C2.2 - nervous transmissions (6c)

Question 1

What is myelination?

Answer 1

• The myelin sheath is formed by Schwann cells wrapped around the axon of a neuron.
  Schwann cells act as insulators as they do not allow ion movement into and out of the axons at these locations.

Question 2

Why does myelination impact nervous transmission?

Answer 2

• In myelinated axons, action potentials can only occur at the nodes of Ranvier, and so they skip from node to node.

This ‘skipping’ both:
- reduces the time takes for an action potential to move along a given axon
- reduces the energy input for generating a resting potential, which only occurs at the nodes of Ranvier

Question 3

What is the relationship between myelination and the speed of nerve impulses?

Answer 3

Myelinated axons conduct nerve impulses faster than unmyelinated axons.

Question 4

What is the diameter of a giant axon in squid?

Answer 4

Giant axons in squid are 500 micrometer in diameter and unmyelinated.

Question 5

What is the diameter of a giant axon in humans?

Answer 5

Unmyelinated axons in the internal organs of humans are only 1 micrometer in diameter.

Question 6

Why does axon diameter impact nervous transmission?

Answer 6

• Larger axons have less resistance to the ions that move in and out of the axon during an action potential, because there is more space for cytoplasmic proteins and vesicles.

Question 7

What is the relationship between axon diameter and the speed of nerve impulses?

Answer 7

Speed of nerve impulses is positively correlated with axon diameter.