Action Potential Conduction

Question 0

What is Propagation time constant?

Answer 0

tau_p = r_i * c_m

tau_p = propagation time constant
r_i = longitudinal resistance per unit length of cytoplasm
c_m = capacitance per unit length of membrane

Question 1

What is the rate-limiting factor in the speed of propagation of the action potential?

Answer 1

Electrotonic conduction

Question 2

The _______ the time constant, the faster each segment will be depolarized to _________. As a result, the current will spread_______.

Answer 2

shorter; threshold; further

Question 3

The ________(larger/smaller) the fiber diameter, the lower the axial resistance. The current will travel _________.

Answer 3

larger; farther

Question 4

What is the proportional formula for conduction velocity?

Answer 4

conduction velocity = (lambda) / (tau_p)

lambda = length constant 
tau_p    = propagation time constant

Question 5

True or False: Action potentials are produced in consecutive adjacent regions down the nerve or muscle?

Answer 5

True

Question 6

If an action potential is excited in the middle, which direction will it conduct?

Answer 6

both directions away from the stimulus point down the fiber

Question 7

If an inactive fiber is excited at its end, where will the action potential travel?

Answer 7

To the other end and stop (because the refractory period prevents backward reverberation)

Question 8

Which cells form myelin? Where do they form myelin? What is the difference between them?

Answer 8

Schwann cells in the PNS
Oligodendrocytes in the CNS

Besides location, Schwann cells only form one segment of myelin on one axon, while oligodendrocytes form myelin segments on many different axons

Question 9

What are the effects of myelination on plasma membrane resistance and capacitance?

Answer 9

INCREASES membrane resistance (increase length constant)
and
DECREASES capacitance (allowing internodal membrane to become more depolarized more rapidly)

Question 10

Where are the nodes of Ranvier? What are properties of the nodes of Ranvier?

Answer 10

• Concentrated Na+ channels

- few, if any, K+ channels

Question 11

Where are internodal regions? What are channel properties of internodal regions?

Answer 11

1. K+ channels are localized

2. Virtually no Na+ channels

Question 12

Describe Saltatory conduction. Are they more or less efficient to unmyelinated nerves? Why?

Answer 12

Action potentials that only happen at the nodes of Ranvier that make it look like APs are jumping from node to node.

Saltatory conduction are more energy efficient than unmyelinated nerves b/c transmembrane currents are restricted to small membrane surface area, use fewer ions and less pumping to maintain gradients.

Question 13

What diseases demyelinate the nerve and where are they located?

Answer 13

Multiple Sclerosis (CNS)

Guillain-Barre Syndrome (PNS)

Question 14

What is Multiple Sclerosis?

Answer 14

The most common demyelinating disease of the CNS. It’s an autoimmune disease.

Question 15

What is Guillian-Barre Syndrome? What makes it different than it’s counterpart in the CNS?

Answer 15

The most common demyelinating disease of the PNS. Triggered by a viral infection.

The difference than Multiple Sclerosis, given proper treatment, patients, most patients recover completely because the PNS has the ability to re-myelinate itself, unlike the CNS.

Question 16

What are the effects of demylination?

Answer 16

1. Slow conduction by a) reducing membrane resistance, thus decreasing length constant and b) increasing membrane capacitance, thus increasing propagation time constant
2. Expose voltage gated K channels
   
   • -> Can cause the AP to be short circuited
   • -> makes K channels accessible to drug therapy with channel blockers (like 4-aminopyridine).