An increased inhomogeneity in the recovery of excitability means that at the same point in time some cells will be fully recovered-the upper example, some will be partially recovered- the middle example, and some will still be totally refractory-the lower example. In those cells with minimal changes in action potential duration, as shown in upper panel, the premature response will have a normal upstroke velocity and will conduct normally. In those with a more prolonged action potential duration, as in the middle panel, a premature response with the same coupling interval as in the upper panel will have a decreased upstroke velocity and will conduct slowly. In those with an even more prolonged action potential, as in the lower panel, the cell will be totally refractory and unable to generate a response.