Working of SA node 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 AV node or the Purkinje fibres, also discharging their
excitable membranes. But the sinus node discharges again before either the AV node or the Purkinje fibres can reach their own
thresholds for self-excitation. Therefore, the new impulse from the sinus node discharges both the AV node and the Purkinje
fibres before self-excitation can occur in either of these.
Thus, the SA node controls the beat of the heart because its rate of rhythmical discharge is faster than that of any other part of
the heart and is virtually always the pacemaker of the normal heart.HEARTBEAT
To pump the blood into blood vessels, heart regularly contracts and relaxes in a rhythmic manner known as ‘heartbeat’.
Each heartbeat includes one systole (contraction phase) and one diastole (relaxation phase) of the heart to distribute and receive
blood to and from the body. The heart of a healthy person beats 72 times per minute. Beating is an inherent capacity of the
heart. The heartbeat is of two types: neurogenic and myogenic. Neurogenic heartbeat is initiated by a nerve impulse coming
from a nerve ganglion (mass of nerve cells) situated near the heart. It is present in the heart of some annelids and most arthropods.
The myogenic heartbeat is initiated by a patch of modified heart muscle itself. It is found in hearts of molluscs and vertebrates.
Origin of heartbeat
The mammalian heart is myogenic that means the heart beat originates from a muscle, however, it is regulated by the nerves.
The heart beat originates from the sinoatrial node (SAN) or pacemaker.
Conduction of heartbeat
Another mass of neuromuscular tissue, the atrioventricular node (AV
node or pacesetter) is situated in the wall of the right atrium. The AV
node picks up the wave of contraction propagated by SA node. A mass
of specialised fibres, the bundle of His, originates from the AV node. The
bundle of His divides into two branches, one going to each ventricle.
Within the mycocardium of the ventricles the branches of bundle of
His divide into a network of fine fibres called the Purkinje fibres. The
bundle of His and the Purkinje fibres convey impulse of contraction from
the AV node to the myocardium of the ventricles.
The conduction of cardiac impulse can be summarised as :
SA node → Atrial syncytium → Junctional fibres → AV node → AV bundle
→ Bundle branches → Purkinje fibres → Ventrical syncytium.
Regulation of heartbeat
When a person is at rest, the heart pumps about 5 litres of blood each minute. During severe exercise, the heart may be required
to pump four to seven times this amount. The basic means by which the volume
pumped by the heart is regulated are of three types:
(i) Intrinsic regulation : Under most conditions, amount of blood pumped per
minute by heart is determined almost entirely by the rate of blood flow into heart from
the veins, i.e., venous return. This intrinsic ability of heart to adapt to increasing
volumes of blood is called Frank-Starling mechanism. It says that the greater the
heart muscle is stretched during filling, greater is the force of contraction and the
greater is the quantity of blood pumped into the aorta, i.e., within physiologic limits,
the heart pumps all the blood that returns to it through veins.
It can be explained as that when an extra amount of blood flows into the ventricles,
the cardiac muscle is stretched to greater length, causing the muscle to contract
with increased force because the actin and myosin filaments are brought to a more