membrane potentials and action potentials

Question 1

definition of Flux

Answer 1

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

Question 2

what are 2 properties of ions

Answer 2

they are charged molecules

opposites attract and like repel

Question 3

3 electrical properties of excitable cells

Answer 3

• voltage (potential difference) = generated by ions to produce a charge gradient
• current = movement of ions due to a potential difference
• resistance = barrier that prevents the movement of ions
  V = I x R

Question 4

STEPS to measure membrane potential

Answer 4

1) place reference electrode outside of the cell = zero volt level
2) another electrode is placed inside the cell - measures a voltage that is negative compared with the outside (ie. reference)

Question 5

properties of ion channels

Answer 5

• lipid (hydrophobic) cell membrane barrier to ion movement and separates ionic environments
• ion channels can open or close

Question 6

what 3 things cause ion channels to open or close

Answer 6

1) transmembrane voltage
2) activating ligands
3) mechanical forces

Question 7

when will movement across the membrane occur

Answer 7

when the concentration of the ion is different on one sided of the membrane and will stop when equilibrium is reached

Question 8

when is the electrochemical equilibrium reached

Answer 8

when the conc gradient exactly balances the electrical gradient

Question 9

what is equilibrium potential

Answer 9

the potential at which electrochemical equilibrium has been reached - it is the potential that prevents diffusion of the ion down its concentration gradient

Question 10

what is the Nersnt equation (to calculate equilibrium potential)

Answer 10

E = RT/zF ln X2/X1
X2 = intracellular ion conc
X1 = extracellular ion conc

Question 11

what are the most important ions in determining resting potential of neurones

Answer 11

Na+ and K+

Question 12

what is typical resting membrane potential

Answer 12

-70mV and each ions contribution to membrane potential is proportional to how permeable the membrane is to the ion at any time

Question 13

GHK equation

Answer 13

Em (mV) = -61 log Pk[k]i + Pna[Na]i + Pcl[Cl]o/Pk[K]o + Pna[Na}o + Pcl[Cl}i

[x] represents conc and the subscript i or o indicates inside or outside the cell

P is permeability or channel open probability (0 = 100% closed, 1 = 100% open, 0.5 = open 50% of the time)

Question 14

what is depolarisation

Answer 14

membrane potential becomes positive towards 0mV

Question 15

what is repolarisation

Answer 15

membrane potential decreases towards resting potential (becomes more negative)

Question 16

what is overshoot

Answer 16

membrane potential becomes more positive

Question 17

what is hyperpolarisation

Answer 17

membrane potential decreases beyond resting potential

Question 18

what is a graded potential

Answer 18

change in membrane potential in response to external stimulation or NTs - and the change in membrane potential is graded in response to the type or strength of stimulation

Question 19

what do graded potentials do

Answer 19

produce the initial change in membrane potential that determines whether an action potential is initiated or prevented

Question 20

what happens to graded potentials over time

Answer 20

graded potentials decay along the axon as small amounts of charge leaks from the axon and so the size of that potential change decreases along its length

Question 21

what are ion channels opened by

Answer 21

membrane depolarisation

Question 22

what are ion channels inactivated by

Answer 22

sustained depolarisation

Question 23

what are ion channels closed by

Answer 23

hyperpolarisation/repolarisation

Question 24

what are the 5 phases of the action potential

Answer 24

phases 1-5

1) resting membrane potential
2) depolarising stimulus
3) upstroke
4) repolarisation
5) after-hyperpolarisation

Question 25

what is phase 1

Answer 25

resting membrane potential permeability for Pk > Pna membrane potential is nearer equilibrium potential for K+ (-90) than that for Na+ (+72)

Question 26

what is phase 2

Answer 26

depolarising stimulus - produces graded potential - the stimulus depolarises the membrane potential - moves in the positive direction towards threshold

Question 27

what is phase 3

Answer 27

upstroke - starts at threshold potential - increased permeability for Na because VGSCs open quickly - increased permeability for K because VGKCs open slowly K+ leaves the cell less than Na+ entering membrane potential moves toward Na+ equilibrium potential

Question 28

what is phase 4

Answer 28

repolarisation - decreased permeability for Na as VGSCs close - entry stops - increased permeability for K as channels are still open so K leaves cell down EC gradient - membrane potential moves towards K+ Eq potential

Question 29

at the start of repolarisation...

Answer 29

- absolute refractory period Na channel activation gate is open inactivation gate is closed new action potential cannot be triggered even with strong stimulus

Question 30

what happens at the peak of an AP

Answer 30

just after all Na+ channels are open they undergo a conformational change called inactivation

Question 31

what is inactivation and what does it do

Answer 31

impedes ion flow through Na+ channel > cannot be reopened | - only when cells repolarise completely - inactivation is removed from Na+ channel > closes and can be reopened

Question 32

what happens later in repolarisation

Answer 32

- absolute refractory period continues - activation and inactivation gates closed - until repolarisation - inactivation is removed - cell channels are primed for reopening (during this time - cell cannot be stimulated)

Question 33

what happens in phase 5

Answer 33

after hyperpolarisation - at rest, VGKCs still open K+ continues to leave down EC gradient - membrane potential moves closer to K+ equilibrium - some VGKCs close - membrane potential returns to resting potential - some Na+ channels open = relative refractory period (some Na+ channels have recovered from inactivation - gate is open) stronger than normal stimulus is required to trigger AP

Question 34

what 2 things determine decay at the site of depolarisation

Answer 34

- internal resistance of axon - membrane insulation alters propagation distance and velocity

Question 35

how does diameter affect decay

Answer 35

- resistance of neurone is determined by diameter the larger the diameter = smaller resistance = slower decay smaller diameter = greater resistance = faster decay

Question 36

what does myelination do to decay

Answer 36

if there is insulation - graded potential decays further along axon due to insulation being provided

Question 37

what 2 factors affect conduction velocity

Answer 37

axon diameter | myelination

Question 38

how does axon diameter affect conduction velocity

Answer 38

as diameter increases = conduction velocity increases | conduction velocity is proportional to the square root of axon diameter

Question 39

how does myelination affect conduction velocity

Answer 39

as myelination increases so does conduction velocity

Question 40

what are 3 factors that influence movement of ions across a membrane

Answer 40

charge of ions conc of ions on both sides voltage across membrane