This is a sample document toshowcase page-based formatting. Itcontains a chapter from aWikibookcalledSensory Systems. None of thecontent has been changed in thisarticle, but some content has beenremoved.Anatomy of the Somatosensory SystemFROM WIKIBOOKS1Oursomatosensorysystemconsistsofsensorsintheskinandsensorsinourmuscles,tendons,andjoints.There-ceptorsintheskin,thesocalledcutaneousreceptors,tellusabouttemperature(thermoreceptors),pressureandsur-facetexture(mechanoreceptors),andpain(nociceptors).Thereceptorsinmusclesandjointsprovideinformationabout muscle length, muscle tension, and joint angles.Cutaneous receptorsSensoryinformationfromMeissnercorpusclesandrapidlyadaptingafferentsleadstoadjustmentofgripforcewhenobjectsarelifted.Theseafferentsrespondwithabriefburstofactionpotentialswhenobjectsmoveasmalldis-tanceduringtheearlystagesoflifting.InresponsetoFigure1:Receptorsinthehu-manskin:Mechanoreceptorscanbefreereceptorsorencapsulated.Examplesforfreereceptorsarethehairreceptorsattherootsofhairs.EncapsulatedreceptorsarethePaciniancorpusclesandthereceptorsintheglabrous(hair-less)skin:Meissnercorpuscles,RuffinicorpusclesandMerkel’sdisks.Hairy skinGlabrous skinEpidermisDermisPaciniancorpusclePapillary RidgesSeptaRuffini’s corpuscleHair receptorMeissner’scorpuscleSebaceousglandFree nerveendingMerkel’sreceptor1The following description is based on lecture notes from Laszlo Zaborszky, from Rutgers University.1Figure2:Mammalianmusclespindleshowingtypicalpositioninamuscle(left),neuronalcon-nectionsinspinalcord(middle)andexpandedschematic(right).Thespindleisastretchreceptorwithitsownmotorsupplycon-sistingofseveralintrafusalmus-clefibres.Thesensoryendingsofaprimary(groupIa)afferentandasecondary(groupII)afferentcoilaroundthenon-contractilecentralportionsoftheintrafusalfibres.rapidlyadaptingafferentactivity,muscleforceincreasesreflexivelyuntilthegrippedobjectnolongermoves.Sucharapidresponsetoatactilestimulusisaclearindicationoftheroleplayedbysomatosensoryneuronsinmotorac-tivity.TheslowlyadaptingMerkel’sreceptorsareresponsibleforformandtextureperception.Aswouldbeexpectedforreceptorsmediatingformperception,Merkel’sreceptorsarepresentathighdensityinthedigitsandaroundthemouth(50/mm²ofskinsurface),atlowerdensityinoth-erglabroussurfaces,andatverylowdensityinhairyskin.Thisinnervationsdensityshrinksprogressivelywiththepassageoftimesothatbytheageof50,thedensityinhu-mandigitsisreducedto10/mm².Unlikerapidlyadaptingaxons,slowlyadaptingfibersrespondnotonlytotheini-tialindentationofskin,butalsotosustainedindentationup to several seconds in duration.ActivationoftherapidlyadaptingPaciniancorpusclesgivesafeelingofvibration,whiletheslowlyadaptingRuffinicorpusclesrespondtothelataralmovementorstretching of skin.NociceptorsNociceptorshavefreenerveendings.Functionally,skinnociceptorsareeitherhigh-thresholdmechanoreceptorsFrom Wikibooks2Rapidly adaptingSlowly adaptingSurfacereceptor/small receptivefieldHair receptor,Meissner’s corpuscle: De-tect an insect or a very fine vibration.Used for recognizing texture.Merkel’s receptor: Used for spa-tial details, e.g. a round surfaceedge or “an X” in brail.Deep receptor /large receptivefieldPacinian corpuscle: “A diffuse vibra-tion” e.g. tapping with a pencil.Ruffini’s corpuscle: “A skinstretch”. Used for joint positionin fingers.Table 1Notice how figure captions andsidenotes are shown in the outsidemargin (on the left or right, dependingon whether the page is left or right).Also, figures are floated to the top/bottom of the page. Wide content, likethe table and Figure 3, intrude into theoutside margins.orpolymodalreceptors.Polymodalreceptorsrespondnotonlytointensemechanicalstimuli,butalsotoheatandtonoxiouschemicals.Thesereceptorsrespondtominutepuncturesoftheepithelium,witharesponsemagnitudethatdependsonthedegreeoftissuedeformation.Theyal-sorespondtotemperaturesintherangeof40–60°C,andchangetheirresponseratesasalinearfunctionofwarm-ing(incontrastwiththesaturatingresponsesdisplayedbynon-noxious thermoreceptors at high temperatures).Painsignalscanbeseparatedintoindividualcompo-nents,correspondingtodifferenttypesofnervefibersusedfortransmittingthesesignals.Therapidlytransmit-tedsignal,whichoftenhashighspatialresolution,iscalledfirstpainorcutaneousprickingpain.Itiswelllocal-izedandeasilytolerated.Themuchslower,highlyaffec-tivecomponentiscalledsecondpainorburningpain;itispoorlylocalizedandpoorlytolerated.Thethirdordeeppain,arisingfromviscera,musculatureandjoints,isalsopoorlylocalized,canbechronicandisoftenassociatedwith referred pain.Muscle SpindlesScatteredthroughoutvirtuallyeverystriatedmuscleinthebodyarelong,thin,stretchreceptorscalledmusclespin-dles.Theyarequitesimpleinprinciple,consistingofafewsmallmusclefiberswithacapsulesurroundingthemiddlethirdofthefibers.Thesefibersarecalledintrafusalfibers,incontrasttotheordinaryextrafusalfibers.Theendsoftheintrafusalfibersareattachedtoextrafusalfibers,sowhen-everthemuscleisstretched,theintrafusalfibersarealsoAnatomy of the Somatosensory System3ForcecontrolsignalDrivingsignalLengthcontrolsignalLoadExternalforcesTendonorgansMuscle forceMusclelengthForce feedbackLength &velocityfeedbackForce (Golgi tendon organ)SpindlesGamma biasLength (secondary muscle-spindel afferents)Length error (primary muscle-spindel afferents)Velocity (primary muscle-spindel afferents)MuscleInter-neuronsFigure3:Feedbackloopsforproprioceptivesignalsfortheperceptionandcontroloflimbmove-ments. Arrows indicate excitatory connections; filled circles inhibitory connections.For more examples of how to useHTML and CSS for paper-basedpublishing, seecss4.pub.stretched.Thecentralregionofeachintrafusalfiberhasfewmyofilamentsandisnon-contractile,butitdoeshaveoneormoresensoryendingsappliedtoit.Whenthemus-cleisstretched,thecentralpartoftheintrafusalfiberisstretched and each sensory ending fires impulses.Musclespindlesalsoreceiveamotorinnervation.Thelargemotorneuronsthatsupplyextrafusalmusclefibersarecalledalphamotorneurons,whilethesmalleronessup-plyingthecontractileportionsofintrafusalfibersarecalledgammaneurons.Gammamotorneuronscanregu-latethesensitivityofthemusclespindlesothatthissensi-tivity can be maintained at any given muscle length.Joint receptorsThejointreceptorsarelow-thresholdmechanoreceptorsandhavebeendividedintofourgroups.Theysignaldiffer-entcharacteristicsofjointfunction(position,movements,directionandspeedofmovements).Thefreereceptorsortype 4 joint receptors are nociceptors.From Wikibooks4