Action Potentials

Question 1

Four types of excitable cells (cells that can experience APs) & what makes them excitable

Answer 1

Neurons

Skeletal muscle

Cardiac muscle

Smooth muscle

Must have enough voltage-gated Na⁺ or Ca²⁺ channels

Question 2

Action potentials are produced by ion diffusion across cell membranes as a result of..

Answer 2

selective and brief changes in membrane permeability to those ions.

(Remember, if membrane permeability to a given ion is increased, then the transmembrane potential will shift twoard the Nernst potential of that ion)

Question 3

conductance (g)

Answer 3

The ease with which ions can flow; an indirect measurement of permeability

Question 4

Rapid depolarization

Answer 4

Na+ channels open, allowing rapid incrase of gNa+ and Na+ influx.

Hodgkin cycle: initial depolarization produces more dpeolarization in a regenerative, positive feedback

Question 5

Overshoot

Answer 5

gNa+ now exceeds gK+ and the action potential overshoots 0mV –> rapidly terminated by Na inactivation

Question 6

Repolarization

Answer 6

• Na inactivation causes gNa+ to fall back to normal
• Delayed increase in gK+ causes Em to move towards Ek as K+ efflux

Question 7

Undershoot/Hyperpolarizing afterpotential

Answer 7

Prevailing elevation of gK+ ->K+ efflux causes Em to remain somewhat hyperpolarized after gNa+ returns to normal level

Question 8

Do K+ channels have inactivation gates like Na+ channels do?

Answer 8

K+ channels do not have inactivation gates, just activation gates - thus, g_K does not inactivate during sustained depolarization

Question 9

States of voltage-gated ion channels during resting state, depolarizing phase, repolarizing phase, and undershoot

Answer 9

Note how the activation gate of both K and Na open in response to depolarization, but the K+ activation gate is delayed and there is no inactivation gate for K

Question 10

Transient vs Steady state inactivation of Na+ channels

Answer 10

Transient inactivation: this is what you see during the action potential; a very sudden depolarization triggers inactivation

Steady state inactivation: the maintained depolarization seen between action potentials, AKA the resting membrane potential

Question 11

Threshold can change - what plays a key role in determining the threshold of depolarization AKA excitability?

Answer 11

Activation & inactivation, esp of Na channels

Question 12

A critical number of Na channels must be recruited to generate a propagating action potential. What is “recruitment”?

Answer 12

As the membrane is progressively depolarized, mroe and mroe Na channels reach threshold and are activated

Question 13

Moderate levels of depolarization that are short lasting will have what effect on excitability?

Answer 13

They increase excitability (lower threshold) by recruiting/activating more Na channels

Question 14

Larger, maintained levels of depolarization have what effect on excitability?

Answer 14

Transiently increases excitability then drops it by elevating the threshold and causing depolarizing blockade

Question 15

What is a depolarizing blockade?

Answer 15

If the membrane is depolarized for a while, more Na channels will be in the inactive state- refractory to another stimulus –> not enough channels available to be recruited for an AP, so firing an AP is impossible no matter how intense the depolarization.

Question 16

Absolute vs relative refractory period

Answer 16

Absolute refractory period: impossible to generate another AP no matter how strong the stimulus because the critical number of Na channels is not available due to closure of their inactivation gates

Relative refractory period follows- a second action potential can be evoked by an especially strong depolarization; occurs because

• Some gates are still closed
• Must overcome the hyperpolarizing afterpotential

Question 17

Where is the absolute and relative refractory period during an action potential?

Answer 17

Question 18

Why is there a small fluctuation of the total current across a cell membrane?

Answer 18

Channels are always either opened or closed, but they can do so randomly even when transmembrane potential is subthreshold

Question 19

The frequency and duration of channel openings is a function of what 2 things?

Answer 19

The degree of depolarization

Concentraiton of Ca++

Question 20

Various excitable cells have APs of different shapes and sizes depending on the

Answer 20

type and properties of voltage-gated channels in that cell membrane

Question 21

Autorhythmicity

Answer 21

Firing at roughly a constant rate

Ex) APs from cells in the SA node

Question 22

Ion channel function can also be modulated by G proteins, neuroransmitters, and intracellular signaling molecules