The work of heart consists of rhythmic contractions and relaxations of the auricles and ventricles.
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A contraction phase of a chamber of the heart is called a systole and a relaxation phase diastole.
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The contractions and relaxations of the different parts of the heart take place in a definite order.
The auricles and ventricles never contract simultaneously, the auricles contract first followed by ventricular systole. Ventricular systole is always followed by ventricular diastole, and so on.
The sequence of one systole followed by one diastole is termed a cardiac cycle. It lasts eight-tenths of a second. Thus the action of heart can be divided in the following three phases:
Phase I:
Simultaneous contraction of both auricles with the blood passing from the auricles into the ventricles which are relaxing.
Phase II:
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Simultaneous contraction of ventricles; the blood is forced into the aorta and the pulmonary trunk, while the auricles are relaxing.’
Phase III:
The ventricles relax and the auricles are also relaxed. This phase of cardiac activity is called the general pause. During the general pause blood enters the auricles from the venous vessels.
All these three phases constitute a single cycle of the heart action. The heart action, thus, follows a cyclic pattern.
The contraction of the heart (systole) and the relaxation of the heart (diastole) constitute heart beat.
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Heart beat can be defined as a propagated wave of muscular contraction. The heart is capable to contract rhythmically for some time even after it has been removed from the body.
Hence the contractions of the heart muscles may be conditioned by processes occurring in the heart itself.
Each time the heart beats, each ventricle pumps out about 70 ml blood. This volume is termed the stroke volume.
The heart beats about 70 times a minute (in man) and this is termed the heart rate. The stroke volume and heart rate on multiplication give the volume of blood pumped out by each ventricle per minute. This termed the cardiac output. This may be represented as follows:
C.O. = H.R. X S.V,
Where C.O. stands for cardiac output, H.R. stands for heart rate and S.V. stands for stroke volume.
The initiation and co-ordination of the heart beat was a subject of controversy for many years and led to extensive comparative investigations.
In many and probably all vertebrates we are now convinced that the beat arises in the heart muscle or myocardium itself and is in no way dependent on nervous tissue.
The most convincing evidence for this view originally was that the embryonic heart begins to pulsate long before any nervous tissue invades the heart.
The studies of Davis, Francis (1946) and Prakash (1957) also suggest that rhythmicity or heart beat is myogenic in origin and they pointed out the presence of specialized impulse conducting cells in the heart.
These cells are quite different from the general cardiac muscle fibres. These cells constitute nodule tissue.
In lower vertebrates such as fishes and amphibians the nodule tissue is located in the sinus-venosus and, therefore, the impulse of heat originates in it.
In higher animals such as birds and mammals, the nodule tissue is located in the sinu-auricular node which has a rich capillary blood supply.
The sinu-auricular node is found at the point where superior vena-cava empties into the right auricle. This region is also known as pace-maker, because it is the region which initiates the heart beat.
At regular intervals a wave of contraction originates at the sinu- auricular node and spreads all along the auricles.
It is picked up by a similar mass of tissue, the auriculo-ventricular node situated in the right auricle near the ventral part of the interauricular system.
Branched fibres extend from the auriculo-ventricular node which for some distance run together as a single bundle (Bundle of His) and then separated into the right and left branches, each branch extending through the wall of the ventricle on the corresponding side and then it reaches the apex of the ventricle whence it gives out branches (Purkinje fibres) forming a network which spreads in the entire walls of the ventricle.
The excitation wave from the auriculo-ventricular node spreads along the bundle of His and Purkinje fibres and thus exciting the muscles of the ventricle with the result the two ventricles with their all parts contract simultaneously.
If the bundle of His (atrio-ventricular bundle) -does not function properly due to any reason, then the heart beat, which originates at the sinu-auricular node, will spread to the auricles and to the auriculo- ventricular node, but will not reach the ventricles.
This condition is termed heart block. The ventricle stop beating, and the circulation of the blood ceases.
If the heart block is restricted to one half of the bundle of His, the condition is termed bundle branch block.
The contraction wave can still spread from the auriculo-ventricular node to one ventricle and this ventricle will contract first.
The contraction wave will then spread by the process of conduction to other ventricle which will contract a short time later.
The rate of conduction of impulse of contraction through bunble of His is very fast about 5 mm/sec.
Recently Mott (1957) observed that in elasmobranchs the sinus- venosus, the auriculo-ventricular junction and truncus arteriosus all show pace making activity.
No distinct areas of pace making cells have been recorded in “the myogenic hearts of invertebrates where rhythmic activity is assumed to be an inherent property of the cardiac muscles (Krijgsman and Divaris, 1955).
Recently A. J. Carlson has established that rhythmicity of heart is not only myogenic in origin but in some cases it is neurogenic in origin. A. J. Carlson conducted so many experiments on the heart of the xiphosuran arthropod, Limulus polyphemus and observed that the heart of this animal has on its dorsal surface a ganglionic mass of nerve cells.
On removing this mass the rhythmicity of the heart ceases. This shows that the heart beat in this animal is neurogenic since it originates in the nervous tissue.
Maynard, 1961 tried to study such pace making ganglion in some other neurogenic hearts but Krijgsman, 1952 in his classical review concluded that Limulus was the only arthropod in which the heart mechanism was clearly understood.
Myogenic and neurogenic hearts:
Two kinds of hearts have been observed in the animals depending upon their mode of contraction.
The hearts in which the wave of contraction starts in the muscle fibres of the heart (nodule tissue) are said to be myogenic heart, while those in which contraction wave takes its origin from the nerve cells or groups of such cells are said to be neurogenic hearts.