Nerve Ke
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- Pages: 41
Excitability Stimulus Mechanical
Thermal
Chemical
Electrical Anode
+ Nerve fiber
_Cathode
Rheobase Chronaxie
Utilization time
Inward rectifier K+ channel
Cl Na+ Cl-
Cl
-
Na+
K+
-
Na+ Cl-
Na Cl-
+
Na+
ClNa+ + + - Na Na Cl K+ ClNa+ ClNa+ Cl-
++++++++++++++++++++++++++++++++++++++++++ +
----------------------------------+ + + K K K K+ K+ + K - K+ K+ K+
P
P
P P P
P P P P
Equilibrium Potential (E): Nernst Equation: E=
+ 61 X log
EK+ = - 61 X log
Ion concentration inside Ion concentration outside [ K+ ] inside
= - 94 mV [ K+ ] outside
ENa = - 61 X log +
[ Na+ ] inside [ Na+ ] outside
= + 61 mV
Goldman Equation:
= - 86 mV. P= permeability C= concentration
Action Potential Potential changes in response to a threshold stimulus Very rapid (msec) Cathode Ray Oscilloscope (CRO)
Recorder (CRO)
Stimulator
Cathode Anode
-ve charge
Nerve fiber
Action potential
Recorder +35 (CRO)
Stimulus artifact
Stimulator
Latent period
-90 Time ( msec ) Cathode Anode
-ve charge
Nerve fiber
Action Potential +35
Reversal of polarity (Over shoot)
0 Depolarization
Na+ entry
-65
Repolarization
K+ exit
Firing level
-90 Hyperpolarization (Under shoot)
(Conductance)
Na+
Outside membrane
Na+-K+ pump Na+ voltagegated channel
K+
K+ voltagegated channel
Inside nerve fiber
Action potential
-65 -90
Local response Catelectrotonus Anelectrotonus
Absolute refractory period
Relative refractory period
Normal zero
Factors affecting membrane potential and excitability: Na+
Role of Na+
K+
Increased excitability
Increased permeability to Na+ e.g. low extracellular Ca++
Decreased Decreased permeability to Na excitability e.g. high extracellular Ca++ Membrane stabilizers +
local anesthetics, TTX
Hyponatremia
Decrease size of AP Little effect on RMP
Factors affecting membrane potential and excitability: Na+
Role of K+
K+
Increased extracellular K+
RMP
excitability
Decreased extracellular K+
RMP
excitability
Familial periodic paralysis
Blockade of K+ channels (TEA)
AP is prolonged No hyperpolarization
Factors affecting membrane potential and excitability: Na+
Role of Na+-K+
K+
Prolonged blockade can affect membrane potential
Accommodation of Nerve Fiber Mechanism: Enough time is available for: Inactivation Na+ gate closure K+ gated channel opening
While Na+ activation gate is still opening
+++++ + + - +++++++
- -- -- - - + - - -- -- -
Antidromic
Orthodromic
Biphasic Action Potential
-
+ + + + + + + + + + + + + + + + + + + + + + + + + + +
Thermal
Chemical
Electrical Anode
+ Nerve fiber
_Cathode
Rheobase Chronaxie
Utilization time
Inward rectifier K+ channel
Cl Na+ Cl-
Cl
-
Na+
K+
-
Na+ Cl-
Na Cl-
+
Na+
ClNa+ + + - Na Na Cl K+ ClNa+ ClNa+ Cl-
++++++++++++++++++++++++++++++++++++++++++ +
----------------------------------+ + + K K K K+ K+ + K - K+ K+ K+
P
P
P P P
P P P P
Equilibrium Potential (E): Nernst Equation: E=
+ 61 X log
EK+ = - 61 X log
Ion concentration inside Ion concentration outside [ K+ ] inside
= - 94 mV [ K+ ] outside
ENa = - 61 X log +
[ Na+ ] inside [ Na+ ] outside
= + 61 mV
Goldman Equation:
= - 86 mV. P= permeability C= concentration
Action Potential Potential changes in response to a threshold stimulus Very rapid (msec) Cathode Ray Oscilloscope (CRO)
Recorder (CRO)
Stimulator
Cathode Anode
-ve charge
Nerve fiber
Action potential
Recorder +35 (CRO)
Stimulus artifact
Stimulator
Latent period
-90 Time ( msec ) Cathode Anode
-ve charge
Nerve fiber
Action Potential +35
Reversal of polarity (Over shoot)
0 Depolarization
Na+ entry
-65
Repolarization
K+ exit
Firing level
-90 Hyperpolarization (Under shoot)
(Conductance)
Na+
Outside membrane
Na+-K+ pump Na+ voltagegated channel
K+
K+ voltagegated channel
Inside nerve fiber
Action potential
-65 -90
Local response Catelectrotonus Anelectrotonus
Absolute refractory period
Relative refractory period
Normal zero
Factors affecting membrane potential and excitability: Na+
Role of Na+
K+
Increased excitability
Increased permeability to Na+ e.g. low extracellular Ca++
Decreased Decreased permeability to Na excitability e.g. high extracellular Ca++ Membrane stabilizers +
local anesthetics, TTX
Hyponatremia
Decrease size of AP Little effect on RMP
Factors affecting membrane potential and excitability: Na+
Role of K+
K+
Increased extracellular K+
RMP
excitability
Decreased extracellular K+
RMP
excitability
Familial periodic paralysis
Blockade of K+ channels (TEA)
AP is prolonged No hyperpolarization
Factors affecting membrane potential and excitability: Na+
Role of Na+-K+
K+
Prolonged blockade can affect membrane potential
Accommodation of Nerve Fiber Mechanism: Enough time is available for: Inactivation Na+ gate closure K+ gated channel opening
While Na+ activation gate is still opening
+++++ + + - +++++++
- -- -- - - + - - -- -- -
Antidromic
Orthodromic
Biphasic Action Potential
-
+ + + + + + + + + + + + + + + + + + + + + + + + + + +
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