The myelin sheathis found surrounding axons of the both the central and peripheral nervous systems. Axons may be myelinated or unmyelinated. In the myelinated axons, the sheath is arranged with small gaps known as theNodes of Ranvier.This is where the action potentials are generated as this is where the majority of the axon’s ion channels are located. This article shall discuss the myelin sheath, its effects on the transmission of action potentials in the nervous system, and the relevant clinical conditions.
The Myelin Sheath
Myelin is a lipid-rich substance that surrounds some axons within the central and peripheral nervous systems. The sheath is formed by wrapping multiple layers of the cellular membrane (mainly lipoprotein) of the myelin-producing cells. In the central nervous system (CNS) these cells are theoligodendrocytes, and in the peripheral nervous system (PNS) these cells are known asSchwann cells.
A single oligodendrocytecan myelinate up to 50 axons, whereasa single Schwann cellis only able to myelinate a single axon. Individual Schwann cells can each cover around 100 micrometers of an axon – meaning that it takes around 10,000 Schwann cells to myelinate a meter of the axon. As mentioned previously, the gaps left between Schwann cells (and between areas of oligodendrocyte-induced myelination in the CNS) are known as theNodes of Ranvier.
Little is known about the exact process ofmyelination.It beginsin utero– early in the third trimester. Although there is very little myelin present at birth, during infancy it progresses rapidly, in line with the development of various cognitive and motor skills. Myelination continues throughout adolescence and into early adulthood, following which the process is largely complete.
Fig 1 – Diagram showing the myelin sheath surrounding an axon in the CNS – with associated oligodendrocyte.
Effects of the Myelin Sheath
The myelin sheath conveys certain properties that increase the speed at which axons are able to conduct the action potentials.
Membrane Resistance
Myelin hasa high membrane resistance:
- Resistance –the degree to which a membrane prevents or facilitates free movement of ions; a low resistance membrane allows lots of ion movement, and a high resistance membrane does not.
This occurs because the myelin sheath inhibits ion movement along the insulated area of the axon, encouraging the diffusion of ions along the axon to reach the next node. At the node, the high concentration of ion channels enables rapiddepolarisationand action potential generation.
Membrane Capacitance
Myelin alsoreduces the capacitance of the axon:
- Capacitance –the ability of an electrical system to store charge or the charge required to initiate an action potential/electrical impulse; the low capacitance conveyed to an axon by myelination means that a lower change in ion concentration is required to initiate an axon potential.
因此有髓鞘的轴突能开展行动ion potentials much faster than unmyelinated axons viasaltatory conduction,在动作电位N之间出现“跳”odes of Ranvier.
Fig 2 – Diagram to show how the myelin sheath results in saltatory conduction of an action potential along an axon.
Clinical Relevance – Guillain-Barré Syndrome
Guillain-Barré Syndrome is a rapid onset of muscle weakness caused byautoimmunedamage to the peripheral nervous system. The cause is unknown, but the underlying mechanism is damage to the myelin sheath of peripheral nerves by the body’s immune system.
Initial symptoms usually include changes to sensation or pain and muscle weakness. These begin distally in the feet and hands and then typically spreadproximallyto the arms and upper body.
The symptoms develop over varying time frames, from hours to over a few weeks. If the respiratory muscles are affected, this disorder can be life-threatening and requiresmechanical ventilation.
Treatment is with supportive care, plasmapheresis, andintravenous immunoglobulins.Whilst the majority of patients undergo recovery, it can take weeks to years. Additionally, around a third of patients will experience some residual weakness.