Thorax
Answers
329.The answer is d.(Moore and Dalley, pp 162–163.). Specialized car-
diac muscle cells, which form the sinoatrial (SA) node, are the pacemakers
of the heart. They have the fastest-paced autorhythmicity of all cardiac
muscle cells and are located in the wall of the right atrium near the open-
ing of the superior vena cava. Specialized cardiac muscle cells forming the
atrioventricular node are also located in the wall of the right atrium, but
near the interatrial wall and the opening of the coronary sinus. The left
atrium(answer c)containsnoknown nodes of pacing cells. Large special-
ized cardiac muscle cells are the Purkinje cells, which make up the bundle
of these that run along the interventricular septum (answer a).These cells
are found in the subendocardial portion of the interventricular wall and
conduct impulses to the ventricular myocytes of both ventricles. The aortic
arch (answer b)contains baroreceptors that control heart rate through a
reflex arc connected to parasympathetic ganglia on the surface of the heart
(answer e).
330.The answer is c.(Moore and Dalley, pp 137–138. Sadler, pp 162–165.)
The heart bends into an S-shape because the caudal regions of the endocar-
dial tubes grow faster than the cranial regions. The heart forms during the
third week by the apposition of left and right endocardial tubes as the head
fold progresses caudally. The endocardial tubes fuse to form a single heart
tube. This fusion begins cranially in the region of the bulbus cordis (outflow
trunks) and proceeds caudally through the ventricles and the atria to the
(answer e)sinus venosus, which is incorporated into the atrium (answer b)
after loop formation. Rapid proliferation of the ventricular region results in
the single-tube heart bending into an S-shaped loop. During this process,
the dorsal mesocardium (answer a)partially breaks down, which leaves the
heart suspended only at the cranial and caudal ends; the discontinuity in the
mesocardium is the transverse sinus. The left and right sides of the heart
(answer d)are established by the subsequent division of the single-tube
heart,notby the apposition of left and right endocardial tubes.
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