Page 352 - Anatomy and Physiology of Farm Animals, 8th Edition

Physiology of the Heart and Circulation / 337

induced by the closures are believed to be the peripheral venous system during either
radiography or ultrasound to assess vascu­
the actual causes of the sounds. Third and
VetBooks.ir fourth heart sounds may be heard in some lar flow within the chambers of the heart
or within the coronary vessels within the
normal horses and cattle with relatively
slow heart rates. The third sound is associ­ cardiac muscle.
ated with the rapid ventricular filling phase
after the initial opening of the A‐V valves,
and the fourth sound is associated with Electrical Activity of the Heart
atrial contractions (Fig. 18‐2).
Heart murmur is a general term for Like skeletal muscle, contraction of each car­
any abnormal heart sound. Murmurs diac muscle cell requires an action potential
typically indicate abnormalities or on the cell membrane (Fig. 18‐4A), and the
pathologies in heart valve function. action potential brings about the release of
Murmurs may occur when a valve fails to calcium from intracellular stores. The cal­
close completely (valvular insufficiency) cium binds to regulatory proteins on the thin
and blood flow goes in the wrong direc- filaments, and movement of the regulatory
tion at the wrong time. Murmurs may proteins permits an interaction between
also occur when a valve fails to open actin and myosin to bring about contraction
completely (valvular stenosis) and blood (see Chapter 9). However, unlike skeletal
is forced through a smaller than normal muscle, each cardiac muscle cell is not inner­
opening. Innocent heart murmurs are vated by a motor neuron that is responsible
nonpathologic sounds made by the for eliciting an initial action potential. In the
blood circulating through the heart. heart, the initial action potential occurs
Often these are termed “functional” or spontaneously in a specialized group of myo­
“physiologic” murmurs and are com- cardial cells found in the sinoatrial (SA)
mon during infancy and will disappear node of the heart (Fig. 18‐5), from which the
in adulthood. action potential is propagated around the
heart to bring about contraction of all cardiac
muscle cells. The cell‐to‐cell propagation in
Imaging the Heart the heart is possible because the intercalated
disks provide an electrical connection
Auscultation is a critical first step to assess­ between myocardial cells (see Fig. 1‐10C).
ing normal function of the heart and lungs.
However, imaging is often used to gain a Sinoatrial Node and Heart Rate
more detailed assessment. Radiographs
are often used to assess heart size and can Figure 18‐4B shows a series of action poten­
be informative regarding the normally air‐ tials from SA node cells. A unique feature of
filled lung tissue, particularly if there is an the electrical activity of these cells is that the
abnormality such as fluid or other dense resting membrane potential is unstable.
tissue. Echocardiography is the use of This instability permits SA node cells to
ultrasound to image the heart and associ­ depolarize spontaneously to threshold,
ated structures. It is a noninvasive proce­ where an action potential is generated. The
dure that provides dynamic images SA node (Fig. 18‐5) is termed the pace-
permitting visualization of structures in maker of the heart because each action
the heart as it contracts and relaxes. This potential that spontaneously develops in the
provides much more information on car­ SA node is propagated around the heart to
diac function than can be obtained with a stimulate action potentials in all myocardial
single static image. Ultrasound technology cells and produce a contraction.
relies on soundwaves, therefore it is not the Both sympathetic and parasympathetic
method of choice for assessing the lungs. nerves innervate the SA node. By their
A contrast media can also be introduced to actions on cells in the SA node, sympathetic