Lecture 4 - Action Potentials

Question 1

what is GNA+?

Answer 1

the conductance of NA+ across the membrane `

Question 2

what is GK+?

Answer 2

the conductance of K+ across the membrane

Question 3

what is EK+?

Answer 3

the equilibrium potential for K+

Question 4

what is ENa+?

Answer 4

the equilibrium potential for Na+

Question 5

what is Vm?

Answer 5

the membrane potential

Question 6

what 2 things does the conductance of an ion across the membrane depend on?

Answer 6

depends on permeability (ion channels) and equilibrium potential (driving force)

Question 7

what determines permeability?

Answer 7

ion channels

Question 8

what determines equilibrium potential?

Answer 8

driving force

Question 9

what is the driving force?

Answer 9

the difference between the actual membrane potential and ions equilibrium potential (indicates how far an ion is from its equilibrium)

Question 10

what is the relationship between a driving force and an ion that is not at equilibrium?

Answer 10

when an ion is not at its equilibrium, an electrochemical driving force acts on the ion, causing the net movement of an ion across the membrane down its electrochemical (conc) gradient

Question 11

what is the effect on polarisation when K+ exits the cell?

Answer 11

K+ hyperpolarises the cell as it exits

Question 12

what is hyperpolarisation?

Answer 12

makes the cell membrane potential more negative

Question 13

what is the effect on polarisation when Na+ enters the cell?

Answer 13

Na+ depolarises. the cell as it enters

Question 14

what is depolarisation?

Answer 14

a sudden change in cell membrane potential that makes the cell more positive (resulting in a less negative charge)

Question 15

what are voltage-gated Na+ channels composed of? (in terms of activation gates)

Answer 15

composed of an activation gate with a voltage sensor that is an ‘on’ switch only specific for Na+ and an inactivation gate that is an ‘off’ switch that is actively closed

Question 16

why is the inactivation gate of Na+ channels actively closed?

Answer 16

at the peak of an AP when enough Na+ has entered the neuron and MP is high enough the channels inactive themselves by closing their inactivation gates

Question 17

what are voltage-gated Na+ channels blocked by?

Answer 17

blocked by tetrodotoxin from pufferfish

Question 18

what is the effect of tetrodotoxin binding to Na+ channels?

Answer 18

binds irreversibly permanently blocking the Na+ channel

Question 19

what are voltage-gated Na+ channels composed of?

Answer 19

composed of 2 beta-subunits and 4 alpha-subunits

Question 20

what model is an inactivation gate of a Na+ channel?

Answer 20

a ball and chain model

Question 21

what is a not-activated voltage-gated Na+ channel?

Answer 21

the pore is closed because it is blocked by a positive charge, closing the Na+ channel
- ball is not blocking the channel

Question 22

what is an activated voltage-gated Na+ channel?

Answer 22

the pore is open as the positive charge is no longer blocking the channel but remains nearby for selectivity

Question 23

what is an inactivated voltage-gated Na+ channel?

Answer 23

the pore is still open but the ball on the end of the chain is blocking it from activating acting as the inactivating particle

Question 24

what are voltage-gated K+ channels composed of?

Answer 24

composed of an activation gate that is specific for K+ that selectively filters and an inactivation gate that is slower than a voltage-gated Na+ inactivation gate

Question 25

what are voltage-gated K+ channels blocked by?

Answer 25

blocked by tetraethylammonium that binds irreversibly

Question 26

what is the absolute refractory period?

Answer 26

the time period where it is impossible to trigger another AP

Question 27

why does the absolute refractory period occur?

Answer 27

because voltage-gated Na+ channels become inactivated by the inactivation gate and remain inactivated until hyperpolarisation occurs

Question 28

what is the relative refractory period?

Answer 28

the time period when another AP is possible to be triggered but it is more difficult to initiate

Question 29

why does the relative refractory period occur?

Answer 29

because after hyperpolarisation occurs the MP is further away from the threshold than for an AP at rest, resulting in a larger depolarization required to reach the threshold triggering the next AP, the Na+ inactivation gate is open but K+ channels are still open

Question 30

how is the speed of conduction increased?

Answer 30

to increase the speed of conduction you must decrease internal axial resistance (Rin) and membrane potential (Rm)

Question 31

how is axon diameter increased to increase the speed of conduction?

Answer 31

axon diameter is increased by decreasing internal axial resistance (Rin) to reduce resistance to ion flow

Question 32

how does myelination increase the speed of conduction?

Answer 32

myelination insulates the axon to increase the membrane potential (Rm) to reduce the leakage of ions

Question 33

how do nodes of Ranvier increase the speed of conduction?

Answer 33

nodes of Ranvier allow saltatory conduction through a high density of Na+ channels that are not present in the internodes allowing the signal to rapidly jump from node to node