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Cardiac
muscle. The frequency of contraction
The
rate at which the heart contracts and the synchronization
of atrial and ventricular contraction required for the efficient
pumping of blood depend on the electrical properties of
the myocardial cells and on the conduction of electrical
information from one region of the heart to another. The
action potential (activation of the muscle) is divided into
five phases (0–4) and is graphed in Figure 9. Each of the
phases of the action potential is caused by time-dependent
changes in the permeability of the plasma membrane to potassium
ions (K+), sodium ions (Na+), and calcium ions (Ca2+). The
resting potential of the myocytes of the ventricle (phase
4) begins with the outside ofthe cell being positive—i.e.,
having a greater concentration of positive ions. Atrial
and ventricular myocytes are normally quiescent (non-rhythmic);
however, when the resting membrane potential is depolarized
to a critical potential (Ecrit), a self-generating action
potential follows, leading to muscle contraction. Phase
0, the upstroke, is associated with a sudden increase in
membrane permeability to Na+. Phases 1, 2, and 3 result
from changes in membrane permeability and conductance to
Na+, K+, and Ca2+.
The
electrical activity of heart muscle cells differs substantially
from that of skeletal muscle cells in that phases 1, 2,
and 3 are considerably prolonged (200 milliseconds versus
five milliseconds, respectively). Another significant difference
in excitability is that heart muscle cannot be tetanized
(i.e., induced to spasm) by the application of repetitive
stimuli (see above Striated muscle), thus ensuring the completion
of the contraction/relaxation cycle and the effective pumping
of blood.
Because
atrial and ventricular cells are normally quiescent, exhibiting
action potentials only after the muscle is depolarized to
the critical membrane potential (Ecrit), the source of the
rhythmic contractions of the heart must be sought elsewhere.
In contrast to atrial and ventricular myocytes, the myocytes
of the sinoatrial (SA) node, the atrioventricular (AV) node,
the bundle branches, and the Purkinje fibre system are made
up of specialized cardiac muscle cells that exhibit a spontaneous
upward drift in the resting potential toward Ecrit, resulting
in thegeneration of the action potential with all of its
phases. The normal rhythmicity of cells from each of these
regions depends on the rate at which spontaneous depolarization
occurs and the resting membrane potential from which it
starts. The region with the fastest intrinsic rate, the
SA node, sets the pace for the whole heart. The pacemaker
activity is propagated to the rest of the heart by means
of the low electrical resistance pathways through the muscle
cells (e.g., intercalated disks) and the presence of specialized
conducting tissue (bundle branches, Purkinjesystem). The
time course of activation and the shape of the action potentials
in different parts of the heart are responsible for the
synchronous activation and contraction of the muscles of
the atrium followed by those of the ventricle.
The
normal rhythm of the heart (i.e., the heart rate) can be
altered by neural activity. The heart is innervated by sympathetic
and parasympathetic nerves, which have a profound effect
on the resting potential and the rate of diastolic depolarization
in the SA nodal region. The activity of the sympathetic
nervous system may be increased by the activation of the
sympathetic nerves innervating the heart or by the secretion
of epinephrine and norepinephrine from the adrenal gland.
This decreases the resting potential of the myocytes of
the SA node while increasing the rate of diastolic depolarization.
The result is an increase in the heart rate. Conversely,
stimulating the parasympathetic nervous system (vagal nerves
to the heart) increases the resting potential and decreases
the rate of diastolic depolarization; under these circumstances
the heart rate slows. The sympathetic nervous system is
activated under conditions of fright or vigorous activity
(the so-called “fight or flight” reaction), where the increase
in force and rate ofheart contraction are easily felt; the
parasympathetic system exerts its influence during periods
of rest.
