The myelin sheath is a layer of fat and protein that covers the axon of a neuron, or nerve cell. The axon, which is a long, thin projection, is designed to carry electrical signals from a neuron to other cells. The purpose of the sheath is to insulate the axon so electrical energy is not lost as it travels along the pathway. It also helps speed up the rate at which the electrical signal is passed; myelinated nerve fibers transmit much more quickly than those that are unmyelinated. The sheath plays a critical role in assuring that the nervous system performs properly.
Glial cells are specialized cells that support the nervous system, and one of their functions is to create the myelin sheath. In the brain and spinal cord, the glial cells are called oligodendrocytes, and they each support multiple neurons. The glial cells in the peripheral nervous system are called Schwann cells, and each one supports a single neuron.
The process which forms the myelin sheath for all neurons is called myelinogenesis. In human beings, it begins while they are still in the uterus. It then continues on for many years, with different parts of the nervous system developing myelin at different times and in a specific sequence. The four main stages of myelinogenesis are axon contact, glial cell gene production, axon ensheathment, and maturation.
Certain diseases can lead to neurons with missing or damaged myelin sheaths. These types of disorders fall into two categories: dysmyelinating and demyelinating. Dysmyelinating diseases, also known as leukodystrophies, are usually genetic disorders that inhibit the development of the myelin sheath early in life, leading to progressively more debilitating symptoms and even death. Demyelinating diseases destroy existing myelin sheaths, leading to various neurological impairments; the most common example of this type of disease is multiple sclerosis. Symptoms of demyelinating diseases may vary from one patient to another and can include numbness or weakness in various parts of the body, vision or speech impairment, or problems with balance.
Due to the number of debilitating disorders that can affect the myelin sheath, many scientists and researchers are engaged in studying remyelination. This is the process of re-growing the myelin sheaths on nerve cells that have been damaged or had their growth inhibited. It is one of the goals of The Myelin Project, an international organization created to research dysmyelinating and demyelinating diseases.