Lecture 4/5 - Graded Potentials & Action Potentials LEARNING OUTCOMES

Define depolarization, repolarization, and hyperpolarization.

• Depolarization: Membrane potential becomes less negative (toward 0 mV).

• Repolarization: Return of membrane potential to resting level after depolarization.

• Hyperpolarization: Membrane potential becomes more negative than resting.

Q: What are the characteristics of graded vs. action potentials?

A:

• Graded potentials: Variable amplitude, decremental, summative, no threshold.

• Action potentials: All-or-none, non-decremental, threshold-dependent, regenerative.

• Difference: Graded potentials are local and modifiable; action potentials are propagated spikes.

Q: What are decremental conduction and summation?

A:

• Decremental conduction: Graded potentials decrease in amplitude over distance.

• Summation: Multiple graded potentials combine temporally or spatially to influence depolarization.

Q: How are EPSPs, IPSPs, and EPPs generated?

A:

• EPSPs: Excitatory neurotransmitters open Na⁺ channels → depolarization.

• IPSPs: Inhibitory neurotransmitters open Cl⁻ or K⁺ channels → hyperpolarization.

• EPPs: ACh at neuromuscular junction opens nicotinic receptors → large graded depolarization.

Q: What is the action potential threshold and how is it generated?

A:
Threshold is the membrane potential where inward Na⁺ current exceeds outward K⁺ current, initiating a self-regenerating action potential via voltage-gated Na⁺ channels.

Q: What are the mechanisms of absolute and relative refractory periods?

A:

• Absolute: Na⁺ channels are inactivated and cannot reopen.

• Relative: Some Na⁺ channels reset; stronger stimulus needed due to ongoing K⁺ efflux and hyperpolarization.

Q: What ionic mechanisms underlie each action potential phase?

A:

• Depolarization: Voltage-gated Na⁺ channels open → Na⁺ influx.

• Repolarization: Na⁺ channels inactivate, voltage-gated K⁺ channels open → K⁺ efflux.

• Hyperpolarization: Prolonged K⁺ efflux before K⁺ channels close.

Flashcard 8
Q: How do local current flow and saltatory conduction support non-decremental conduction?

A:
Local current depolarizes adjacent membrane segments, regenerating APs. In saltatory conduction, APs jump between nodes of Ranvier, increasing speed and preserving amplitude.

Flashcard 9
Q: How does the Na⁺/K⁺ ATPase pump contribute to resting membrane potential?

A:
Directly by moving 3 Na⁺ out and 2 K⁺ in, generating a small net negative charge.
Indirectly by maintaining ion gradients that determine passive K⁺ diffusion, the main driver of resting potential.

Q: What factors determine conduction velocity in neurons?

A:

• Axon diameter: Larger diameter = faster conduction.

• Myelination: Increases speed via saltatory conduction.

• Temperature: Higher temperature enhances ion channel kinetics.