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Contraction of the heart muscle

The heart muscle , also known as heart or myocardium, is an involuntary contraction muscle located in the mid-thorax region. It is one of the three types of muscle of our body, in addition to the smooth, characteristic of the viscera, and the skeletal, which together with the bones constitutes the locomotor system. The cells that make up the heart muscle contain a single nucleus, unlike the two remaining muscle tissue types whose cells can have up to four.

The heart muscle is responsible for pumping all existing blood through the circulatory system through the process of contraction. A striking feature of this muscle is that it is able to function without the need for nerve stimulation: it is a myogenic muscle, that is, it decides when it has to contract and when it has to dilate (processes known as systole and diastole, respectively).

What is the heart muscle made of?

Contraction of the heart muscle

The heart muscle, like any other muscle in the body, is able to contract. However, unlike other muscles, the heart is able to read and respond to information provided by tissues in the form of hormones , this being one of the main characteristics of its functioning. In the same way, the cardiac muscle is composed of:

Striations

The heart muscle has a striated appearance because it is made up of thick interlaced protein segments of actin and myosin , such as striated skeletal muscle. The difference between these two muscle types lies in the arrangement of the actin and myosin fibers: while in skeletal muscle they adopt a linear and parallel arrangement, in the cardiac muscle the fibers intertwine, adopting an irregular disposition.

Tubules T

Another difference between the cardiac muscle and skeletal muscle is the T-tubules , which are no more than extensions and ramifications of the plasma membrane of muscle cells. In the cardiac muscle the T tubules are larger and wider. In addition, the cells that make up cardiac muscle tissue have fewer T-tubules than those of skeletal muscle.

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Intercalary discs

They are the union systems of cardiac muscle cells. These structures are located in the regions of the membrane where the ends of two cells meet.

How is the contraction of the heart muscle carried out?

Contraction of the heart muscle

The coupling process between excitation and cardiac contraction is constituted by a series of events based on the action potential. This process is of vital importance, because it allows the heart to beat in a controlled manner and without the need for external nervous stimulation.

The heart muscle coupling is the sequential result of the contraction of the muscles of the heart, which contracts between 60 and 100 times per minute even when the body is at rest . However, this velocity can be altered by the sympathetic and parasympathetic nerves, which can modulate the heart rate. Next we explain the phases that take place during the process of cardiac contraction.

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The initiation

The cells located that constitute the sinoatrial node (SA) generate action potentials spontaneously. This structure is the natural pacemaker of the heart , it depends on the start of the cardiac contraction and, therefore, the heartbeat frequency.

The action potentials generated extend along the membrane of cardiac muscle cells , also known as sarcolemma, in the form of impulses. The impulses pass from one cell to another through structures known as gap junctions.

The driving speed of the action potential will vary according to the part of the heart in which it is located. First, the upper chambers, known as the atria, are contracted, and shortly after, the lower chambers, the ventricles, contract. This slight delay allows the storage of blood within the ventricles before its entire contraction. If it did not exist, the heart would explode.

Calcium-induced calcium release

Different regions of the sarcolemma penetrate inside the cells. They carry calcium gradients that depolarize the membrane and initiate the transmission of the action potential generated in the sinoatrial node.

The increase in calcium in the cell leads to the activation of a receptor called ryanodine type 2 (RyR2), located in the sarcoplasmic reticulum (SR) membrane, which acts as a calcium retainer inside the T tubules.

In conclusion, the activation of the sarcoplasmic reticulum causes the release of more calcium within the cell by this organelle.   For this reason the process is called calcium-induced calcium release (ICRC).

Muscle contraction

Contraction of the heart muscle

The increase of calcium within the cell produced by the release of calcium by the sarcoplasmic reticulum allows the indirect activation of the actin and myosin fibers, leading to muscle contraction.

But how does this happen? Calcium binds to a protein present in the actin filaments called troponin. After this union, troponin undergoes a conformational change with the objective of joining the region furthest from the adjacent myosin fiber. The union between the actin and myosin heads is known as a cross bridge.

Adenosine triphosphate (ATP), a molecule produced in the mitochondria, is used as an energy source for the mobilization of the myosin head. On the other hand, actin slides through the myosin filament causing a shortening of the muscle , that is, what we know as muscle contraction. This process is a consequence of the increase in intracellular calcium concentration.

Completion of the contraction

The contraction of the heart muscle ends when the intracellular calcium concentration decreases. In this way, troponin returns to its original state, blocking the binding sites of the actin and preventing the formation of crossed bridges.

The decrease in intracellular calcium concentration is a consequence of the opening of ion transporters present in the sarcoplasmic reticulum membrane, which stores the calcium to release it with the arrival of the next action potential and, therefore, the next contraction.

Contraction of the heart muscle

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