Action Potential | Part 1/3

#actionpotential

Action Potential | Part 1/3

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▬▬▬▬▬▬▬▬▬▬ Contents of this video ▬▬▬▬▬▬▬▬▬▬

(0:00-4:08)
Action Potentials are electrochemical changes occurring in cell membranes of excitable cells which rapidly propagate; Signaling mechanism of neurons and other cells.
Mechanical stimulus is translated into action potentials in neurons; ultimately carrying the signal to CNS.
Sensory neuron; axon connecting the skin sensors with the cell body of neuron.

(4:10-9:05)
Resting Membrane Potential (RMP); Na-K ATPases; pushing 3Na+ out and taking 2K+ in. In resting state, membrane is permeable to K+ but not Na+.
K+ diffuses out through K leaky channels trying to achieve the RMP = -90mV to -70mV.
Equilibrium potential for K+ = -85mV. RMP is ideally a little less negative than K+'s RMP.
Na+ channel is now in its "Resting State".

(9:09-17:08)
Sub-threshold Stimulus: Stimulation of neuron; Touch sensitive/Mechanically operated Na+ channels: Distortion with touch stimulus will open these channels and Na+ will start to trickle in.
Very low inward moving current of cations (Na+); Membrane potential will become slightly less negative BUT does not reach the threshold for opening of Voltage-gated Na+ channels; hence these will remain closed for now.
Ultimately this potential difference will die out; since K+ moving out will bring the potential back.

(17:10-23:30)
Threshold Stimulus: Opening of Threshold Voltage-sensitive Na+ channels; at -60mV activation gates of millions of Na-channels flip open; Na+ rushes in; membrane suddenly becoming very permeable to Na+ at the site of stimulus.
Inner electronegativity of membrane is rapidly neutralized; crosses 0 and rapidly moves towards Na+ equilibrium potential(+65mV).
Na+ Channel has now reached its "Active State".

(23:33-31:25)
Closure of Inactivation Gate: Activation channels were triggered and opened rapidly to reach Na+ equilibrium potential of +65mV but before they do, Inactivation Gates begin to close down. [When one gate opens the other closes]; however, the activation gate opens a little faster than the inactivation gate closes.
As soon as the Inactivation Gate closes, the channel is said to be in "Inactive State", and it will remain "trapped" in this state until the membrane repolarizes again.
The threshold potential has not been reached yet and Na+ channels are closed/Inactive.

(31:26-40:41)
Opening of Voltage-Sensitive/Gated K+ Channels: Open around at the time when Na+ channels have just closed. So, the membrane which has just become impermeable to Na+ suddenly becomes "permeable" to K+.
K+ rushes out along the concentration gradient (K+ efflux), leaving the inner side of the membrane more and more electronegative; back towards the RMP; this is REpolarisation.
Membrane losing K+ in an attempt to reach the K+ equilibrium potential but due to high K+ permeability, it overshoots the K+ equilibrium potential and is thus hyperpolarized.
Closing of Voltage-Gated K+ channels, ultimately reduces the K+ permeability to the level maintained by K+ leaky channels, thus bringing the potential back up to RMP.
Recap.

(40:45-52:32)
Propagation of local Depolarizing current to adjacent parts of the membrane.
A wave of depolarization followed by a wave of repolarization sweeps over the membrane; Na+ influx followed by K+ efflux.

(52:34-57:50)
Role of Na+K+ATPases in restoring chemical imbalances created by a propagating wave of action potential.
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