Action potentials

Question 1

How far can an electrical signal travel passively in a neuron

Answer 1

only a few mm

axons are very poor cables

Question 2

What happens to current measured inside a cell hooked to an electrode but without ion channels?

Answer 2

Steadily increases

Question 3

What happens to current measured inside a cell hooked to an electrode with ion channels?

Answer 3

Increases to a point and then plateaus

Question 4

What are 2 important properties of action potentials?

Answer 4

1. membrane potential must be depolarized to same level before action potential occurs EVERYTIME
2. one action potential has begun: same SIZE and same SHAPE every time for the same neuron
   ALL OR NONE EVENT

Question 5

Does the action potential attenuate as it travels down the cell?

Answer 5

No

Question 6

Is the cell membrane more permeable to Na+ or K+? What is the typical resting membrane potential?

Answer 6

K+

-70mV

Question 7

How does the cell membrane change relative to Na during depolarization?

Answer 7

Becomes relatively more permeable to Na

Question 8

What determines the threshold voltage?

Answer 8

It is the point at which the Na and K voltages are exactly equal and opposite

Question 9

Once the threshold is reached, how is the action potential generated?

Answer 9

AP generated only if inward Na+ momentarily exceeds inward K+ - if it does not, no AP

Question 10

Describe the positive feedback loop generated by Na+ during depolarization

Answer 10

Na entering the channels begin conducting -> further depolarize cell

With increased depolarization -> more Na channels start to open

ase more Na channels open -> more Na enters cell and cell becomes more depolarized

Question 11

Why does repolarization occur?

Answer 11

1. P Na declines back to resting level

2. P K undergoes transient increase

Question 12

When is the incactivation gate closed? When is it open?

Answer 12

inactivation gate is open at rest, closes at depolarization

Question 13

When is the activation gate open? When is it closed?

Answer 13

Activation gate is open at depolarization, closes at rest

Question 14

What are the relative speeds of the Inactivation and activation gates?

Answer 14

Inactivation gate slower than activation gate

Question 15

When are K channels open? When are they closed? How do they respond relative to Na channesl

Answer 15

K channels are closed at rest, open at depolarization, but respond with delay compared to Na

Question 16

What causes the refractory period?

Answer 16

Na leaves cell, K enters, membrane potential builds back up

Question 17

Describe the negative feedback that occurs during the refractory period

Answer 17

depolarization opens K channels-> K enters -> re-polarizes cell-> induces K channels to shut

Question 18

What is the benefit of the voltage gated potassium channels?

Answer 18

Faster re-polarization -> more action potentials in a give period of time

Question 19

What is the absolute refractory period?

Answer 19

Time when no stimulus can evoke an AP (inactivation Na gates are closed)

Question 20

What is the relative refractory period?

Answer 20

Time when stronger than normal stimulus is reqired to evoke AP

Question 21

What is the function of the Na/K pump?

Answer 21

Restore proper ion balance

• depends on surface to volume ratio
• big axons run down slowly

Question 22

What causes the initial depolarization?

Answer 22

synaptic transmission

Question 23

How does local anesthetic work?

Answer 23

Blocks sodium channels-> cannot open
- signal may spread passively by cannot travel length of axon to propagate signal

=> cannot reach threshold to propagate signal and send “pain” to brain

Question 24

Explain the safety factor

Answer 24

the membranes of neurons contain 10-15 x the number of Na+ channels necessary to reach threshold depolarization

WHY:

1. Branching axons: at each branch point the safety factor decreases
2. Sodium channel inactivation: Na+ channels cannot propagate new signal until refractory period complete- with more Na channels, some did not depolarize, and axon is ready for next AP sooner
3. Velocity: excess Na channels produce extra current -> next ring of membrane will be depolarized more quickly.

Question 25

What does conduction velocity in an unmyelinated neuron depend on?

Answer 25

ONLY axon diameter:

• small diameter = depolarize shorter length ahead of active locus
• large diameter = depolarize longer length ahead of active locus

Question 26

What is the effect of myelin on nerve conduction?

Answer 26

Myelin increases the effective resistance between axoplasm and extracellular fluid =>

ELECTRICAL INSULATOR!
- speeds signal propagation

Question 27

Explain saltatory conduction

Answer 27

AP jumps from node (of ranvier) to node -> of myelinated axons (increased velocity

Question 28

What does the conduction velocity in a myelinated neuron depend on?

Answer 28

directly proportional to diameter

=> 2x diameter = 2x velocity

Question 29

What is the length constant?

Answer 29

Lambda
= pont (distance) by which AP has lost 37% of its original value
= 0.5x square root of (membrane resistance/internal resistance)
umyelinated = ~1mm
(Rm is low, Ri is high)

Question 30

How does internal resistance Ri affect current propagation?

Answer 30

Increased internal resistance (Ri) = decreased spread of current

Question 31

How does membrane resistance Rm affect current propagation?

Answer 31

increased membrane resistance (Rm) = increased spread of current

Question 32

How does membrane capacitance affect current propagation?

Answer 32

increased membrane capacitance (Cm) = decreased spread of current

Question 33

Why are axons poor passive conductors?

Answer 33

low Rm (less current spread)
high Ri (less current spread)
high Cm (less current spread)

Question 34

How does lidocaine work?

Answer 34

Binds intracellular portion of voltage-gated Na+ channels, blocking Na+ entry

=> Channel must be open so it can enter cell
- most effective on smaller (low safety factor) & more active (open channel) axons

Preferentially affect pain over touch axons*