All our activities — eating, walking, talking — are controlled by our brains, the center of the nervous system. The brain receives huge amounts of information from outside our body via our five senses (vision, sound, taste, touch, and smell), integrates this information, and orders our muscles to take action. How is all that accomplished so efficiently? The answer lies in a membrane structure called myelin.
Information Transmission in the Body
All information both to and from the body must be coordinated and transmitted simultaneously and very quickly. The brain itself requires extremely fast speeds to operate at even at the simplest level. How do the biological tissues of our body support such rapid coordination of the brain, limbs, and sensory input? They do so with nervous system tissue that imitates electrical wiring.
The nervous system consists of two primary cell types: neurons and glial cells. These cells communicate with each other to perform important tasks in the nervous system. The glial cells support neurons structurally and maintain their long-term neuronal integrity, and neurons regulate glial cell behavior. In this support of neurons, glial cells have become highly specialized. Glial cells, which can be divided into several types, have various important functions, such as providing structural support, growth support, and insulation around the axon.
Why must glial cells support neurons? Neurons are specialized cells that receive and send signals to other cells through fragile and thin cellular extensions called axons. These axons extend over distances long and short to reach their target, ultimately connecting neurons with other nerve tissue, muscle tissue, or sensory organs. For example, some motor neurons in the spinal cord have axons that exceed 1 m in length, connecting the spine to the lower limb muscles. These axons transmit signals to the target muscle in the form of electric impulses called action potentials. However, the axons alone are not enough to produce rapid conduction of the electric current necessary for this signal to be sent. Glial cells are the key element for supporting the messages neurons send and receive all over the body. Much like the insulation around the wires in electrical systems, glial cells form a membranous sheath surrounding axons called myelin, thereby insulating the axon. This myelination, as it is called, can greatly increase the speed of signals transmitted between neurons (known as action potentials). Indeed, the evolution of myelin allowed vertebrates to achieve efficient nervous systems.
Removing Myelin Disrupts Neural Communication
With knowledge of myelin’s role in neural communication, researchers aimed to find out what happens when myelin is disrupted. In the 1980s, researchers used animal models to assess how electrical nerve signals are altered after axons were stripped of the myelin (demyelinated). When researchers chemically induced myelin loss in the spinal cords of cats, they found that signals moved more slowly along the nerve fiber and often failed to make it to the end of the axon.
Around the same time, scientists also made breakthroughs in identifying many of the components of myelin, like the major protein elements of the myelin sheath and the genes that encode them. Researchers developed mouse models that had defective myelin proteins, resulting in a myelin deficiency. One such mouse is the “shiverer” mouse, named after its body tremors. Mice like the shiverer mouse have provided researchers with a model system for studying myelin’s function in the healthy nervous system and its dysfunction in demyelinating diseases.
Myelin Loss in Disease
Loss of myelin is a problem for many CNS disorders, including stroke, spinal cord injury, and, most notably, multiple sclerosis (MS).
MS is a chronic, disabling disease of the CNS that affects more than 2.3 million people worldwide. MS results from the accumulation of damage to myelin and the underlying nerve fibers it insulates and protects.
Current research indicates that MS involves an autoimmune response. Scientists think that immune cells, which normally defend the body against bacteria and viruses, mistakenly attack the myelin sheath, stripping it away and exposing the nerve fibers underneath. In addition, recent research suggests that axon damage occurs early on in the course of the disease. Once damaged, the ability of nerve cells in the brain and spinal cord to communicate with each other and with muscles is compromised, leading to a variety of unpredictable symptoms that vary from person to person. These symptoms, which can be temporary or permanent, range from fatigue, weakness, and numbness to blindness and even paralysis.
References: Myelin: An Overview