However, in some individuals, there are tracts of atrio ventricular muscle bundles which are remnants of the embryonic atrio ventricular connections. These accessory pathways span the fibrous skeleton of the heart and are referred to as AV nodal bypass tracts. They provide an alternative pathway to normal AV nodal conduction. In the example shown, the tracts are on the lateral wall of the left atrium. However, the tracts may be located on the right side of the heart and may span the AV ring at any location, anterior, posterior, the lateral surface, and in the septum. Impulses traversing these bypass tracts may be a cause of abnormal ECG wave forms and of abnormal cardiac rhythms.