Download Setting the Heart`s Tempo

Setting the Heart’s Tempo
Heart, or cardiac, muscle differs from other types of muscle. Like skeletal muscle,
cardiac muscle appears striated (grooved or ridged) when viewed under a microscope. But
unlike skeletal muscle, cardiac muscle displays a branching pattern. The greatest difference
stems from the ability of this muscle to contract without being stimulated by external nerves.
Muscle with this ability, called myogenic muscle, explains why the heart will continue to beat,
at least for a short time, when removed from the body.
The heart’s tempo or beat rate is set by the sinoatrial (SA) node. This bundle of
specialized nerves and muscles is located where the venae cavae enter the right atrium. The
SA node acts as a pacemaker, setting a rhythm of about 70 beats per min for the heart. Nerve
impulses are carried from the pacemaker to other muscle cells by modified muscle tissue.
Originating in the atria, the contractions travel to a second node, the atrioventricular (AV)
node. The AV node serves as a conductor, passing nerve impulses via two large nerve fibres,
called Purkinje fibres, through the septum toward the ventricles. The Purkinje fibres run
along the septum that separates the right and left ventricles, carrying impulses from the AV
node to the bottom tip of the heart. From here, these branching fibres carry impulses up
along the outer walls of the ventricles back toward the atria. A wave of cardiac contraction
follows the nerve pathway. Both right and left atria contract prior to the contraction of the
right and left ventricles.
One of the greatest challenges for surgeons performing open-heart surgery is to make
incisions at the appropriate location. A scalpel placed in the wrong spot could cut fibres that
conduct nerve impulses. Heart rate is influenced by autonomic nerves. Two regulatory
nervous systems—the sympathetic and parasympathetic nervous systems—conduct impulses
from the brain to the SA node. Stimulated during times of stress, the sympathetic nerves
increase heart rate. This increases blood flow to tissues, enabling the body to meet increased
energy demands. Conditions in which the heart rate exceeds 100 beats per min are referred
to as tachycardia. Tachycardia can result from exercise or from the consumption of such
drugs as caffeine or nicotine. During times of relaxation, the parasympathetic nerves are
stimulated to slow the heart rate.
Diagnosing Heart Conditions Doctors can use electrocardiographs, which map
electrical fields within the heart, to make tracings to diagnose certain heart problems.
Electrodes placed on the body surface are connected to a recording device. The electrical
impulses are displayed on a graph called an electrocardiogram (ECG). Changes in electrical
current reveal normal or abnormal events of the cardiac cycle. The first wave, referred to as
the P wave, monitors atrial contraction. The larger spike, referred to as the QRS wave,
records ventricular contraction. A final T wave signals that the ventricles have recovered. A
patch of dead heart tissue, for example, will not conduct impulses and produces abnormal
line tracings. By comparing the tracings, doctors are able to locate the area of the heart that
is damaged.
Define the following words:
Myogenic Muscle, SA node, Av Node, Purkinje fibers, Sympathetic nervous system,
Parasympathetic nervous system, septum, Pulmonary circulatory system, systemic circulatory
system, atria, ventricles, Atrioventricular valves, semilunar valves, Aorta, coronary arteries,