Heart Sound | Autorhythmicity of heart beat

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Heart Sound

• The beating heart produces characteristic sounds which can be heard by placing the ear against the chest or by using stethoscope (an instrument which magnifies sounds and conducts them to ear).
• In a normal person, two sounds are produced per heart beat.

• First sound. This is caused partly by the closure of the bicuspid and tricuspid valves and partly by the contraction of the muscles in the ventricles.
• The first sound, ' lubb ' is low pitched, not very loud and of long duration.

• Second sound. This is caused by the closure of the semilunar valves and marks the end of ventricular systole.
• The second sound ' dup ' is highly pitched, louder, sharper and shorter in duration.
• The two sounds have been described in words as " lubb dup " and their quality indicates the state of the valves.
• Damage to the bicuspid or tricuspid valve effects the quality of the first heart sound.
• When the semilunar valves are injured, a soft hissing noise " lubb shhh ” is heard in place of the second sound.
• This is called a heart murmur. It may be caused by the syphilis, rheumatic fever or any other disease which injures the semilunar valves and affects their working.
• Thus, the blood can leak back from the pulmonary trunk and aorta into the ventricles.

Autorhythmicity of Heart beat

• The automatic rhythmicity of the heart is its ability to contract spontaneously and at a regular rate. As we know, human heart is myogenic (myo - muscle, genic- originating from), the cardiac impulse normally originates from the SA node (a node of specialized cardiac muscle fibres).
• Although the origin of cardiac impulse is myogenic, the rate of its formation and conduction may be changed by the action of nerves.
• For example, the sympathetic nerve fibres increase the activity of the SA node to accelerate the heart beat while the vagus 10th cranial nerve (carries parasympathetic nerve fibres) decreases the rate of impulse formation from the SA node and its conduction.
• Thus vagus cranial nerve decreases the heart beat.
• AV node, bundle of His and Purkinje fibres together provide autorhythmicity to the heart.

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