Lecture 14: Action Potentials Part II

Action potentials arise from changes in what?

changes in Na+ and K+ conductances

Briefly describe the current during hyperpolarization.

• transient capacitative current

• no ionic current

Briefly describe the current during depolarization.

• transient capacitative current

• large, time-dependent ionic current

• inward, then outward current

What does tetraethylammonium do?

blocks K+ current—revealing the Na+ current

What does tetrodotoxin and saxitoxin do?

blocks Na+ current—revealing the K+ current

Where does TTX come from?

the pufferfish

Where does STX come from?

“red tide” dinoflagellates

True or False: Toxins block channel pores and thus ion flow.

true

Briefly describe the voltage-dependence of Na+ currents.

• biphasic time course

• before the peak: activation

• after the peak: inactivation (channel is impermeable)

Briefly describe the voltage-dependnece of K+ currents.

• outward only

• activates with a delay

• inactivation can be absent or present

What are some assumptions about the macroscopic Na+/K+ I-V curves?

• idealized channels, always assumed to be fully open, irrespective of V_m

• same unitary conductance (gamma)

What are the equations for I_x?

• I_x = G_x (V_m - E_x)

• I_x = n x i_x x P_o

What is the equation for i_x?

i_x = gamma_x (V_m - E_x)

What does a P_o of 0.5 mean?

half of the channels are open and half are closed, and the curves are the steepest

What does P_O depend on?

voltage (V_m)

True or False: The I-V curve for K+ channels is shifted to the right.

true

Does the number of channels change with V_m?

no, it does not

Does the P_o depend on V_m?

yes, for voltage-gated channels

Briefly describe the absolute refractory period.

a second response is not possible regardless of strength or duation of the stimulus

Briefly describe the relative refractory period.

a second response can be elicited but requires a stronger stimulus

Regarding the Na+ inactivation parameter, what does a “high h” mean?

low probability of inactivation

What is the absolute refractory period due to?

Na+ channel inactivation

What is the relative refractory period due to?

delayed K+ channel opening

What does a slower rate of Na+-channel inactivation lead to?

a longer AP

What does delayed K+ channel opening lead to?

increased outward K+ conductance hyperpolarizes V_m, a larger stimulus is required to reach threshold again

True or False: Action potentials are actively propagated down a membrane (axon or muscle fiber) with no change in amplitude or shape. Progressive regeneration of AP response by voltage-gated channels.

true

True or False: V_m depolarizations cross threshold and open Na_v channels in adjacent region. The AP is regenerated and moves down the membrane.

true

What are the Nodes of Ranvier?

sites of high density voltage-gated Na+ channels

What does myelin provide for the axon?

insulaton

What does myelin prevent?

myelin prevents leakage of current across parts of the membrane that are covered by the sheath

True or False: High membrane resistance in internode region forces current to jump from node to node.

true

What does the length constant determine?

conductivity

What is conductivity?

how far the electronic spread of a local change in V_m can extend

What is the equation for the length constant?

lambda = [(aR_m) / (2R_i)]^1/2

What does V equal?

V = V₀e^(-x/lambda)

What is conductivity proportional to?

the length constant

Large diameter axons have a ____ radius and ____ internal resistance of axoplasm.

large, low

True or False: Current will follow the path of least resistance.

true

Does low internal resistance lead to high or low conductivity?

high conductivity

Do smaller or larger diameters have low conductivity?

smaller diameters have low conductivity

What does myelination increase?

R_m

What does R_m increase?

conductivity

What does R_m decrease?

the capacitance of an axon

What does capacitance decrease? What does this produce?

decreases time constant (tau)—producing faster conduction velocity