Action potentials

Question 1

Describe paracrine cell communication. Name a 3 molecule that uses this type of cell communication

Answer 1

-Acts locally -Neurotransmitters released from nerves - Hormones which act as neurotransmitters (adrenalin)

Question 2

Describe endocrine cell communication

Answer 2

Hormones released into the blood- have a long lasting effect

Question 3

Describe how the resting potential is created inside the cell

Answer 3

At rest there is a small potential (voltage) difference. Inside the cell is negative, outside the cell is positive. This is caused by the selective diffusion of ions across the membrane. Sodium and potassium ion channels allow facilitated diffusion of ions along concentration gradient. K+ ions leave cell through leak K+ channels. This causes the cell to become negatively charged as proteins within the cell are negatively charged. Na+ enters cell along concentration gradient. Na+ channels are not as leaky as K+ channels. Less Na+ enters Na+/K+ pump exchanges 3 Na+ out of cell for 2 K+ into cell (against there concentration gradients). This maintains the concentration gradient to enable diffusion to occur.

Question 4

What are the 3 mechanisms which contribute to the resting potential of the cell?

Answer 4

1. passive ion diffusion (leak channels) 2. Active ionic diffusion (ATP is used to transport the K+ out of the cell) 3. Gibbs-Donnan equilibrium effect (

Question 5

Describe the normal distribution of Na+ and K+ in and out of the cell

Answer 5

Na+ is higher outside of the cell K+ is higher inside of cell

Question 6

What is the potential (voltage) difference across a normal membrane

Answer 6

Inside is slightly negative Outside is slightly positive

Question 7

What is Gibbs-Donnan equilibrium (effect)

Answer 7

The concept that proteins inside the cell which are negatively charged are impermeable to the membrane so can not leave. This means there will be asymmetric distribution of permeable charged ions as they move to ensure concentration ratios are equal at equilibrium.

Question 8

What is the effect of the sodium potassium pump on the resting potential?

Answer 8

exchanges 3 Na+ ions from intracellular space for 2 K+ ions from extracellular space so: 1. makes Na+ concentration high in extracellular space and low in intracellular space 2. makes K+ concentration high in intracellular space and low in extracellular space 3. Result = extracellular space has positive voltage with respect to intracellular space

Question 9

What is used to measure charge across a membrane?

Answer 9

Voltmeter

Question 10

How much mV does the Na+/K+ pump contribute to the resting potential?

Answer 10

4 mV

Question 11

What is the over all mV (volts) of the resting potential?

Answer 11

70mV

Question 12

What is the Nernest equation?

Answer 12

Assesses the voltage potential across the membrane in the instance of the net diffusion of ions (across the cell due to a concentration gradient) stops.

Question 13

What does the Nernest equation relate?

Answer 13

the membrane potential to the concentration gradient

Question 14

Em=61.5 log [C]o _ [C]i For a monovalent cation at 37 degrees What does each part of this equation mean?

Answer 14

61.5- the constant for cl-. [C]o- concentration outside the cell [C]i concentration inside the cell 37 is simply the temperature

Question 15

Examples to try: [Na+]o = 150 mM [Na+]i = 15 mM Constant is 61.5

Answer 15

log 150/15 = log 10 = 1 ENa+ = 61.5 x 1 = 61.5 mV inside positive

Question 16

Another example to try [K+]i = 150mM [K+]o = 5mM Constant is 61.5

Answer 16

EK+ = -61.5 x 1.477 = -91mv inside negative

Question 17

What is the measured mV? It should be the same as Ek+ why is it not?

Answer 17

measured -70mV It is close to EK+ which is -91 mV but not the same because small amounts of Na+ leak across the membrane making the Em less than EK+

Question 18

What is the relative permeability of K+ and Na+? What does it mean?

Answer 18

P K+ : P Na+ = 50 -100:1 The neuronal plasma membrane is 100 times more permeable to K+ than it is to Na+

Question 19

What is the difference between the facilitated diffusion leak channels for Na+ and K+?

Answer 19

K+ has more leak channels and the leak channels have a bigger diameter

Question 20

What happens in excitable nerve and muscle cells?

Answer 20

They undergo rapid changes in voltage across the membrane (action potential) This is caused by the presence of voltage dependant ion channels

Question 21

What is an action potential? What causes it to occur?

Answer 21

It is the rapid change in voltage across a membrane

It occurs due to voltage dependant ion channels

Question 22

Draw and label neurone

Answer 22

Question 23

What is the trigger region for the generation of an action potential?

Answer 23

Axon (Axonal), and especially at axon hillock

Question 24

Name the two types of voltage dependant ion channels

Answer 24

–Na+ channels

–K+ channels

Question 25

How many gates are there for Voltage-sensitive Na+ channels? Name them

Answer 25

2 gates- Activation gate

Inactivation gate

Question 26

What are the features of the activation gate in Voltage-sensitive Na+ channels

Answer 26

–closed in resting state

• Open in response to depolarisation
• Opening very fast <0.1 ms
• Allows about 6000 Na+ ions into cell

Question 27

What are the features of the inactivation gate in Voltage-sensitive Na+ channels

Answer 27

–open in resting state

• Closes in response to depolarisation
• Closes slow (around 1ms after depolarisation)

Question 28

What is the consequnce of the inactivation gate of Voltage-sensitive Na+ channels closing slowly?

Answer 28

Na+ is still able to enter during depolarisation before the inactivation gate fully closes

Question 29

What happens to Voltage-sensitive Na+ channels after repolarisation?

Answer 29

They return to normal

Question 30

Describe the features of Voltage-sensitive K+ channels

Answer 30

At rest it stays closed.

Delayed opening is triggered at the threshold in responce to deloarisation- (-70mV to 30mV). Opens slightly slower than Na channels close

• They stay open during entire depolarisation
• Increase P K+ compared to > resting P K+
• Produce repolarisation and undershoot (hyperpolarisation)

Question 31

What is undershoot?

Answer 31

Hyperpolarisation (30mV to -80mV)

Question 32

Draw and label an action potential graph

Answer 32

Question 33

What is the threshold for an action potential?

Answer 33

55mV

Question 34

Describe the stages of an action potential

Answer 34

Stage 1 resting- all gated Na+ and K+ channels are closed (Na+ activation gates closed and inactivation gates open)

Stage 2-Depolarisation- Na+ channels open

Stage 3- repolarisation- Na+ channels inactivating, K+ channels open

Stage 4- Hyperpolarisation- K+ channels remain open, Na+ channels reset

Question 35

Describe the all or nothining principal of action potentials

Answer 35

–Either depolarization does not reach threshold = therefore no action potential

–or it reaches threshold and elicits an action potential that is always the same

Question 36

What causes the threshold of an action potential to be reached?

Answer 36

•Threshold for an action potential is reached when the opening voltage-gated sodium channels stimulate other channels to open in a positive feedback loop

Question 37

What is an absolute refractory period?

Answer 37

– Channels Cannot open regardless of size of the applied stimulus

–Due to inactivated Na+ channels

–1-2 ms

Contributes to all or nothing property

Question 38

Describe the relative refractory period

Answer 38

–Before closure of K+ channels PK+ is large

–Due to continued outward diffusion of K+

When k+ is low enough during the repolarisation phase, another action potential can be activated if the stimulus is suprathreshold

–Therefore larger stimulus needed to make PNa+> PK+

–Increased threshold

–3-15 ms

•make it possible to encode information by converting the strength of a stimulus into the frequency of action potentials

Question 39

Label the absolute and relative refractoyr periods on a graph

Answer 39

Question 40

What is a suprathreshold?

Answer 40

graded potential can produce a higher number of action potentials within a given period