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A nerve sheath is a kind of insulation which surrounds the chain-like portion of a nerve cell, or neuron, known as the axon. The function of the nerve sheath is to enhance the neuron’s ability to transmit signals along its axon. It is made from a substance called myelin, which is produced by the glial cells and is composed of fat and protein. The loss of this myelin nerve sheath, as occurs in a number of diseases including multiple sclerosis and Guillain-Barre syndrome, can delay the transmission of nerve signals, resulting in impaired movement, speech, and cognition.
Neurons consist of three main parts: the soma, the axon, and the dendrites. The soma is the neuron’s “body,” and the site at which neural signals are received. Protruding from the soma is the axon, a chain-like structure of connected “links” along which outward signals sent by the soma travel. At the end of the axon are the dendrites, branch-like channels through which a signal is at last delivered to the somas of adjacent neurons.
It is these neural signals that dictate all of the body’s movements and functions, from the conscious — raising one’s hand to wave hello — to the unconscious — digesting one’s dinner. To ensure that the body runs smoothly, each neural signal must be delivered to the relevant bodily target at an incredibly fast speed. This is where the nerve sheath comes in. It hugs the axon like a sleeve, preventing signals from escaping or losing strength as they pass along each axon link and thus ensuring their quick transmission.
The myelin from which the nerve sheath is formed consists primarily of fat, although it also contains some protein. Myelin is manufactured by structures known as glial cells, which support the formation and function of the neurons. It is the pale color of the myelin that gives certain organs, such as the inner brain, their ashen hue.
Certain autoimmune and genetically inherited diseases, such as multiple sclerosis and Guillain-Barre syndrome, are marked by the destruction of the nerve sheath. Once the sheath has been damaged or destroyed, the neuron’s ability to efficiently transmit signals weakens significantly. Consequently, sufferers of these diseases often experience symptoms such as impaired movement, speech, and cognition. Unfortunately, the body is not capable of naturally replacing damaged myelin. As of 2010, medical researchers are working to establish a method for myelin replacement so that diseases affecting the nerve sheath can be effectively treated.