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An Action potential in axon
- -signal for sending information over distances along axons
- -all or none,peaks~40ml
- -doesn't diminish in size (different from EPSP in dendrite)
- -same duration,~1msec
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From 0mv to -80mv by opening K+ channels
- a all channels closed
- b K+ channels open Gk>0 vm negative
- c equlibrium potential vm =Ek=-80 no net K+ current
- d staring at vm=Ek=-80 gk>>gNa
- e Na+channels open gNa>>gK vm more postive (rising phase) close to Ena
- f Na+ channels close gK>>gNa (falling phase) k+ out
- g resting membrane restored gK>>gNa
- f
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Voltage gated sodium channels
- -one long polypeptide
- -fomer domains (I-IV)-form a pore
- -each domain has six transmembrance helices (s1-s6)
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Voltage sensor
s4 helic positively charged amino acids
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What happens when vm at -40mv or above ?
- -voltage sensor moves
- -chemical changes conformation
- -pore opens up
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Selectivity filter
- -partially hydrated Na+ions fit
- -patially hydrated K+ ion are too large
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Patch clamp technique
- -glass pipette forms a tight seal w/ membrane
- -used to record the current through single ion channels
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Patch clamp recording of voltage gated sodium channel which are?
- -depolarize membrane to -40mv
- -inward current
- -inactivate quickley, can't open again before reture to rmp
- -local anaesthetics blcok voltage gated Na+ channels
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steps of voltage gated sodium channels
- 1 at -65 mv channel closed
- 2 membrane voltage =-40mv channel opens
- 3 inactivation by gloloular portaion of proton (ball)
- 4 pore closes when voltage is -65mv ball is away
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Many different kinds of potassium channels
- 1 leak channels open all the time
- 2 voltage gated K+ channels(delayed rectifiers)
- -open w/ delay
- -outward current
- -contribute to falling phase of action potential (together w/ leak channels)
- different time scale then sodium
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How sodium and potassium channels produce an action potential?
- -Na+ channels open immediately when voltage reaches threshold
- -inward Na+ cuurrent brief b/c Na+ channels inativate
- -delayed recitfiers (voltage gated K+ channels open w/ delay
- -outward K+ current
- -brief inward current followed by delayed outward current
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Action potential conduction down axon
- positive charges spread causing membrane just ahead to depolarize and reach threshold
- action potential only propagates in one direction b/c retractory
- period-Na+ channels inactivated until membrane potential returns to rest
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myelin and nodes of ranvier
- positive charges spread futher w/ myclin
- voltage gated Na+ channels are only present in nodes of ranvier
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saltatory conduction
- Saltatory means "skipping", action potential skips from on node of ranvier to another
- -speeds up action potential conduction b/c it doesn't have to be regenerated as often
- -saves energy Na/K pump restores ion concentration gradients after each action potential
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