Lecture 4: Action Potentials

Question 1

Characteristics of action potentials

Answer 1

1. All or nothing: fixed amplitude
2. Regenerative: once started goes through full cycle
3. Time limited: fixed duration
4. Charge-only movement: equilibrium potentials and ion concentrations are relatively unchanged.

Question 2

How are timing and strength of a stimulus encoded into APs?

Answer 2

Timing is encoded by the timing of the APs. Strength is encoded by the frequency.

Question 3

What affects conductance speed of an AP?

Answer 3

Longer length constant λ = faster conductance. Membrane and internal resistances determine how much current moves along the axon and how much leaks out.

Question 4

How does multiple sclerosis impact the body?

Answer 4

MS is an autoimmune demyelinating disease, reducing membrane resistance and decreasing conductance speed.

Question 5

Myelin

Answer 5

Fatty substance that insulates neurons. Results in saltatory conduction between Nodes of Ranvier.

Question 6

How does myelin affect neurons?

Answer 6

Increases speed of conduction by increasing membrane resistance, reduces metabolic cost by requiring less ion movement to propagate an AP, saves room by enabling the body to use thinner axons.

Question 7

Action potential steps

Answer 7

1. Maintained resting membrane potential
2. Depolarizing stimulus brings local membrane to threshold
3. Voltage gated Na+ channels open causing Na+ influx and rapid depolarization
4. Na+ inactivation gate + slower K+ channel opening rapidly stops depolarization
5. Outward current from K+ leak channels repolarizes membrane
6. Slow-closing K+ gated channels cause hyperpolarization as Na+ channels go from inactivated to closed
7. Closure of gated K+ channels allows membrane to return to resting potential.

Question 8

Refractory periods

Answer 8

Refractory periods keep APs unidirectional and distinct. Absolute is when the Na+ channels are inactivated (no depolarization can reopen them). Relative is during hyperpolarization/as Na+ channels transition to closed stochastically (harder to trigger new AP)

Question 9

How does local anesthesia affect neurons?

Answer 9

Local anesthetics like procaine and lidocaine block voltage-gated Na+ channels to prevent AP firing.