HEART BEAT





To pump the blood into blood vessels, heart regularly contracts and relaxes in a rhythmic manner, It is known as heart beat.

Each heart beat 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 heart of a resting human being pumps about 5 litres of blood per minute.









Types of heart beat





The heart beat is of two types: neurogenic and myogenic. The neurogenic heart beat is initiated by a nerve impulse on mass of nerve cells) situated near the heart. It is present in the heart of some annelids and most arthropods. The myogenic heart beat is initiated by a patch of modified heart muscles itself. It is found in hearts of molluscs and vertibrates.









Origin of heart beat





The mammalian heart is myogenic. It 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.









How nerve impulses travel so rapidly throughout the heart?





Cardiac muscle fibres are made up of many individual cells connected in series and in parallel with another. At each intercalated

disc the cell membranes fuse with one another in such a way that they form permeable communlcating · Junctions (gap Junctions) that allow almost totally free diffusion of Ions. Therefore, from a functional point of view, ions move with ease in the Intracellular fluid along the longitudinal axes of the cardiac muscle fibres, so that action potentials travel easily from one cardiac muscle cell to the next, past the intercalated discs. Thus, cardiac muscle ls syncytium of many heart muscle cells in which the cardiac cells are so interconnected that when one of these cells becomes excited, the action potential spreads

to all of them, spreading from cell to cell throughout the latticework interconnections.





Ectopic pacemaker





Occasionally, some either parts of the heart develops a rhythmical discharge rate that is more rapid than that of sinus node. For instance, this sometimes occur in the A-V node or in the purkinje fibres when one of these becomes abnormal. I'm either case, the pacemaker of the heart shiftsbform the sinus node of the A-V node or to the excited Purkinje fibres. Under rarer conditions, a place in the atrial or ventricular muscle develops excessive excitability and becomes the pacamaker.

A pacemaker elsewhere than the sinus node is called an “ectopic pacemaker". An ectopic pacemaker causes an abnormal sequence of contraction of the heart and can cause significant debility of heart pumping.





Artificial pacemaker





SA node may become defective due to various reasons. It then fails to generate cardiac impulse at the normal rate, the near Dean becomes slow and irregular and tissues receive less blood. This disorder may be corrected by implanting an artificial pacemaker in the patient's chest. This instrument stimulates the heart electrically at regular intervals to beat at normal rate.



