BISC 421 1st Edition Lecture 5 Outline of Current LectureI. Finish up The Membrane and Action Potentials II. Ion Channels Current LectureThe Membrane and Action Potentials - continuationMathematical reconstruction of the action potential •Representation of Na+ and K+ currents •Hyperpolarization exists because the voltage gated K+ channels are still open-‐ delayed closing •Action potential made up of two diferent types of currents •Na+ inward current to depolarize the cell •K+ outward current repolarizes the cell‐Coordinated opening and closing of Na+ and K+ channels In the resting state, most Na+channels areclosed. Non-gated K+ channels are open. Negative-inside. If Na+ channels open, the resulting influx of Na+ overwhelms the efflux of K+. Net inward movement of (+)-charges. Now the cytosolic face of the membrane has a net (+)-charge.Depolarized.MovedtowardsENa. At peak, gated- K+ channels openand causes an efflux of (+)-charge. Na+ channels begin to close due to their intrinsic properties.Axonrepolarizes.•Na+ channels open faster than the K+ channels•Moving points along the length of the axon•Cause initial depolarization with stimulationif signifcant enough to cross a threshold we will activate Na+ voltage gated channelsinflux of positive charge into the cellpassive difusion of Na+ down the axonas they spread out they cause adjacent channels to become activated and allow more influx of Na+ in•At same time the slower K+ channels will open and allow the positive charge to flow back outPropagation of an action potential•Once an action potential starts, it will go through the axon without stopping-‐ all or nothing•Green line is resting potential-‐ most cells have a threshold around -‐50mV and if this threshold is reached an action potential will occur•If you could stick electrodes along the length of the axon, the action potentialwill be the same-‐ meaning that the information doesn’t change as the length changes•Want to make sure the action potential propagates in one directionRefractory Period•Refractory period is the time is takes to fre another action potential•Because of the hyperpolarization, we cannot fre another action potential right afer starting the frst one. There is a waiting period•Prevents the AP from going backwards•Essentially the Na+ channels have an inactivation period•Refractory periods can be diferent between neuronsRegenerative nature of the AP•All or none feature of the action potential is called the regenerative nature because it generates these APs in a cycle•The fast cycle of Na+ is followed by the slow cycle of K+•Act like a yin/yang sort of systemPassive current fow in an axon Voltage dissipatesbecause:1. Thecytoplasmhas highresistanceand is a poorconductor.2. The membrane is not totally impermeable and some charge (ions) is lost due to leak.The length constant !"#$#%&'()*+#)(#),-#•How diferent neurons have diferent conduction velocities•Why don’t all neurons fre at the same speed? Because diferent speeds provide us more information•Myelination determines how fast the AP will go•There is also passive current flow in the axon-‐ if we have electrodes along the length of the axon and stimulate it but not to threshold then the stimulation will die off by the end due to this passive difusion of Na+•The cytoplasm is not an empty vacuum so the Na+ will have to work its way through-‐ the resistance of the cytoplasm is relatively high and therefore the Na+ will not be able flow as well•The duration of the current depends on the length•Initially have a voltage change but over distance this change will slowly decline because of passive difusion•Measure of how far along the axon we can get a change in membrane potential•The larger the length constant the more passive charge flow•If we have a large length constant that means that the initial stimulation was more eficient (will be able to go longer)•For efcient conduction, want a large length constant•The membrane is a capacitor that separates and holds charge•There is a time that is needed to charge the membrane•Depends on the membrane resistance and capacitance (the more resistance or capacitance of the membrane the larger the time constant)•So for a rapidly fring neuron we want a small time constantTime is needed to charge the membraneHow does an axon conduct the AP faster•Want large length constant and small time constant-‐ do this by increasing the diameter ofthe axon•Smaller diameters are not as fast at conducting•Why does changing the diameter help? Because this lowers the resistance•The other way to make things go faster is by increasing the resistance of the membrane-‐ do this by myelination so charge wont flow outMyelinated neurons conduct faster•Makes neurons much more energy eficient•Ions wont flow out so charge won't be lost•Allow ions to come in through the nodes of ranvier-‐ areas of the axon that are notcovered by a myelin sheath•These nodes have lots of Na+ channels and K+ channels to allow charge in, this is more efcient because will not lose many of the ions but allow charge in•This charge will go further faster and the nodes are perfectly alignedSaltatory AP conduction along a myelinated axon•Faster conduction due to insulation and nodes•Needs fewer proteins to create AP-‐ more energy efcientIon Channels and TransportersIon Channelsy•Focus primarily on K+ and Na+ channels but there are other channels in the membrane (ones that are selective and non selective)•K+ and Na+ channels are electrically and chemically gated•They use difusion and concentration gradients to open and closeTypes of Voltage Gated Ion Channel•Plus sign is important-‐ reason they are voltage gated is because the channels are positively chargedTypes of Ligand-‐gated ion channels•There are also ligand gated channels-‐ ligand binds, changes structure and opens channel•There are channels that are activated by a variety of things-‐ number of diferent classes of channelsMeasuring Ion Channel Activity•Ability to measure channel activity•Can record from one little patch of membrane-‐ take glass electrode and make it into a fne tip and can make a contact with the membrane where you can form a tight seal (nothing will pass in between)•Can have two diferent types of recordingso 1. Cell recording-‐ diferences between the membrane potential