LECTURE 10: ACTION POTENTIALS

Question 1

Membrane Potential Generation = 6

Answer 1

1. K+ efflux causes charge separation
2. Negative inside
3. Positive outside
4. Charge separation produces voltage
5. Open K+ channels, generate resting membrane potential.
6. Efflux of K+ down its concentration gradient.

Question 2

Membrane Potential Generation:

Answer 2

Em Equation

slide 5

in mV

Question 3

Understanding Action Potentials = 3

Answer 3

1. The action potential is a rapid reversal of membrane potential in response to a critical level of depolarization: i.e. Threshold
2. Time-course < 5ms
3. Longer in muscle
   ~10 ms skeletal
   ~250 ms cardiac

Question 4

Ionic Basis of Action Potential:

Three states of Na+ channels

Answer 4

1. Resting - CLOSED BUT CAPABLE OF OPENING
2. Activated - RAPID OPENING TRIGGERED AT THRESHOLD
   - OPEN (activated)
3. Inactivated - SLOW CLOSING TRIGGERED AT THRESHOLD
   - CLOSED and not capable of opening (inactivated)

Question 5

Ionic Basis of Action Potential

Two states of K+ channels

Answer 5

1. Resting - closed
2. Activated - delayed opening triggered at threshold.
   - open

Question 6

Ionic Basis of Action Potential = 2

Answer 6

1. Voltage gated Na+ and K+ channels
2. No direct involvement of pump or carriers, though indirect role for Na+/K+ ATPase pump

Question 7

Action Potential Generation = 2

Answer 7

1. Action potential determined by changes in Na+ and K+ conductance (gNa and gK)
2. Opening and closing of voltage gated K+ channels is slower than for Na+ channels – gives rise to after hyperpolarization

Question 8

Action Potential Function = 4

Answer 8

1. Signal transmission over long distance
2. Coding of stimulus features
3. Trigger muscle contractions
4. Neurotransmitter/hormone release

Question 9

Frequency Coding: 3

Answer 9

1. Action potential all or none
2. Size independent of strength of input or distance travelled
3. Frequency of firing increases with stimulus intensity

Question 10

Passive Signal vs Action Potential

Answer 10

1. Passive current flow
   - Decays exponentially with distance
   - Length constant ()
2. Action potential
   - All-or-nothing response
   - Fixed amplitude
   - Travels long distances (>1 m) without decay

Question 11

Propagation of Action Potential

Answer 11

Propagation :

1. Passive flow —>
2. Depolarisation —>
3. Opening Na+ channels
4. REPEAT BACK TO PASSIVE FLOW

Question 12

PROPAGATION OF ACTION POTENTIAL… NA+

Answer 12

Na+ enters the neuron

1. Active area at peak of action potential
2. Adjacent inactive area into which depolarisation is spreading; will soon reach threshold.
3. Remainder of axon still at resting potential

Local current flow that depolarises adjacent inactive area from resting potential to threshold potential.

Question 13

PROPAGATION OF ACTION POTENTIAL… K+

Answer 13

1. Previous active area returned to resting potential; no longer active; in refractory peroid.
2. Adjacent area that was brought to threshold by local current flow; now active at peak of action potential
3. NEW ADJACENT inactive area into which depolarisation is spreading; will soon reach threshold.
4. remainder of axon still at resting potential.

Question 14

Unidirectional AP Propagation…WHAT IS REFACTORY PERIOD.

Answer 14

Refractory period:
Period in which new AP can’t be initiated Inactivation of Na+ channel

Question 15

Explain the 3 stages of Refractory period.

Answer 15

1. Resting
   Activation gate CLOSED
   Inactivation gate OPEN
2. Activated
   Activation gate OPEN
   Inactivation gate OPEN
3. Inactivated
   Activation gate OPEN Inactivation gate CLOSED

Question 16

Absolute vs Relative refractory period

Answer 16

• Inactivation of Na+ channels
• After hyperpolarization – requires stronger stimulus to reach threshold

Question 17

FORWARDS VS BACKWARD CURRENT…

Answer 17

BACKWARD CURRENT

• flow does not reexcite previously active area because this area is in its refractory period.

FORWARD CURRENT flow excites new inactive area

Question 18

Action Potential Propagation

Answer 18

Depolarization at distance depends on length constant ….

Large …. means the potential change spreads further

Question 19

Action Potential Propagation EQUATION

Answer 19

SLIDE 18

Question 20

UNDERSTANDING Saltatory Conduction = 3

Answer 20

1 * Role of myelin

2 * Improves passive current flow by insulation

3 * Action potential generation only at nodes of Ranvier (high density of Na+ channels)

Question 21

WHAT IS CONDUCTION VELOCITY?

Answer 21

Compare speed of action potential (conduction velocity) between unmyelinated and myelinated axons.

Effect of axon diameter:

Question 22

UNDERSTANDING Cardiac Action Potential

Answer 22

Action potentials can differ depending on properties of ion channels involved

1. Phase 0, Na+ enters tge cell = depolarisation
2. Phase 2, Ca++ enters the cell, Initiation of contraction.
3. Phase 3, K+ exits the cell re-polarisation.
4. Resting potential

Question 23

Cardiac Pacemaker Potentials = 4

Answer 23

1. Autorhythmic cells: pacemaker activity
2. Funny Na+ channels: open when membrane potential becomes more negative (ie
   hyperpolarization): net Na+ inward current.
3. Slow closing of K+ channels
4. Transient (T-type) Ca2+ channels; long-lasting (L-type) Ca2+ channels

Question 24

SUMMARY = 7

Answer 24

1. Depolarisation (inside potential approached outside) of neurons opens voltage gated Na+ channels
2. Sodium entry drives the membrane potential positive
3. Sodium channels inactivate and voltage gates K+ channels
   open restoring the resting potential
4. Inactivation of Na+ channels produces the refractory period and prevents retrograde propagation of AP.
5. Large axon diameter or myelin increase conduction velocity by allowing graded potentials to spread further.
6. Cardiac AP are sustained by Ca2+ entry
7. Cardiac pacemaker cells have and unstable resting potential due to closure of K+ channels and Na+ leak