Action Potential (LA)

Question 1

What is the name of the potential difference that exits across the membrane of axons (and all cells)?

Answer 1

resting membrane potential

Question 2

What is the range of the resting membrane potential?

Answer 2

(-) 20-90mV

Question 3

How does the change of the ICF compare to the ECF at the resting membrane potential?

Answer 3

ICF is negative compared to ECF (so RMP is -ve)

Question 4

How do the numbers of positive and negative charges differ between the ICF and ECF?

Answer 4

numbers of -ve and +ve charges are equal in ICF and ECF

Question 5

How is the membrane polarised despite there being equal numbers of positive and negative charges on either side at RMP?

Answer 5

directly on either side of the membrane, positive charges surround the outside (ECF) while negative charges surround the inside (ICF)

Question 6

What is used as the reference for resting membrane potential?

Answer 6

-70 mV

Question 7

Compare the ECF and ICF composition of Na+

Answer 7

ECF 145 mM
ICF 15 mM

Question 8

Compare the ECF and ICF composition of K+

Answer 8

ECF 4 mM
ICF 150 mM

Question 9

Compare the ECF and ICF composition of Cl-

Answer 9

ECF 110 mM
ICF 10 mM

Question 10

What is the diffusion gradient of Na+ at RMP (in theory as Na+ channels are closed)?

Answer 10

ECF (145mM) -> ICF (15mM)

Question 11

What is the diffusion gradient of K+ at RMP?

Answer 11

ICF (150mM) -> ECF (4mM)

Question 12

Describe the movement of Na+ at RMP

Answer 12

No movement (or negligible leakage) - resting membrane is impermeable to Na+ as Na+ channels are closed

Question 13

Describe the movement of K+ at RMP

Answer 13

K+ moves from ICF -> ECF via facilitated diffusion through open K+ channels - resting membrane is very permeable to K+

Question 14

What is the effect of the impermeability of Na+ and the permeability of K+ on the membrane potential?

Answer 14

movement of K+ from ICF to ECF results is an excess of -ve charge inside the cell and an excess of +ve change outside the cell. Na+ remains at a high concentration in the ECF (negative RMP results)

Question 15

What is the main cause of the RMP?

Answer 15

diffusion of K+ from the cell interior to exterior through open K+ channels

Question 16

What happens to the small amount of Na+ that leaks into the cell during RMP?

Answer 16

expelled by the Na+/K+ pump

Question 17

Aside from the outwards diffusion of K+, what other factor contributes to the RMP?

Answer 17

Na+/K+ pump contributes by exchanging unequal numbers of Na+ and K+ (more positive pumped out)

Question 18

Describe the action of the Na+/K+ pump

Answer 18

moves 3 Na+ outwards and 2K+ inwards using energy from ATP (expels any Na+ leaked in, maintains K+ diffusion gradient?)

Question 19

What word can be used to describe the Na+/K+ pump?

Answer 19

electrogenic (generates charge)

Question 20

What direction does the active transport of Na+ and K+ occur across the membrane in comparison to their diffusion gradients?

Answer 20

active transport is against (opposite) diffusion gradients (Na+ actively transported out, K+ transported in)

Question 21

What is the threshold for an action potential?

Answer 21

-55mV

Question 22

What causes the ‘rising’ phase of the AP diagram?

Answer 22

Na+ influx (voltage-gated Na+ channels open at threshold)

Question 23

What causes the ‘falling’ phase of the AP diagram?

Answer 23

K+ efflux (voltage-gated K+ channels open at AP peak (+35mV) / when Na+ channels close)

Question 24

When is Na+ permeability high?

Answer 24

during the upstroke of the AP

Question 25

When is K+ permeability high?

Answer 25

during the downstroke of the AP (permeability continues throughout refractory period and RMP)

Question 26

How can gated ion channels be activated?

Answer 26

by ligand (involves receptor) or voltage

Question 27

What is meant by an ion channel being specific?

Answer 27

it is selective for a single ion e.g. Na+ channel, K+ channel, Ca++ channel

Question 28

What gate(s) does a voltage-gated sodium channel have?

Answer 28

m-gate and h-gate

Question 29

What are the 3 possible states of a voltage-gated Na+ channel?

Answer 29

- channel closed (RMP) - channel open (threshold met) - channel closed (refractory)

Question 30

What are the states of the gates when the voltage-gated Na+ channel is closed?

Answer 30

m-gate closed, h-gate open

Question 31

What are the states of the gates when the voltage-gated Na+ channel is open?

Answer 31

m-gate open, h-gate open

Question 32

What are the states of the gates when the voltage-gated Na+ channel is closed (refractory)?

Answer 32

m-gate open, h-gate closed - during hyperpolarisation

Question 33

How many gates does a voltage-gated potassium channel have?

Answer 33

1

Question 34

What are the possible states of a voltage-gated potassium channel?

Answer 34

either open (repolarisation) or closed (depolarisation) - only one gate

Question 35

Describe the state of the ion channels when a stimulus is applied but the depolarisation does not reach the threshold?

Answer 35

voltage-gated Na+ channels closed (m-gate closed) and voltage-gated K+ closed

Question 36

What happens when the membrane potential reaches the threshold (-55mV)?

Answer 36

- voltage-gated Na+ channels start opening - Na+ influx - more depolarisation (more channels reach threshold due to Na+ influx) K+ channels still closed

Question 37

What happens when all the Na+ channels are open?

Answer 37

- maximum Na+ influx - MP overshoots 0mV

Question 38

What membrane potential is reached at the AP peak?

Answer 38

+ 35mV

Question 39

What happens when the MP reaches +35mV?

Answer 39

- Na+ channels close (h-gate closes) - K+ channels open - K+ efflux begins (repolarisation)

Question 40

Which Na+ gate is referred to as an inactivation gate and is associated with the refractory period?

Answer 40

h-gate

Question 41

What happens during the AP down stroke?

Answer 41

- K+ efflux continues as K+ channels still open - Na+ channels shut due to h-gate (-> refractory period)

Question 42

What happens once the membrane potential has returned to resting level (-70mV)?

Answer 42

- ion channels return to resting state - excitability restored - Na+ closed and gates have rearranged (m-gate closed, h-gate open) - voltage-gated K+ closed

Question 43

What is the significant of the refractory period?

Answer 43

neurone cannot generate another AP until the first one has ended and excitability is restored

Question 44

What is meant by the AP being an all-or-nothing response?

Answer 44

once the threshold is reached, the amplitude of the AP is independent of the stimulus

Question 45

What is the name of the type of feedback involved in depolarisation?

Answer 45

positive feedback - opening of Na+ channels allows more Na+ influx which opens more channels and creates a greater Na+ influx

Question 46

What is the refractory period?

Answer 46

period of inexcitability - neurone cannot generate another AP until the first one has ended

Question 47

What causes the period of inexcitability (refractory period)?

Answer 47

voltage-gated Na+ channels are inactivated due to the closure of the inactivation / h gates so Na+ cannot diffuse into the neurone

Question 48

What are the consequences of the refractory period?

Answer 48

- limits max firing frequency of AP - ensures unidirectional propagation - prevents summation of APs - prevents summation of contractions in cardiac muscle

Question 49

What is the significance of the refractory period for cardiac muscle?

Answer 49

refractory period prevents summation of contractions in cardiac muscle - the cardiac AP lasts as long as the ventricular contraction

Question 50

How does the propagation of APs remain unidirectional?

Answer 50

Na+ channels after the AP are inactivated (refractory period) so only Na+ channels ahead of the AP can be activated

Question 51

Outline how an AP propagates

Answer 51

- AP is initiated in one section of axon - Na+ influx depolarises adjacent resting parts of the axon (reach threshold) - the positive Na+ ions move towards the negative ICF in the adjacent part of the axon - AP is regenerated further along axon - more current flows (Na+) - next region of axon is activated

Question 52

In what form do APs travel along the axon?

Answer 52

as waves of depolarisation

Question 53

How does axon diameter affect speed of AP propagation?

Answer 53

as axon diameter increases, speed of AP propagation also increases. Large axons conduct impulses more rapidly than small ones

Question 54

What has evolved to greatly increase AP propagation speed without having extremely large diameter axons?

Answer 54

myeline

Question 55

What does myelin consist of?

Answer 55

many layers of cell membranes wrapped around the axon

Question 56

Which glial cells lay down myelin?

Answer 56

Schwann cells in PNS, oligodendrocytes in CNS

Question 57

How does myelin increase speed of AP propagation for a given axon diameter?

Answer 57

myelin forms an insulating layer which reduces leakage of current (Na+) from axon. And results in saltatory conduction

Question 58

What is the name of the intervals where the myelin sheath is interrupted?

Answer 58

nodes of Ranvier

Question 59

What happens at the nodes of Ranvier?

Answer 59

the axon membrane is exposed to the ECF so the exchange of Na+ and K+ for AP occur

Question 60

What is the jumping of AP from one node of Ranvier to the next referred to as?

Answer 60

saltatory conduction

Question 61

How does saltatory conduction increase conduction speed?

Answer 61

- current spreads further along axon - fewer regeneration steps per unit length of axon - so AP propagates more rapidly compared to unmyelinated axons

Question 62

Name the different connective tissue layers that offer protection for a peripheral nerve

Answer 62

endoneurium, perineurium, epineurium

Question 63

What is the endoneurium?

Answer 63

protective CT layer that surrounds an axon

Question 64

What is the perineurium?

Answer 64

protective CT layer that surrounds a bunch of axons (can be a mix of myelinated an unmyelinated axons within the perineurium). Surrounded by blood vessels

Question 65

What is the epineurium?

Answer 65

connective tissue that joins and protects different nerves

Question 66

What does a peripheral nerve consist of?

Answer 66

different nerves (encased by perineurium), blood vessels / artery and vein, and fat all protected by a epineurium layer

Question 67

How may the axons found within the same peripheral nerve differ?

Answer 67

Can differ in size: axon diameter and conduction velocity. Can differ in function: sensory or motor.

Question 68

What are the types of axons present in cutaneous nerves?

Answer 68

AB, Ad, C fibres

Question 69

Which axons / fibres are myelinated?

Answer 69

AB and Ad fibres (A fibres)

Question 70

Which axons / fibres are unmyelinated?

Answer 70

C fibres

Question 71

What is the function of AB fibres?

Answer 71

mechanoreceptors

Question 72

What is the function of Ad fibres?

Answer 72

- mechanoreceptors - thermoreceptors (cold) - nociceptors - chemoreceptors (taste)

Question 73

What is the function of C fibres?

Answer 73

- mechanoreceptors - thermoreceptors (hot and cold) - nociceptors

Question 74

What features affects an axon's susceptibility to LA?

Answer 74

- axon diameter (smaller diameter increases susceptibility) - whether it is myelinated or not (myelination increases susceptibility?)

Question 75

What types of axons are found in the coronal pulp?

Answer 75

unmyelinated C fibres, myelinated Ad and AB fibres

Question 76

How does the proportion of myelinated and unmyelinated fibres in the coronal pulp differ?

Answer 76

majority are unmyelinated C fibres, minority are myelinated Ad and AB fibres

Question 77

Why does the pulp contain both myelinated (Ad and AB) and unmyelinated C fibres?

Answer 77

allows the pulp to respond to different stimuli and transmit different messages to the brain - e.g. sharp pain (Ad) vs dull ache (C - slower)