse, overuse Receptive Field The spatial domain to which an applied stimulus will trigger signaling along a particular sensory fiber How many receptors and are they deep or superficial Superficial receptors- receptive field is focused, well localized Deep receptors – receptive field is more diffuse, not as specifically localized Meissner’s Pacinian Sensory Fiber Classification • Peripheral fibers conveying non-proprioceptive somatosensory afferent signals are often classified alphabetically while proprioceptive afferents from muscle are usually listed numerically • Large A-alpha and A-beta convey mechanoreceptor afferent from the skin • Smaller A-delta and unmyelinated C convey pain and temperature MECHANORECEPTORS Skin • A mechanical stimulus causes activation • Superficial – Merkel’s disk (in epidermis) – Meissner’s corpuscle – transmit via Aa (Type 1) – minimal or no end organ – multiple receptors converge input to one primary afferent • Deep – Ruffini corpuscle – Pacinian corpuscle – Transmit via Ab (type 2) – Are embedded within an end organ – If thin skin- also have nerve root plexus Kandel & Schwartz Fig. 23-1 see also: Cramer & Darby Fig. 9.1 Mechanoreceptors Other cutaneous mechanoreceptors (hairy skin, nonglabrous) Iggo Dome receptors- Merkel’s discs grouped together, epithelium protrudes outward, “dome”, very sensitive Free nerve endings – for very light touch, ex: cornea Hair Follicle Receptor/Peritrichial endings- free nerve ending wrapped around the hair follicle, detects movements across the skin Cutaneous mechanoreceptors are classified as slowly or rapidly adapting The combinations of deep, superficial, slow, and rapid adapting reports different characteristics of the applied stimulus Slowly adapting reports continuous stimulus Rapidly adapting reports on/off, changing stimulus Summary Merkel’s Disc –superficial, small receptive field, Aa/1, slow-adapting (SA1) Meissner’s Corpuscle- superficial, small receptive field, Aa, rapidly adapting (RA1) “light touch” Ruffini’s End Organ- deep, large receptive field, Ab/2, slow adapting (SA2) Pacinian Corpuscle –deep, large receptive field, Ab/2, rapidly adapting (RA2) “pressure” Kandel & Schwartz Fig. 21-8 What we perceive is derived from the collective signaling from sensory receptors – → a characteristic distribution of these four types of mechanoreceptors gives rise to a varying discrimination of touch and feel between the fingertips and the palm of the hand Kandel & Schwartz Fig. 23-4 ᠆ when our hand contacts an object, reporting from the four types of receptors is conveyed via “parallel channels of sensory information” that each encode a specific aspect of this contact ᠆ the information arriving from these separate channels is combined within the sensory cortex to form our perception “of the whole” Feel/Grasping Objects • Somatotopic mapping from receptive field and labeled lines→ size and shape of object, point of contact with object • Mixture of receptor types → surface texture of objects, movement of object in relationship to the surface of fingers and hand • Result- can grasp a wide range of objects Mechanoreceptors Mechanoreceptors can be superficial, deep, slow adapting, fast adapting Many are in the skin and fascia Thermoreceptors- warm and cool Thermoreceptors are most sensitive to changes in temperature Most adapt rapidly to sustained temperatures Unaware of slow changes in skin temp between 88-97 F (skin normally 90 F) Free nerve endings Thermoreceptors Cutaneous thermoreceptors respond to changes in surface/skin temperature over a certain range Skin temperature (90F)/core temperature (98F) Free nerve endings with thermal sensitivity, temperature change activates ion channels on the receptor membrane TRP (transient receptor potential) channels Thermoreceptors Thermoreceptors are sensitive to NONpainful changes in temperature as well as painful changes (nociceptive thermoreceptors) Individual receptors respond to either heat or cold but not to both Hypothalamus also has temperature sensitive receptors to maintain core temperature Heat receptors, 95-110 F/35-43.3 C, transmitted by C fibers Cold receptors, 60-85 F/15.5-30 C, transmitted by Ad fibers and C fibers Thermoreceptors- warm and cool Transient receptor potential channels TRP Warm receptors report increases in temperature above 97°F, but are less rapidly adapting that cold receptors, are also much less sensitive to temperature change than cold receptors, providing less sensitivity to warming than to cooling low threshold cold receptors detect rapid drops in temperature below 88°F; high threshold cold receptors/nociceptors detect rapid drops in temperature in the lower range (down to and below 32°F) Dual function-change in temperature, warning danger→nociceptor Thermoreceptors- can act as nociceptors Why would a temperature gated channel also be sensitive to chemicals? These chemicals mimic endogenous chemicals released with tissue damage Separate cold and heat nociceptors generate the perception of pain to either cold [Ts below 60°F] or heat [Ts above 110°F] Paradoxically, some chemicals like capsaicin can desensitized pain fibers and deplete substance P in pain fiber nerve terminals (analgesic effect) Nociceptors Warning- any potential damage to tissue Chemical, mechanical, thermal, etc Nociceptors Free nerve endings 2 classes -A fibers-Strong, mechanical stimulus like pinprick, some thermal -C fibers- Polymodal receptors. Respond to nociceptive stimulus that is thermal, mechanical, and/or chemical nociceptors transduce noxious (painful) stimuli arising from all three somatosensory modalities: • mechanical → both sharp and sustained pressure • thermal → hot (Ts > 110°F) and cold (Ts < 60° F) • chemical → irritants, e.g. acid, inflammatory molecules (Note: Mechanoreceptors and Nociceptors are also found in muscle, fascia, joints, viscera) what differentiates a nociceptor from a non-nociceptive somatosensory receptor is the receptor’s stimulus threshold … Nociceptors have a higher stimulus threshold, compared to non-nociceptive receptors → a greater stimulus intensity is therefore required to initiate signaling from nociceptors nociceptive signaling is conveyed from the periphery into the CNS via Ad and C fibers • Ad fibers arise from mechanical and thermal nociceptors; convey sharp (short lasting) sensation of pain • C fibers arise from polymodal receptors; convey dull or diffuse pain e.g. in response to a painful stimulus: • an initial sharp pain is conveyed by Ad fibers → “first pain” • a longer duration, duller sensation of pain is subsequently conveyed by C fibers → “second pain” Kandel & Schwartz Fig. 24-1