action potentials icpp

Question 1

Give 3 properties of action potentials

Answer 1

threshold value must be reached to induce AP
all or nothing principle
propagated without loss of amplitude

Question 2

Where is an AP initiated?

Answer 2

axon hillock

Question 3

How do we know Na is important in AP?

Answer 3

when concentration of Na is change outside the size of the AP changes parallel to change in ENa

Question 4

Explain the changes in ionic permeability in an action potential

Answer 4

Na voltage gates open to influx of Na 
less negative membrane potential 
Na gates inactivate and K voltage gates open 
hyperpolarisation 
slowly close k gates 
membrane repolarised

Question 5

What is the absolute refractory period?

Answer 5

nearly all Na voltage channels are inactivated no AP can be generated

Question 6

What is the relative refractory period?

Answer 6

Na voltage channels recovery number of channels inactivated as decreased
AP can be induced by greater stimulus required

Question 7

Why is the refractory period important?

Answer 7

ensure that AP is propagated in one direction only

Question 8

What is the molecular structure of a sodium voltage gated ion channels?

Answer 8

1 alpha subunit with 4 repeats
S5/6- of each repeat = pore
S4= voltage sensor
S1/S6 of repeats3/4- inactivation particle

Question 9

How does the voltage sensor in the various ion channels work?

Answer 9

S4 =series of positively charge aa residues in the voltage filed
change in voltage field results in a change in conformation and the opening of the pore

Question 10

What is the molecular structure of the k voltage ion channel ?

Answer 10

4 x alpha subunits
s4 = voltage sensor
s5/6= pore

Question 11

Name a local anaesthetic

Answer 11

lidocaine

Question 12

What are the theories of how local anaesthetics work?

Answer 12

1. blocking occurs at closed state- poorly understood
2. open channel block pathway, protonated form enters open pore and blocks it strongest block at inactivation state
3. hydrophobic block pathway

Question 13

Explain local current theory.

Answer 13

depolarisation causes local membrane depolarisation.
the further away the point of AP initiation the smaller the depolarisation
any region that depolarisation is above threshold induces an AP, those Na channels closest to initial AP

Question 14

What is the length constant? What does it depend on?

Answer 14

the distance it takes for the potential to fall to 37% of its original value.
depends on capacitance and membrane resistance

Question 15

How does conductance velocity change with diameter?

Answer 15

increase with diameter
increase membrane resistance
decrease capacitance

Question 16

What cells produce myelin?

Answer 16

schwann cells

Question 17

What are the regions where there is a high concentration of Na voltage channels called in a myelinated axon?

Answer 17

nodes on ranvier

Question 18

How does a myelin sheath improve conductance velocity?

Answer 18

myelin is a good insulator
local currents depolarise the next node above threshold
AP jumps from node to node

increase membrane resistance
decrease capacitance
increasing length constant
increasing conduction speed

Question 19

What is they type of conduction seen in a myelinated axon?

Answer 19

saltatory conduction

Question 20

What is the effect of demyelination?

Answer 20

stop saltatory conduction
reduced insulation and not enough Na channels in myelin covered area to induce AP in next node
AP stopped

Question 21

Name a condition where myelin is affected in the CNS

Answer 21

multiple sclerosis

Question 22

Name a condition where myelin is affected in the PNS

Answer 22

Laudry- Guillain-Barre syndrome

Question 23

How do we know Ca implicated in NT release?

Answer 23

decrease in extracellular Ca causes decrease in end plate potentials

Question 24

Describe molecular structure of Ca channel?

Answer 24

1 alpha subunit 4 repeats

very similar to Na voltage channel

Question 25

What type of Ca voltage channel is predominant in muscle, heart, lungs and neurons? Which blocker is it sensitive to?

Answer 25

L-type

dihydropyridines

Question 26

What can the alpha subunits of Ca and Na voltage channels be combined with and what is the effect?

Answer 26

other subunits can be added to modify the behaviour of channel

Question 27

How can Ca channels be inactivated?

Answer 27

1. voltage dependent inactivation

2. Ca dependent inactivation

Question 28

What does Ca bind with to bring vesicle closer to the membrane?

Answer 28

synaptotagmin

Question 29

What does the SNARE complex make ?

Answer 29

fusion pore through which nt can be released

Question 30

What receptors are present at the NMJ for ACh?

Answer 30

nicotinic acetylcholine receptors

Question 31

What are the two types of neuromuscular blocker?

Answer 31

competitive blocker

depolarising blocker

Question 32

Name a competitive blocker. How do competitive blockers work?

Answer 32

d-tubocurarine
compete for ACh binding site on nAChR
increasing concentration of ACh can overcome block

Question 33

Name a depolarising blocker. How do they work?

Answer 33

succinylcholine
activate nAChR intial AP
no broken down by AChesterase and therefore maintenance of AP causes inactivation of Na channels so that a AP cannot be induced by ACh

Question 34

What is myasthenia gravis? What is the pathophysiology?

Answer 34

autoimmune disorder where antibodies are produced against nAChR.
nAChR destroyed by complement mediated lysis
reduced end plate potential resulting in muscle weakness

Question 35

How is myasthenia gravis diagnosed ?

Answer 35

weakness induced by repeat facial movement
inject AChesterase inhibitor
if MG recovery is quick

Question 36

How do organophosphates act as depolarising blockers?

Answer 36

form stable irreversible covalent bonds with AChesterase

Question 37

Why does it often take weeks to recover from organophosphate poisoning?

Answer 37

need to wait for synthesis of enzyme