Human Body Function 102: Action Potential

Study Notes

Students should be able to discuss action potential and its ionic basis
Students should be able to explain excitability changes during the action potential
Students should be able to deduce the effect of changes in plasma ionic concentrations on the action potential

Diagram of a neuron with labeled parts: dendrite, nucleus, soma, axon, myelin, Schwann cell, node of Ranvier, axon terminal

Definition: The difference in potential between the outer and inner surface of the membrane of excitable tissues (nerves and muscles) under resting conditions
The inside of the membrane is negatively charged relative to the outside

Diagram illustrating the measurement setup: electrode outside, electrode inside, CRO (oscilloscope)

Passive Forces (93%):
- Selective permeability of the cell membrane: The membrane is impermeable to negatively charged proteins, organic phosphates, and sulfates present inside the cell.
- Permeability to potassium (K+) ions is 50-70 times greater than permeability to sodium (Na+) ions at rest. At rest, K+ concentration is higher inside the membrane than outside, and the opposite is true for Na+.
Active Force (7%):
- Na+-K+ Pump: This electrogenic pump (with a 3/2 coupling ratio) transfers more positive charges to the outside, aiding in maintaining the RMP.

The interactive question asks about the names and functions of labeled components in a diagram showing ion movement across a cell membrane.

Definition: A transient reversal in the membrane polarity of an excitable cell (nerve or muscle) in response to a threshold stimulus.
Diagram showing the different phases (depolarization, repolarization, hyperpolarization) of the action potential with associated voltage changes.

1. Latent period:
- The time interval after stimulus application until depolarization begins.
- The time the stimulus takes to travel along the axon.
2. Depolarization Phase:
- The membrane potential becomes more positive, reaching approximately +35 mV, as an initial slow depolarization is followed by a rapid depolarization triggered when the membrane potential reaches -55mV from a resting -70mV.
- Reversal of polarity occurs: The inner membrane surface becomes positive relative to the outer surface.
Ionic basis: Voltage-gated Na+ channels start to open at an increasing rate, and inward Na+ influx occurs, causing further depolarization through a positive feedback mechanism.
3. Repolarization Phase:
- The membrane potential returns to the resting level.
- Ionic basis: The voltage-gated Na+ channels become inactivated (h gate closes); K+ efflux through voltage-gated K+ channels (n gate opens) causes repolarization, and the net diffusion of positive charges out of the cell completes repolarization.

1. Absolute Refractory Period (ARP):
- The nerve is completely unresponsive to stimuli during depolarization and early repolarization
2. Relative Refractory Period (RRP):
- The nerve is partially responsive to stronger stimuli during the remaining part of repolarization.
3. Supranormal Period:
- The excitability of the nerve is greater than normal, and a weaker stimulus can excite the nerve.
4. Subnormal Period:
- Nerve excitability is below normal; stronger than normal stimulus is needed to excite the nerve.

Hypernatremia: No effect on RMP; facilitates depolarization
Hyponatremia: No effect on RMP; delays depolarization, decreases action potential magnitude
Hyperkalemia: Decreases RMP, initial increase then decrease in excitability due to Na+ channel closure, and slower repolarization
Hypokalemia: Decreases excitability of the nerve
Hypocalcemia: Increases excitability by increasing Na+ influx
Hypercalcemia: Decreases excitability and stabilizes the membrane

Tetrodotoxin (TTX): Blocks voltage-gated Na+ channels, preventing action potentials (m-gate closure).
Local anesthetics: Block voltage-gated Na+ channels (h-gate closure), are lipid-soluble and cross the membrane.