Nodal tissue is a unique type of
tissue that has both
muscular and
nervous characteristics, and can be found at two points in the
heart: the
sinoatrial node (SA node) and the
atrioventricular node (AV node). It is responsible for coordinating the
cardiac cycle (heartbeat).
The SA node is found at the upper dorsal wall of the right
atrium. It initiates the
heartbeat, by sending out an excitation
impulse every 0.85 seconds, causing the
atria to contract.
When this impulse reaches the AV node, which is located at the base of the right atrium very near the
septum, it signals the
ventricles to
contract by way of two large fibers terminating in the many small
Purkinje fibers.
The SA node is commonly refered to as the
pacemaker, because it keeps the heartbeat regular. If for some reason it fails to work properly, the heart will still beat, but
irregularly. In recent years though, artificial pacemakers have come a long way, and it's now fairly easy to correct any problems.
Unlike my
procrastinating self, the heart is intrinsically motivated, which is to say that it mostly directs itself, rather than having the brain control it. It is these nodes which are source of this.