Graded local potentials Proportional to the stimulus size Act locally starting from the stimulus site Attenuate with distance Spread in both directions Take place in many types of cells 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 Learning objectives To contrast between local and action potentials To explain the uni directionality of action potential transport To predict the propagation of the action potential in experimental conditions To contrast between propagation in unmyelinated and myelinated axons Action potential can be generated and propagated ONLY in Neurons only at the axon Muscles Why only there Function follows 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 Activation gate closed Inactivation gate closed Outside Activation gate Inside Inactivation gate Both gates open Potassium channel The key players Na K At a resting state Na activation gate closed Na inactivation gate open K gate closed Upon depolarization and ONLY after reaching a threshold Gates change their status Na activation a fast gate Na inactivation and K slow gates a 0 5 ms delay 2 Na act gates 3 Na inact gates K Stimulus Na 3 2 1 4 K 1 Channels are inactivation Na gates are Resting membrane potential 4 Na act K And all of this 3ms How would this graph look like if we added a drug that blocks the potassium voltage gated channels Draw your graph on a notecard Poll If we inhibit the K voltage gated channels the action potential in a typical neuron will reach its peak at 70mV a 55mV b c 30mV d 65mV Poll Local potential will spread but will die out a True b False 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 Remember the graded potential Once Na ions entered the cell they spread locally and depolarize the nearby regions Graded potentials die ou t LC Action potentials do not die or attenuate as they progress Why 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 If the ions can spread to both sides why does AP move only in one direction Let s start with the trigger zone Refractory periods refractory period Na channels have not yet reset to their resting positions A new AP can NOT be generated refractory period Some Na channels have reset to their resting positions Propagation of the action potential Learning objectives To contrast between local and action potentials To explain the uni directionality of action potential transport To predict the propagation of the action potential in experimental conditions To contrast between propagation in unmyelinated and myelinated axons Poll 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 Size Does Matter The larger the diameter the lower the resistance the faster the impulse Learning objectives To contrast between local and action potentials To explain the uni directionality of action potential transport To predict the propagation of the action potential in experimental conditions To contrast between propagation in unmyelinated and myelinated axons