lecture 2 : membrane potentials , action potentials

Question 1

what is an action potential?

Answer 1

• the change in electrical potential associated with the passage of an impulse along the membrane of a muscle or a nerve cell

Question 2

what happens when P ion increases?

Answer 2

• the ion crosses the membrane down the electrochemical gradient
• this moves the membrane potential towards the equilibrium potential for that ion

Question 3

what are the amplitudes and duration for action potentials?

Answer 3

• the amplitude is up to 100mV
• the duration is normally a few ms

both of these aspects depend on the neurone type

Question 4

where are action potentials generated?

Answer 4

they are generated in the axon hillock

Question 5

what is phase 1?

resting membrane potential

Answer 5

the resting membrane potential :
Pk»PNa
- the sodium channel activation gate is closed
- the sodium channel inactivation gate is open
- the potassium channel is closed

at rest the voltage gated channels (Na+ and K+ are closed)

Question 6

what is phase 2 ?

depolarising stimulus

Answer 6

• depolarisation causes the opening of the voltage gated sodium channels which allows sodium to flow into the cell
• this means the membrane potential equilibrium changes to be more in the direction of the sodium
• the stimulus needs to be above the threshold so to generate an action potential

Question 7

what is phase 3?

depolarising phase

Answer 7

• the pNA increases as the voltage gated sodium channels open quickly
• when the membrane potential reaches the threshold potential the upstroke starts
• Na+ ions enter the cell down the electrochemical gradient
• Pk also increases as the potassium voltage gated channels open as well
• K+ ions leave the cell down the gradient but there are fewer leaving than the Na+ ions entering

therefore overall the membrane potential is moving towards the sodium
Na channel activation gate is open
NA channel inactivation gate is closed
potassium channel is closed

Question 8

what is stage 4

the repolarisation phase

Answer 8

• the sodium voltage gated channels inactivate so no NA is entering
• the potassium voltage gated channel is open so that potassium ions leave the cell

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Question 9

what is stage 4

the repolarisation phase

Answer 9

• the sodium voltage gated channels inactivate so no NA is entering
• the potassium voltage gated channel is open so that potassium ions leave the cell
• therefore the overall effect is that the membrane potential moves towards K+

Question 10

what happens during early repolarising?

Answer 10

• the Na channel activation gate is open
• the Na channel inactivation gate is closed
• the potassium channel is open

Question 11

what is the absolute refractory period?

Answer 11

• this term is used to denote the sodium channel inactivation gate being closed
• this means a new action potential cannot be generated
  even with a very strong signal

Question 12

what happens during late re- polarisation?

Answer 12

• sodium channel activation gate is closed
• sodium channel inactivation gate is closed
• the potassium gate is open

Question 13

what happens during stage 5

hyper polarisation?

Answer 13

• the sodium channel activation gate is closed
• the sodium channel inactivation gate is open
• the potassium channel is open
• the membrane potential moves closer to the K+ equilibrium position until the potassium voltage gated channels close.

overall the membrane enters a relative refractory period
it is not absolute but a stronger stimulus is needed to open the voltage gated sodium channels due to the membrane potential already being more negative than normal. ( hyper polarisation)

Question 14

what is a threshold?

Answer 14

it is the change in membrane potential required to open the voltage gated sodium channels

Question 15

what is the all or nothing nature?

Answer 15

only once the threshold has been reached can a full sized action potential be produced.

Question 16

what is the refractory state?

Answer 16

this is being unresponsive to stimulus

Question 17

what happens if the potential is less than the threshold?

Answer 17

then no depolarisation happens and the graded potential returns to resting potential

Question 18

what happens if the potential is more than the threshold?

Answer 18

• positive feedback behaviour starts
• the opening of the voltage gated sodium channels
• the Na+ permeability increases
• this increases Na+ entry into the cell
• this causes depolarisation

Question 19

what are the ion movements during the action potentials?

Answer 19

• there are very small changes in ion concentration during an action potential
• the ion pumps are not directly involved in the ion movement
• the electrochemical equlibrium is restored after the action potential by ions moving through non voltage gated ion channels
• some ions move through pumps but this is a slow process

Question 20

what aspects affect the speed of the action potential?

Answer 20

• the diameter of the neurone

- the myelination of the axon

Question 21

what is the absolute refractory period?

Answer 21

• this is the blocking of the voltage gated sodium channels
• the section of the membrane which is hyper polarised cannot be depolarised again
• this makes sure the action potential does not travel in the wrong direction

Question 22

what are nodes of ranvier and what do they allow?

Answer 22

• areas of unmyelinated axon
• fast conduction of the signal
• this is where all the voltage gated channels are

Question 23

how does myelination speed up the action potential?

Answer 23

• this prevents the loss of charge by acting as an insulator
• faster propagation

Question 24

how does the diameter of the neurone affect the conduction velocity?

Answer 24

• the larger the diameter the lower the resistance this means the faster the conduction speed due to more electrically charged ions

Question 25

name 3 medical conditions that effect the conduction velocity?
slow it down

Answer 25

• reduced axon diameter (e.g. injury)
• reduced myelination (multiple sclerosis)
• cold anoxia, drugs