C h a p t e r 1 7
Control of Cardiovascular Function
383
contain gap junctions that serve as low-resistance path-
ways for passage of ions and electrical impulses from
one cardiac cell to another (Fig. 17-6B). Thus, the myo-
cardium behaves as a single unit, or
syncytium
, rather
than as a group of isolated units, as does skeletal muscle.
When one myocardial cell becomes excited, the impulse
travels rapidly so the heart can beat as a unit.
As in skeletal muscle, cardiac muscle cells con-
tain actin and myosin filaments, which interact and
slide along one another during muscle contraction.
A number of important proteins regulate the interaction
between the actin and myosin filaments. These include
the tropomyosin and the troponin complex, which
consists of three subunits (troponin T, troponin I, and
troponin C) that regulate calcium-mediated muscle con-
traction (see Chapter 1, Fig. 1-19). In clinical practice,
the serum levels of specific cardiac forms of troponin
T and troponin I, released from injured heart muscle,
are used in the diagnosis of myocardial infarction
(see Chapter 19).
Pleura
Subclavian vein
External
jugular vein
Internal
jugular vein
Aortic arch
Left atrium
Left coronary
artery
Right coronary
artery
Left
ventricle
Left
ventricle
Anterior
Posterior
Right
ventricle
Right ventricle
Interventricular
septum
Pericardium
Superior
vena cava
Right
atrium
A
B
C
FIGURE 17-4.
(A)
The heart in relation to the sternum, ribs, and lungs.
(B)
Anterior view of the heart
and great vessels (note that the lungs, which normally fold over part of the heart’s anterior, have
been pulled back).
(C)
Cross-section of the heart showing the increased thickness of the left ventricle
compared to the right.
