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ANPS 019 Beneyto-Santonja 11/07/12 Somatosensation What sensations does the brain detect? Special senses o Olfaction (smell) o Gustation (taste) o Audition (hearing) o Vestibular (balance) o Vision (sight) Somatosensations o Superficial cutaneous senses: light touch and pressure, vibration, hot, cold, pain, itch, tickle o Deep senses: position sense, pain, intense pressure o Visceral senses: visceral pain, hunger, nausea, thirst Somatosensation utilizes many receptors Somatosensation= senses detected through body Many different receptor types o Nociceptors (pain) Free nerve endings sense tissue damage, chemical signals, temperature extremes o Thermoreceptors free nerve endings sense temperature o Chemoreceptors respond to chemicals in intersititial fluid (H+, O2, CO2) o Mechanoreceptors Superficial cutaneous – highly sensitive fine touch and pressure for specific localization Deep cutaneous – less sensitive crude touch and pressure for less specific localization Barorecptors detect internal pressure (blood vessels, bladder, GI) Proprioceptors/Joint receptors (monitor limb position) Receptor characteristics Receptive field – part of the body from which the receptor can be stimulated (e.g., area of skin for touch sensation) o Small receptive fields can discriminate finer sensations o When receptors synapse in spinal cord, many receptors converge; therefore, spinal cord neuron receptive fields are much more complex Adapation – reduction in sensation with repeated stimulation; some receptors adapt whereas other do not o Slowly adapting receptors respond through the stimulus duration o Rapidly adapting receptors only respond at the beginning of the stimulus Receptor Potentials differ from Action Potentials: o Action Potentials are All-or-None o Receptor Potentials are Graded and Summed o Amplitude of receptor potential is due to intensity of the stimulus o Once of sufficient intensity the receptor fires an action potential o Action potential firing rate codes stimulus intensity Superficial Cutaneous Mechanoreceptors Monitor small areas of skin (small receptive field) to detect very fine sensations Meissner’s corpuscle – rapidly adapting, low frequency vibration Merkel cell – slowly adapting, pressure Deep Cutaneous Mechanoreceptors Monitor larger areas of skin (large receptive field) to detect crude sensations Pacinian corpuscle – rapidly adapting, vibration Ruffini ending – slowly adapting, pressure Muscle Spindle; Muscle Receptor Monitors muscle length and rate of change Organized in parallel with extrafusal muscle fibers Extrafusal vs. Intrafusal Muscle Fibers Sensory receptor is associated with intrafusal muscle Requires two types of motor neurons: o Alpha motor neurons innervate extrafusal muscle (muscle bulk) o Gamma motor neurons innervate intrafusal muscle o Both motor neurons must fire simultaneously! Golgi Tendon Organ (GTO); Joint Receptor Monitors muscle tension Organized in series with extrafusal muscle fibers What happens after a receptor is activated? Stimulus Detected by Receptor Activates First-order neuron in PNS Activates Second-order neuron in CNS (Axon of second order neuron crosses to opposite side) Activated Third-order neuron in thalamus Activates Neuron in primary sensory cortex provides Conscious perception of stimulus Ultimately, somatosensory info is transmitted to the Postcentral Gyrus in the Parietal Lob The 3 main somatosensory pathways can be understood by receptor types Mechanoreceptors (Joint & muscle receptors) o Spinocerebellar Cuneocerebellar= unconscious sensations o Dorsal Column-Medial Lemniscal= conscious sensations Nociceptors (pain) & Thermoreceptors (temperature) o Anterolateral (aka Spinothalamic)= pain & temperature Chemoreceptors (H+, O2, CO2)