Lecture 7: Action Potentials

Question 1

Opening and closing of any gate is _____.

Answer 1

probabilistic

ie. at intermediate voltage (like -50mV), some fraction of K-channels will have open n-gates, and some will not

Question 2

Does m gate or n gate opens faster?

Answer 2

’m’ gate (Na) opens faster than the ‘n’ gate (K)

Question 3

When is each gate (m, h, n) more likely to be open?

Answer 3

m: positive potentials
h: negative potentials
n: positive potentials

Question 4

Why does the K+ conductance not need to have an inactivation gate of its own?

Answer 4

K+ itself is negative feedback – just by going through its channels will make its own channels inactivate because it’s making the membrane potential more negative, which closes the n gate

because K+ is moving the membrane potential in the opposite direction of Na+, it does not need an inactivation gate because it is its own activation gate

Question 5

Why does the Na+ conductance need to have an inactivation gate of its own?

Answer 5

the more Na+ that enters, the more depolarized the membrane potential will be – need an external mechanism (h gate) to stop Na+ from entering

Question 6

Can we explain each phase of a real world AP?

Answer 6

yes

• depolarization is about extra Na+ conductance
• repolarization is about Na+ conductance going away, and K+ conductance starting
• hyperpolarization is about K+ conductance staying on for some time, even though it returns to RMP and threshold potential

Question 7

What’s happening at VTh to cause accelerating depolarization?

Answer 7

accelerating depolarization is positive feedback cycle for Na+ channels – every few channels that open, open more channels for current to flow

Question 8

Why is there a refractory period after an AP is fired (before another one can be generated?)

Answer 8

refractory period is explained by features of the h gate

Question 9

How does that all work so quickly?

Answer 9

• not able to explain with H&H’s data

- only channels can explain how so much charge can move across membrane so quickly – transporters are not enough

Question 10

What is the problem with action potentials?

Answer 10

action potentials are very large depolarizations, and when triggered they move a lot of ions (both Na+ and K+) across the membrane

• those ions eventually have to be pumped back the other way, and that’s energetically expensive
• they also reduce rich analog, graded information to a binary ‘fire or don’t fire’
• are metabolically wasteful

Question 11

What is myelin?

Answer 11

substance containing high levels of lipid and proteins, produced in myelinating glia, which form outgrowths that wrap around neural axons in CNS and PNS (up to 16-20 layers)

Question 12

What is the function of myelin?

Answer 12

provides electrical insulation for axons

Question 13

What does myelination produce? What does it reduce?

Answer 13

very tiny extracellular space between myelin and axon membrane

severely reduces the expression/function of ion channels below it

Question 14

What does myelination interfere with?

Answer 14

layers of insulation interferes with ability of axon membrane to act as capacitor (ie. storing charge when current flows)

Question 15

What does myelination allow?

Answer 15

allows electrotonic current to spread more efficiently, increasing depolarization rate

Question 16

How does myelination affect Rm and Ri, and the length constant?

Answer 16

increases Rm (due to fewer leaking channels) without affecting Ri, therefore λ will greatly increase

• current will spread much more efficiently, compared to an unmyelinated axon
• this allows the AP related depolarization to travel much further by electrotonic conduction before needing to be regenerated

Question 17

What are myelinated stretches of axons punctuated by?

Answer 17

VG-channel dense Nodes of Ranvier

Question 18

Where are voltage-gated sodium channels in a myelinated axon found?

Answer 18

centre of nodes of Ranvier

Question 19

Where are voltage-gated potassium channels in a myelinated axon found?

Answer 19

outer parts of nodes of Ranvier

Question 20

What is saltatory propagation?

Answer 20

AP appears to jump abruptly from node to node because new APs can only be generated where there are VG-Na channels (ie. not in the internode regions)

Question 21

When does AP regenerate during saltatory conduction?

Answer 21

at each node – even before the potential has finished spreading in the internode

Question 22

Where does current decay during saltatory conduction?

Answer 22

in the internodal (myelinated) region

Question 23

How does the magnitude of AP change throughout saltatory conduction?

Answer 23

magnitude of the AP-associated current decreases gradually between nodes then abruptly jumps back to full strength as the AP is regenerated at the next node

Question 24

Is there a trade-off to myelination?

Answer 24

you now have to grow and look after a whole new class of cells, in addition to neurons

ie. multiple sclerosis (MS) – autoimmune disease that causes immune system to attack oligodendrocytes within CNS, leading to losses of myelination of CNS axons

Question 25

How is myelination affected due to multiple sclerosis (MS)?

Answer 25

• immune system attacks oligodendrocytes within CNS, leading to losses of myelination of CNS axons
• demyelinated axons are no longer insulated in the former internode regions (leaky), which can slow APs or even lead to complete failure of AP transmission in the affected axons if the VGNCs at the next node can’t get depolarized enough to reach threshold and regenerate the AP