Week 9 Ionic basis of the action potential

Question 1

What is the general membrane potential of neurons?

Answer 1

• Membrane potential of neurons -65 mV (Membrane is polarised)
-hyperpolarisation or depolarisation mean change-

Question 2

Who were the main scientists working on action potentials?

Answer 2

• E.Overton (1902) – sodium ions responsible for action potential
• 1939- Hodgkin and Huxley- squid giant axon to record action potential
- both Recorded action potentials from squid giant axons
- published paper before WW2- key breakthrough

Question 3

What are the key things we can learn from Hodgkin and Huxely’s and Overton’s research?

Answer 3

• Trigger for initiation of action potential is membrane depolarisation
• Threshold potential must be reached for membrane depol. Etc
• Action potentials either happen or not (big/small don’t exist).

Question 4

What are the stages of an action potential?

Answer 4

1. Rising phase (depolarisation)
2. Falling phase (repolarisation)
3. After-hyperpolarisation

Question 5

What did the 1st recreation of an action potential tell?

Answer 5

1. Action potential not just a collapse of resting pot to 0 - there exists an overshoot (goes down before it goes to 0)
2. Phase of hyperpolarisation - due to ion movements

Question 6

What experiment was conducted to test the role of Na ions in action potential and what were the results?

Answer 6

1. Normal seawater is gathered
2. Replace sodium chloride with choline chloride ( choline could not pass through membrane)
3. No action potential - the water was returned to seawater
   +Result - Extracellular sodium essential for action potential
   • Experiment confirmed Overtons hypothesis about sodium. Also showed rising phase of action potential due to membrane becoming more permeable to sodium ions.
   -Goldman equation- a 500 fold increase in membrane permeability to account for change in voltage ( rising phase). (based on the experiment)

Question 7

What happens during the falling phase of an action potential?

Answer 7

• If falling state simply due to return of state before Na ions, potential would return to -70 mV but this doesn’t happen
• So? After-hyperpolarisation- membrane potential approaches Ek ( equilibrium potential ) which suggests permeability to K ions.
• Goldman equation – 10fold increase of permeability of potassium ions.

Question 8

What does the theory of ionic basis of action potential dictate?

Answer 8

1. Increase in permeability Na ions
2. Increase in permeability K ion and decrease in permeability Na ions
3. Return to resting state permeabilities.

Question 9

How can the ionic basis of action potential theory be tested?

Answer 9

• this theory can be tested with an experiment called voltage-clamping where:
  Voltage activated Na and K ion channels mediate the changes in membrane permeability to Na and Potassium ions that occur during action potential. ( like pores)

Question 10

Describe the propagation of an action potential.

Answer 10

• Recording axon in three places we observe the depolarisation along the axon from one place to another.
• Action potentials ALWAYS go from one direction. -Important property of neurons and axons that makes sure this happens- is called refractory period-. Reduced or impossible chance of action potential to occur where it just occurred.
• Relative refractory period- we CAN have another action potential but reduced chance.

Question 11

Why is there a refractory period?

Answer 11

1. Ensures action potential is unidirectional.
2. Prevents action potential ‘bouncing back’ when they reach axon terminals
3. Prevents summation of action potentials.

Question 12

What causes the refractory period?

Answer 12

Na+ channels that mediate the rising phase of

the action potential are transiently inactivated.

Question 13

Describe the mechanism of action potential propagation.

Answer 13

• current reaches patch in membrane, activates
• current flows through activated patch in membrane, depolarizes adjacent patch
• Adjacent patch reaches threshold, current flows, depolarizes next adjacent patch
• repolarized patch is refractory so AP (action potential) travels in one direction
• after refractory period, again ready to be activated
• process continues

Question 14

What are the action potential parameters?

Answer 14

1.Amplitude - is invariant (“all-or-nothing”) for a given neuron but varies between neurons.
2. Frequency - is variable.
It is the frequency of action potentials that encodes information in neurons. However, the absolute refractory period determines the maximum number of action potentials that can be generated in 1 second
(~ 300/sec)
3. Velocity - is invariant for an individual
neuron but there is variability between neurons.
e.g.
Cat motor nerve = 30-120 m/s
Fish motor nerve = 3-36 m/s
Cockroach motor nerve = 2 m/s
Sea anemone motor nerve = 0.1 m.s
Typical squid motor axons = 4 m/s
Squid giant axons = 40 m/s (these are examples not necessary to know ALL maybe 1 or 2)

Question 15

What is the relation between velocity of an AP and the axon diameter?

Answer 15

-Velocity of action potential propagation is proportional to the square root of the axon diameter.
u = k x (square root of d)
Where:
u = velocity
k = constant
d = axon diameter
(i.e. 100x increase in d to get 10x increase in u)
e.g. squid “ordinary” motor axons = 10 µm squid giant motor axons = 1 mm

Question 16

What is Myelination?

Answer 16

• a mechanism for increasing the velocity of action potential propagation without increasing axon diameter
• Myelination enables rapid “saltatory” conduction of action potentials in myelinated axons.