Myelin & Nerve Conduction Speed

Myelin & Nerve Conduction Speed

We show three situations: a bare plasma membrane, a non-myelinated axon, and a myelinated axon.

Bare plasma membrane

For bare plasma membrane, the voltage arrow rapidly thins as it travels along the axon, because charged ions leak across the membrane.
Thus, voltage decreases as it travels until it eventually disappears.

Non-myelinated axon

For the non-myelinated axon, the voltage rapidly decreases, but at every voltage-gated ion channel, it is regenerated since new ions are let into the cell to replace the charge that leaks out.

Although the stimulus is regenerated, the action potential conduction speed is slow because it takes time to open new channels to allow ions in.

Myelinated axon

For myelinated axons, we show that the stimulus does not degrade as rapidly as the bare plasma membrane or the non-myelinated axon: the myelin sheath acts as an insulator.
The current does not need to be regenerated nearly as often so it can travel farther and faster.
We label the nodes of Ranvier between the myelin sheaths and draw voltage-gated ion channels in them.

At the nodes of Ranvier, more voltage-gated ion channels open and a new action potential is generated (making the action potential appear to "jump" from node to node, a mechanism called saltatory conduction).

We draw curved arrows to help visualize this "jumping" of the action potential from node to node.

Summary

To recap, we dash a line down a single time point across the three axons to see that the myelinated stimulus has the greatest voltage at this point; the non-myelinated stimulus is set to regenerate; and the bare plasma membrane continues to decay.

Clinical Correlation: Multiple Sclerosis

Gasser Classification of Nerve Fibers

According to the Gasser classification scheme, peripheral nerve fibers are categorized as Group A, B, or C based on their degree of myelination, diameter, and resultant conduction speed.

(A second classification scheme also exists, the Lloyd scheme, but it applies to sensory nerves, only).

Group A

Group A fibers have the largest diameter, thick myelin sheaths, and conduction speeds of up to 120 m/s.

Group B

Group B fibers have intermediate diameters, are lightly myelinated, and have conduction speeds of around 10 m/s.

Group C

Group C fibers have the smallest diameters, are not myelinated, and have conduction speeds of 1 m/s or less.

Peripheral Nerve Myelination

Unmyelinated cell

In an unmyelinated cell, we see:

The Schwann cell nucleus lies intermixed with axons within the Schwann cell cytoplasm.
The mesaxon is the zone of apposition for the Schwann cell membrane and the axon.
Multiple axons can pass through a Schwann cell, when the nerve fiber is unmyelinated.

Myelinated cell

In a myelinated cell, we see:

The Schwann cell nucleus lies off to the side of the axon, which is enveloped in concentric circles of myelin: a myelinated sheath.

Schwann cells vs oligodendrocytes

Unlike oligodendrocytes, which can myelinate up to 50 individual axons, a Schwann cell only myelinates one axon, as shown here (specifically only one peripheral nervous system internode).

Additional Images