Layers of the Heart Wall
The heart is surrounded by a double membrane called Pericardium. This tough, fibrous coating serves to protect the heart. The outer layer covers the origins of the major vessels and has discrete attachments to the spinal column, diaphragm and inner membranous coat by ligaments. The inner layer connects to the cardiac muscle itself.
Cardiac muscle is a type of muscle found only in the heart, whose muscle fibres branch, forming junctions with each other to enable the rapid transmission of electrical signals throughout the heart. The transmission of electrical impulses causes the muscle to contract and synchronize heartbeat. These contractions are spontaneous as the heart is a myogenic organ, initiating contraction from within the muscle itself, independent of an external stimulus such as nerve innervation. The heart wall consists of three layers:
- Epicardium = Protective outer layer, made of connective tissue covered by a thin epithelium.
- Myocardium = Muscular middle layer, made of cardiac muscle, whose fibres contract spontaneously to produce the heartbeat.
- Endocardium = Inner layer (continuous with the inner lining of blood vessels). It is composed of both epithelial and connective tissue, and the Purkinje fibres, involved in the transmission of electrical signals from the Atrio-ventricular node, are also located here.