Organisation of the mouse sinoatrial node: structure and expression of HCN channels

In the mouse, (i) the SAN is structurally complex with a densely-packed head and loosely-packed tail; (ii) HCN4 is the only HCN isoform detectable and is present throughout the SAN; and (iii) there is a specialised interface between the SAN and surrounding atrium that may be necessary for the SAN to drive the more hyperpolarized atrial muscle.