In terms of resistance and capacitance why does myelination

In terms of resistance and capacitance, why does myelination increase conduction velocity? Are these the same reasons for why the squid giant axon has a fast conduction velocity?

Solution

Myelin increases conduction velocity by increasing the length constant and decreasing capacitance. The length constant is the distance an ion travels before it permeates out of the cell and is related to the resistance. The greater the membrane resistance, the greater the length constant and hence farther is the movement of ions before leaking out.
During its movement down the axon, the cations (Na+, K+) are attracted to the external negative charges on the membrane. Myelination acts as an insulator. It increase the distance of external negative charges from the internal positive charges and decreases capacitance. The positive charges move along the myelinated segments (internode). At unmyelinated regions (Node of Ranvier) they slow down. They charge the capacitor-link across the membrane with their negative counterpart, until saturation, and conducted rapidly down the myelinated section where the external charges are distanced from the internal charges. This process, saltatory conduction, charges the capacitor and elicits \"jumping\" by the myelin segment.

So the myelin sheath act as an insulator for the axons, just like the rubber coating on a copper wire and prevent the electrical impulses from dissipating into the surrounding tissue. The non insulated areas, Nodes of Ranvier, permit ion exchange. By insulating the axon for particular lengths and then having nodes for ion exchange, the myelin sheath increases the speed of transmission of the impulse. In neurons without this insulating layer, impulses travel at approx. 5m/sec. With a myelin sheath, impulses can travel upto 100 m/sec.

The squid giant axon is the very large (0.5 -1 mm in diameter) axon, which controls the water jet propulsion system in squid with which it makes very fast movements through the water. Between the tentacles of the squid is a siphon through which water can be rapidly expelled by the fast contractions of the body wall muscles. This contraction is initiated by action potentials in the giant axon. Action potentials travel faster in a larger axon than a smaller one. The increased diameter of the squid axon decreases the internal resistance of the axon, (resistance is inversely proportional to the cross sectional area). This increases the space constant ( = r_m / r_i), leading to faster local depolarization and a faster action potential (E =E₀e^-x/ ).While the squid axon is very large in diameter it is unmyelinated, which decreases the conduction velocity substantially. The conduction velocity of a typical 0.5 mm squid axon is about 25 m/s.