Control of cardiac muscle

1. Neural influence. Cardiac muscle does
not require neural input for activation, but its
activity is modulated by the sympathetic (increased
contractile activity) and parasympathetic (decreased
activity) divisions of the autonomic nervous system
(ANS).
2. Cardiac conduction system. Intrinsic
(endogenous) control of cardiac muscle function is
mediated by the impulse conducting system,
which consists of conductile cardiac myocytes
arranged in the following order: S-A node,
internodal fibers, AV node, AV bundle with
right and left branches, and subendocardial
branches. Conductile myocytes, as their name
implies, are specialized for conduction rather than
contraction. In this respect, these unique cardiac
muscle cells are more like nerve cells in function.
a. Sinu-atrial (S-A) node (also
sinoatrial node, sinus node) is located in the right
atrium at the junction with the superior vena cava; it
contains specialized muscle cells that spontaneously
depolarize rhythmically to initiate each heart beat.
This area is known as the cardiac pacemaker.
i. Gap junctions provide electrical
coupling at intercalated disks to propagate
depolarizations.
ii. Internodal tracts of conductile
cardiocytes spread pacemaker impulses to atrial
myocytes as well as to the A-V node.
b. Atrioventricular (AV) node is
located in the right atrium near the AV junction
between the tricuspid valve and the orifice of the
coronary sinus. The impulse is then forwarded to
the ventricles after a short delay of about 0.1
seconds.
c. Ventricular fibers
i. AV Bundle (bundle of His).
From the A-V node, a trunk of conduction fibers run
through the fibrous skeleton of the interventricular
septum where they divide into two branches.
ii. Right and left bundles (bundle
branches) are smaller branches of the main AV
bundle.
iii. Subendocardial branches,
which transmit impulses to ventricular contractile
myocytes, are the terminal branches of the
conductile apparatus and consist of much larger
diameter conductile fibers that ramify to the
ventricles from the respective right and left bundle
branches. These fibers, which are called Purkinjefibers, are composed of cells that are characterized
by their large amount of cytoplasm containing fewer
myofibrils and abundant mitochondria and glycogen.
N.B.: some authors use the term Purkinje fiber
loosely to describe all of the conductile myocytes
from the nodal fibers to their terminations.
However, the usage here will be traditional and
restrict the term Purkinje fibers to the wide diameter
conductile myocytes. Actually, the other conductile
myocytes, from S-A node to AV bundles are typically
smaller in diameter than normal myocytes.
Location of cardiac muscle. Cardiac muscle is
found mainly in the myocardium of the heart.
However, it also occurs in the walls of some veins
associated with the heart, e.g., coronary sinus
(Coakley, JB & Summerfield King, T. J Anat 93:30-35, 1959).
Regeneration and repair. Until recently, it has
been widely accepted that cardiac muscle tissue is in
not capable of renewal or repair (e.g., Soonpaa, MH &
Field, LJ. Circ Res 83:15-26, 1998). This dogma has been
questioned by recent studies, which have provided
evidence that cardiac myocytes, under certain
circumstances, can exhibit limited regeneration (e.g.,
Arbustini, E et al. Am J Cardiol 72:608-614, 1993; Quaini, F et al.
Circ Res 75:1050-1063, 1994; Kajstura, J et al. Proc Natl Acad Sci
USA 95:8801-8805, 1998; Beltrami, AP et al. N Engl J Med
344:1750-1757, 2001).
a. Stem cells. Whether regeneration of
cardiac muscle results from 1) terminally
differentiated cardiac myocytes becoming mitotically
active, 2) activation of a quiescent population of
cardiac stem cells (from within heart muscle, CT or
blood vessels), or 3) from blood-borne precursors
(e.g., bone marrow stem cells) remains to be
determined.
b. Clinical implications. Although the
notion of cardiac stem cells has generated
considerable excitement, it is still clear that the
limited regenerative capacity of the heart cannot
prevent such conditions as heart failure. However,
the concept of stem cell renewal opens the door for
potential treatments in the future.