Action Potential in Muscle and Nerve

Study Notes

Action potential is a rapid depolarization and repolarization of the cell membrane in excitable cells (nerve and muscle).
Depolarization: Membrane potential becomes less negative (cell interior becomes less negative).
Hyperpolarization: Membrane potential becomes more negative (cell interior becomes more negative).
Inward current: Flow of positive charge into the cell, depolarizes the membrane potential.
Outward current: Flow of positive charge out of the cell, hyperpolarizes the membrane potential.
Threshold: Membrane potential at which an action potential is inevitable. Net inward current exceeds outward current, causing self-sustaining depolarization.
All-or-none response: If net inward current is less than outward current, no action potential occurs.

Resting membrane potential is approximately -70 mV.
This is due to high resting conductance to potassium (K+), driving the membrane potential toward K+ equilibrium potential.
At rest, sodium (Na+) channels are closed, and Na+ conductance is low.
Depolarization: Inward current of Na+ depolarizes the membrane potential to threshold.
Opening voltage-gated Na+ channels causes rapid increase in Na+ conductance, leading to a positive feedback loop.
Membrane potential is driven towards Na+ equilibrium potential (+65 mV), but doesn't reach it due to short-lived increase in Na+ conductance.
Repolarization: Na+ channels close. K+ channels open slowly and increase K+ conductance to levels higher than at rest.
Increased K+ conductance results in an outward current causing membrane repolarization.
Undershoot (hyperpolarization): K+ conductance remains higher than at rest for a period after Na+ channel closure.
Membrane potential is driven close to K+ equilibrium potential.
Voltage-gated K+ channels cause the action potential to return to resting membrane potential. (negative feedback)

Absolute refractory period: Another action potential cannot be elicited regardless of stimulus strength, coinciding with depolarization and part of repolarization. Na+ channels are inactivated.
Relative refractory period: Action potential can be elicited with a larger than usual inward current (after absolute refractory period). K+ conductance is high; membrane potential is close to K+ equilibrium potential.

Action potentials propagate by local currents to adjacent areas of the membrane, depolarizing them to threshold and generating new action potentials.
Conduction velocity increases with: Larger fiber diameter (less resistance), myelination (insulation).
Myelinated nerves exhibit saltatory conduction (action potentials jump between nodes of Ranvier).
Action potentials in skeletal muscle share similarities with nerve action potentials, but with quantitative differences in timing and magnitude.