2.2 Action Potential

[ previous lecture ], [next lecture ]

신경세포: 정보를 받고, 전달, 비교, 타세포의 조절 기능한다,
신경세포의 활동시 action potentials이 나타난다
(brief positive-going changes in the membrane potential)

Time Courses of Action Potentials(APs)
Fig. 2.4: APs of varous mammalian tissues
- abrupt rise from the negative RP to a positive peak near +30 mV.
- return to RP level: at different rates
nerve: 1ms, skeletal muscle: 10 ms, cardiac muscle: more than 200 ms

Fig. 2.5: Several phases of the AP
1. upstroke(depolarization phase):0.2-0.5ms duration, positive-going change
2. overshoot: postivie part of the AP.
3. repolarization: restore to the RP level.
4. afterpotentials: muscle: depolarizing afterpotentials
nerve: hyperpolarizing afterpotential

The Origin of the Action Potential
Threshold and excitability
막이 RP로 부터 약 -50 mV까지 depolarize되면 AP이 발생한다.
threshold: the potenitial at which depolaization gives rise to an AP
threshold에서 exicitation이 일어남: reversal of polarity, upstroke, 1 ms
Excitable cell: AP을 일으키는 세포: 각 세포 마다 특징적
"all-or-none" law of excitation
Membrane conductance
AP 발생시 ion들의 막투과성에 변화가 생긴다.
Equilibrium potential에서 driving voltage와 net current는 zero이다.
Eion: refence potential, E: departure of the membrane potential
Iion: current, gion: membrane conductance
gion = Iion/(E-Eion)

Ionic currents during the action potential
RP: Ek와 거의 비슷
AP시 세포내가 더 positive 하게 되어진다는 것은 gNa가 증가 했음을 의미.
왜냐하면 Na+만이 ENa가 +60 mV를 나타냄으로.
(막밖의 Na+이 높으면 AP 생성, 막밖의 Na+가 부족하면 AP 생성 불가)
따라서 AP 생성에 있어서 excitation은 MP이 threshold level로
depolrize되면서 gNa가 증가함에 의한다.
AP시 gk 또한 증가한다. Tetraehylammonium (TEA) 투여하면
AP의 repolarization이 상당히 늦어진다.
AP시 Na+ inflow 후, K+ outflow가 있다.


Kinetics of Ionic Currents during Excitation
Membrane-current measurement
신경세포의 흥분--->여러 이온들의 conductance의 변화--->potential change
Votage clamp 실험 (Dr. Neher, 1978년)
Fig. 2.6.
MP을 stepwise로 변화시킨후 myelinated axon에서 membrane current 측정
RP: -95 mV로 유지
2.6A,C: 0 ms에 potential을 -60, -30, 0, +30, +60 mV로 변화시키면서 current 측정
Complex current (sum of Na+ & K+ current components)
-60 mV에서는 subthreshold: no appreciable changes in the ionic currents
voltage를 증가시키면서 negative에서 positive로 변화
2.6B: TEA (blocker of the potassium currents)하에서 sodium current를 측정
+60 mV에서 positive-going
qucickly reaches a maximum current, then returns to zero(inactivation)
Inactivation: small depolarization에서 slow, large depol--> fast
2.6D: tetrodotoxin (TTX, Na+ current blocker)하에서 K+ current의 변화를 측정
-positive over the whole range of test potentials
-K+ flows out of the nerve (EK 약 -100 mV)
-depolarization current에 비례해서 증가
-0.5 ms delay, reaches a plateau within 5ms, no inactivation

Na and K conductances during the action potential
reconstruction of the course of the AP from the measured voltage
dependences of the amplitudes and time courses of gNa and gK.
Fig. 2.7: gNa rises sharply at threshold, reaches maximum before the paek of AP
gK rising slowly with some delay after depolarization
reaches is maimum late, when repolarization is hlaf completed.
decline again
accelerates the second phase of the repolarization &
causes the AP to be followed by a hyperpolarizing afterpotnetial

Inactivation of the Na+ Current
AP후 RP에 되돌어 와도 Na+는 부분적으로만 활성화될 수 있다.
Fig. 2.8: Potential-dependence of sodium-system inactivation
X-axis: departure of the MP from the RP
Y-axis: maximal Na current
maximal Na+ current (INa max)를 일으키기 위해서는 axon membrane이
20-40 mV 정도 hyperpolarize 되어있어야 한다.
prolonged depolarization can prevent excitation
cells with a potential more positive than -60 to -50 mV become inexcitable.
Refractory periods.
Na+-system inactivation의 결과---> 막이 refractory
Fig. 2.9:
absolute refratory period: no AP generation, 약 1 ms duration
---> limits the maximal frequency (500-1000/sec)
relative refractory period: large depolarization에 의해 AP genration가능
several msec duration
Ionic Currents during the Afterpotentials
Brief hyperpolarizing afterpotential
-overshooting reporlarization
repolarizing phase가 RP에 이르렀을 때 gK 가 resting level에
못 미치며, gNa보다 상대적으로 높다.
따라서 MP이 EK에 더 가깝다.
Prolonged hyperpolarizing afterpotnetials
- summate when excitation is repeated at a high rate
- pronounced in the very thin nerve fibers of vertebrates
- produced by an electrogenic Na+ pump
Stabilization of the resting potential by [Ca2+]o
Fig. 2.10: Dependence of maximal Na+ permeability (PNa) on the magnitude of a
stepwise depolarization
As [Ca2+]o decreases, the threshold depolarization for eliciting
a rise in PNa is reduced---> lowering threshold for AP genration
(i.e., raising excitabiltiy)
Increase [Ca2+]o---> stabilize the membrane potential
at synapse: activity of a cell increase --->Ca2+ influx --->
reduce [Ca2+]o ---> more excitable<