Lecture 9- The action potential

Question 1

3 features of an AP

Answer 1

1) All or nothing (only occurs if threshold is reached) 2) Non-overlapping 3) Unidirectional

Question 2

AP are different in

Answer 2

different structures

Question 3

if a stimuli does not depolarise the membrane above the threshold…

Answer 3

the AP will not fire down the hillock of the axon

Question 4

hillock of axon

Answer 4

The axon hillock is the last site in the soma where membrane potentialspropagated from synaptic inputs are summated before being transmitted to the axon

Question 5

when cells are depolarised which channels are open

Answer 5

sodium

-moves Vm (membrane potential) closer to E_Na

Question 6

when the cell is polarising the …… are open

Answer 6

potassium

• moves Vm back to E_k

Question 7

Vm

Answer 7

membrane potential

Question 8

voltage clamp

Answer 8

enables membrane currents to be measured overtime at a set membrane potential

Question 9

Time course of conductance changes during an action potential

Answer 9

During the course of an action potential, the permeability to different ions changes considerably.

At the beginning of an AP Na+ conductance is high and K+ conductance is low. Mid way through the AP Na+ conductance considerably reduces and K+ conductance increases. By the end of the AP when the cell is at resting potential, the conductance of both sodium and potassium will be 0.

Question 10

Summary of sodium and potassium channel activity during AP

Answer 10

1. Summary of sodium and potassium channel activity during AP
2. NaV (voltage gated) channels open
3. Na+ influx into cell
   
   • Positive feedback to trigger opening of other NaV channels
4. Membrane depolarisation
5. At ENa NaV channels inactivate and KV channels open
   
   • Na+ influx stops
   • K+ efflux
6. Membrane repolarises

Question 11

Is Na/K ATPase pump involved in the repolarisation of the AP?

Answer 11

NO

Question 12

the refractory period is the

Answer 12

Recovery period after AP has fired- the amount of time it takes for an excitable membrane to be ready for a second stimulus.

Question 13

during the refractory peroid

Answer 13

New AP will not fire

Question 14

types of refractory period

Answer 14

ARP- absolute refractory period

RRP- relative refractory period

Question 15

absolute refractory period

Answer 15

Is the period of time during which a second action potential ABSOLUTELY cannot be initiated, no matter how large the applied stimulus is (i.e. during active AP).

Nearly all Na+ channels are in inactivated states

Question 16

relative refractory period

Answer 16

Is the interval immediately following the Absolute Refractory Period during which initiation of a second action potential is INHIBITED, but not impossible. As voltage-gated potassium channels open to terminate the action potential by repolarizing the membrane, the potassium conductance of the membrane increases and the K+ ions move out of the cell and bring the membrane potential closer to the equilibrium potential for potassium and this can lead to membrane hyperpolarization.

Na+ channels are recovering from inactivations- excitability returns to normal as the number of channels in the inactivated state decreases and as the number of open voltage KV channels closing increases

Question 17

inacgtivated voltage channels

Answer 17

cannot open

Question 18

active voltage gated channels

Answer 18

may not be open, but have the potential to

Question 19

hyperpolarisation

Answer 19

Occurs when the membran potential becomes more negative than the normal resting potential.

Until K+ conductance returns to the resting value, a greater stimulus will be required to reach the threshold for another AP

Question 20

what marks the end of the relative refractory period

Answer 20

Return of equilibrium resting potential

• E_k

Question 21

molecular properties of votlage gated Na+ channel

Answer 21

Question 22

molecular properties of votlage gated K+ channel

Answer 22

Question 23

different types of cells have

Answer 23

different conduction velocities

Question 24

the larger the axon the

Answer 24

the lower the cytoplasmic resistance and the higher the conduction velocity

Question 25

classes of peripheral axons (6)

Answer 25

Aα Aβ Aγ A𝛿 B C

Question 26

which class of peripheral axon has the created conduction velocity

Answer 26

Aα- widest

Question 27

Which class of peripheral axons has the smallest conduction velocity

Answer 27

C- narrowest

Question 28

what type of fibres are Aα

Answer 28

sensory fibres fron muscle spindles, motor neurone to skeletal muscle

Question 29

what type of peripheral axon is A𝛿

Answer 29

sensory fibre from pain and temp receptors (sharp localised pain)

Question 30

what type of peripheral axon is B

Answer 30

preganglionic neurones of the autonomic nervous sstem (myelinated)

Question 31

what type of peripheral axon is C

Answer 31

sensory fibres from pain, temp and itch receptors (diffuse pain)

Question 32

local current theory

Answer 32

Injection of current into an axon will cause the resulting charge to spread along the axon and cause an immediate local change in the membrane potential. A change in MP in one part can spread to adjacent areas of the axon

Question 33

conduction velocity is determined by

Answer 33

how far along the axon these local currents can spread

Question 34

name 3 properties of an axon that leads to high conduction velocity

Answer 34

* High membrane resistance * Low membrane capacitance * Large axon diameter- leads to low cytoplasmic resistance

Question 35

capacitance, C, is simply the

Answer 35

ability to store charge - property of the lipid bilayer

Question 36

high capacitance

Answer 36

voltage changes more slowly in response to a current injection- decrease in spread of local current

Question 37

low capacitance

Answer 37

increase conduction velocity

Question 38

membrane resistance dpends on the number of

Answer 38

ion channels open

Question 39

low resistance

Answer 39

the more ion channels open and more loss of the local current across the membrane

Question 40

high resistance

Answer 40

increase conductance- change in voltage spreads further along the axons

Question 41

what can increase conduction velocity of a neurone

Answer 41

myelination

Question 42

effect of myelination

Answer 42

reduce capacitance and increase the resistance of the axonal membrane

Question 43

which types of neurones are myelination - larger or smaller diameter?

Answer 43

larger e.g. motorneurons

Question 44

what process does myelination faciliate

Answer 44

saltatory conduction

Question 45

what is the myelin sheath made out of in the peripheral NS

Answer 45

schwanna cells

Question 46

what is myelin sheath made out of in the CNS

Answer 46

oligodendrocytes

Question 47

density of sodium channels in myelinated neruones

Answer 47

high density in nodes of ranvier- quick conduction of AP

Question 48

density of sodium channels along non-myleinted axon

Answer 48

evenly distributed

Question 49

structure of mylelin sheath

Answer 49

Layers of Schwann cells tightly folded around each other - Shwanna cells surround the outside of the neurone

Question 50

saltatory conudction

Answer 50

Myelin sheath acts as a good insulator causing local circuit currents to depolarise the next node above threshold and initiate an action potential Action potnetial “jumps “from node to node allowing much faster conduction velocity- AP only occurs at nodes.

Question 51

what occurs when demyelination happens

Answer 51

Action potential threshold cannot reach threshold in regions of demyelination and stop saltatory conduction

Question 52

what is the most common demylinating disease

Answer 52

multiple sclerosis

Question 53

multiple sclerosis causes

Answer 53

all CNS nerves to become demyelinated

Question 54

what is a less common demyelinating disease of the CNS

Answer 54

Devic's disease

Question 55

Devic's disease

Answer 55

demylination of optic and spinal cord nerves only

Question 56

demylinating disease of the PNS

Answer 56

* Landry-Guillain-Barre syndrome * Charcot-Marie-Tooth disease

Question 57

name two widely used anaesthetics

Answer 57

lidocaine and procain

Question 58

local anaesthetics can be

Answer 58

protonated or unprotonated - unprotonated can pass the membrane

Question 59

anaesthetics work by

Answer 59

blocking Na+ channels Easier when the channel is open and have a higher affinity to inactivated state of Na+ channel

Question 60

how does lidocaine act as a loacl anaesthetic

Answer 60

Act by binding to and blocking Na+ channels This stops AP from occurring

Question 61

Local anaesthetics block conduction in nerve fibres in the following order:

Answer 61

* Small myelinated axons * Non-myelinated axon * Large myelinated axon