Nerve impulses are normally set off when sodium is absorbed into a nerve cell. This opens sodium channels in the cell which allow in more sodium. This reaction propagates down the cell to trigger the next nerve cell in the chain. However, in some cases, the feedback loop can continue unabated despite the absence of a normal stimulus. In most cases, this is the result of overstimulus of the nerves, creating a sensation similar to that of phantom pain. The sensation can be quite intense, requiring painkillers, and can continue for hours after the original stimulus.