What exactly is multiple sclerosis?

From Cognitive Neuroscience (pg. 26):

J.C. was 32 years old when she stumbled while walking down her level, clean driveway. Embarrassed at her clumsiness, she laughed and told her friend that she should have the driveway repaired. Two weeks later she fell while walking across her living room. Six months after these seemingly trivial instances of clumsiness, some minor but annoying visual problems developed but disappeared over the next 6 months. During the next year J.C. experienced some strange numbness in her hands and weakness in her left leg, which prompted her to seek medical treatment. Following a series of tests, J.C. was diagnosed with a neuromuscular disease called multiple sclerosis (MS). What is this disease, why did it lead to the varied symptoms, and why was J.C. sometimes clumsy?

Moment to moment, on a millisecond scale, the human nervous system processes sensory information and executes motor responses. Some of these are under voluntary control; perhaps the vast majority are reflexive or automatic in one fashion or another. With respect to standing, for example, not only do we decide voluntarily whether to stand or sit, but we also use reflexive systems to maintain balance and posture. To accomplish all this in real time takes speed and accuracy of timing. Thus, in many ways the nervous system is a high-speed machine. The timing of this machine is compromised in some disease states when the integrity of the neurons and their component parts is destroyed.

MS is one disease among many that manifests itself, in part, as a loss of coordination among information transactions within specified neural systems. Specifically, MS is manifest as damage of the myelin sheaths surrounding axons in the central nervous system or peripheral nervous system, or both. Through mechanisms not completely understood (likely some form of autoimmune reaction of the body against the molecules in the myelin itself), the myelin is damaged, and in a spotty fashion it may be broken down completely. The result for the patient can be mild or very severe. The symptoms of MS depend on which axons are affected by demyelination. If the affected axons are in the optic tract leading from an eye to the brain, then visual problems will be encountered. If axons in peripheral nerves are demyelinated, losses of sensation or muscular control and strength may result. If the demyelination occurs in white matter tracts interconnecting regions of cerebral cortex involved in higher function, the damage may affect cognition or personality.

Why does damage to myelin lead to these problems? Damage to myelin can lead to slowing or complete disruption of neural signaling and, hence, a loss of function in the portion of the neural circuitry affected. In addition, however, inflammation that occurs as a result of the demyelination can lead to damage of the axons themselves and ultimately to their destruction. So the symptoms of MS may have two causes: demyelination and neuronal damage.