To receive information about the world, the CNS contains a wide variety of sensory receptors, each of which is specialized •Fig. 5.11 Comparison of Action Potential Conduction in an Unmyelinated Axon and in a Myelinated Axon. Attheinitialtime(A andC),anactionpotentialisbeinggeneratedattheleftsideofeachaxon.Notethattheinwardcurrentintheunmyelinatedaxon(A) isdepolarizinganadjacentportion,whereastheinwardcurrentinthemyelinatedaxon(C) isdepolarizingallofthemembranetothenextnode.Atthesecondinstantintime(B andD),theactionpotentialintheunmyelinatedaxon(B) hasbeengeneratedintheadjacentportion,whereastheactionpotentialinthemyelinatedaxon(D) hasbeengeneratedatsubsequentnodesandisalreadydepolarizingthelastnodetotheright.(RedrawnfromCastroA,etal:Neuroscience: An Outline Approach. Philadelphia:Mosby;2002.)
Insomediseasesknownasdemyelinating disorders, themyelinsheathdeteriorates.Inmultiple sclerosis, scatteredprogressivedemyelinationofaxonsintheCNSresultsinlossofmotorcontrolandsensorydeficits.Theneuropathycommoninseverecasesofdiabetesmellitusiscausedbythedemyelinationofperipheralaxons.Whenmyelinislost,thelengthconstantbecomesmuchshorter.Hence,theactionpotentiallosesamplitudeasitiselectrotonicallyconductedfromonenodeofRanviertothenext.Ifdemyelinationissufficientlysevere,theactionpotentialmayarriveatthenextnodeofRanvierwithinsufficientstrengthtofireanactionpotentialatthatnode,leadingtopropagationfailure.
to detect a particular type of energy (stimulus). When a stimulus activates a sensory receptor, it initiates a process called sensory transduction by which information about the stimulus (e.g., its intensity and duration) is converted into local electrical signals. These local signals are called
TheactionpotentialsofmyelinatedaxonsmaynothaveahyperpolarizingafterpotentialoranextendedrelativerefractoryperiodbecausetheirK+ channelsaredisplacedfromthenodesintothepartlyexposedflankingparanodes.Thatincreasestherateatwhichthesefast-conductingaxonscanfire.Myelinatedaxonsarealsomoremetabolicallyefficientthanunmyelinatedaxons.Na+,K+-ATPaseextrudestheNa+ thatentersthecellandcausestheK+ thatleavesthecelltoreaccumulateduringactionpotentials.Inmyelinatedaxons,ioniccurrentsarerestrictedtothesmallfractionofthemembranesurfaceatthenodesofRanvier.Forthisreason,farfewerionstraverseaunitlengthoffibermembrane,andmuchlessionpumping—andenergyexpenditure—isnecessarytomaintainthegradients.