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 cells Action potential can be generated and propagated ONLY in Neurons only at the axon Muscles Why only there Function follows form Voltage gated channels are found mainly on the axon and the axon hillock Axon hillock trigger zone Axon Dendrites Cell body Sodium voltage gated channels have two gates Outside Activation gate Inside Activation gate closed Inactivation gate closed Inactivation gate Both gates open Potassium channel It looks a little more complex than this The key players K Na At a resting state Na activation gate closed Na inactivation gate open K gate closed Upon depolarization Gates change their status Na activation a fast gate Na inactivation and K slow gates a 0 5 ms delay 2 Na act gates open depolarization 3 Na inact gates close K open Repolarization 3 2 Stimulus Na 1 4 K 1 Channels are closed inactiv Na gates are open Resting membrane potential 4 Na act close K open Hyperpolarization And all of this 3ms The Na voltage gated channels provide a positive feedback cycle A positive feedback loop Once the potential reaches a threshold opening the Na activation gate it is an all or none process The action potential is not proportional to the size of the stimulus stimulus Threshold Action potential propagates along the axon Remember 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 proceed A single action potential does not actually move through the cell Formation of AP in one site generates AP in the next site and so forth Let s start with the trigger zone Refractory periods Absolute refractory period Na channels have not yet reset to their resting positions A new AP can NOT be generated Relative refractory period Some Na channels have reset to their resting positions Propagation 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 cells Myelin sheaths provide faster propagation Saltatory conduction Saltare to leap Nodes of Ranvier Saltatory conduction Conduction along myelinated axons Continuous conduction Conduction along unmyelinated axons Multiple sclerosis Jacqueline du Pr 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 43 What happened Size Does Matter The larger the diameter the lower the resistance the faster the impulse Summary The plasma membrane is polarized in a resting state Chemically gated channels generate local graded potentials Voltage gated channels allow generation of action potentials Action potentials propagate along the axons Conduction speed is dependent on size and degree of myelination