Biology Today - May 2018

To prevent the AV valves from being pushed up into the atria when the ventricles are contracting (a condition called prolapse), the

valves are fastened to muscular projections (papillary muscles) of the

ventricular walls by fibrous strands chordae tendineae. The papillary

muscles do not open or close the valves. They act only to limit the valves’

movement and prevent the backward flow of blood.

The aortic and pulmonary artery semilunar valves function quite differently

from the AV valves. The high pressure in the arteries at the end of systole

causes the semilunar valves to snap to the closed position, in contrast to

the much softer closure of the AV valves. Because of smaller openings, the

velocity of blood ejection through the aortic and pulmonary valves is far

greater than that through the much larger AV valves.

HISTOLOGY OF HEART
The heart consists of an outermost smooth coelomic epithelium – visceral
pericardium, the middle thick muscular layer – the myocardium, which is composed of cardiac muscle cells and the innermost
layer – the endothelium consisting of the simple squamous epithelial cells.
Cardiac Muscle Cells
Contractile muscles
The cardiac muscle cells of the myocardium are arranged in layers that are tightly bound together and completely encircle the blood-
filled chambers. There are dark areas crossing the cardiac muscle fibres called intercalated discs; they are actually cell membranes
that separate individual cardiac muscle cells from one another. When the walls of a chamber contract, they come together like a
squeezing fist and exert pressure on the blood they enclose. This causes pumping of blood.
Similar to smooth and skeletal muscle, it is an electrically excitable tissue that converts chemical energy stored in the bonds of ATP
into force generation. Action potential propagates along cell membranes, Ca2+ enters the cytosol and the cycling of force-generation
cross-bridges is activated.
But unlike skeletal muscle cells, which can be rested for prolonged periods and only a fraction of which are activated in a given muscle
during most contractions, every heart cell contracts with every beat of the heart. Beating about once every second, in an
average life span, cardiac muscle cells may contract almost 3 billion times without resting. Remarkably, despite this enormous
workload, the heart has only a limited ability to replace its muscle cells. Recent experiments suggest that only about 1 percent of
heart muscle cells are replaced per year.
Autorhythmic muscles
Approximately 1 percent of cardiac cells constitute a network known as the
conducting system of the heart (do not function in contraction rather
excitation). They constitute the nodal tissue which initiate heart beat and
spread the impulse. A patch of this tissue is present in the right atrium near
the opening of superior vena cava called the sino-atrial node (SAN).
Another mass of this tissue is seen in the right atrium called the atrio-
ventricular node (AVN). A bundle of nodal fibres, atrioventricular
bundle (AV bundle) continues from the AVN which passes through the
atrioventricular septa to emerge on the top and immediately divides into a
right and left bundle. These branches give rise to minute fibres throughout
the ventricular musculature of the respective sides and are called Purkinje
fibres which alongwith right and left bundles are known as bundle of His.

The nodal musculature has the ability to generate action potentials without any external stimuli, i.e., it is autoexcitable. 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). The cardiac muscles contract in much the same way as skeletal muscle, except that the duration of contraction is
much longer. Conversely, the specialised autorhythmic muscles contract only feebly because they contain few contractile fibrils.

LeftAV

(bicuspid) valve

Aortic semilunar

valve

Openings to

coronary arteries

Pulmonary

semilunar valve

RightAV

(Tricuspid) valve Fig.:Valves of the heart

Purkinje fibres

Right bundle

branch

Left posterior

division

Left interior

division

Left bundle

branch

Bundle of His

Atrioventricular

(AV) node

Sinoatrial

(SA) node

Fig.: Conducting system of the heart