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Chapter 2
Electrical signals of nerve cells
細胞內的電位記錄
玻璃電極,直徑小於 1 um
刺激電極
記錄電極
HyperpolarizationDepolarizationThreshold potential
離子的流動如何造成電訊號 ?
1. Create different ionic concentrations across cell membrane2. Membranes are selectively permeable to ions
Electrochemical equilibrium: electricity & concentration gradient
只對 K+ 通透的膜
濃度差十倍時 膜電位差為 58 的倍數
單一離子平衡電位的計算方式
• Nernst equation: EX = RT/zF ln [X]2/[X1]
• = 58/z log [X]2/[X1]
• z is the electrical charge
• EK = 58 log 1/10
• = -58 mV
• side 2 is a reference compartment, defined as 0 potential
問題
1. Side 1: [Na+] = 1 mM Side 2: [Na+] = 10 mM
2. Side 1: [Ca2+] = 1 mM Side 2: [Ca2+] = 10 mM
3. Side 1: [Cl-] = 10 mM Side 2: [Cl-] = 1 mM
4. Side 1: [K+] = 100 mM Side 2: [K+] = 1 mM
答案
1. ENa = 58/1*log 10/1 = 58 mV
2. ECa = 58/2*log 10/1 = 29 mV
3. ECl = 58/-1*log 1/10 = 58 mV
4. EK = 58/1*log 1/100 = -116 mV
膜電位對離子流動的方向及大小的影響 p42
• Connect a battery across the two sides to make side 1 more negative than side 2
• Side 1 = -58 mV, no net K+ ion flow
• Side 1 more negative than -58 mV, K+ ion will flow form side 2 to side 1
Electrochemical equilibrium in a multi-ion environment1. Permeability p432. Goldman equation
Intracellular recording replace cytoplasmlarge synapse
The ionic basis of the resting membrane potential
若休息膜電位
對
K+的通透性最好,則改變細胞外
K+濃
度將影響膜電位
The ionic basis of action potential
移除N
a+
同時影響了上升速度及強
度
Squid giant axonFrog motor neuron
axon Cell bodyCell body
Inferior olive Purkinje
Action potential form and nomenclature
Hodgkin & Katz 尚無法證實 membrane 是如何改變其 permeability 以 generate action potential
Chapter 3
Voltage-dependent membrane potential
Ionic currents across nerve cell membrane
• How the increase in Na+ permeability occurs?
• Voltage clamp method: Kenneth Cole, 1940s
1.2.
3.
4.
Alan Hodgkin & Andrew Huxley, 1940s當膜電位改變時,是否有離子進出細胞膜?
Capacitive current: voltage clamp 輸入的電流
Squid neuron: ENa = 55 mV
To test whether the earlycurrent is Na+ or not
Toxins that poison ion channels: tetrodotoxin Na+
tetraethylammonium ions K+
證明造成這兩種離子通透性改變的機制,
是互相獨立的。
Toxins that poison ion channels
減緩 Na+ channels inactivation
降低 threshold
Two voltage-dependent membrane conductances
• V = IR = I * 1/g = I/g
• Ix = gV = g (Vm-Ex)
• Conclusions:
• (1) both Na+ and K+ conductances are voltage-dependent Fig. 3.6
(2) the Na+ and K+ conductances change over time
Reconstruction of the action potential
Rate of depolarization falls:(1) electrochemical driving force of Na+ decreases(2) Na+ conductance inactivates(3) K+ conductance increase
Until Na+ conductance inactivation
Restore the membrane potential to the resting levelby inactivate K+ channels
Threshold: (1) the point at which the amount of heat supplied exogenously = the amount of heat that can be dissipated (2) Na+ inflow = K+ outflow
Long-distance signaling by means of action potentials
Passive current flow: current leak neurons are poor conductors than a wire
How do action potentials propagate along such a poor conductor?
(1) the amplitude of AP(2) the time delay of AP
Conduction velocity
Two types of voltage-dependent channels
Voltage-dependent channels close
Refractory period(1) the number of AP/ time(2) propagate back
AP propagation:(1) passive flow(2) active flow by voltage- dependent channels
Increased conduction velocity as a result of mylination
Improve passive flow, increase velocity:(1) increase diameter, decreases the internal resistance(2) increase insulation Mylination:
CNS oligodendrocytePNS Schwann cells
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