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The sinus node artery in man.

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The Sinus Node Artery in Man '
Wayne State University, School of Medicine, Department of Anatomy,
Detroit, Michigan
Thirteen sinus node arteries from human material ( 2 months post
partum to 68 years) were studied by various histologic techniques for connective tissue,
muscle, and nerve distribution and arrangement. Its relationship to the sinus node
is also emphasized. The outer longitudinal muscle coat of the artery is replaced with
a thick adventitia. This loss of smooth muscle is not typically found i n other coronary
arteries. In addition, many nerve fibers, arranged both circularly and longitudinally,
present the possibility of a pressoreceptor mechanism related not only to coronary
blood pressure changes but also to the neurogenic control of the heart beat.
The sinus node is considered to be the
pacemaker of the heart. An integral part of
this node is the sinus node artery. The
importance of this artery was emphasized
by James ('64) who observed its virtual
occlusion i n a patient with Marfan's syndrome who developed atrial fibrillation during his terminal illness. I n man, according
to James ('58, '60, '61), the sinus node
artery arises from the proximal 2 or 3 cm
of the right coronary in 55% of cases studied and from the proximal 1 cm of the
circumflex branch of the left coronary i n
about 45% of cases. Other origins are rare
and constitute about 2% (James and
Burch, '58; James, '60, '61). Gross ('21),
from his study of 100 human hearts, states
that the sinus node artery originates from
the right coronary in 60% of cases. From
either origin the sinus node artery courses
along the anterior atrial wall toward the
termination of the superior vena c a w ,
which it encircles, thus coursing through
the sinus node (James, '61). Although the
artery communicates with other atrial
arteries, the primary supply of the node
is almost always unilateral (James, '61).
This study deals with the histology of
the sinus node artery in man, how it differs
from the rest of the coronary arteries, and
its possible functional relationships to the
sinus node and coronary blood pressure.
Thirteen sinus node arteries obtained
from hearts of human autopsied material
were prepared using formalin fixation,
paraffin embedding and serial sectioning
ANAT REC.,153: 23-30.
at 8 .I.! The specimens ranged i n age from
two months (post partum) to 68 years.
From the younger hearts the entire
artery including the sinus node and some
surrounding tissue was prepared. From
the older hearts, segments of 4 to 5 m m
were prepared. From one heart serial sections of the artery were made alternating
a block for cross-sectioning with a block
for sectioning in the longitudinal axis.
For the various tissues, Weigert resorcin
fuchsin elastic stain, H & E, Luxol fast blue
periodic acid-Schiff, and Holmes' silver
stain were found most useful. The Weigert
stain was used as a general combination
stain to demonstrate the relationship of
collagen, elastic fibers, and muscle in the
arterial wall. H & E was used as a standard stain. Luxol fast blue was used for
myelin and Holmes' silver stain for nerve
The intima shows the usual arrangement
as that found in medium-sized arteries.
The media is composed of a n inner circular and outer longitudinal smooth muscle
layer, and the adventitia contains thin collagenous and elastic fibers. The adventitia
immediately adjacent to the media is predominately composed of elastic fibers and
the remaining part is composed mainly of
collagenous fibers (fig. 2).
The muscle bundles of the outer longitudinal layer are arranged in a segmental
pattern of origin and insertion separated
by elastic and collagenous fibers. Elastic
and collagen septa extend from the adIThis study was supported by a, Michigan Heart
Association qrant and a U. S. Publlc Health Service
Grant Sol-FR-05384-02.
ventitia to surround the outer longitudinal
muscle bundles (fig. 2). These septa become continuous with the elastic framework which surrounds the inner circular
muscle layer (figs. 2 and 3). Some muscle
bundles, in addition to their longitudinal
arrangement, assume a circular course
and their nuclei become ellipsoid in shape.
This pattern was also observed by Boucek
('63) in the coronary arteries of the dog.
The length of the artery before entering
the sinus node is about 4-6 cm in the adult
heart. The length varies according to its
origin (right or left coronary), the manner
in which the artery encircles the base of
the superior vena cava, and with the size
of the heart.
Before the nodal artery approaches the
sinus node, it loses its outer longitudinal
muscle layer of its media over a distance
of 1 mm (fig. 4). It is replaced with a
greatly enlarged elastic and collagenous
adventitia. This thickened adventitia extends about 1 mm in a two-month infant
to about 3 mm in length in the adult before
it enters the sinus node. The artery then
communicates with other atrial arteries
and proceeds to enter the sinus node with
its adventitia now replaced by the sinus
node (fig. 51.
The nerve fibers observed present a
unique arrangement in relation to the
thickened adventitia. Within the adventitia nerve fibers course in both circular
and longitudinal directions (fig. 6 ) . In
the peripheral portion of the adventitia
large nerve bundles were found running
in a longitudinal direction (fig. 7).
Boucek ('63) pointed out that the tunica
media of the branches of the coronary
arteries in the dog are continuous or discontinuous at their origin according to
their degree of angulation. Furthermore,
he stated that the tunica media of the
nodal artery of the dog is continuous with
the coronary at its origin. In this study
we have also found a similar arrangement
in the human sinus node artery. In addition, Boucek felt that the sinus node artery was the most richly innervated cardiac artery in the dog.
As James ('61) points out, the origin of
this vessel is no more than a few centimeters from the aorta thus placing it in
an ideal location to monitor pressure
changes in the aorta. In addition, James
and Nadeau ('63) observed a non-specific
bradycardia which occurs during injection
of any solution directly into the sinus node
artery in the dog.
As the artery approaches the sinus node
the loss of its outer longitudinal muscle
layer would allow the artery to be more
sensitive to blood pressure changes. In
addition, the finding of a large and dense
adventitia with the presence of nerve fibers which has replaced the outer longitudinal muscle layer is highly suggestive
of a pressoreceptor mechanism as that
found in the wall of the carotid sinus
(De Castro, '28). The loss of this outer
muscular layer and its replacement with
a thickened adventitia is not typically
found in other coronary arteries. It is
possible that the bradycardia which James
and Nadeau ('63) observed in the dog
may be the result of stimulation of this
possible pressoreceptor mechanism. Figure 1 shows a reconstruction of the sinus
node artery and its relation to the sinus
Mizeres ('58) has shown that in the
dog, sympathetic fibers reach the right
atrial wall through two pathways. One is
through the right stellate cardiac nerves
and the other is through the craniovagal
and caudovagal cardiac nerves. His description of the cardiac nerves in man
('63) also shows sympathetic branches
fusing with cardiac branches of the right
vagus nerve coursing to the right atrial
wall. James ('63) points out that bilateral
vagotomy in the dog does not prevent
sinus bradycardia when the sinus node
artery was injected. It is thus possible
that a neural mechanism for the conduction of impulses from or to the sinus node
artery or sinus node may be carried by
either the sympathetic or parasympathetic
system. There is still the possibility of an
intrinsic neural reflex arc existing between the sinus node and the sinus node
artery. Thus it may be necessary to do an
adventitial stripping of the artery to eliminate any neurogenic mechanism.
Fig. I Reconstructive drawing showing the sinus node artery encircling the base of the superior
vena cava. The artery (from right to left) first acquires a thickened adventitia and then anastomoses with another artery at the beginning of the sinus node. ( a ) Longitudinal nerve bundles.
( b ) Circular nerve bundles embracing the artery and running through the adventitia. ( c ) Sinus
node. ( 1 ) adventitia, ( 2 ) outer longitudinal smooth muscle, ( 3 ) inner circular smooth muscle.
The authors wish to express their appreciation to Dr. T. N. James and Dr. W.
Lipp for their valuable advice.
Boucek, Robert J., Ruth Takashita and Rita
1963 Relationship between microanatomy and functional properties of the coronary arteries (dog). Anat. Rec., 147: 199-207.
De Castro, F. 1928 Trab. Lab. envest. biol.
Univ. Madr., 25: 331.
Gross, L. 1921 The Blood Supply to the Heart.
Paul B. Hoeber, Inc., New York, p. 37.
James, T. N. 1960 Anatomy, normal and pathologic, as related to the functions of the heart.
Chap. 2 in Clinical Cardiopulmonary Physiology, 2nd ed., B. Gordon, ed., Grune and Stratton, New York.
1961 Anatomy of the Coronary Arteries. Paul B. Hoeber, Inc., New York, pp.
103-13 1.
1961 Anatomy of the human sinus
node. Anat. Rec., 141: 109-115.
James, T. N., and G. E. Burch 1958 The atrial
coronary arteries in man. Circulation, 17:
James, T. N., and R. A. Nadeau 1963 Sinus
bradycardia during injections directly into the
sinus node artery. Am. J. Physiol. 204: 9-15.
James, T. N., B. Frame and I. J. Schatz 1964
Pathology of cardiac conduction system in
Marfan’s syndrome. Orch. Intern. Med., 114:
Mizeres, N. J. 1958 The origin and course of
the cardioaccelerator fibers in the dog. Anat.
Rec., 132: 261-280.
1963 The cardiac plexus in man. Am.
J. Anat., 112: 141-152.
R. Ryback and N. J. Mizeres
Photomicrograph of a longitudinal section of the sinus node artery of a 20 year old
male. (a) Adventitia, (b) outer longitudinal muscle layer, ( c ) inner circular muscle
layer, and ( d ) septum. Weigert resorcin fuchsin elastic stain. x 465.
Cross section of the adult nodal artery showing the extent of a septum. ( a ) outer
longitudinal layer, (b) inner circular layer and ( c ) septum. Weigert resorcin fuchsin
elastic stain. x 800.
R. Ryback and N. J. Mizeres
A series of photomicrographs from one adult nodal artery showing the replacement of
the outer longitudinal layer ( a ) over a distance of 1 mm with an enlarged adventitia
( b ) . Fuchsin elastic ( a ) and Holmes’ silver stain (b). X 75.
R. Ryback and N. J. Mizeres
A series of photomicrographs showing the thickened adventitia of the nodal artery,
( a ) before joining a n atrial artery, ( b ) at the beginning of the sinus node, and ( c )
entering the sinus node. Hematoxylin and Eosin, Weigert elastic. x SO.
R. Ryback and N. J. Mizeres
Section of the adult nodal artery showing longitudinal and circular nerve bundles
within the adventitia. Holmes’ silver stain. x 600.
A longitudinal nerve bundle in the peripheral part of the thickened adventitia in the
infant nodal artery before it entered the sinus node. Hematoxylin and Eosin. X 90.
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artery, node, man, sinus
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