Physiology: Myelin and Chemical Synapse

Question 1

What is the role of myelin?

Answer 1

Reduces leakage and capacitance
Faster conduction
Less energy consumption

Question 2

Name the strategies to increase conduction velocity.

Answer 2

Increase diameter of axon
Myelination of axon
Nodes of Ranvier
Increase temperature

Question 3

Describe how increasing the diameter of an axon increases conduction velocity.

Answer 3

• Reduce resistance to flow along axon
• Proportionally more charge flows along axon
• Position B gets to threshold more quickly
  But this strategy has physical limitations

Question 4

Describe how myelination increases conduction velocity.

Answer 4

• Few channels under myelin, less leak
• Proportionally more charge flows within axon
• “B” gets to threshold more quickly
• Less capacitance (greater charge separation)
• Even with myelin, still some leak and capacitance
• Action potential will not reach end of nerve, if not regenerated.
• Nodes of Ranvier solve this problem.

Question 5

Describe how the nodes of Ranvier increase conduction velocity.

Answer 5

· Nodes are spaced so that potential still above threshold for AP
· Nodes contain voltage gated channels, support new AP (saltatory conduction)
AP at each node is the same, AP at end of axon is the same size as at beginning

Question 6

List examples resulting from the consequence of axon demyelination.

Answer 6

Guillian Barre, multiple sclerosis, trauma

Question 7

Describe what happens due to the consequences of axon demyelination.

Answer 7

· Increase in capacitance, increase in leak = decrease in conduction velocity (takes longer to get next intact node to threshold)
Few voltage gated channels in demyelinated region of axon = conduction BLOCK (potential from previous node decays to below threshold at next intact node.

Question 8

Describe the process of the propagation of an action potential across a chemical synapse.

Answer 8

AP arrives at pre-synaptic bouton and activates voltage gated Na channels to open.
Causes the influx of Na and depolarization.
This causes voltage gated Ca channels to open.
Influx of Ca into pre-synaptic bouton.
Ca dependent enzymes can act on actin and other proteins to cause fusion of neurotransmitter vesicles.
Neurotransmitter vesicles diffuse across the synapse and bind to direct gated (ionotropic) and indirect gated (metabotropic) ion channels causing influx of the Na in the post-synaptic membrane.
This causes an excitatory post-synaptic potential and the AP is propagated.

Question 9

Name the three things that terminates the transmission of a neurotransmitter.

Answer 9

• Diffuses into the blood and glial cells where it is broken down
• Bind to ACh esterase at NMJ basement membrane where it is broken down
Taken back up by the presynaptic membrane where it is packaged back into vesicles.