serve as low-resistance pathways between
cells, permitting cell-to-cell conduction of
electrical (ionic) currents. Therefore, if one
cardiac myocyte is electrically stimulated,
cell-to-cell conduction ensures that the elec-
trical impulse will travel to all of the intercon-
nected myocytes. This arrangement allows
the heart to contract as a unit (i.e., as a syn-
cytium). In contrast, individual skeletal mus-
cle cells are innervated by motor neurons,
which utilize neuromuscular transmission to
activate individual muscle fibers to contract.
No cell-to-cell electrical conduction occurs in
skeletal muscle.
The cardiac myocyte is composed of bun-
dles of myofibrils that contain myofilaments
(Fig. 3.1). When myocytes are viewed micro-
scopically, distinct repeating lines and bands
can be seen, each of which represents differ-
ent myofilament components. The segment
between two Z-lines represents the basic con-
tractile unit of the myocyte, the sarcomere.
The length of each sarcomere under physi-
ologic conditions ranges from about 1.6 to
2.2 pm in human hearts. As described later
and in Chapter 4, the length of the sarcomere
is an important determinant of the force of
myocyte contraction.
The sarcomere contains thick and thin
filaments, which represent about 50% of
the cell volume (see Fig. 3.1). Thick fila-
ments are comprised of myosin, whereas thin
filaments contain actin and other associated
proteins. Chemical interactions between the
actin and myosin filaments during the pro-
cess of excitation-contraction coupling (see
the next section) cause the sarcomere to
shorten as the myosin and actin filaments
slide past each other, thereby shortening the
distance between the Z-lines. Within the sar-
comere, a large, filamentous protein called
titin exists. It connects the myosin filament to
the Z-lines, which helps to keep the thick fila-
ment centered within the sarcomere. Because
of its elastic properties, titin plays an impor-
tant role in the passive mechanical properties
of the heart (see Chapter 4). In addition to
titin, myosin, and actin, a number of other
proteins form the cytoskeleton of myocytes,
connecting the internal and external cell
Myosin is a large molecular weight protein.
Within each sarcomere, myosin molecules
are bundled together so that there are about
300 molecules of myosin per thick filament.
Each myosin molecule contains two heads,
■ FIGURE 3.1 Structure of cardiac myocytes. Myocytes are joined together by intercalated disks to form a
functional syncytium (right side of the figure). Myocytes are composed of myofibrils, each of which con-
tains myofilam ents that are composed largely of actin (thin filam ents) and myosin (thick filam ents) (left
side of the figure). Myosin is anchored to the Z-line by the protein titin. The sarcomere, or basic contractile
unit, lies between tw o Z-lines.
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