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UNC-Chapel Hill BIOL 252 - Nervous system fundamentals

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PowerPoint PresentationSlide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26•Action potentials do/are NOT- Proportional to the stimulus size- Act locally- Attenuate with distance- Spread in both directions- Take place in many types of cellsAction potential can be generated and propagated ONLY in:-Neurons (only at the axon) -Muscles Why only there?Function follows formAxonCell bodyDendritesAxon hillock (trigger zone)Voltage - gated channels are found mainly on the axon and the axon hillockBoth gates openActivation gate closedSodium voltage - gated channels have two gatesInactivation gate closedActivation gateInactivation gatePotassium channelOutsideInsideIt looks a little more complex than this:At a resting state-Na+ activation gate: closed-Na+ inactivation gate: open-K+ gate: closedUpon depolarization-Gates change their statusThe key playersK+Na+-Na+ activation: a fast gate-Na+ inactivation and K+ : slow gates (a 0.5 ms delay)1) Channels are closed (inactiv. Na gates are open)Resting membrane potential2) Na act. gates opendepolarization4) Na act. close, K openHyperpolarization3) Na inact. gates close, K open RepolarizationStimulusNaK(1)(2)(3)(4)And all of this: <3msThe Na+ voltage-gated channels provide a positive feedback cyclestimulus- Once the potential reaches a threshold (opening the Na+ activation gate), it is an all-or-none processThreshold- The action potential is not proportional to the size of the stimulusA positive feedback loopAction potential propagates along the axonRemember the graded potential- Once Na+ ions entered the cell, they spread locally and depolarize the nearby regions- Graded potential die out- When it comes to the axon, Na+ voltage-gated channels allow the action potential to proceedA single action potential does not actually move through the cellFormation of AP in one site, generates AP in the next site, and so forthLet’s start with the trigger zoneAbsolute refractory periodRelative refractory periodRefractory periodsNa+ channels have not yet reset to their resting positions.A new AP can NOT be generatedSome Na+ channels have reset to their resting positionsPropagation of the action potential•Action potentials do/are NOT- Proportional to the stimulus size- Act locally- Attenuate with distance- Spread in both directions- Take place in many types of cellsNodes of RanvierMyelin sheaths provide faster propagationSaltatory conduction(Saltare = “to leap”)Saltatory conductionConduction along myelinated axonsContinuous conductionConduction along unmyelinated axonsMultiple sclerosisJacqueline du Pré is considered to be one of the greatest cellist in our times. Her career was cut short by Multiple sclerosis, which forced her to cease performing at the age of 28, and led to her premature death at the age of 43What happened?Size Does Matter!The larger the diameter, the lower the resistance, the faster the impulseSummaryThe plasma membrane is polarized in a resting stateChemically-gated channels generate local, graded potentialsVoltage-gated channels allow generation of action potentialsAction potentials propagate along the axonsConduction speed is dependent on size and degree of myelination--------+ +++


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