excitable cells

Question 1

ion movements during action potential

Answer 1

Na+ influx, K+ efflux; only very small no. ions cross and change membrane potential; ion pumps not involved in ion movements during action potential, instead restore electrochemical eqm

Question 2

5 phases of action potential

Answer 2

resting membrane potential, stimulus, depolarising phase, repolarising phase, after-hyperpolarising phase

Question 3

stage 1: resting potential

Answer 3

K+ efflux as more permeable to K+ than Na+

Question 4

stage 2: depolarising stimulus

Answer 4

depolarises membrane potentia →, moves it in the +ve direction towards threshold potential

Question 5

stage 3: upstroke

Answer 5

starts at threshold potentia: permeability of Na+ massive because voltage-gated Na+ channels open rapidly → Na+ enter down gradient → permeability of K+ slower as voltage-gated channels start opening slowly → K+ leave cell down gradient → less Na+ entering → membrane potential moves toward Na+ eqm potential

Question 6

stage 4: repolarisation

Answer 6

permeability of Na+ massively reduces as voltage-gated Na+ channels inactivate → Na+ influx stops → permeability of K+ increases as more voltage-gated K+ channels open and remain open → K+ leaves cell down gradient → membrane potential moves toward K+ eqm potential

Question 7

start of repolarisation: ball and chain hypothesis

Answer 7

despite Na+ channel activation gate open, inactivation gate protein then very rapidly blocks Na+ entry into cell as membrane potential depolarises; underlying mechainsm for absolute refractory period

Question 8

absolute refractory period: meaning and necessity

Answer 8

activation gate open then closes, inactivation gate closed; new action potential cannot be triggered even with very strong stimulus; occurs immediately after neuronal stimulation; safety net as must repolarise first for inactivation gate protein to be removed

Question 9

what do refractory periods ensure

Answer 9

action potential is unidirectional

Question 10

later in repolarisation

Answer 10

inactivation gate and activation gate remain closed

Question 11

stage 5: after-hyperpolarisation

Answer 11

at rest all voltage-gated K+ channels still open → K+ continues to leave down gradient → membrane potential moves closer to K+ eqm → some voltage-gated K+ channels then close → membrane potential returns to resting potential

Question 12

during after-hyperpolarisation

Answer 12

Na+ channel activation gate closed, Na+ channel inactivation gate open; mechanism for relative refractory period

Question 13

relative refractory period: meaning and consequence

Answer 13

activation gate closed, inactivation gate open; stronger than normal stimulus required to trigger an action potential as must overcome larger change in potential to get to threshold

Question 14

size of action potential

Answer 14

+40mV

Question 15

timescale of action potential

Answer 15

4ms