Neuro Lecture #3 PDF

Lecture 012125 Action Potential – The need for action potentials Reading: Ch 3 – Discovery of Mechanism – How does the AP propagate? – Passive Membrane Properties – Summation Action Potentials (APs) Why? To carry information rapidly over distance, in axons, and in muscle...

Lecture 012125 Action Potential – The need for action potentials Reading: Ch 3 – Discovery of Mechanism – How does the AP propagate? – Passive Membrane Properties – Summation Action Potentials (APs) Why? To carry information rapidly over distance, in axons, and in muscle Frequency coding - intensity of axonal AP frequency denotes the information Recruitment - number of axons firing AP denotes information Place Code - identity of axon carrying the AP denotes the information How? Voltage gated Na+, K+, Ca++ channels - rapid, positive or negative feedback - faithful propagation of AP over distance  Some neurons do not use action potentials electrically inexcitable graded potential is passive short distance to terminal NT secretion Example: retinal bipolar neuron Most neurons use action potentials Example: lower motor neuron of electrically excitable spinal cord action potential is regenerative long distance to terminal NT secretion  Action potentials define excitable cell activity Resting VM VM near EK+ ENa+ Rising Phase: Threshold VM Active process overshoots 0 mV VM approaches ENa+ Falling phase: Active process Undershoot: EK+ refractory / recovery VM approaches EK+ Absolute refractory period - no second AP possible. No recovery from inactivation Relative refractory period - second AP possible, as sodium channels recover.  VM typical term, for membrane potential Investigating the EION typical term, for equilibrium potential action potential VM (EM) =  {PION x EION} What factors promote change in VM during the action potential? What biological events could cause PION or EION to change? Why use a Squid Axon to figure this out?  Basis for Action Potential (1952) Nobel prize laureates - Hodgkin and Huxley first recording of action potential, squid giant axon measured ionic currents of action potential hypothesized voltage-dependent “gating particles” But Daddy I don’t I told you I don’t WANT to go to the WANT to play lab again today! catch, Hodgkin! Nobel nominee - Presenting the squid, Loligo pealii! Giant axon is used for rapid escape Squid giant axon preparation Silver wire inside axon Hodgkin and Huxley Curtis. and Cole First recording of VM - yes it is negative! But what got them the Nobel Prize…. stimulate axon, depolarize VM - active response (AP) AP is voltage-dependent conductance of membrane altered rapidly threshold depolarization needed for this event action potential overshoots 0 mV, undershoots resting VM action potential about 2 msec how does AP occur? But first………Let’s review the ionic basis of resting potential alter extracellular K+ – will change EK Membrane potential VM follows EK But: alter extracellular Na+ – will change ENa Membrane potential independent of ENa VM EK Is there are role for Na+ in electrical signaling? But of course…. Hodgkin – Huxley experiments with squid axon 1 = control; 6 = post control Ionic basis of action potential 2 - 5: lowering Of [Na+]OUT Sodium hypothesis: rising phase - overshoot approaches ENa alter extracellular Na – will change ENa no change in resting potential (previous slide) height of AP follows change in ENa H = Na+OFF, Mechanism M= Na+ON N = K+ON 1. First hypothesis. voltage dependent transport of Na+ (pump) (but…..time course of AP too rapid) 2. Second hypothesis. voltage dependent permeability to Na+ (postulate existence of “gating particles”) Gates: M = Na+ON, H = Na+OFF, N = K+ON  Action potential - voltage dependent positive feedback cycle 1. Depolarization of nerve cell membrane “Hodgkin cycle” 2. M gate opens - gNa increase - active depolarization 3. Result = further depolarization (1, 2, 1, 2…..) 3. H gate shuts - gNa decrease - active repolarization 2. 4. N gate opens - gK increase - active repolarization 1. VM = EM 4. PION = GION Ionic basis of action potential - gates are voltage sensors in ion channels KV channel NaV channel Sequence of sodium channel: S4 + + + + + Why don’t they (Na+, K+) cancel each other out? Ion channels of the action potential - genes cloned - look for gates Stimulus Single channel recordings Na+ and K+ channel genes cloned m h Voltage (patch) clamp identifies currents Search for m, h gates (Na+), n gate (K+) n Search for pore region, ion selection region Structure function studies - mutagenesis of interesting regions does mutation disrupt function? m first to respond, then h and n (different kinetics so Na and K don’t cancel)  domains “form” a pore Current view on channel structure and function 1. pore - common 6 segment domains in Na, K, Ca channels 2. voltage sensor - common S4 segments (+) in these channels 3. inactivation mechanism - differing cytoplasmic regions 4. ion selectivity filter - differing pore lining regions S4 segment voltage sensors - outward push with + VM opens pore S5-S6 linkers (P loops) fold into pore and select for ion Inactivation particle - cytoplasmic cork Voltage Sensors for each domain are on the periphery  Pore Voltage Sensing Ion Channel Module Module Structure 6 TM S5 – S6 S1 – S4 Segments OUT S4 S2 S3 S1 IN S1 to S3 negative charges stabilize S4 positive charges as S4 responds to alteration of membrane potential Na+ ions _ _ + + _ _ + + + + + + _ _ _ _ _ _ Electrotonic (passive) membrane properties - second look Length constant l - distance along axon, from which an initial voltage change declines to 1/e (37%). How far does dendritic or axonal depolarization affect VM? Same applies to voltage increase. Time constant t - Time it takes for an initial voltage change, to decline to 1/e (37%). Same applies to voltage increase. How long till inward current causes AP? Passive decay of depolarization along axon in space (left) or in time (right) Why the fuss about passive membrane properties?  I. Action potential propagation AP reproduced from axon hillock to terminal 1. passive membrane properties to allow spread of current from AP Length constant - passive decay of AP 2. voltage- dependent channels along axon at sites prior to the limit of l, where decay approaches threshold. 2 3 3. voltage- dependent channels inactivate and limit frequency of action 1 potentials to follow Voltage dependent channels and length constant l But: AP - depolarization of membrane at first site opening each channel in turn passive decay to next site (area of channels) is a relatively slow process. if above threshold, new AP passive decay to next site (area of channels) if above threshold, new AP  II. Action potential (conduction) velocity Myelin increases action potential conduction velocity - “saltatory conduction” Internode myelin Node Na+ channels K+ channels Myelin: high length constant keeps VM above threshold until next node high RM, low RA MS case study! Two ways to increase conduction velocity: l = sqrt (RM / RA) Each increases length constant l Increase RM Decrease RA OR myelin large diameter axons Node-specific channels Fast AP Slow AP Comparison of AP conduction velocity in non-myelinated versus myelinated axons small diameter versus large diameter axons  Electrotonic properties - third look, and thinking ahead about synapses….. Passive membrane properties and decision making - summation of inputs 1. Sign summation: EPSPs (+) and IPSPs (-): channels open to depolarize or hyperpolarize 2. Temporal summation: time constant t allows change in VM to persist in time 3. Spatial summation: length constant l allows change in VM to persist over distance Temporal summation: Spatial summation: Sign summation decay of signal before decay of signal before - excitatory or inhibitory? addition of next? reaching axon hillock? Summation is how neurons make decisions Review slide: Action Potential At rest K+ “leak” channels are open A few Na+ channels are open With depolarization of VM: First, Na+ channels open (m) Second, K+ channels open (n) Second, Na+ channels inactivate (h) Repolarization of VM: Na+ channels inactivated VM is brought all the way to EK (this called the undershoot) NaV NaV OFF KV Why? because there is essentially no gNa until the Na channels recover from inactivation KLeak Does it matter? But of course……