1.2.1 The myocardium
The myocardium actively contracts to pump blood into the great arteries and actively relaxes in early diastole to ensure emptying of the veins and filling of the ventricles. It is surrounded by the epicardium and lined by endocardium.
The bulk of the myocardium is made up of muscular fibres which contract when they are depolarised. They have the important property that, unlike skeletal muscle, they have a refractory period which prevents tetanic contraction. As a result, electrical stimulation is required for each contraction. Some myocardial cells are specialised for transmission of electrical signals through the heart, forming a specialised conduction system. The electrophysiology of these tissues is different from that of the contractile part of the myocardium and is discussed in more detail below.
Each myocyte is made up of many myofibrils which are aligned along the long axis of the cell. In turn, each myofibril is made up of sarcomeres which are joined in series. The sarcomeres contain interdigitating myofilaments made up of contractile proteins. The cell surface is covered with sarcoplasmic reticulum, which forms invaginations between each of the sarcomeres. This is the source of calcium, which is released when the membrane is depolarised. The contractile fibres contain four proteins. Two proteins form filaments which overlap; the thicker filament is myosin, the thinner, actin. Two regulatory proteins, troponin and tropomyosin, are involved in the formation of cross-bridges between the two filaments. When the cell is stimulated by an excitatory impulse, a complex movement of ions, particularly the release of calcium stored in the sarcoplasmic reticulum, results in an increase in the overlap of the myofilaments and a consequent reduction in the length of the filament and cell, i.e. contraction. Together, the cells form two functional syncytia, divided at the atrioventricular junction, which allow the spread of depolarisation wavefronts throughout the atrial and ventricular myocardium.
The heart is divided into four chambers: two atria which act as priming chamand two ventricles, which are the main pumps. In the horse, the heart is positioned almost vertically on the sternum, with the long-axis in a dorso-ventral direction.
The cardiac chambers are adapted for maximum efficiency for the requireof pumping blood through a high pressure and a low pressure circuit connected in series. The left ventricle (LV) drives blood through the high preshigh resistant systemic circulation. It is constructed like a piston, forming a cylindrical chamber suited to high pressure pumping. The inflow tract is formed by the mitral valve and the caudal wall of the ventricle, the outflow tract by the cranial wall (interventricular septum) and the centrally positioned aortic valve. These tracts are arranged in a ‘V’ shape (Figure 1.1). The right ventricle (RV) has a large surface area in comparison to its volume and is more suited to low pressure pumping. The inflow tract is on the right cranial side of the heart, the outflow tract is on the left cranial side of the heart (Figure 1.2). Together they form a ‘U’ shape (Figure 1.3). The RV wraps around the cranial aspect of the LV and its contraction is intimately related to that of the LV. Under normal cirthe interventricular septum, which divides the ventricles, contracts as part of the LV.
The atria are situated dorsal to the ventricles and act as receiving chambers, storing returning venous blood until it can empty into the ventricles in early diastole. Blood returns to the right atrium from the caudal vena cava, cranial vena cava and the sinus venosus. Pulmonary veins feed the left atrium with blood returning from the lungs. The lining of the atria is trabeculated, suiting them to their task of storing blood prior to ventricular filling. The atria are relatively thin walled because they pump against low pressure in the normal heart. The right atrium is larger than the left atrium, and at post-mortem examination is usually less tightly contracted.
The coronary arteries are divided into two main trunks. The right coronary artery usually takes more blood flow than the left. The arteries leave the aorta from the sinuses of Valsalva, where their openings are guarded by the valves. Their position in the sinuses ensures that coronary blood flow is not obstructed by the valve leaflets. The majority of coronary blood flow occurs during diastole.