Action potential, Graded potential

 

GRADED POTENTIAL 

-resulting change in membrane potential is small 

-vary in size

-can be positive and negative

 

 ACTION POTENTIAL

—70mv~threshold voltage of -55mV triggers the action potential at the axon hillock, which then travels down the axon

 

 

 

 

 

 

1. Dendrites/cell body 에서 graded potential 접수 by ligand-gated/ mechanically-gated ion channel

2. Axon hilock 에 있는 voltage-gated channel 에서 Action potential 전달 to axon

ACTION POTENTIAL IS CAUSED BY VOLTAGE GATED CHANNEL

GRADED POTENTIAL PRECEDE ACTION POTENTIAL

Graded potentials must occur to depolarize the neuron to threshold before action potentials can occur.

 

 

Why does graded potential cannot generate action potential? (= cannot transmit along the axon far distance?)

-Leakage or loss of charge across the cell membrane

-cellular membrane will always default and go back to the RMP as the ions are free to move and diffuse across the plasma membrane. Also the cytoplasm of the cell provides resistance to electric charge flowing across the body of the cell. 

Thus graded potentials will lose their strength as they move some distance.

 

 

 

 

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*we have Three different types of Ion channels

 

1. Voltage-gated ion channel - only open when there's action potential occurs (reach the threshold)

2. Ligand-gated ion channel - open when they are bound by specific molecule

3. Mechanically-gated ion channel - open in response to physical forces such as changes in length or changes in pressure.

 

Each ion needs each channel

 

 

 

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*Graded potential 이 일어나서 membrane potential 이 증가했을때 (근데 threshold는 안넘었을때),

Na+k+atpase pump가 다시 resting membrane potential(-70mV) 로 돌려놓는다

 

*When the outside stimulus is large enough to bring the membrane potential in the neuron body up from -70mV, to the threshold voltage -55mV or higher, this triggers the action potential at the axon hilock, which then travels down to the axon 

 

 

 

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https://www.youtube.com/watch?v=KTTeD2AMiPA 

 

PROCESS WITH CHANNEL-with inactivation activation gates

1. resting potential -> CLOSED

 

 

 

2. membrane potential increased, reaches threshold -> voltage sensor remove the gate -> OPEN

-> Na+ influx by electrochemical gradient

 

 

 

3. When the membrane potential reaches +30mV -> INACTIVATED 

when the inactivated channel closed, it is called ABSOLUTE REFRACTORY PERIOD

-> Inactivation channel close and block voltage sensor so can not be stimulated

ABSOLUTE REFRACTORY PERIOD

 

 

 

 

 

3.  At the same time voltage gated K+ channel opens and K+ efflux by electrochemical gradient

-> increased positive outside of membrane stimulates the voltage sensor in Na+ channel and make it close (same as RMP state) And this is called relative refractory period

RELATIVE REFRACTORY PERIOD

During the relative refractory period, inactivation gate is opened so action potential can potentially occur

BUT larger than normal stimulus is needed to cause another action potential 

BECAUSE K+ channel is still opened and K+ is effluxing making membrane potential negative(lower than -70mV), so it is harder than usual to reach the threshold

 

 

 

 

 

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SODIUM (voltage-gated) CHANNEL 3STATES

AND ACTION POTENTIAL

-At rest : sodium channel closed

-Once the membrane reaches the theshold voltage : sodium channel open

-> influx of sodium ion because of the electrochemical gradient (depolarisation)

-> when membrane potential reaches 30mV (overshoot)

 

-As the membrane potential becomes 30mV : sodium channel inactivated

-> this stops sodium flows into the cell

-> inactivated channel blocks from other stimulus and this is absolute refractory period

-At the same time, this change in membrane potential opens K+ voltage gated channel

-> K+ efflux by the electrochemical gradient (repolarisation)

 

-> increased positive in outside of the membrane makes sodium channel closed (same shape as RMP) and this is relative refractory period

 

-Because the K+channel is slow in opening and closing, when it is closing, for a brief period, K+ still efflux

(hyperpolarisation) During this period, K+channel closed

-> Through out this period, Na+K+ atpase pump still working (3Na+out, 2K+in)

 

 

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ABSOLUTE REFRACTORY PERIOD

(Hit the threshold~depolarise~start to repolarise) 

(Membrane cannot be stimulated to go through another action potential event)

 

During DEPOLARISATION, the inactivated sodium channels won’t respond to any stimulus at all.

During this time, the neuron is in its absolute refractory period, the period of time when a nerve cannot fire another action potential, no matter how strongly it’s stimulated. 

The absolute refractory period prevents action potentials from happening again too quickly and prevents action potential from traveling backwards along the axon.

 

 

 

RELATIVE REFRACTORY PERIOD

(If we have above stimulus threshold, we could potentially elicit another action potential relative to the strength of the stimulus being supra-threshold(above threshold) where we can have another depolarisation and repolarisation phase)

 

During HYPERPOLARIZATION, the sodium channels are closed, and the inactivation gate opens. There’s no change in sodium flow but now they could be opened again. Because, while the sodium channels could open, it would take a larger than usual stimulus to reach threshold. Because the cell is hyperpolarised due to the potassium still leaving the cell

 

 

 

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FREQUENCY of the Action potential 

 

-Maximum voltage in  one neuron during an action potential, never changes.

-An action potential doesn’t get bigger with a bigger stimulus. ALL OR NOTHING

(It either happens, or it doesn’t happen.)

-What can change is the frequency of the action potential

 

 

 

SPEED OF TRANSMISSION

 

-myelin sheath -> saltatory conduction

:Action potential signal jumps to the axon covered by the sheath

(jump to node to node)

 

 

 

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neuron fires at the action potential