Membrane Potential and Action Potentials

Question 1

Ion Flux

Answer 1

The number of molecules that cross a unit area per unit of time ( number of particles )

Question 2

Voltage ( potential difference )

Answer 2

Generated by ions to produce a charge gradient

Question 3

Current ( amps)

Answer 3

Movement of ions due to a potential difference

Question 4

Resistance

Answer 4

Barrier that prevents the movement of ions

Question 5

What are changes in membrane potential caused by

Answer 5

Ion transporters

Question 6

When is an electrochemical equilibrium achieved

Answer 6

When electrical force prevents further diffusion across the membrane . So when the concentration gradient exactly balances the electrical gradient .

Question 7

What does the Nernst equation tell you

Answer 7

The equilibrium potential ( E)

Question 8

What is R in the Ernst Equation

Answer 8

Gas constant ( 8.31)

Question 9

What is T in the Ernst Equaton

Answer 9

Temperature in Kelvin ( 37 degrees or 310K)

Question 10

What is z on the Ernst Equation

Answer 10

Charge on the ion ( eg. -1 for Cl - etc)

Question 11

What is F in the Ernst Equation

Answer 11

Faradays number - charge per mol of ion

Question 12

What do you do to the Ernst equation when calculating

Answer 12

Assume T = 37 degrees or 310 K
Convert natural log to common log
State E in mV
Male compartment 2 the inside and 1 outside

Question 13

X 2 and X1 in Ernst Equation

Answer 13

X2 is the intracellular ion conc and X 1 is theextra cellular ion conc

Question 14

Typical concentrations of sodium

Answer 14

150 mmol outside and 10 mmol inside

Question 15

Typical concentrations of potassium

Answer 15

5 mmol outside and 150 mmol inside

Question 16

The version of the Ernst equation we need

Answer 16

E = -61/z x log ( X inside /X outside )

Question 17

Why do membrane potentials not rest at E k or Ena

Answer 17

Because there are always some channels open at all times

Question 18

What does the GhK questions do

Answer 18

Describes the resting membrane potential ( look at the examples on OneNote )

Question 19

What is the supposed membrane potential supposed to be

Answer 19

A typical membrane resting potential is -70 mmol and not -90 mmol which is Ek as some of the other ion channels are always open . As even at rest, the membrane has some finite permeability to some Na + and so the membrane potential is higher

Question 20

Typical concentrations of Cl-

Answer 20

110 extra cellular and 5 intracellular

Question 21

What does depolarisation mean

Answer 21

Membrane potential moves towards 0 mV

Question 22

What does repolarisation mean

Answer 22

Membrane potential decreases towards the resting potential

Question 23

Overshoot

Answer 23

When membrane potential becomes more positive

Question 24

What does hyperpolarisation mean

Answer 24

When membrane potential decreases beyond resting potential

Question 25

What are graded potentials

Answer 25

They have decrement all speed and strength and they occur at synapses and in sensory receptors , they also contribute to initiating or preventing action potentials

Question 26

Action potentials and graded potentials

Answer 26

Actions potentials occur when a graded potentials reaches a threshold for the activation of Na + channels resulting an ‘all or nothing’ principal

Question 27

Where do action potentials occur

Answer 27

Excitable cells ( mainly neurone and muscle cells but also in some endocrine tissues)

Question 28

What does the permeability of ions depend on in action potentials

Answer 28

Permeability depends on conformational state of ion channels, they are opened by membrane depolarisation , inactivated by sustained depolarisation and closed by membrane hyperpolarisation / repolarisation

Question 29

Changes in membrane potential during the action potential are due to

Answer 29

Ion channels and not pumps . The sodium potassium potential manga is the concentration gradient on which the movement of the ion depends on

Question 30

Phase 1 of action potential

Answer 30

Resting membrane potential . Permeability for potassium ions are greater than sodium ions, the membrane potential is nearer the equilibrium potential for potassium ( -90mV) than that for sodium ( +72mV)

Question 31

Phase 2 depolarising stimulus

Answer 31

The stimulus depolarises the membrane potential and moves it in the positive direction towards the threshold

Question 32

Phase 3

Answer 32

Upstroke .starts at threshold potential, sodium channels open quickly and potassium channels open slowly , so potassium is moving out less than sodium is entering and membrane potential moves towards the sodium equilibrium potential

Question 33

Phase 4 repolarisation

Answer 33

The sodium ion channels close and sodium entry stops and potassium ion channels opens and remains open so membrane potential moves towards the potassium equilibrium potential

Question 34

At the start of repolarisation

Answer 34

Absolute refractory period , activation gate is open but inactivation gate is closed in sodium so new action potential cannot be triggered even with very strong stimulus

Question 35

Later in repolarisation

Answer 35

Absolute refractory period continues and so the activation and inactivation gates are closed

Question 36

After hyperpolarisation

Answer 36

At rest the potassium channels are still open and so potassium continues to leave the cell down the electrochemical gradient . Membrane potential moves closer to the potassium equilibrium - some voltage gated potassium channels then close . Membrane potential returns to resting potential

Question 37

After hyperpolarisation what happens to sodium channels

Answer 37

Relative refractory period and so the inactivation gate is open and a stronger than normal stimulus is required to trigger an action potential

Question 38

What affects how quickly the graded potential decays away from the site of initiation

Answer 38

Internal diameter of the axon Internal resistance of axon How well the axon is insulated

Question 39

Where are voltage gated channels mostly located

Answer 39

Nodes of Ranvier

Question 40

What affects the conduction velocity

Answer 40

Axon diameter and myelinated Large diameter , myelinated axons travel very quickly while small diameter non myelinated axons travel slower. Cold anoxia compression and drugs ( anaesthetics ) also decrease conduction velocity . Reduced myelination can be due to multiple sclerosis and diphtheria)

Question 41

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

Answer 41

Concentration of ion on both sides of the membrane, the change of the ion and the voltage across the membrane

Question 42

Why is potassium ion equilibrium negative and sodium positive when they are both positive ions

Answer 42

There is more K+ inside the cell than outside to tend to flow out of the cell, while more Na+ outside the cell than in therefore tend to flow into the cell. A potential to -70mV is needed to attract K+ and stop net flow outwards while a positive charge of +40mV is needed to repel Na+ from entering the cell.

Question 43

Which ion is imp for the upstroke and which is imp for the falling portion of the action potential ?

Answer 43

The upstroke is mediated by the sodium ions moving down their conc gradient into the cell . The falling portion of the action potential is by potassium ions going down their conc gradient and exiting the cell

Question 44

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

Answer 44

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 the conduction velocity and myelin thickness

Question 45

What causes a graded potential

Answer 45

Local changes in ionic conductance ( e.g. synaptic or sensory that produces a local current) that spreads along a stretch of membrane becoming smaller