Contractile muscles made from Cardiac Muscle cells
• The cardiac muscle cells of the myocardium are arranged in layers that are tightly bound together (form syncytium) and completely encircle the blood-filled chambers.
• There are dark areas crossing the cardiac muscle fibres called intercalated discs; they are actually cell membranes that separate individual cardiac muscle cells from one another.
• When the walls of a chamber contract, they come together like a squeezing fist and exert pressure on the blood they enclose. This causes pumping of blood.
• Approximately 1 percent of cardiac cells do not function in contraction but have specialized features that are essential for normal heart excitation. These cells constitute a network known as the conducting system of the heart and are in electrical contact with the cardiac muscle cells via gap junctions.
• The conducting system initiates the heartbeat and helps spread the impulse rapidly throughout the heart. They constitute the nodal tissue.
• A patch of this tissue is present in the right upper corner of the right atrium called the sino-atrial node (SAN).
• Another mass of this tissue is seen in the lower left corner of the right atrium close to the atrioventricular septum called the atrio-ventricular (AV node). A bundle of nodal fibres, atrioventricular bundle (AV bundle) continues from the AVN which passes through the atrioventricular septa to emerge on the top of the interventricular septum and immediately divides into a right and left bundle.
• These branches give rise to minute fibres throughout the ventricular musdature of the respective sides and are called Purkinje fibres. These fibres alongwith right and left bundles are known as bundle of His.
The nodal musculature has the ability to generate action potentials without any external stimuli, i.e., it is auto excitable. The SAN can generate the maximum number of action potentials, i.e., 70-75 min-1, and is responsible for and initiating and maintaining the rhythmic contractile activity of the heart. Therefore, it is called the pacemaker.
Why does SA node work as pacemaker ?
The AV nodal fibres when not externally stimulated discharge at 40-60 times/min and Purkinje fibres discharge at approx. 15 to 40 times/min in contrast to the normal rate of SA node i.e., 70-80 times/min. The discharge rate of SA node is considerably faster than the natural self excitatory discharge rate of either of them, Each time the sinus node discharges, its impulse is conducted into both A-V node or the Purkinje fibres, also discharging their excitable membranes. But the sinus node discharges again before either the A-V node or the Purkinje fibres can reach their own thresholds for self- excitation. Therefore, the new impulse from the sinus node discharges both the A-V node and the Purkinje fibres before self-excitation can occur in either of these.
Thus, the sinus node controls the beat of the heart because its rate of rhythmical discharge is faster than that j of any other part of the heart and is virtually always the pacemaker of the normal heart.