Ranvier lacing ring

A Ranvier lacing ring is a ring-shaped interruption of the fat or myelin sheath surrounding nerve fibers. In the course of “saltatoric excitation conduction” it serves to increase the speed of nerve conduction. Saltatoric, from Latin: saltare = to jump refers to the “jump” of an action potential that takes place when it encounters a Ranvier lacing ring as it spreads along the axon (a sheathed nerve fiber).

Function

Different nerve fibers have different excitation conduction speeds. For example, the particularly fast A-alpha nerve fibers, which conduct their impulses to the skeletal muscles, have a speed of almost 120 m/s, i.e. over 400 km/h! This fast transmission is only possible by the action potential jumping from time to time, more precisely, always from one Ranvier-lacing ring to the next.

Quasi like a sprinter, who additionally makes long jumps during his sprint. The Ranvier lacing rings are located at regular intervals of 0.2 – 2mm on the axon, each between two Schwann cells. These cells form the myelin sheath, a fatty layer that surrounds and isolates the nerve fiber.

The density of voltage-dependent sodium channels is particularly high in the area of a Ranvier constriction ring. An incoming signal (i.e. action potential) from the preceding Ranvier constriction ring is processed, amplified and immediately transmitted by the strong influx of sodium from outside the nerve until it meets the next constriction ring. There the same game starts again. The stretch between the lacing rings serves only as a forwarding stretch and is therefore well insulated. The lacing ring works like an amplifier on the nerve fiber.

Demyelination

Demyelination is the loss of myelin sheath between the Ranvier’s cord rings. Although there is still a transmission of the action potential at the lacing ring, this is too weak from a certain degree of the disease onwards to reach the next lacing ring. This is due to the fact that the path between the lacing rings is no longer myelinated (i.e. isolated). Known demyelinating diseases are multiple sclerosis or diabetic neuropathy (“diabetic foot“).