Exam 1 Week 2: ppt 7 Action Potential Conduction

Question 1

How far might an AP need to be conducted?

Answer 1

•Action potential may need to be conducted over a distance of a meter or more

Question 2

Self-propagating mechanism

Answer 2

AP must be self-propogating so it generates a “local circuit” of current to each adjacent resting membrane, leading to a depolarization there that reaches threshold causing the AP to move down the membrane. This continues till the AP reaches the axon bouton.

Action potential may need to be conducted over a distance of a meter or more so they have to be self-propagating. This self-propagating mechanism is begun by the action potential generating a “local circuit” of current in the adjacent resting membrane

Question 3

Explain of the self-propagating mechanism (answer here is in 5 steps)

Answer 3

1. –AP generates “local circuit” of current to adjacent resting membrane
   
   1. • charges are carried by the inward-rushing Na+ through the depolarized membrane for some distance in both directions along the axon, causing the inside of the cell to become more positive wherever Na+ travels(B)
2. • charges (from the Na+) depolarize the membrane for a distance along the axon in both directions where the inside of the cell becomes more positive
3. –If this depolarization raises membrane potential above threshold, another action potential is created (C)
4. –These action potentials then creates more local currents in the new part of the membrane
5. –This theoretically could propigate the AP in both directions (D)
   
   • However, regenerative propagation cannot reverse direction because of membrane behind the excited areas is refractory from the AP that just happened there, so the AP is only propigated in one direction (forward directon).

Question 4

What can increase the speed of conduction in a nerve? (2)

Answer 4

1. –Increasing diameter of the axon
2. –Insulting with a myelin sheath

Question 5

What two neuroglial cells form myelin sheath, and where?

Answer 5

–Schwann cell - PNS
–Oligodendrocytes - CNS

Question 6

can nerve contain both myelinated and unmyelinated axons?

Answer 6

Yes. I think most if not all nerves have both.

Question 7

How much does schwan cells decrease inon flow through the membrane?

Answer 7

~5000 fold

Question 8

What does schwant cells contain?

Answer 8

sphingomyelin

Question 9

What are Nodes of Ranvier?

Answer 9

Nodes of Ranvier are the gaps between adjacent Schwann cells is where Na+ ions can still flow producing threshold depolarizing currents and thus action potentials

Question 10

What is Saltatory conduction?

Answer 10

increased conduction in myelinated axons is called “Saltatory” conduction and can occur at a rate of 150 m/sec in the largest myelinated axons.

_______________________________________________

–Subsequent action potentials are produced 1-3 mm apart vs 1-2 micrometers in unmyelinated axons

It describes how myelin and nodes of ranvier speed the rate of conduction in axons

So with myelination subsequent action potentials are produced 1-3 mm apart vs 1-2 mm in unmyelinated axons. This increased conduction in myelinated axons is called “Saltatory” conduction and can occur at a rate of 150 m/sec in the largest myelinated axons.

Question 11

what is the distance between action potentials

in unmeyelniated fibers

&

in myelinated fibers

Answer 11

action potentials are produced 1-3 millimeters apart in meylinated axons

vs 1-2 micrometers in unmyelinated axons

Question 12

What does the actual word “Saltatory” mean? and how does this relate to what it is descriibing?

(describe how saltatory conduction works)

Answer 12

1. •“Saltatory” means jumping
2. –But the action potential is not really jumping from node to node
3. –Na+ actually enters at one node and diffuses along the axon to the next node
   
   • There is a decline in Na+ concentration and charge as Na+ diffuses along the axon, but there is still enough Na+ and charge to reach next node
   • –So not a jump of charge but a continuous flow of Na+ and charge moving along the axon
4. With meyelination, 5-6 times as much current can flow at the next node than is needed which provides a “safety factor”
   
   • –The safety factor is diminished with demyelination
5. –So demyelination of a myelinated axon produces conduction block
   
   • –Seen in Multiple Sclerosis & Guillain–Barré syndrome

Question 13

what are two examples of diseases that involve demielyination?

Answer 13

Guillain-Barre (PNS)

Multiple Sclerosis (CNS)