Neurons are the nervous system’s communication cells, transmitting signals that control thought and movement. Many are myelinated, meaning they are covered in a protective coating called the myelin sheath. This layer is comparable to the plastic insulation on an electrical wire, allowing signals to travel with speed and precision.
The Structure of a Myelinated Neuron
A neuron has a long projection called an axon that carries electrical impulses from the cell body. In myelinated neurons, the axon is enveloped by myelin, a fatty substance made of lipids and proteins. The myelin sheath is not continuous but is formed in segmented sections wrapped around the axon.
The myelinated segments are separated by short, uninsulated gaps known as the Nodes of Ranvier. The sections of myelin between these nodes are called internodes. This structure is produced by glial cells, which are support cells in the nervous system.
The cells creating the myelin sheath differ by location. In the central nervous system (CNS)—the brain and spinal cord—oligodendrocytes produce myelin, and one cell can myelinate multiple axons. In the peripheral nervous system (PNS), Schwann cells perform myelination, with each cell wrapping a portion of a single axon.
The Function of Myelination
The myelin sheath’s main purpose is to increase the speed of nerve impulses. It acts as an electrical insulator, preventing electrical charge from leaking from the axon so the signal maintains its strength. In contrast, the signal in unmyelinated neurons travels continuously along the axon, a much slower process.
Myelinated neurons use an efficient transmission method called saltatory conduction, where the nerve impulse jumps from one Node of Ranvier to the next. This process skips the insulated myelinated segments. The Nodes of Ranvier contain ion channels that regenerate the signal at each gap, enabling this leap.
This process increases the speed of nerve signal transmission. A myelinated axon can conduct an impulse at speeds up to 120 meters per second, while an unmyelinated axon transmits signals at only 0.5 to 2 meters per second. This high-speed communication allows for rapid reflexes, complex motor control, and high-level cognitive functions.
Myelination Development and Repair
Myelination, the formation of myelin sheaths, is a developmental process that begins in the fetal period and continues long after birth. The process accelerates during infancy and continues through adolescence into early adulthood. In some brain regions, myelination is not complete until the third decade of life, and its progression corresponds with maturing physical and cognitive abilities.
Different areas of the nervous system myelinate in a sequence. Motor pathways myelinate before sensory ones, and brain regions for higher-order functions, like the prefrontal cortex, are among the last to complete the process. New myelin can also form in adulthood, suggesting a role in learning and memory.
The body can repair damaged myelin through remyelination, where new sheaths replace those lost to injury or disease. This repair is not always complete, and its efficiency declines with age. In chronic neurological conditions, this process is often insufficient to restore full function.
Impact of Demyelination
Demyelination is the loss of the myelin sheath, which disrupts nervous system function by causing nerve signals to slow, distort, or stop completely. This interference can lead to a wide range of neurological symptoms, depending on which nerves are affected.
Multiple Sclerosis (MS) is a demyelinating disease of the central nervous system. The immune system attacks the myelin in the brain and spinal cord, leading to scar tissue, or lesions. This can cause symptoms like muscle weakness, vision problems, and issues with coordination and balance, and the condition often involves periods of relapse and remission.
In the peripheral nervous system, Guillain-Barré syndrome (GBS) is a demyelinating condition where the immune system targets myelin. It often develops rapidly after an infection, causing muscle weakness and tingling that starts in the legs and moves to the upper body. While many people recover, GBS can severely impact motor function.