The common myocardium is formed of a fibrous skeleton with attached bundles of muscle tissue (the actual myocardium, or heart muscle). It is the intermediate tunic of the thickest heart wall. It is characterized by involuntary but striated muscle tissue (usually the smooth muscle is involuntary and the striated muscle is voluntary), it is not innervated by the nervous system but has its own conduction system (specific myocardium) that allows the autoritmic contraction. The thickness of the myocardium (and consequently the wall of the heart) is variable, and is different between atria and ventricles and also between right and left. Let's see how the muscles of atria and ventricles are organized.
Muscles of the atria :
It's composed by:
- own bundles → surround the vein outlet orifices and run vertically up to the fibrous skeleton .
- common beams → bundles of fibers that extend transversely from one hall to another.
Overall, it is a fairly thin muscle layer, except at the side wall of the right atrium where the combed muscles are present, which constitute a thickening.
Muscles of the ventricles :
- own bundles : they form the intermediate layer of the myocardium; they originate on the atrioventricular fibrous rings ( fibrous skeleton of the heart ) and reach the apex and then go back up to the fibrous ring. They form two cone-shaped bags.
- common beams : they are inserted in the orifices (fibrous part) and in the fibrous trigons, descending towards the apex going deeper than their own bundles; some of them form the papillary muscles.
Three layers can be distinguished in the musculature of the ventricles:
- superficial (formed by common descending beams)
- intermediate (formed by own beams)
- deep (formed by common ascending beams).
Functionally connects the muscles between atria and ventricles (electrically isolated from fibrous skeleton ); it is a variety of myocardial tissue formed by cells that lead but do not contract (only a few bundles of myofibrils are present). Hence the stimuli that propagate to the common myocardium, allowing the contraction of atria and ventricles and therefore in general the functioning of the cardio-circulatory system . There are two particular formations within the specific myocardium:
Also called a natural pacemaker (it is made up of cells that generate the electrical impulse autonomously, that is, which are able to depolarize), located near the outlet of the superior vena cava (anterior wall of the right atrium). From this the stimulus to the contraction of the heart starts; the stimulus, obviously in electrical form, is distributed to the entire surface of the atria through the internodal pathways and from these directly to the atrial myocardial fibers (common myocardium). As a consequence of this the atriums contract simultaneously (we have seen that the common beams crosswise connect the two atria).
The internode pathways also connect the sinoatrial node (SA) to the atrioventricular node (AV).
It is located near the outlet of the coronary sinus (which is located below the foramen ovale) medial wall of the right atrium, at the interatrial septum. From the node the impulse passes to the beam of His (continuation of the atrioventricular node) which is divided into two branches for the two ventricles. The two branches are subdivided in turn to form the network of Purkinje fibers that extend up to the papillary muscles and the walls of the ventricles; the stimuli are distributed first to the papillary muscles (so that the systole muscles are already stretched and the right and left atrioventricular valves are closed for not reflux) then the walls of the ventricles, causing the contraction and the pumping of blood in the system of distribution (blood vessels).
⇑ READ ALSO: Structure of the heart
⇓ READ ALSO: Pericardium