Heart Beat

The contraction of atria and ventricles makes a heart beat. The initiation of heart rhythm may be myogenic (originating within the heart muscle itself) or neurogenic (generated by the nerves). 

1.0Differences between Neurogenic and Myogenic Heart

The conducting system of myogenic heart

It is made up of myocardium that is specialised for initiation and conduction of the cardiac impulse. 

The conducting system has the following parts

S.A. Node (Pacemaker)

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Inter nodal pathway

↓

A.V. Node

↓

Bundle of His

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Purkinje fibres (Right & Left)

Speed of conduction is fastest in Purkinje fibres and slowest in AV node.

Sinoatrial node (SA node) : It is known as the “pacemaker” of  the heart. It is present in right upper corner of the right atrium. It generates impulses at the rate of about 72 per minute and initiates heart beat.

Internodal pathway : Connects the SA node to the AV node.

Atrioventricular node (AV Node) : It is situated in the lower left corner of the right atrium close to the atrio-ventricular septum. It is capable of generating impulse at rate of about 40–60/min. 

Bundle of His (AV Bundle) :  It is the connection between the atrial and ventricular musculature. It begins at the AV node and then divides into left and right branches as it descends down towards ventricles. 

Branches of the AV bundle descends on the interventricular septum and is distributed to the ventricle after dividing into Purkinje fibres.

The Purkinje Fibres: These are distributed through the endocardium of the ventricles and propogate the impulse in the entire ventricle musculature. (18-25 per min.)

The SAN can generate the maximum number of action potentials, i.e., 70-75 min^–1, and is responsible for initiating and maintaining the rhythmic contractile activity of the heart. Therefore, it is called the pacemaker. Our heart normally beats 70-75 times in a minute (average 72 beats min^–1).

Rhythmic contraction and relaxation of heart is called heartbeat. Actually, contraction and relaxation occur separately in atria and ventricles. However, ventricular movements are quite prominent and forceful. Therefore, heart beat is synonym with ventricular or apex beat. It increases temporarily with activity and disease. In animals heart beat is connected with size. In mammals, smaller animals have higher heartbeat. 

Adult human - 72 per min.

Note : Heart rate is higher in women, children and infants and lower in aged persons. 

2.0Regulation of Heartbeat 

Nervous Control 

The "Cardiac centre"(neural centre) which regulates heart beat is found in Medulla oblongata of the brain, it can moderate the cardiac function through ANS.

This cardiac centre has two units :

Cardio accelerator centre

Cardio inhibitory centre

From the cardio-acceleratory centre, a pair of sympathetic nerves go into the S.A. node. Neural signals through the sympathetic nerves can increase the rate of heart beat, the strength of ventricular contraction and thereby cardiac output.

While the cardio-inhibitory centre sends impulses to the S.A. node through cardiac branch of Vagus-nerve. From the parasympathetic nerve-fibres, acetylcholine is secreted which decreases the heart rate, speed of conduction of action potential and the cardiac output.

Hormonal control

Adrenal medulla hormone (adrenaline, nor adrenaline) increases heart rate and the cardiac output.

3.0Heart Sound

During each cardiac cycle, two prominent sounds are produced i.e. "Lubb" and "Dub".

These "Lubb" and "Dub" sounds of the heart can be heard with the help of an instrument called" Stethoscope"(Invented by laennec).

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