Membrane Potentials and Action Potentials

Question 1

Voltage

Answer 1

Potential difference
Units: Volts
Generated by ions to produce a charge gradient

Question 2

Current

Answer 2

Unit: Amps

Movement of ions due to a potential difference

Question 3

Resistance

Answer 3

Units: Ohms

Barrier that prevents the movement of ions

Question 4

How to measure membrane potential

Answer 4

A reference rod is placed outside the cell.
This is the zero-volt level

Another electrode is placed inside the cell . It measures a voltage difference that is negative compared with the outside

All cells have a membrane potential

Question 5

How cell membrane maintains potential

Answer 5

Lipid (hydrophobic) cell membrane is a barrier to ion movement and separates ionic environment.

Cell membrane can selectively change its permeability to specific ions

Permeable pores in the membrane (ion channels) open and close depending on transmembrane voltage, presence of activating ligands or mechanical force

Question 6

Ion channels

Answer 6

Ion channels can be selective for different types of ions

Movement across the membrane will occur when the concentration of the ion is different on one side of the membrane and ceases upon equilibration

Due to diffusion through a selectively permeable membrane

Question 7

Nerst Equation

Answer 7

E = [(RT)/(zF)] * ln { [X2]/[X1] }

Question 8

Concentration of ions

Answer 8

Na+ :
Extracellular 150 nM
Intracellular 10 nM

K+ :
Extracellular 5 nM
Intracellular 150 nM

Ca2+ :
Extracellular 2 nM
Intracellular 10^(-4) nM

Cl- :
Extracellular 110 nM
Intracellular 5 nM

Organic phosphates :
Extracellular 3 nM
Intracellular 130 nM

pH :
Extracellular 7.4
Intracellular 7.1

Osmolarity:
Extracellular = Intracellular = 285 mosmol/L

Question 9

Why do membrane potentials not rest at E(K) or E(Na)?

Answer 9

E(K) = -90mV
E(Na) = +72mV
Typical E(Membrane) = -70mV

Because there are always some channels open at all times

Question 10

Calculation for real membrane potential

Answer 10

K+, Na+ and Cl- concentrations all contribute to the real membrane potential

Size of each ion’s contribution is proportional to the real membrane potential

E = -61 log {P(K)[K in] +…} / {P(K)[K out] +…}

Question 11

Depolarisation

Answer 11

Membrane potential moves towards 0mV

Question 12

Repolarisation

Answer 12

Membrane potential decreases towards resting potential

Question 13

Overshoot

Answer 13

When membrane potential becomes more positive

Question 14

Hyperpolarisation

Answer 14

When membrane potential decreases beyond resting potential

Question 15

Action potential

Answer 15

Occur in excitable cells (mainly neurons and muscle cells but also in some endocrine tissues)
In neurons they are also known as nerve impulses and allow the transmission of information reliably and quickly over long distances

Play a central role in cell-to-cell communication and can be used to activate intracellular processes

Question 16

Ionic basis of action potentials

Answer 16

Permeability depends on conformational state of ion channels:

• -Opened by membrane depolarisation
• -Inactivated by sustained depolarisation
• -Closed by membrane hyperpolarisation/repolarisation

When membrane permeability of an ion increases it crosses the membrane down its electrochemical gradient
Movement changes the membrane potential toward the equilibrium potential for that ion

Changes in membrane potential during the action potential are not due to ion pumps

Question 17

5 phases of the Action Potential

Answer 17

Phase 1. Resting membrane potential

Phase 2. Depolarising stimulus

Phase 3. Upstroke

Phase 4. Repolarisation

Phase 5. After-hyperpolarisation

Question 18

Phase 1. Resting membrane potential

Answer 18

Permeability for P(K) > P(Na)

Membrane potential nearer equilibrium potential for K+ (-90mV) than that for Na+ (+72mV)

Question 19

Phase 2. Depolarising stimulus

Answer 19

The stimulus depolarises the membrane potential

Moves it in the positive direction towards the threshold

Question 20

Phase 3. Upstroke

Answer 20

Starts at threshold potential

Increase in Na+ permeability because voltage-gated Na+ channels open quickly
[Na+ enters the cell down electrochemical gradient]

Followed by increase in K+ permeability as the voltage-gated K+ channels start to open slowly
[K+ leaves the cell down electrochemical gradient]

Less than Na+ entering
Membrane potential moves towards the Na+ equilibrium potential

Question 21

Phase 4. Repolarisation

Answer 21

P(Na) decreases because the voltage-gated Na+ channels close - Na+ stops entry

P(K) increases as more voltage-gated K+ channels open and remain open

K+ leaves the cell down the electrochemical gradient

Membrane potential moves towards the K+ equilibrium potential

Question 22

Absolute refractory period

Answer 22

Occurs at the start of repolarisation

Activation gate is open
Inactivation gate is closed

New action potential cannot be triggered even with very strong stimulus

Absolute refractory period continues later in repolarisation

Activation and inactivation gates closed

Question 23

Phase 5. After-hyperpolarisation

Answer 23

At rest voltage-gated K+ channels are still open

K+ continues to leave the cell down gradient

Membrane potential moves closer to the K+ equilibrium - some VGKC then close

Membrane potential returns to the resting potential

ATPase restores Na+ and K+ conc.

Question 24

Relative refractory period

Answer 24

Happens after-hyperpolarisation

Inactivation gate is open

Stronger than normal stimulus required to trigger an action potential

Question 25

Regenerative relationship between P(Na) and E(m)

Answer 25

Once threshold potential reached, action potential triggered

APs are ‘all-or-nothing’ events, once triggered a full-sized AP occurs

Positive feedback causes the cycle of depolarisation, opening of VGSC, increase in P(Na), influx of Na+, to continue

Cycle continues until VGSC inactivate

Following AP there is a absolute refractory period where membrane is unresponsive to threshold depolarisation

Membrane remains in the refractory period until VGSC recover from inactivation

Question 26

Passive propagation of AP

Answer 26

Only K+ channels open

Internal (or axial) membrane resistance alters propagation distance and velocity

Question 27

Active propagation of AP

Answer 27

Local current flow depolarizes adjacent region toward threshold
This happens due to passive propagation

The adjacent region is the stimulated to form a new AP

This cycle continues hence propagating the signal down the axon

Question 28

Nodes of Ranvier

Answer 28

Gaps between myelin sheaths on an axon

Question 29

Saltatory conduction

Answer 29

Voltage-gated channels are mostly located at nodes

Myelin sheaths insulate the axons preventing leakage of the AP

This means that an action potential generated at one node of Ranvier elicits current that flows passively within the myelinated segment until the next node is reached.

Current flow then generates an action potential in the next Node of Ranvier.

The cycle is repeated along the length of the axon.

Current flows across the neuronal membrane only at the nodes.

This is saltatory propagation, meaning that the action potential jumps from node to node.

Question 30

Conduction velocity

Answer 30

Both axon diameter and myelination influence conduction velocity

Action potential travels quickly

Small diameter, non-myelinated axons : 1m/s

Large diameter, myelinated axons : 120m/s

Question 31

Factors affecting conduction velocity

Answer 31

Reduced axon diameter (i.e. regrowth after injury)

Reduced myelination (i.e. multiple sclerosis, diphtheria)

Cold

Anoxia

Compression and drugs (some anaesthetics)

Question 32

What are the 3 main factors that influence movement of ions across the membrane

Answer 32

Concentration gradient of ions
Charge on ions
Voltage across the membrane

Question 33

Which ion is important for the upstroke and which is important for the falling portion of the AP?

In which direction do these ions flow

Answer 33

The upstroke mediated largely by Na+ ions moving down their conc. gradient into the cell.

The falling portion dominated by K+ ions going down conc. gradient and therefore exiting the cell

Question 34

Why is E(K) negative (-70mV) and E(Na) positive (+40mV) when both are positive ions.

Answer 34

There is more K+ inside the cell than outside, tend to flow out of the cell.

More Na+ outside the cell, tend to flow in

Potential to -70mV is needed to attract K+ and stop the net flow outwards while a positive charge of +40mV is needed to repel Na+ from entering the cell

Question 35

What factors influence the speed of propagation of an action potential along an axon?

Answer 35

Larger axons have lower resistance, so ions move faster - conduction velocity is proportional to the square root of the axon diameter.

There is a linear relationship between conduction velocity and myelin thickness