The nerve fibers of the spinal cord carry the electrical impulses that carry information from and to the brain to support the complex control system of the human body. Each microscopically thin nerve fiber is insulated by a regular pattern of myelin sheaths along its length that allow for the efficient conduction of electrical impulses

In MS, or following a spinal cord injury, some of these myelin sheaths can be damaged and permanently lost. In MS the myelin is believed to be destroyed by the body’s own immune system whereas in spinal cord injury the demyelination occurs as a result of physical trauma to the spinal cord. In either case, nerve impulses "short circuit" in demyelinated axons, much like electricity in a wire whose insulation is stripped. Thus, even though a demyelinated axon is alive, it is unable transmit motor or sensory impulses.

Fampridine-SR's major action is to block specialized potassium channels on axons.

It is these exposed channels that "leak" potassium ions, causing the axon to "short circuit". By closing the exposed potassium channels, Fampridine-SR permits the axon to transmit impulses again, even in a demyelinated state.