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Western University
Physiology 4710A/B

Lecture 7TouchHow touch is electrically encodedAs in vision we can use touch to distinguish edges feel textures read letters and recognize objects as complex as facesWe do this with very few receptor typesas in visionThere are five receptors sensitive to touch oThere are also receptors that sense pain and temperature Transforming mechanical energy into electrical activityPacinian1 Mechanical stimulus eg Pressure deforms the receptors onion like membrane 2 Channels open and Na flows through membraneoinside of the receptor depolarizes voltage becomes more 3 If the graded potential summed at the initial segment is above threshold AP is generated and propagated down the axon oMost touch afferents have myelinated axons in which AP hop from gap to gap thus speeding up conductionHow is the magnitude of the stimulus encodedStimulus magnitude is encoded in part by a frequency codeoGreater the pressure the more the receptor depolarizes oWhen above threshold more APssecond are generated The relationship between number of APsecond and pressure is nonlinear oIt saturates at high pressures The response to the stimulus adaptsTheof Apssec adapts because the receptor potential adaptsReceptor potential adapts in part because the onionlike laminae slip back closing the channelsoLayers slip back to normal position so the pore closes and potential goes to normal The adaptation enhances the detection of changes in pressureoConstant pressure such as that exerted by your clothes is not important Other Receptors for touchIn any one part of your skin you will find 4 receptors 1Hair receptors back of hand or Meissener palm of hand2Merkel13Ruffini4PacinianWhich receptors are rapidly adapting RA or slowly adapting SA1Hair receptors or meissner RA1 surface but not both 2Merkel SA1 surface3Ruffini SA2 deep4Pacinian RA2 deep receptors Thus both surface and deep layers of the skin contain both RA and SA receptors Label 1receptors at the epidermis near the skin surfaceLabel 2receptors deeper in the dermis Which receptor has the largest receptive field sizeRF size increases with depth in the skinPacinian corpuscles have the largest receptive fieldWe need a variety of touch receptors to code a large variety of touch stimuli Without 5 different types of touch receptors touch would be like being color blindPain and temperature receptorsThere are 2 types of free nerve endings that are sensitive to pain stimuli 1Fast conducting myelinated fiber that signals an early localized intense painaAlso mediates sensation of itching2Slow conducting unmyelinated fiber that signals a later poorly localized long lasting dull pain Also two types of free nerve endings sensitive to temperature stimuli1Fast conducting myelinated fiber that fires most for hot but not burning stimuli 2A fast conducting myelinated fiber that fires for cold but not freezing stimuli Burning or freezing stimuli activate pain receptors Touch afferent fibers have large diameters Pressure first blocks the conduction of AP in large fibers oYour limb falls asleepBut the sense of temperature and pain mediated by small diameter fibers is often preserved The Gate control theoryPatrick Wall and Ronald MelzackoSuggests that pain sensation is dependent on the balance between input from large nerve fibers touch and that from small nerve fibers painoIf there is more large than small there should be little or no pain oIf there is more small than large then one will sense pain oBy activated large fibers through rubbing one can alleviate pain Labeled linesFor the brain to recognize that a stimulus is a vibration that is coming from the surface of the skin the brain must label the afferent type that has been activated as an RA1 afferent 2Each type of touch sensor has its own private linea labeled line Labeled linea label attached to each afferent fiber as to what submodality it is oBecause of this there is no reason for encoding and decoding each packed of info oCortex learns from experience what travels along the fiber o Similar coding in the auditory systemWe sense the frequency of a sound from where the fiber is originating on the basilar membrane Information here is encoded in two ways 1The firing frequency of a particular neuron2Which neuron this is Pathway to the primary sensory cortexPathway for transmission of touch is the dorsal column medial lemniscal system Path for labeled line to the cortexIn the spinal cord the dorsal column is the first stage in the development of somatotopic organizationoIn the lower segments only afferents from the leg are foundoAs one moves up the spinal cord new afferents enter laterally oThus in high segments of the spinal cord from medial to lateral leg trunk armThe signal crosses over to the other side of the brain and synapses the ventral posterior nucleus of the thalamus 3
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