Conducting System of the Heart

The heart receives its sympathetic supply through cardiac branches arising from the cervical and upper thoracic ganglia of the sympathetic trunk.

Its parasympathetic supply is from cardiac branches of the vagus nerves.

These branches join together to form the cardiac plexuses, networks of fine nerves on the base of the heart. The plexuses supply the coronary vessels supplying the heart and the intrinsic conducting system of the heart.

The conducting system of the heart is composed of:

1. Pacemaker cells

   • Small cardiac muscle cells that contain few contractile proteins which can conduct impulses.

2. Purkinje fibers

   • Elongated cardiac muscle cells lacking contractile proteins which can conduct impulses.

   • This intrinsic conducting system functions to control the heart rate and regulates contractions of the atria and ventricles.

The conducting system of the heart can be summarized as:

1. Cardiac muscle fibers contract rhythmically and synchronously without external stimulation.

2. Sinoatrial (SA) node or pacemaker

   • Pacemaker cells located within the walls of the right atrium at the upper end of the crista terminalis near to the entry point of the superior vena cava.

   • The intrinsic muscular rhythms are regulated by pacemakers. The SA node (pacemaker of the heart) is regulated by inputs from the sympathetic and parasympathetic nerves of the cardiac plexuses.

3. The contractile wave passes from the SA node through atrial cardiac muscle fibers, causing them to contract.

   • The atria contract from above downwards, driving blood through the tricuspid and bicuspid valves into the ventricles.

4. This impulse eventually reaches the AV node.

5. Atrioventricular (AV) node

   • Pacemaker cells located in the floor of the right atrium, above the opening of the coronary sinus.

6. The wave of contraction passes from the AV node through the AV septum and into the AV bundle and conduction is carried to the ventricles.

7. Atrioventricular bundle

   • As it leaves the atrioventricular node, it crosses the atrioventricular junction to enter the interventricular system. This is the only electrical connection between the atria and ventricles.

   • Once in the septum, the bundle divides into two branches (right and left), one for each ventricle.

   • The right branch passes down the right side of the septum to the moderator band crossing the ventricle. It forms a Purkinje plexus in the walls of the right ventricle.

   • The left branch runs down beneath the surface of the ventricular septum. It often branches before terminating to form Purkinje fiber plexuses among the papillary muscles of the left ventricle.

8. There is a delay of about 40 milliseconds between activity reaching the AV node and beginning of contraction of the ventricles, giving the ventricles time to fill completely before they contract.

9. Contraction of the ventricles begins at the apex and proceeds upwards towards the pulmonary trunk and aorta, thus driving blood into the pulmonary and systemic systems.

Clinical Notes:

1. Referred pain occurs because afferent pain fibres follow sympathetic nerves via the cardiac plexus to T1 -T4 spinal nerves. As such, cardiac pain is referred to T1-T4 dermatomes which radiates down the left arm.