action potential II

Question 1

The distance that current will propagate depends on:

Answer 1

Ri – internal resistance
Higher Ri -> smaller spread of current
Rm – membrane resistance
Higher Rm -> larger spread of current
Cm - membrane capacitance
Higher Cm -> slower spread of current

Question 2

Axons are poor cables because

Answer 2

• Rm is low, Ri is high, and Cm is high
• Therefore, for a typical axon, λ is small: ~ 1mm
• However, axons are well-built for active propagation

Question 3

active propagate using local current flow depends on

Answer 3

depends on voltage gated Na+ channels

Question 4

_____ increases conduction velocity

Answer 4

myelin

Question 5

myelin increases conduction velocity by

Answer 5

1. increases membrane resistance (and therefore length constant)
2. decreases membrane capacitance
3. allows for salatory conduction

Question 6

conduction velocity ranges from

Answer 6

0.5- 120 m/s

Question 7

conduction velocity depends on

Answer 7

myelination

axon width

Question 8

axons carrying the most time sensitive intro are

Answer 8

larger and more myelinated

Question 9

demyelinating disease

Answer 9

MS

Guillain Barre

Question 10

MS

Answer 10

damages myelin in CNS

Question 11

Guillain Barre is

Answer 11

immune response against myelin in axons innervating muscles

Question 12

In demyelinating diseases, myelin sheaths are

Answer 12

damaged, which slows and/or blocks conduction (therefore nervous system function)

Question 13

demyelination may play a role in

Answer 13

chronic pain

Question 14

how does demyelination playa role in chronic pain?

Answer 14

• Nodes of Ranvier are disrupted

* Isolated clusters of voltage-gated Na+ channels

Question 15

Safety factor

Answer 15

Density of voltage-gated Na+ channels is 5-10 times higher than necessary for AP
propagation (providing additional “safety” that propagation will occur)

Question 16

Why the redundancy? Allows for:

Answer 16

1) Branching of axons

2) Shorter refractory period

Question 17

Inhibitors of AP

Answer 17

• Action potential generation and conduction depends on voltage-gated Na+ channels
• If these channels are blocked, current can only be propagated passively

Question 18

Neurotoxins

Answer 18

• Extracellular blockers of voltage-gated Na+ channels – abolish action potentials
– Tetrodotoxin (e.g., from pufferfish) – Saxitoxin (e.g., from clams)
• Modulators of voltage-gated Na+ channel function – may shift voltage dependence; alter ion selectivity
– Batrochotoxin (e.g., from frogs)

Question 19

Local anesthetics

Answer 19

• Example: Lidocaine
• Binds to intracellular portion voltage-gated
Na+ channels, blocking Na+ entry – Requires channel to be open to enter the cell
• Greatest effect on axons that are: – Smaller (lower safety factor)
– More active (channels must open)
• Preferentially affect pain over touch axons

Question 20

Axons are poor cables for

Answer 20

passive current propagation

Question 21

Axons are well built for fast active propagation because

Answer 21

– Myelination, Nodes of Ranvier

– High safety-factor

Question 22

AP’s are critical for survival

Answer 22

– Blocking APs can be fatal (although local blockade is
useful)
– Demyelinating diseases reduce conduction velocity and degrade information transmission

Question 23

Important APs

Answer 23

• Nervous system: Representation of sensory information and motor plans
• Muscle: Excitation-contraction coupling
• Heart: Pacemaker potential and control of contraction

Question 24

To “boost” the signal,

Answer 24

a fresh injection of charges is required.

Question 25

This boost in positive charges comes from

Answer 25

Na+ flowing into the cell through voltage gated sodium channels.

Question 26

The sodium channels sense the voltage of the propagating electrical signal and the activation gate _____, allowing an influx of _____.

Answer 26

opens sodium into the cell This is the signal “boost” which allows another electrical signal to be propagated along an axon.

Question 27

Know why action potential propagation is much slower than the velocity of light.

Answer 27

The system of action potential relies on the diffusion of ions through channels that are gated. The process of diffusion, while fast, is still slower than the speed of light. More importantly, there are millisecond delays in the gating mechanisms of the voltage gated channels. The activation gate of the Na gate has a relatively fast response, but it is still a mechanical, conformational change in a protein that requires time to occur. This yields conduction speeds up to 200 m/s, far, far slower than the speed of light.

Question 28

Addition of layers of myelin increases ______ and decreases _______, allowing for a faster conduction speed.

Answer 28

membrane resistance membrane capacitance

Question 29

After producing an action potential an axon cannot generate another one for a few ms, because of

Answer 29

because the delay in the opening of the sodium inactivation (h) gates.

Question 30

Myelin increases the speed of conduction of nerve fibers, when it is lost, ____.

Answer 30

fibers that were previously fast can become slow or lose the ability to conduct action potentials

Question 31

Increasing axon diameter increases conduction velocity by increasing_______.

Answer 31

cross-sectional area and thus the number of Na channels

Question 32

Hyperkalemia can be caused by

Answer 32

redistribution of K ions out of cells by insulin deficiency or resistance, acidosis or impaired movement of K into cells (Beta receptor blockade). or by impaired ability for the kidneys to secrete K, a side affect of drugs. Meals very high in K, combined with renal disease and limb ischemia also cause increased K in the ECF.

Question 33

Cardiac arrhythmias due to hyperkalemia are a result of

Answer 33

the membrane potential being shifted closer to threshold (i.e. slightly depolarized).

Question 34

myelin reduces the leak of charges out of the axon, so ________.

Answer 34

very little current is lost between the nodes Since little charge is lost, the next node is depolarized to threshold very rapidly (fewer negative charges are needed to maintain the membrane potential, so few Na ions required), resulting in action potentials “jumping” from node to node.

Question 35

Activation gates may swing open, but with the h gates closed,

Answer 35

Na will still be unable to flow through the channel.

Question 36

The potassium channels also contribute to the refractory period because after a cell repolarizes, there is a

Answer 36

delay in the closing of K channels.

Question 37

All cardiac muscle cells are capable of ______, but the cells of SA node fire fastest and set the tempo of heart beats. When other cells begin firing out of sync, or cease firing when they should, cardiac arrhythmias result.

Answer 37

spontaneous depol

Question 38

The higher K conductance makes it more difficult for a stimulus to depolarize the axon because ______.

Answer 38

K is still flowing out of the cell

Question 39

Myelin prevents ions from ______ which in turn increases membrane resistance and decreases capacitance.

Answer 39

leaking out of cells,

Question 40

If axons become demyelinated, they can slow action potentials due to decreased ______ or lose the ability to conduct because too many of the charges (ie current) are lost between nodes, preventing another action potential.

Answer 40

resistance

Question 41

Decreasing axon diameter results in ____.

Answer 41

slower conduction

Question 42

Large axons reach threshold at _____ strengths than small axons.

Answer 42

lower stimulus

Question 43

Safety factor is also increased for

Answer 43

large diameter axons compared to smaller axons.