LA symposium - 1 action potentials mechanisms

Question 1

What cells have membrane potentials

Answer 1

all

Question 2

what is more negative ICF or ECF

Answer 2

ICF is negative with respect to the ECF

Question 3

There are equal numbers of +ve and -ve charges in ECF and ICF, so why is ICF negative with respect to ECF

Answer 3

ion/charge distribution is ‘polarised’

Question 4

What are the rough concentrations of ions (Na, K and Cl) in ECF and ICF

Answer 4

• Na+ (ECF = 145mM, ICF = 15mM)
• K+ (ECF = 4mM, ICF = 150mM)
• Cl- (ECF=110mM, ICF=10mM)

Question 5

if the ion concentrations of Na and K are such that Na wants to come in and K wants to come out why don’t they?

Answer 5

the membrane doesn’t let them because they are not lipophilic

Question 6

Is the resting membrane potential permeable to Na

Answer 6

no

Question 7

is the resting membrane potential permeable to K

Answer 7

yes

Question 8

what ion’s diffusion gradient results in an excess -ve charge inside the cell (the resting membrane potential)

Answer 8

the diffusion of potassium from cell interior through potassium channels

Question 9

what happens to the small amount of Na that leaks into the cell (at resting membrane potential)

Answer 9

it is expelled by the Na/K pump

Question 10

What creates the resting membrane potential

Answer 10

1. mainly diffusion of K from cell interior

2. also Na/K pump exchanging unequal numbers of Na and K

Question 11

How does the Na/K pump work

Answer 11

3 Na outwards
2 K inwards
ADP --> ATP
(active transport)
The Na/K pump is electrogenic

Question 12

What is the action potential threshold

Answer 12

-55mV

Question 13

What is the resting membrane potential value

Answer 13

-70mV

Question 14

what is the rising phase of the action potential due to

Answer 14

Na influx via voltage gated Na channels

Question 15

what is the highest value the action potential reaches

Answer 15

+35mV

Question 16

what is the falling phase of the action potential due to

Answer 16

K efflux via voltage gated K channels

Question 17

What ion channel has more than 1 gate

Answer 17

sodium

Question 18

what are the gates in the sodium channel called

Answer 18

• M gate

- H gate

Question 19

When the voltage gated sodium channel is closed (not in refactory period), are m and h gates open or closed

Answer 19

m gate - closed

h gate - open

Question 20

When the voltage gated sodium channel is closed during the refactory period, are the m and h gates open or closed

Answer 20

m gate - open

h gate - closed

Question 21

when is the h gate open only

…closed???

Answer 21

during the refactory period

Question 22

what happens at 35mV to the voltage gated sodium channel

Answer 22

h gate closes first, then m gate

Question 23

how many gates does the voltage gated potassium channel have

Answer 23

1

Question 24

Describe the stages of an action potential

Answer 24

1. Stimulus applied
   - depolarisation
   - MP moves towards ‘threshold’
   - gated ion channels closed
2. MP reaches ‘threshold’ (-55mV)
   - Na channels start opening
   - Na influx
   - more depolarisation
   - K channels remain closed
3. • all Na channels open
   • max Na influx
   • MP overshoots 0mV
4. When MP reaches +35mV
   - Na channels shut (h gate closes)
   - K channels open
   - K efflux begins
5. AP down stroke
   - recovery phase
   - Na channels shut (-> refactory period)
   - K channels open
   - K efflux continues
6. MP returns to ‘resting’ level
   - ion channels return to resting state
   - excitability restored

Question 25

What would happen if there was no refactory period

Answer 25

- signal could move in direction not allowed | - wouldn't have control

Question 26

Summarize action potentials

Answer 26

- all or nothing events - amplitude independent of stimulus - At 'threshold': voltage-gated Na+ channels open, Na diffuse in, further depolarisation, positive feedback involved here - at 'peak' Na channels close, voltage gated K channels open, K diffuse out, repolarisation - return to resting membrane potential

Question 27

What is the refactory period

Answer 27

- after an AP is initiated, the neuron cannot generate another AP until the first one has ended - this period of inexcitability is called the refactory period

Question 28

what is responsible for the refactory period

Answer 28

- inactivation of voltage gated sodium channels | - the inactivation (h) gates shut, and so Na cannot diffuse into the neuron

Question 29

what are the consequences of refractory period

Answer 29

- limits max firing frequency of ap's in axons - ensures unidirectional propagation of action potentials - prevents summation of action potentials - prevents summation of contractions in cardiac muscle - the cardiac AP lasts as long as the ventricular contraction

Question 30

if you were going to block sodium channels where would you need to block them from and why

Answer 30

from the inside of the cell as gates are positioned intracellularly

Question 31

Describe action potential propagation

Answer 31

- an AP in one section of axon sets up longitudinal current flow - this depolarises adjacent 'resting' parts of the axon - the AP is regenerated further along the axon - more current flows, and the next region of axon is activated - action potentials travel along the axon as waves of depolarisation

Question 32

in what direction does action potential propagation move along the length of the axon

Answer 32

- current flows in ICF and ECF from +ve to -ve regions - this current flow alters the membrane potential in adjacent regions, and the AP 'creeps' along the axon (in this way the AP travels along the length of the axon)

Question 33

Does the speed of AP propagation increase or decrease with the axon diameter

Answer 33

increase

Question 34

Do large or small axons conduct impulses more rapidly than the other

Answer 34

large is more rapid

Question 35

what greatly increases the speed of AP propagation

Answer 35

myelin

Question 36

What does myelin do to AP propagation

Answer 36

increases speed

Question 37

how does myelin arrange on an axon

Answer 37

many layers of cell membranes wrapped round the axon

Question 38

what cells lay myelin down

Answer 38

glial (schwann) cells

Question 39

how does myelin speed up AP propagation

Answer 39

forms an insulating layer, reducing leakage of current from axon

Question 40

what are the intervals which interupt myelin

Answer 40

nodes of ranvier

Question 41

what happens at the nodes of ranvier

Answer 41

the axon membrane is exposed to the ECF, and ion flow can occur

Question 42

describe saltatory conduction

Answer 42

- in myelinated nerve, the passive currents spread further along the axon - there are fewer regeneration steps per unit length of axon - thus, the AP propagates more rapidly than in unmyelinated axons

Question 43

apart from axons, what do we have high concentrations of in peripheral nerves

Answer 43

- lipid - fat pads - blood vessels - mix of myelinated and unmyelinated axons

Question 44

how do axons within peripheral nerves differ

Answer 44

Size - axon diameter - conduction velocity Function - sensory - motor

Question 45

What axon types are prevent in 'cutaneous' nerves and what are their functions

Answer 45

``` A beta (myelinated) - mechanoreceptors ``` A delta (myelinated) - mechanoreceptors - thermoreceptors (cold) - nocieptors - chemoreceptors (taste) C (unmyelinated) - mechanoreceptors - thermoreceptors (hot (and cold?)) - nocieptors

Question 46

Of the mental and canine nerve, which has less myelination and why

Answer 46

less myelination in canine nerve - as nerve enters the tooth they loose myelination - has an affect on how LA works Mental is near tooth Canine nerve inside pulp

Question 47

explain the last slide of the lecture

Answer 47

find out what he said