12. The Action Potential Part 4: General Properties of Action Potentials

What determines the threshold of an action potential?

• Requires a large enough depolarization (ex: -40 mV).

• Depends on voltage sensitivity of ion channels

• Occurs when Na+ permeability > K+ permeability

• Threshold = point where enough voltage-gated Na+ channels open to counter K+ efflux through leak channels

How do we know the ionic conductances that occur during an action potential?

• Determined using voltage clamp experiments

• Measure macroscopic ionic currents

• Use impermeant ions in solution to isolate specific ion effects

• Can also apply ion channel blockers (e.g., TTX for Na+, TEA for K+)

What other questions are important about action potentials?

• What factors affect AP propagation?

• Are there different types of APs (e.g., skeletal vs cardiac)?

How is a toilet flush similar to an action potential?

• Threshold: minimum force (depolarization) required to trigger flush (AP)

• All-or-none: pushing harder doesn’t make a bigger flush (AP size fixed)

• Refractory period: can’t flush again until tank refills (Na+ channels recover)

• Direction: water flows one way; AP travels in one direction

How is the threshold for an action potential determined experimentally?

• Inject current and observe membrane potential change

• Only when Vm reaches threshold → AP occurs

• Example:

  • Stimulus 1 & 2 = subthreshold (no AP)

  • Stimulus 3 = threshold reached → AP produced

  • Near-threshold stimuli may cause aborted or local responses

    • **Note: For this class, if it reaches threshold consider it AP

How does signal summation affect reaching threshold?

• Neurons receive many inputs (excitatory & inhibitory)

• Summation at axon hillock determines AP generation

  • No summation: subthreshold signal, no AP

  • Temporal summation: repeated graded potentials over time adds up

  • Spatial summation: simultaneous graded potentials from multiple neurons add up

  • Integration: combines excitatory (+) and inhibitory (–) signals

What does the all-or-none nature of action potentials mean?

• Subthreshold currents: no AP generated

• Once threshold is reached, AP always occurs

• Size and shape of AP remain the same, regardless of stimulus size (as long as above threshold)

• Difference is in speed of AP

What happens if stimulus amplitude increases beyond threshold?

• AP size stays constant (all-or-none)

• But frequency of APs increases with stimulus strength

• Neurons can fire up to 200–300 APs per second (Hz)

What is the refractory period, and why does it matter?

• Refractory period: Right after an AP, a second AP cannot of the same size occur, no matter how strong the stimulus.

  • Lasts for a short recovery time (~10 ms in squid axons).

• Reason: Voltage-gated Na⁺ channels are inactivated and cannot reopen immediately.

• Partial or “early” APs: During recovery, small or incomplete APs may appear.

  • These are due to some Na⁺ channels starting to recover from inactivation, but not enough to generate a full AP.

What are the absolute and relative refractory periods?

• Absolute refractory period (1–2 ms):

  • No new AP possible

  • Na+ channels are inactivated

• Relative refractory period (3–4 ms):

  • Stronger stimulus can trigger smaller amplitude AP

  • Na+ channels recovering; K+ permeability still high

  • Duration varies by neuron and hyperpolarization depth

Why is an absolute refractory period necessary?

• Ensures discrete, separate APs (prevents overlap)

• Controls maximum firing rate of neuron

• Enables one-way propagation of APs along the axon

How does an action potential propagate down an axon (regenerative)?

• Voltage-gated ion channels along the axon allow AP to regenerate

• Each new section of membrane depolarizes the next

• Unidirectional movement due to Na+ channel inactivation behind the AP

What affects conduction velocity of an action potential?

• Conduction velocity = time for signal to travel neuron length

• Larger axon diameter → less internal resistance → faster conduction (volume > SA = current flows more easily)

• Explains why squid giant axon is so large (rapid escape response)

How does myelination affect conduction velocity?

• Myelin insulates axon → reduces current leak

• Allows saltatory conduction: AP jumps between Nodes of Ranvier

• Increases speed and efficiency of AP propagation

How do myelinated and unmyelinated axons differ in conduction speed?

• Myelinated axons: faster conduction (less leak, saltatory jumping)

• Unmyelinated axons: slower, continuous conduction along membrane

• Axons evolved to increase conduction velocities by 

  • (1) Insulating

  • (2) Allowing diffusion without electrostatic decay instead of relying on the slower speeds of opening/closing dynamics

How does conduction velocity differ among organisms?

• Varies by species and axon structure

• Larger diameter or more myelinated neurons → faster signals

• Adapted to the organism’s needs and environment (e.g., escape response)

How does a skeletal muscle action potential differ from a neuron’s?

• Similar in mechanism (voltage-gated Na+ and K+ channels)

• Directly triggers muscle contraction

• AP is shorter than contraction

• AP acts as electrical “trigger” for muscle movement

• Ex: High frequency APs → sustained tension (seen in tetanus/muscle spasms)

How is the cardiac action potential different from skeletal muscle?

• Depolarization: voltage-gated Na+ and Ca2+ channels open (inward flow. Na+ channels quickly inactivate)

• Plateau phase: Ca2+ influx balanced by K+ efflux (maintains depolarization)

• Repolarization: Ca2+ influx decreases, K+ efflux increases

• Creates longer AP duration than skeletal muscle

How is the cardiac action potential linked to muscle contraction?

• Cardiac AP matches the duration of contraction

• Due to sustained Ca2+ influx during the plateau phase

• AP duration influences strength and timing of cardiac muscle contraction

• Prevents tetanus (continuous contraction) → essential for heart function