action potentials

Question 1

what is the action potential

Answer 1

excitable cells create an electrical signal via the movements of ions across a membrane

Question 2

what are ionic gradients

Answer 2

required for the movement of ions across the membrane, changing the membranes permeability to different ions allowing the cells membrane potential to be changed

Question 3

what are the stages of propagation

Answer 3

initial stimulus
depolarisation
repolarisation
hyperpolarisation

Question 4

what happens at initial stimulus stage

Answer 4

initial depolarisation that triggers the action potential
due to the movement of Na+ because of activation of receptors at local spread or depolarisation from an adjacent action potential

Question 5

what happens at depolarisation

Answer 5

-55mV threshold is met
voltage gated Na+ channels open resulting in rapid depolarisation

Question 6

what happens at repolarisation

Answer 6

after membrane is fully depolarised it becomes more negative again as VGSCs are inactivated and voltage gated K+ channels open

Question 7

what happens at hyperpolarisation

Answer 7

cell overshoots repolarisation phase due to movement of K+ resulting in brief period of hyperpolarisation before returning back to resting membrane potential

Question 8

what is the absolute refractory period

Answer 8

cells cannot generate an action potential at all as all Na channels are in the inactivated state

Question 9

what is the relative refractory period

Answer 9

cell can generate another action potential as VGSCs are beginning to recover although some are still inactive
period ends when all channels have recovered

Question 10

what happens at the neuromuscular junction

Answer 10

depolarisation causes VGCC to open resulting in Ca influx
Ca binds to pre-synaptic membrane bringing ACh vesicles to membrane
ACh is released by exocytosis
ACh diffuses across cleft and binds to nicotinic ACh receptors on sarcolemma

Question 11

what happens after ACh binds to receptors on sarcolemma

Answer 11

receptors are activated resulting in Na influx and depolarisation of end plate
ACh is degraded by acetylcholinesterase and taken up for recyclign
local spread of depolarisation causes VGSCs to open resulting in AP generation

Question 12

resting membrane potential of skeletal muscle

Answer 12

-90mV

Question 13

resting membrane potential of neurone

Answer 13

-70mV

Question 14

resting membrane potential of smooth muscle cell

Answer 14

-60mV

Question 15

how is resting membrane potential maintained

Answer 15

if concentration of K is greater inside the cell compared to outside, then K will move out of the cell
as K leaves, cell is less positive and extracellular solution becomes more positive so electrical gradient increases
chemical and electrical gradients are equal and opposite so they cancel
net negative charge remains which is resting membrane potential