2.2.4. Action Potentials

Question 1

Cells that can produce Action Potentials

Answer 1

1. Cardiac muscle
2. Skeletal muscle
3. Neurons (most)
4. Smooth muscle (some)

Question 2

The duration of an action potential is longest in which type of cell?

Answer 2

Skeletal muscle

1-2 ms compared to 0.5 ms in Neurons and Cardiac muscle

Question 3

What is largest peak amplitude of an action potential?

Answer 3

~150 mV

Question 4

Sodium Conductance Changes during an AP

Answer 4

1000-fold increase and automatically returns to resting levels

Question 5

Potassium Conductance Changes during an AP

Answer 5

30-fold increase and returns to resting levels (NOT automatically)

Question 6

What establishes the resting potential?

Answer 6

Non-gated Na and K channels (Na leaks into the cell and K leaks out of the cell)

Question 7

Do non-gated Na and K channels change during an action potential

Answer 7

NO…Don…NO

Question 8

How many gates does the voltage-gated Na channel have?

Answer 8

Two:

1. M-gate: is closed at the resting potential, but opens RAPIDLY with depolarization
2. H-gate: open at resting potential, but closes SLOWLY with depolarization

Question 9

How many gates does the voltage-gated K channel have?

Answer 9

Just one and it is closed at resting potential, but open VERY SLOWLY with depolarization

Question 10

Driving force and Conductance at Resting Potential of Na vs. K

Answer 10

Na: Driving force is large and conductance is small

K: Driving force is small and conductance is medium

Question 11

Na Channel during depolarization

Answer 11

For a brief period both gates are open and sodium enters the cell

Question 12

K Channel during depolarization

Answer 12

Begins to open very slowly (very little K leaves the cell at this time)

Question 13

Na Channel near peak of AP

Answer 13

Some of the slowly closing gates have closed, but most are still open (conductance is still large, but driving force is low AKA inward Na current is decreased but still present)

Question 14

K Channel near peak of AP

Answer 14

A large fraction of the slowly opening gates are now open (driving force is large, but conductance is small).

Outward K current is large (stops depolarizations and begins to repolarize cell back toward the resting potential of -90 mV)

Question 15

Na Channel past peak of AP

Answer 15

Rapidly open gate remains open but inward Na current decreases because slowly closing gate is closed

Question 16

K Channel past peak of AP

Answer 16

All of the slowly opening gates are now open

Question 17

Na Channel during repolarization

Answer 17

Both gates are closed

Question 18

K Channel during repolarization

Answer 18

Some of the gates are beginning to close (outward current decreases but still continues)

Question 19

Na Channel Repolarized

Answer 19

Slowly closing gate reopens. Rapidly opening gate does not reopen b/c membrane potential is below threshold

Question 20

K Channel Repolarized

Answer 20

Most slowly opening gates are closed

Question 21

Both Channels at the Resting Potential

Answer 21

Na: gates are unchanged (rapidly opening is closed, and slowly closing is open)

K: all gates closed

Question 22

Control of Sodium Conductance

Answer 22

Depolarization leads to increased conductance which leads to more sodium entry into the cell which leads to further depolarization which leads to increased conductance….

Question 23

What stops the Sodium Conductance Cycle?

Answer 23

When the bottom gate closes (slowly closing gate), it cuts off sodium conductance

Question 24

Control of Potassium Conductance

Answer 24

Depolarization leads to increased conductance of K which leads to K efflux, resulting in hyperpolarization

Hyperpolarization and Depolarization ARE NOT CONNECTED

Question 25

Sodium Potassium ATPase Pump

Answer 25

During each action potential cells gain a small amount of Na and loose a small amount of K

Na-K ATPase pumps: pump sodium out of the cell while pumping potassium back into the cell (consuming energy in the process)

THIS IS KEY: NA-K PUMP DOES NOT REPOLARIZE THE CELL DURING AN ACTION POTENTIAL (cell is repolarized by K leaving the cell)

Question 26

Action Potential Propagation

Answer 26

AP propagates to the left and not to the right (in the area ahead of the AP, Na channels are ready to be activated on depolarization)

In the area just to the right of the peak of the AP, Na channels are in various states of inactivation (they cannot be reactivated until being reset by hyperpolarization)

Further to the right, the Na channels are reactivated (prepared to produce a new AP when activated)

Question 27

What determines how soon the new AP is created?

Answer 27

Primarily a function of axon diameter and myelination

Question 28

Non-Myelinated Axon

Answer 28

The amount of current leaking out, in part, determines how far along the axon the new AP is generated (leaker = closer the new AP is to the original = slower conduction velocity)

Question 29

Conduction Velocity

Answer 29

Increases with increasing axon radius (vertebrates generally use myelination)

Myelinated CV is proportional to axon radius
Non-myelinated CV is proportional to the square root of axon radius

Question 30

Conduction in a Myelinated Axon

Answer 30

Even though there are some voltage-gated Na and K channels under they myelin, neither can pass current since they are blocked

Question 31

What channels are concentrated at the Nodes of Ranvier?

Answer 31

Voltage-gated Na channels

Question 32

Saltatory Conduction

Answer 32

AP jumps from node to node (typically in groups of 3-5)

Question 33

Multiple Sclerosis (MS)

Answer 33

Most common demyelinating disease. Current leaks out between nodes AND voltage-gated K channels (normally blocked by myelin) are exposed

At best, AP is slower. At worst, AP is blocked

Question 34

Nodes of Ranview

Answer 34

Small, periodic gaps in the insulation that expose the axon membrane to the extracellular fluid (only place where APs are produced)