Membrane Potential and Ion Channels

Question 1

Synaptic Transduction

Answer 1

Question 2

Excitability

Answer 2

Excitability refers to the ability of a cell to generate action potentials, which means a transient depolarization and repolarization of the cell’s membrane potential.

Question 3

Free Energy of Transport Equation

Answer 3

Question 4

Cellular vs Blood Ions

Answer 4

Question 5

The asymmetry in Na+ and K+ concentrations in the blood and cytoplasm is due to the activity of. . .

Answer 5

Na⁺ / K⁺ ATPase:

Transports 3 Na+ ions out of the cell and 2 K+ into the cell.

Question 6

The asymmetry in Ca²⁺ concentration between the SR and cytoplasm is due to the activity of. . .

Answer 6

SERCA pumps

Question 7

Balance of Electrical and Concentration Gradients Across a Selectively Permeable Membrane

Answer 7

Question 8

You set up a selectively permeable membrane, only permeable to potassium, and allow potassium to diffuse. If you measured the potassium concentrations in the two compartments after opening the channels, what would the difference be?

Answer 8

Almost nothing! It would hardly be detectable. Note that it only takes a limited number of potassium ions to move across the membrane to develop a difference in voltage.

Question 9

Resting voltage for a selectively potassium permeable membrane

Answer 9

Question 10

Resting voltage for a selectively sodium permeable membrane

Answer 10

Question 11

Nernst Potential equation for ion transport

Answer 11

The Nernst potential for any given ionic species is the membrane potential at which the ionic species is in equilibrium; i.e., there is no net movement of the ion across the membrane. Therefore, the Nernst potential for an ion is referred to as the equilibrium potential (Veq.) for that ion.

REMEMBER: For ion transport, the negative sign is removed from the Nernst equation. Bear in mind whether your ion is positive or negative!!!!!! That will determine the sign of the z variable. It is assumed to be POSITIVE as a default, as in concordance with the standard assumption of positive current flow in physics.

Question 12

By convention, membrane potential is typically descirbed as. . .

Answer 12

The inside to outside potential

In other words, always use [X]_out/[X]_in

Question 13

Goldman equation

Answer 13

Where P_x is the permeability of ion x.

Note that this is simply a summation of Nernst potentials for the ions, and that whether it is in/out or out/in depends on the ion’s charge and therefore sign.

Question 14

A sudden change in the extracellular potassium concentration can cause life-threatening arrhythmias and may lead to a complete loss of neuronal excitability. Use the Goldman Equation and the data given regarding the intracellular and extracellular concentrations of each ion as well as their membrane permeability to calculate the transmembrane potential as the extracellular concentration of potassium is raised from 4.5 to 100 mM in increments of 5 mM; contrast this to a similar calculation on raising the extracellular sodium concentration from 135 mM to 235 mM in increments of 5 mM.

Question 15

RT/F

Answer 15

0.02699

Question 16

In most cells at rest, ___ channels are open, but ___ channels are not.

Answer 16

In most cells at rest, potassium channels are open, but sodium channels are not.

Question 17

_____ channels that set the resting membrane potential.

Answer 17

Potassium channels that set the resting membrane potential.

These channels are open at rest and are not voltage-gated. They set the resting membrane potential near the Nernst potential for potassium (typically -60-80 mV).

Question 18

_____ channels initiate an action potential.

Answer 18

Ligand-gated sodium channels initiate an action potential.

Question 19

These channels open in response to depolarization of the membrane

Answer 19

Voltage-gated sodium channels: These channels open in response to depolarization and have a unique feature of inactivating themselves after a short period. These channels are essential for propagating an action potential.

Voltage-gated potassium channels: These channels also open in response to depolarization, but more slowly than the sodium channels. These channels help restore the membrane potential back to the resting potential. Like sodium channels, they also inactivate themselves over time.

Voltage-gated calcium channels: (In muscle) These channels allow calcium influx from either the outside of the cell or the sarcoplasmic reticulum in response to the action potential (depolarization). These channels also inactivate themselves over time.

Question 20

Depolarization

Answer 20

When the membrane potential is moving closer to 0.

Question 21

Action potential diagram

Answer 21

V_T: the threshold voltage that triggers opening of the voltage gated sodium channel

V_Na: the Nernst potential for sodium

V_K: the Nernst potential for potassium

Question 22

Sodium and potassium channel conductances during action potential

Answer 22

The membrane repolarizes when g_K is greater than g_Na

Note that this diagram only shows the conductance of the voltage gated channels; it does not show the conductance of the potassium channels that are open in the membrane at rest.

Question 23

Voltage gated sodium channel mechanism

Answer 23

At resting potential in the membrane, the channel is closed. Upon depolarization to the threshold voltage, the channel opens (1 below). While the membrane is depolarized, the channel then undergoes inactivation (2 below). This is a different conformation than the initial closed state (note bottom gate is closed). To open again, the channel has to cycle back to the resting state (3 below).

Note the critical importance of the inactivation step. If the channel did not inactivate, the membrane would remain permeable to sodium, and the cell would remain depolarized.

Question 24

Cholinergic Synapse Diagram

Answer 24

Question 25

Muscarinic receptors are ____. Nicotinic receptors are ____.

Answer 25

Muscarinic receptors are metabotropic. Nicotinic receptors are ionotropic.

Question 26

Nicotinic Receptor Structure

Answer 26

Question 27

Nicotinic Receptor Gating

Answer 27

Note that channel opening and closing are much slower events than binding of ACh to the receptor.

Question 28

Neuromuscular Junction

Answer 28

Question 29

Quantal Release of Acetylcholine Triggering Action Potentials

Answer 29

Q or MEPP (miniature end-plate potential) is a small depolarization event caused by the release of a small quantum of acetylcholine (degranulation). Multiple MEPPs concurrently or in quick succession may generate an action potential.

Question 30

Excitation-Contraction Coupling

Answer 30

Question 31

Lidocaine

Answer 31

Na⁺ Channel antagonist

Prevents propagation of an action potential, but does not change the resting membrane potential.

Question 32

Nerve voltage-gated sodium channels are ____ while the heart voltage-gated sodium channel is ____.

Answer 32

Nerve voltage-gated sodium channels are Na_v 1.7 and 1.8 while the heart voltage-gated sodium channel is Na_v 1.5.

Question 33

Fasciculations

Answer 33

Non-coordinated muscle contractions caused by chemical interaction with contraction machinery.

Initial response to a depolarizing neuromuscular blocker

Question 34

Succinylcholine

Answer 34

Succinylcholine blocks the transmission of the signal WITHIN THE MUSCLE FIBER.

Think about the muscle ADJACENT TO and DOWNSTREAM OF the cell of the neuromuscular endplate.

Question 35

Plasma pseudocholinesterase

Answer 35

The enzyme that degrades succinylcholine as it diffuses out into the blood.