left bundle branch (heart, anatomy)

Last reviewed 01/2018

The left bundle branch of the cardiac conducting system is the continuation of conducting fibres from the atrioventricular bundle of His. It forms within the interventricular septum at the junction of the membranous and muscular parts. It runs to the left within the wall as an increasingly broad sheet of cells made up of multiple fine fascicles. Reaching the wall of the left ventricle, the sheet heads towards the apex of the chamber in the subendocardial layer of the muscular septum.

After a few centimetres, the left main branch divides into two broad groups of fibres:

• the anterior hemibundle which supplies the:
• anterior papillary muscle
• Purkinje network of the anterior and superior surfaces of the left ventricle
• the posterior hemibundle which supplies the:
• posterior papillary muscle
• Purkinje network of the inferior and posterior surfaces of the left ventricle

The first contacts between Purkinje cells and contractile myocytes which permits contraction is within the papillary muscles. Prior to this point, the bundles are covered by an insulating layer of connective tissue. From the apical regions, functional contacts are made in a recurrent manner within the subendocardial layer back to the base of the chamber. This means that:

• the tension within the papillary muscles, and consequently the valve leaflets, is beginning to increase before ventricular contraction
• ventricular contraction passes from the apex to the base of the chamber
• contraction occurs in the endocardium before the epicardium

Histologically, the cells of the left bundle branch are predominantly rapidly-conducting Purkinje myocytes.