Session 4 - Action Potentials

Question 1

What is an action potential?

Answer 1

A change in voltage across a membrane

Question 2

What does an action potential depend on to be generated?

Answer 2

Ionic gradients and relative permeability

Question 3

What is required to be reached for an action potential to be generated?

Answer 3

A threshold voltage level

Question 4

What is meant by the “all or nothing” theory of action potential generation

Answer 4

Action potentials are either generated or they are not, there are no half or double action potentils

Question 5

What is the basis of the all or nothingsystem?

Answer 5

The positivefeedback loop, whereby the membrane depolarizes, causing more Na+ channels to open, which means more Na+ enters cell which means the membrane depolarizes further leading to more Na+ channels opening - and so on.

Question 6

Outline the sodium hypothesis

Answer 6

Once the membrane has been depolarised to threshhold voltage, voltage gated Na+ channels open, allowing Na+ influx as ions attempt to move to their equilibrium potential of +61 mv. This influxdeploarises the membrane further, causing more voltage gated Na+ channels to open.

Question 7

What two events occur to cause repolarisation?

Answer 7

During maintained depolarisation, Na+ channels close by a mechanism called inactivation. Voltage gated K+ channels are opened by depolaristion, causing a K+ ion efflux as K+ attempts to move towards it own equilibrim potential.

Question 8

What is A?

Answer 8

Na+ channels open

Question 9

What is happening at B?

Answer 9

Reducing Na in cell makes ENa more negative

Question 10

What is C?

Answer 10

Na+ channels close due to inactivation

Question 11

What is D?

Answer 11

K+ channels open

Question 12

Why can an action potential not be stopped halfway?

Answer 12

Because once threshold voltage reached, positive feedback of voltage gated sodium ion channels guarantees action potential synthesis.

Question 13

What is the absolute refractory period?

Answer 13

Nearly all Na+ channels in inactivated state. Excitability is 0.

Question 14

What is relative refractory period?

Answer 14

Na+ channels re recovering from inactivation, the excitbility returns towards normal as the number of channels in the inactive state decreses.

Question 15

Howare Na+ channels inactivated?

Answer 15

By N-peptide binding

Question 16

What is accomadation?

Answer 16

The longerastimulus is, the larger the depolarisation necessary to initiate an action potential.This is because Na+ channels become inactivated.

Question 17

Describe the structure of voltage gated Na+ and Ca2+ channels

Answer 17

1 peptide with 4 homologous repeats Each repeat consists of 6 transmembranous domains 1 domain is able to sense the voltage field across a membrane Function required 1 subunit

Question 18

Describe the structure of voltage gated K+ channels

Answer 18

Four non=linked peptide repeats Six Transmembrane Domains One Domain is Voltage Sensitive Function requires Four subunits

Question 19

How do local anaesthetics prevent action potential generation?

Answer 19

Bind and block Na+ channels, stopping action potential generation

Question 20

In what order do local anaesthetics block nerve fibres?

Answer 20

1.Small myelinated axons 2.Non-myelinated axons 3.Large myelinated axons

Question 21

Why do local anaethetics effectneurones in this order?

Answer 21

Because they effect sensory neurones before motor

Question 22

At what point do LA’s block Na+ channels?

Answer 22

When the channel is open

Question 23

What can extracllular recording of action potentials give information about?

Answer 23

Conduction velocity under various conditions

Question 24

How can extracellular recording be used to measure conduction velocity?

Answer 24

Electrodes are used to raise the membrane potential to threshold to generate an action potential. Changes in potential between stimulating (Cathode, -‘ve) and recording (Anode,+’ve) electrodes along an axon.

Question 25

How can conduction velocity be calculated?

Answer 25

Using the equation conduction velocity = distance between electrodes /time gap between the stimulus and the action potential being generated

Question 26

How are axons raised to threshold?

Answer 26

Arrival of a stimulus causes voltage dependent Na+ channels to open

Question 27

What is the local circuit theory of propagation?

Answer 27

Change in membrane potential at one part of axon spreads to adjacent areas via local current spread. Local current spread causes depolarisation of adjacent membrane and opening of voltage gated sodium ion channels, causing the propagation of the action potential.

Question 28

What is the relationship between the distance the local current spreads and the speed of the concuction velocity of the axon

Answer 28

Further the spread, the faster the conduction velocity of the axon

Question 29

What are three properties of an axon which lead to high conduction velocity?

Answer 29

• A high membrane resistance - A high axon diameter (low cytoplasmic resistance) - A low Membrane capacitance

Question 30

How does a high membrane resistance lead to high conduction velocity?

Answer 30

???? Can anyone clarify this in their notes? The lower the resistance, the more ion channels open. Lower the resistance > more ion channels open > more loss of local current across the membrane > limits local current spread

Question 31

How does low membrane capacitance led to high conduction velocity?

Answer 31

Measure of the ability of the lipid bilayer to store charge. High capacitance requires more current to charge (or a longer time for a given current). Can cause a decrease in local current spread.

Question 32

What is increased by myelination?

Answer 32

Conduction velocity

Question 33

What type of axons are myelinated?

Answer 33

Large diameter axons such as motor neurones, smaller ones such as sensory neurones are not.

Question 34

How does myelination increase conduction velocity?

Answer 34

Reduces capcitance and increases Membrane Resistance. This is achieved via saltatory propagation.

Question 35

Where do action potentials occur on a myelinated neurone?

Answer 35

Nodes of ranvier

Question 36

What is saltatory propagation?

Answer 36

AP jumps between nodes of ranvier because myelin sheath acts as good insulator, so local circuit currents depolarise next node along and generate further AP

Question 37

What is the density of Na+ ion channels at the nodes of ranvier? How does this differ from sensory neurones?

Answer 37

High Sensory neurones have even distribution

Question 38

What two types of cells are responsible for myelination?

Answer 38

Schwanncells and oligodendrocytes

Question 39

What are the consequences of demyelination, as in MS?

Answer 39

Insufficient number of Na+ channels at demyelinated regions, which reduce or block conduction velocity of action potentials.

Question 40

Give a local anesthetic example

Answer 40

Procine blocks na chanalls

Question 41

Synaptic tramesion types of synpase

Answer 41

Axo axonal Axodendretic Axosomatic