Signal Propagation on Different Scales

In the previous section we discussed how an action potential arises. Now we want to under-stand, how the electrical signal propagates through the tissue. For this purpose, we describe here the anatomical structure of cardiac muscle fibers.

Cell to Cell Coupling

The lower part of Fig. 2.4 shows a schematic picture of cardiac fibers, which are encased by thesarcolemma. The cardiac cells possess a nucleus and they are separated by intercalated discs (also shown in a photomicrograph of cardiac muscle in the upper left part). The in-tercalated discs consist of gap junctions, that actually allow the propagation of ions from one cell to a neighboring cell and desmosomes, which bind the cells together and ensure in this way the mechanical contraction of the whole muscle. Furthermore, the large mito-chondria (around 25% to 35% of the volume in cardiac cells, whereas in comparison≈2%

in skeletal muscle) provide the adenosine triphosphate (ATP) supply of the cell, and make the cardiac cells resistant to fatigue. Themyofibrils (the parts of the cell which provide the actual mechanical contraction) constitute most of the remaining space of the cell. The elec-trical signal is propagating intracellular (from cell to cell) as well as extracellular along the sarcolemma. The t-tubules are basically (transverse) invaginations of the sarcolemma, and

Figure 2.5: The arrangement of cardiac fibers in the heart. The subfigure (a) depicts a sketch of the fiber alignment in atria and ventricular muscle. Blue arrows indicate the defined direction, corresponding to the conductivitiesσ_` and σ_p for one exemplary point of the tissue. Reprinted by permission of Pearson Education, Inc., New York, New York[1].

The rotation of fiber sheets from the epicardium to the endocardium is shown in (b). As in (a), blue arrows indicate the conductivities which correspond to the longitudinal direction along the fibers (σ`), the perpendicular direction, orthogonal to the fiber direction but within the fiber sheet (σ_p) and the transverse direction, orthogonal to the first two directions, with a transmural direction (σ_t). Reprinted from [26], with the permission of AIP Publishing.

allow the quick propagation of the extracellular signal into the inner part of the cell. Here, the electrical signal can enter the intracellular domain by a large number of ion channels.

The last constituent of the cardiac cell depicted here is thesarcoplasmic reticulum, basically a huge storage of calcium, which is essential for the mechanical contraction of the cell (see section 2.1.3 on page 20).

Arrangement of Cardiac Fibers

After discussing how an action potential propagates from cell to cell, we now want to clarify how the electrical signal spreads on a global scale, thus in the whole organ. In particular, the question arises: does the propagation of the electrical signal occur homogeneous and/or isotropic? In fact, cardiac muscle fibers are arranged in such a way, that the pumping function is optimized in a highly efficient way.

In Fig. 2.5(a) the direction of the fibers is sketched. The sophisticated structure and ar-rangement causes a screw-like contraction, which pushes the blood out of the heart. In detail, (tube-like) cardiac cells are arranged in layers or sheets, parallel to the surface of the heart. Three distinct directions can be defined at every point of the tissue (also marked in Fig. 2.5(a) and (b)): the direction within the layer and along the fibers (longitudinal direc-tion), the direction perpendicular to the fiber, but within the layer (perpendicular direction) and the direction which is orthogonal to the first two directions, and thus transverse to the layer (transverse direction). Since the sheets of tissue are aligned parallel to the surface

2.1. Complexity of the Heart

Figure 2.6: The conduction system of the heart. The pathway of the electrical signal is ini-tiated by the sinoatrial node (1). It propagates through the atria (causing their contraction) and along the internodal pathway to the atrioventricular node (2). After a delay of about 0.1 s, the signal continues to travel in the atrioventricular bundle (3), which splits up into the left and right bundle branches (4). At the end, the electrical signal propagates through the Purkinje fibers (5), where it connects to the cardiac cells and initiates the contraction of the ventricles. Reprinted by permission of Pearson Education, Inc., New York, New York [1].

of the heart (except e.g. in the septum), the transverse direction gives in general also the transmural direction. Furthermore, neighboring layers of tissue are rotated against each other in such a way, that the fiber direction (longitudinal direction) rotates monotonously 120^◦ from the outside (epicardium) to the inside of the heart (endocardium) (Fig. 2.5(b)).

Due to the anatomical heterogeneity of the tissue, the electrical conductivities in these three directions (σ`, σp and σt) can differ significantly in their magnitude from each other and additionally can depend on the region of the heart, too.

Electrical Conduction System of the Heart

The frequency of contractions of the heart muscle is determined by the autonomic nervous system. In particular, the sympathetic and the parasympathetic nervous system may in-crease or dein-crease the heart rate, respectively, depending on the actual condition and need of the whole body. The actual process of a mechanical contraction, however, is triggered by the heart itself.

The heart initiates intrinsically the contraction from the sinoatrial (SA) node (an au-tonomous pacemaker, see Fig. 2.6) and the electrical signal is then distributed inside the heart muscle, using a sophisticated electrical conduction system. From the SA node, the electrical activity spreads throughout the atria, causing a contraction here and travels along theinternodal pathwayto theatrioventricular (AV) node. The propagation of the electrical

Figure 2.7: The structure of a moyofibril. Myofibrils are composed of sarcomeres, which are separated by Z discs. The sarcomeres consist mainly of thin (actin) filaments, and thick (myosin) filaments. In the case of a contraction of the muscle, the actin filaments are moving (with the Z discs) towards the myosin filaments (thus in the direction of the H zone). Reprinted by permission of Pearson Education, Inc., New York, New York [1].

signal is delayed here for around 0.1 s, which allows the atria to complete the contraction.

The atrioventricular node is (in the case of a healthy heart) the only electrical connection between the atria and the ventricles. From here, the impulse travels along the atrioventricu-lar bundleinside the septum of the heart, and splitting then up into the left and right bundle branches, heading to the apex of the heart. The Purkinje fibers represent the final part of the conduction system. Only here, the electrical signal connects to the cardiac muscle and induces the mechanical contraction of the ventricles.