Oral Biology- Action Potentials and LA

Question 1

What is RMP due to?

Answer 1

Mainly due to diffusion of K+ from cell interior through K+ channels

Question 2

What is the diffusion potentionals of sodium and potassium?

Answer 2

Na+ would diffuse into cell, but ‘resting’ membrane is impermeable to Na+

K+ would diffuse out of cell, and ‘resting’ membrane is very permeable to K+

Question 3

How does the sodium/potassium pump contribute to the resting membrane potential?

Answer 3

Exchanges unequal numbers of K+ and Na+

Pump moves 3 Na OUT
And 2 K IN

Question 4

What is the AP thresholds?

Answer 4

-55mV

Question 5

What is the ‘rising’ phase AP due to?

Answer 5

Na influx

Question 6

What is falling AP stage due to?

Answer 6

K efflux

Question 7

What are the 3 voltage-gated sodium channel positions?

Answer 7

Channel closed
- m-gate closed, h-gate open

Channel open
- m-gate open, h-gate open

Channel closed
- m-gate open, h-gate closed (refractory period)

Question 8

What is the first stage of AP?

Answer 8

Stimulus applied– depolarisation
MP moves towards ‘threshold’
Gated ion channels closed

Question 9

What happens when MP reaches threshold?

Answer 9

MP reaches -55mV

• Na channels start to open
• Na influx
• More depolarisation

K channels remain closed

Question 10

What happens after MP reaches 0mV

Answer 10

MP overshoots 0mV

Question 11

What happens when MP reaches +35mV?

Answer 11

Na channels shut
Inactivation (‘h’) gate closes

K channels open
K efflux begins

Question 12

What happens after K efflux begins?

Answer 12

AP downstroke
Refractory period

MP returns to ‘resting’ level
Ion channels return to resting stage

Question 13

What is independent of stimulus?

Answer 13

Amplitude is independent of stimulus

Question 14

What causes refractory period?

Answer 14

Due to inactivation of voltage-gated sodium channels

Question 15

What are 4 consequences of the refractory period?

Answer 15

1. Limits maximum firing frequency of AP in axons
2. Ensures unidirectional propagation
3. Prevents summation of APs
4. Prevents summation of contractions in cardiac muscle- the cardiac AP lasts as long as ventricular contractions

Question 16

What lays down myelin?

Answer 16

Glial cells

Question 17

What is the function of myelin sheaths?

Answer 17

Forms insulating layer, decreases leakage of current from axon

Question 18

Name the interruptions in myelin sheath

Answer 18

Nodes of Ranvier

Question 19

What are the 3 axon types present in ‘cutaneous’ nerves?

Give function

Answer 19

Aβ- myelinated- mechanoreceptors

Aδ- myelinated- mechanoreceptors, thermoreceptors, nociceptors, chemoreceptors

C- UNmyelinated- mechanoreceptors, thermoreceptors, nociceptors

Question 20

How do LAs work?

Answer 20

Blocks voltage-gated Na+ ion channels, preventing generation of AP

Question 21

What are the 3 connective tissue layers surrounding nerves?

Answer 21

Epineurium- outer sheath

Perineurium- sheath surrounds bundles (fasicles) of nerves

Endoneurium- sheath around individual fibres within nerve

Question 22

What is the order of block in different fibres?

Answer 22

Aδ
C
Aβ
Aα

Question 23

What is the mechanism of LA actions?

Answer 23

LA binds to site in Na channel
Blocks channel and prevents Na influx

This blocks AP generation and propagation

Question 24

What are the 3 components of LA molecule

Answer 24

Organic molecules:

Aromatic region (hydrophobic)
Ester or amide bond
Basic amine side chain (hydrophilic)

Question 25

Why is LA partly dissociated

Answer 25

Active in ionised form | Can only cross membrane in un-ionised form

Question 26

Why is the LA base present as and why?

Answer 26

Hydrochloride Increases solubility in aqueous solution

Question 27

What 4 other components are added to LA preparations?

Answer 27

Reducing agent (e.g. Sodium metabisulphide) Preservative(s) Fungicide Vasoconstrictor

Question 28

Name 3 ester LAs

Answer 28

Cocaine Procaine Benzocaine*

Question 29

Name 6 amide LAs

Answer 29

``` Lignocaine (lidocaine)* Prilocaine* Articaine* Mepiracaine Bupiracaine* Ropiracaine ```

Question 30

Name 2 types of vasoconstrictors in LA

Answer 30

Adrenaline | Felypressin (synthetic vasopressin)

Question 31

What receptors does adrenaline act on and what effects are there?

Answer 31

Act on receptors on vascular smooth muscle α receptors: vasoconstriction β2 receptors: vasodilation β1 receptors: cardiac muscle - positive chonotropic effect (increases HR) - positive inotropic effect (increases force of heart)

Question 32

What receptors does felypressin act on?

Answer 32

ADH receptors

Question 33

What effect does adrenaline have locally?

Answer 33

Vasoconstrictor (α receptors)

Question 34

What effect does adrenaline have systemically?

Answer 34

Lowers TPR (β > α)

Question 35

What effect does adrenaline have on CO?

Answer 35

Increases CO

Question 36

What effect does adrenaline have on arterial blood pressures?

Answer 36

Little or no effect

Question 37

What effect does noradrenaline have locally?

Answer 37

Vasoconstrictor effect

Question 38

What effect does noradrenaline have systemically?

Answer 38

Increased TPR (α > β)

Question 39

What effect does noradrenaline have on CO?

Answer 39

Increases

Question 40

What effect does noradrenaline have on arterial BP?

Answer 40

Increases

Question 41

What causes inactivation of LA?

Answer 41

'Washout' from tissues by blood supply

Question 42

How are ester type LAs broken down and how long is their action?

Answer 42

Broken down my tissue esterases Action quite brief

Question 43

How are amide type LAs broken down and how long is their action?

Answer 43

Broken down by liver amidases Longer duration of action

Question 44

Name 6 modes of administration of LA

Answer 44

``` Surface application ('topical')* Injection Local infiltration* Regional nerve block* Root nerve block (spinal, epidural) IV ```

Question 45

What are the 2 LA preparations for lignocaine and prilocaine?

Answer 45

2% lignocaine HCl 2% lignocaine HCl + 1:80,000 adrenaline 4% prilocaine HCl 3% prilocaine HCl + felypressin (0.03 U/ml)

Question 46

What is max dose of lignocaine?

Answer 46

Approx 4mg per kg body weight

Question 47

What is max adrenaline dose?

Answer 47

500 μg

Question 48

How much adrenaline is in a cartridge?

Answer 48

27.5 μg

Question 49

What usual values are the resting membrane potential?

Answer 49

20-90 mV