Chapter 13 Touch Components of Touch 1 Tactile mechanical displacement of skin 2 Temperature 3 Pain including itch and tickling 4 Body sensations where body parts are Proprioception Perception mediated by kinesthetic and vestibular receptors Somatosensation A collective term for sensory signals from the body also includes vestibular system Skin sensory organ for touch Largest sensory organ About 1 8 square meters 19 square feet About 4 kg 9 pounds Touch receptors Embedded in outer layer epidermis and underlying layer dermis of skin Each touch receptor can be categorized by three criteria 1 Type of stimulation to which the receptor responds 2 Size of the receptive field 3 Rate of adaptation fast versus slow 4 Types of mechanoreceptors Merkel cell neurite complex SA I Located at boundary between epidermis and dermis Responds best to steady downward pressure Small receptive field size Used to detect fine spatial detail Sensitive to very low frequency of vibrations 5 Hz Slowly adapting SA I Important for texture and pattern perception e g for Braille reading or to determine the orientation of a screw head without looking When nerve is electrically stimulated people report feeling pressure Meissner corpuscles FA I Located at boundary between epidermis and dermis Sensitive to low frequency vibrations between 5 Hz and 50 Hz Fast adapting FA I Small receptive field size Important to detect slip of objects across skin e g to correct grip around object When stimulated electrically people feel wobble or flutter Ruffini Endings SA II Embedded deeply in the dermis Responds best to sustained downward pressure and lateral skin stretch Responds to finger position and grasp Slowly adapting SA II Large receptive field size When stimulated people experience no tactile sensation More than one SA II fiber need to be stimulated in order to be detectable Pacinian Corpuscles FA II Embedded in subcutaneaous tissue Respond best to high frequency vibrations of 50 700 Hz Active when object makes first contact with skin e g mosquito landing on skin or hitting key on keyboard Fast adapting FA II Large receptive field size When electrically stimulated people report buzz Kinesthetic receptors Play important role in sense of where limbs are what kinds of movements are made Muscle Spindles Convey the rate at which the muscle fibers are changing in length Receptors in tendons provide signals about tension in muscles attached to tendons Receptors in joints react when joint is bent to an extreme angle Importance of kinesthetic receptors Strange case of neurological patient Ian Waterman o Cutaneous nerves connecting Waterman s kinesthetic mechanoreceptors to brain destroyed by viral infection o Lacks kinesthetic senses dependent on vision to tell limb positions Thermoreceptors Sensory receptors that signal information about changes in skin temperature Two distinct populations of thermoreceptors warmth fibers cold fibers Body is constantly regulating internal temperature Thermoreceptors respond when you make contact with an object warmer or colder than your skin Nociceptors trigger pain sensations Sensory receptors that transmit information about noxious painful stimulation that causes damage or potential damage to the skin Two groups of nociceptors crushing and heat A delta fibers fast transmission to brain respond to strong pressure initial and quick sharp burst of pain at injury time C fibers slower response sustained stimulation throbbing sensation that evolves after initial surge of pain Benefit of pain perception Case of Miss C Sensing dangerous objects hot pots in the kitchen Born with insensitivity to pain Died at age 29 from untreated infection could not protect herself did not sneeze or cough This is a HUGE problem for diabetic patients who often loose sensation of their feet and become invalids because of untreated minor injuries diabetic feet rot and gangrene because no pain sensation make injuries such as blisters or cuts or twisted ankles go UNTREATED Diabetics lose ability to walk Touch sensations travel as far as 2 meters to get from skin and muscles of feet to brain Information must pass through spinal cord First Synapse Axons of various tactile receptors combine into single nerve trunks Several nerve trunks from different areas of body Once in spinal cord two major pathways Spinothalamic slower evolutionary older heat and pain multiple synapses slower Dorsal column medial lemniscal faster Tactile and proprioceptive information fewer synapses fast transmission Touch sensations are represented somatotopically in the brain Primary somatosensory cortex called S1 secondary somatosensory cortex Analogous to retinotopic mapping found in vision Adjacent areas on skin connect to adjacent areas in brain Homunculus Maplike representation of regions of the body in the called S2 brain Brain contains several sensory maps of body in different areas of S1 and also in S2 Phantom limb Perceived sensation from a physically amputated limb of the body Parts of brain listening to missing limbs not fully aware of altered connections so they attribute activity in these areas to stimulation from missing limb Remember how close the hand was to the face Patients report feeling their Real PAIN might be felt if they perceive their phantom limbs to be in missing hand OVER their face uncomfortable positions Pain Pain sensations triggered by nociceptors Responses to noxious stimuli can be moderated by anticipation religious belief prior experience watching others respond and excitement Example Wounded soldier in battle who does not feel pain until after battle Analgesia Decreasing pain sensation during conscious experience Soldier in above example Experienced effect because of endogenous opiates chemicals released in body to block release or uptake of neurotransmitters transmitting pain sensation to brain Endogenous opiates may be responsible for certain placebo effects Externally produced substances have similar effect Morphine heroin codeine Gate control theory from the brain A description of the system that transmits pain that incorporates modulating signals Feedback circuit located in substantia gelatinosa of dorsal horn of spinal cord Gate neurons that block pain transmission can be activated by extreme pressure cold or other noxious stimulation applied to another site distant from the source of pain Haptic perception Knowledge of the world that is derived from sensory receptors in skin muscles tendons and joints usually