Action Potentials

Question 1

What is action potential?

Answer 1

Electrical signal generated by excitable cells due to voltage-dependent changes in membrane permeabilities

Question 2

4 Facts about Action Potential

Answer 2

• Can propagate along the axon (ie. can spread)
  
  • No change in amplitude
• Triggered by depolarization to threshold (-40mV) which causes voltage-gated channels to open
• ** Neurotransmitter binding **
• Sensory input through opened ion channels (not voltage-gated) which causes ions to flow through & membrane potential changes

Question 3

Action Potential “All or None” Phenomenon

Answer 3

Action Potential does NOT happen at the same instant
-Below threshold no action potential is generated

• Above threshold an action potential is generated
• Amplitude and duration is not affected by the “strength” of the depolarization
• Starts at cell body, spreads down axon to terminal

Question 4

3 Changes in membrane permeability create action potentials

Answer 4

• An event (ie. sensory stimuli, neurotransmitter binding) triggers membrane depolarization
• Membrane depolarization beyond a certain threshold triggers several events related to the voltage-gated ion channels
• Changes membrane resistance- CHANNELS OPEN
  
  • More changes in membrane potential because IONS FLOW THROUGH CHANNELS

Question 5

Sequence of membrane permeability events when threshold is reached (3)

Answer 5

1. ) Opening of the voltage-gated Na+ channels (↑ permeability)
   
   • Triggered by depolarization
   • Na+ flows in
2. ) Inactivation of the Na+ channels (↓permeability)
3. ) Opening of the voltage-gated K+ channels (↑ permeability)
   
   • Triggered by same depolarization, but take longer to open than Na+ channels
   • K+ flows out

Question 6

Lidocaine

Answer 6

• Lidocaine “blocks” voltage-gated Na+ channels

- NO ACTION POTENTIAL happens, so NO MESSAGES being conducted back to the brain (pain info cannot be communicated)

Question 7

How are action potentials propagated?

Answer 7

• Sodium ions move into the neuron (site of action potential) and flow across the axon length
• Capacitive (electrotonic) current = local movement of ions
  
  • Cations accumulate along the inner membrane, repelling cations on the other side of the membrane
• Change in the distribution of charges depolarizes it, reaching threshold, opening the sodium channels and allowing for a “new” action potential in this region of the axon

Question 8

Why don’t action potentials travel in both directions?

Answer 8

• Movement of ions across the membrane depolarizes the axon both upstream and downstream of the action potential, so theoretically you could get an action potential upstream and downstream… HOWEVER…
• Membrane upstream is still in the refractory period (recovery period)
  
  • Na+ channels are still inactivated = can’t open to generate new action potential
  • K+ channels are still open- membrane hyperpolarized = need more depolarization to reach threshold than in the downstream region of neuron

Question 9

What specifically propagates the action potential?

Answer 9

Local circuits of currents

Question 10

Can the propagation of action potentials be reversed?

Answer 10

No reverse propagation of action potentials

• Downstream = action potential possible
• Upstream = no action potential possible

Question 11

1st Factor controlling action potential velocity (speed)

Answer 11

1. ) Axon diameter
   - ↑ action potential velocity with ↑ increased -diameter
   - More ion channels per unit length with increasing diameter = less resistance to ion flow into the axon = easier to depolarize

Question 12

2nd Factor controlling action potential velocity (speed)

Answer 12

2. ) Temperature
   - Ion channels are temperature dependent
   - Increasing temperature exponentially increases conduction velocity
   - Warm blooded animals (vs. reptiles) can maintain a high conduction velocity without having large diameter neurons

Question 13

3rd Factor controlling action potential velocity (speed)

Answer 13

3. ) Myelination
   - ↑ action potential velocity
   - Action potential “jumps” from Node of Ranvier to Node of Ravier- Saltatory conduction
   
   • Myelin = insulates the axon
     - No ions can move in/out portions of the axon where there is the myelin wrapped around
   • Only need to depolarize the Node of Ranvier regions, which is a relatively small portion of the axon, therefore speed is much faster in myelinated axons than unmyelinated axons