Cardiac Conduction System

Conduction of AP

The cardiac conductionThe transmission of nerve impulses along neurons. system is a network of circuitry in the heart. Its purpose is to conduct action potentials. This is nervous tissue, not cardiac muscle tissue or the fibrous skeleton. This nervous tissue also does not have a direct connection to the brain. It does its own thing, though it can adjust its activity based on signals from the brain. This is a network of non-contractile cellsThe basic structural and functional units of life.. These are neuronsThe functional cells of the nervous system that transmit signals., not muscle fibers, and we refer to them as fibers and nodes. A node is a cluster of neurons. A fiber is essentially an axon. We call them fibers in the heart. The nodes generate their own action potentials. They do not take their direction from the brain. However, the brain can influence their activity.

SA Node

There are two nodes: the SA node and the AV node. The SA node, or sinoatrial node, is located in the right atrium. It sets the sinus rhythm. This makes it the heart’s master pacemaker. It fires action potentials at about 80 beats per minute when left to its own devices. Epinephrine in the blood returning to the heart via the vena cava can influence the SA node to fire faster, increasing the heart rate

Autorhythmic Cells

Nodes contain autorhythmic or pacemaker cells, which exhibit spontaneous depolarizationThe loss of electrical charge across a membrane, triggering an action potential.. This is also called automaticity.  These cells do not maintain a resting potential but gradually depolarize until they reach a thresholdThe minimum voltage needed to trigger an action potential., triggering an action potentialA rapid, temporary electrical charge that travels along neurons, allowing signal transmission..  Essentially, the heart has cells that spontaneously depolarize. This unique property allows the heart to beat independently of external signals. These pacemaker cells differ from cardiac muscle fibers, which exhibit a plateau phase in their action potential graph.

Internodal Pathways

There are a few fibers that connect the SA node to the AV node.  These fibers innervateTo invade a tissue with nervous tissue is to innervate it. the myocardium of the right atrium.  They originate from the pacemaker cells in the SA node and converge on the AV node in the interatrial septum.  There are so many of them. If one of these pathways gets damaged, maybe from the opening of a PFO, it’s no problem. There are so many other pathways that can be taken.  I guess this is kinda like insurance. 

AV Node

The AV node, or atrioventricular node, is located closer to the ventricles in the interatrial septum. Let’s say that those pacemaker cells in the SA node stop working.  No problem because the AV node also has its own pacemaker cells.  The AV node sets a junctional rhythm at 50 beats per minute if the SA node is nonfunctional. The AV node introduces a slight delay in signal transmission, allowing the atria to contract before the ventricles.  This is really important, this delay.  If the delay weren’t in place, the ventricles would contract at the same time as the atria. Blood would flow nowhere.  This small delay lets the heart squeeze blood through. It’s similar to how you squeeze toothpaste from an almost-empty tube.

AV Bundle and Branches

The SA and AV nodes may fail. In that case, the AV bundle and branches take over. They set a rhythm of 40 beats per minute. This isn’t a great heart rate, but you can live on it.  The AV bundle is the only connection that conducts action potentials from the atria to the ventricles. This bundle and its branches run through the interventricular septum. They can be damaged in a VSD or ventricular septal defect.  In factA statement based on direct observation that is repeatedly confirmed., attempts to correct a VSD can cause more damage to the AV bundle and branches.. Purkinje fibers, which branch out from the AV bundle branches, help distribute the action potential throughout the thick myocardium.