Lecture 14: Action Potentials Part 2

During hyperpolarization, what is the capacitative current and ionic current look like?

transient capacitative current, no ionic current

During depolarization, what is the capacitative current and ionic current look like?

transient capacitative current, large time-dependent ionic current (inward, then outward)

What does tetraethylammonium (TEA) do?

blocks K⁺ current, revealing the Na⁺ current

What does tetrodotoxin (TTX) do?

block Na⁺ current, revealing the K⁺ current

What is true of the total ionic current at a clamped membrane of -45 mV?

small inward current

What is true of the total ionic current at a clamped membrane of -30 mV?

large inward current

What is true of the total ionic current at a clamped membrane of 0 mV?

some inward current and a little outward current

What is true of the total ionic current at a clamped membrane of +30 mV?

small inward current and large outward current

What is true of the total ionic current at a clamped membrane of +45 mV?

no inward current, large outward current

What is true of the -45 mV time course when TEA was added to Na⁺ current?

little inward current

What is true of the -30 mV time course when TEA was added to Na⁺ current?

• large inward current

• biphasic time course

• activation before peak

• inactivation on intracellular side

What is true of the 0 mV time course when TEA was added to Na⁺ current?

some inward current with inactivation

What is true of the +30 mV time course when TEA was added to Na⁺ current?

very little inward current with inactivation

What is true of the +45 mV time course when TEA was added to Na⁺ current?

little outward current with inactivation

What is true of the total ionic current at a clamped membrane of -45 mV?

little inward current

What is true of the total ionic current at a clamped membrane of -30 mV?

large inward current

What is true of the total ionic current at a clamped membrane of 0 mV?

some inward current, small outward current

What is true of the total ionic current at a clamped membrane of +30 mV?

little inward current, some outward current

What is true of the total ionic current at a clamped membrane of +45 mV?

no inward current, large outward current

What is true of the -45 mV time course when TTX was added to K⁺ current?

no current

What is true of the -30 mV time course when TTX was added to K⁺ current?

no current

What is true of the 0 mV time course when TTX was added to K⁺ current?

delayed-rectifying current, small outward current

What is true of the +30 mV time course when TTX was added to K⁺ current?

delayed-rectifying, some outward current

What is true of the +45 mV time course when TTX was added to K⁺ current?

delayed-rectifying current, large outward current

What is true of idealized channels?

• always assumed to be fully open, irrespective of V_m

• assumed to have the same unitary conductance (gamma)

What is the formula for unitary conductance of a single channel?

i_x = y_x (V_m - E_x)

What is true of P_o for both K⁺ and Na⁺ channels?

P_o depends on voltage (V_m)

What is true of Na⁺ inactivation?

a high h means a low probability of inactivation

What were Hodgkin and Huxley able to do with the K⁺ and Na⁺ current parameters?

They were able to create a predicted action potential showing both absolute and relative refractory periods

Describe the Nodes of Ranvier.

sites of high density of voltage-gated Na⁺ channels

What is true of myelin?

provided insulation for the axon and prevents leakage of current across parts of the membrane that have myelin

What does high membrane resistance in the internode region cause the current to do?

jump from node to node

What does the length constant determine?

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

What is the equation for length constant

lambda = square root [(aR_m)/(2R_i)]

How can you increase or decrease lambda?

increase: increase R_m and/or a

decrease: increase R_i

What is true of conductivity?

it is proportional to length constant

Why do axons need myelination?

• large diameter, bigger radius, and lower R_i

  • lower R_i = high conductivity

  • smaller diameter, low conductivity

• myelination increases R_m and decreases capacitance

  • a decrease in capacitance decreases the time constant and produces a faster conduction velocity