Session 4- AP and its properties

Question 1

What effect on Vm does an increase in conductance of a particular ion have?

Answer 1

The Vm will move closer to the membrane potential for that ion

Question 2

What happens to Na+ conductance during the upstroke?

Answer 2

It increases

Question 3

What happens to K+ conductance during depolarisation?

Answer 3

In increases

Question 4

What is the use of a voltage clamp?

Answer 4

It prevents Vm from changing in response to changes in membrane current, so allows ionic currents to be measured.

Question 5

What is the threshold potential?

Answer 5

The level to which Vm must be increased in cause an action potential

Question 6

What is the axon hillock?

Answer 6

Area where AP is initiated from synaptic inputs

Question 7

What is the “all or nothing response”?

Answer 7

If threshold potential is exceeded then an AP will be completely generated regardless of starting voltage. This is due to +ve feedback on voltage gated Na+ channels

Question 8

What happens during the upstroke?

Answer 8

V-gated Na+ channels open
Na+ enters cell
Membrane depolarises
More Na+ channels open (+ve feedback)

Question 9

What happens during the downstroke?

Answer 9

Na+ channels are inactivated
K+ open
Efflux of K+
Na+ influx stops
Repolarisation

Question 10

What is the absolute refractory period?

Answer 10

Most (nearly all) Na+ channels are inactivated. Another AP cannot occur during the ARP

Question 11

What is the relative refractory period?

Answer 11

Na+ channels are recovering and the electrical excitability of the membrane returns to normal as fewer Na+ channels are inactivated

Question 12

What is accommodation?

Answer 12

The longer the stimulus, the larger the depolarisation needed to initiate an AP - shown by the threshold curve

Question 13

What is the function of the S4 region of K+ and Na+ channels?

Answer 13

Voltage sensitivity

Question 14

What is the function of the P region of Na+ and K+ channels?

Answer 14

Contributes to ionic selectivity

Question 15

What is the MoA of local anaesthetics such as procaine?

Answer 15

Blocks Na+ channels, preventing Na+ influx, so preventing an AP from being generated

Question 16

How can the conduction velocity of axons be measured?

Answer 16

By recording changes in potential between the stimulating (Cathode –‘ve) and recording (Anode, +’ve) electrodes along an axon, conduction velocity can be calculated using the equation Conduction Velocity = Distance/Time

Question 17

What is the local current theory?

Answer 17

Depolarisation of a small region of membrane produces a local current which then propagates by activating nearby v-gated Na+ channels causing the propagation of an AP

Question 18

What factors affect the propagation of the current?

Answer 18

Membrane resistance
Axon diameter (so membrane area)
Membrane capacitance

Question 19

What is the effect of myelination on conduction velocity?

Answer 19

Increased resistance, and decreased capacitance so increases conduction velocity

Question 20

What is saltatory conduction?

Answer 20

AP jumps from one node of ranvier to the next; the mechanism of AP conduction in myelinated neurones

Question 21

What are some examples of myelinated neurones?

Answer 21

Motor neurones

Question 22

What are some examples of unmyelinated neurones?

Answer 22

Small diameter sensory fibers e.g C-type neurones

Question 23

What are some examples of demyelinating disease?

Answer 23

Multiple Sclerosis : autoimmune demyelination in some areas of the CNS

Guillain-Barre syndrome = peripheral nerve autoimmune demyelination, usually triggered by an infection