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Reconstruction of the human atrioventricular node.

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Reconstruction of the Human Atrioventricular Node '
D e p a r t m e n t of Anatomy, T e m p l e University School o f Medicine,
Philadelphia, Pennsylvania
The right atrioventricular junction and A-V node area in each of
five human hearts was studied histologically in serial sections. The A-V node of a
54-year old female was reconstructed in four colors to provide a three-dimensional
model of cardiac relationships, blood supply, and nodal configuration. The nodal fibers
demonstrated two laminations. A superficial layer was composed of longitudinally
oriented fibers, whereas a deeper layer was composed of oblique and transversely
directed fibers. The atrioventricular bundle fibers were always continuous with nodal
fibers of the deeper portion of the A-V node. Atrionodal fiber junctions occur along
the superior, endocardia1 and inferior borders of the A-V node and impart to these
surfaces a spiked appearance. The potential significance of A-V node structure to
atrial cardiac conduction is presented.
The existence of discrete sinoatrial and
atrioventricular nodes, an atrioventricular
bundle and its bundle branches in the human heart has been substantiated anatomically and physiologically. Microscopic
characteristics of the conduction system
components in man and other animals has
been reviewed thoroughly by several investigators (Todd, '32; Glomset and Glomset, '40; Truex and Copenhaver, '47; Copenhaver and Truex, '52; Lev, '60; F. Allodi,
'60; Truex, '61; Robb, '65; Truex and
Smythe, '65). Dissections of the atrioventricular node and junction have been made
with the aid of a dissecting microscope at
high magnification (Walls, '45; Widran
and Lev, '51; Truex et al., '58a, '64; Titus
et al., '63a). More recently wax model reconstructions of the dog and human cardiac conduction components have clarified
many of the three dimensional anatomic
features of this unique fiber system (Baird
and Robb, '50; Truex et al., '60, '64).
The application of such information to
elucidate the anatomic relationships of
the conduction system in anomalous human hearts has been reported (Reemtsma
and Copenhaver, '58; Truex et al., '58b,
'60; Lev, '59; Titus et al., '63b). A variety
of diagrams, word schemas and injection
methods also help visualize the intricate
fiber relationships of the nodal areas and
bundle branches (Hoffman, '61; James, '61;
Truex et al., '54, '64, '66).
All of the above studies provided basic
information pertaining to the shape, size,
ANAT. REC., 158: 11 20.
location and blood supply of the human
atrioventricular node (AVN). It was
shown to have a compact portion lying
adjacent to the right fibrous trigone, and
to be covered superficially by a thin layer
of subendocardial atrial muscle fibers and
fat (figs. 3, 4). Discrete bundles composed of small nodal fibers issue from its
superior and right borders (fig. 1). Such
fascicles can be traced for longer or shorter
distances into the interatrial septum and
around the ostium of the coronary sinus.
The node itself is composed of tightly interlaced strands of small nodal fibers, most
of the fibers having diameters of 5 to 8 u.
The compact node is penetrated by and
often subdivided into superficial and deep
portions by one or more small arteries and
veins (fig. 4). The atrioventricular bundle
generally leaves the deep surface of the
A-V node to penetrate the fibrous ring enroute to the top of the interventricular septum. Numerous fine nerve fibers ramify
among the nodal fibers, but their ultimate
terminations usually are not demonstrated
by light microscopy.
Continuity between atrial muscle fibers
and the fibers of the A-V node have been
observed (Cohn, '09; Taussig, '31; Blair
and Davies, '35; Baird and Robb, '50).
The larger atrial fibers (i.e. diameters of
9-25 u ) diminish in size and frequently
become continuous with the smaller nodal
ZThis investigation was supported by U.S.P.H.Research grant H-7047, and Career Award 5-K6-GM14,092.
a Research Assistant.
significance of such A-V junctional zones
in normal and abnormal cardiac cmduction, the present study was undertaken.
Fig. 1 Diagram of atrioventricular node and
right atrial relationships. Rectangular lines indicate tissue block from which model was reconstrueted. Numbers a t top of block indicate plane
of section and location of serial sections shown
in figures 3 and 4. The relations of the coronary
sinus ( C S ) , left bundle branch (LBB) and right
bundle branch (RBB) are included for orientation.
Fig. 2 Reduced tracing of serial section number 1736 shown in figure 4 to illustrate those
tissues which were incorporated in reconstructed
model (stippled and hatched areas). The endocardium, fat, and atrial muscle of the interatrial
septum (clear area) were omitted to expose the
borders and muscle junctions of the A-V node.
The top of the muscular interventricular septum
(IVS) was included in the model.
fibers along the endocardial and superior
surfaces of the A-V node (figs. 3, 4). In
rare instances a strand of atrial fibers may
penetrate into the A-V node before its
component fibers diminish in size and
blend with the smaller nodal fibers. Although such connections between atrial
and nodal fibers have been observed frequently in single histologic sections, no
morphologic study to date has attempted to
visualize these mutliple junction sites of
the A-V node by a three dimensional reconstruction. In view of the physiologic
The right atrioventricular junction and
A-V node area of five human hearts form
the basis of the present morphologic investigation. Each specimen was cut in
paraffin serial section at 10 u. Alternate
slides were prepared by the hematoxylineosin, Masson's trichrome, and the Holmes'
silver staining procedures. The atrioventricular junction and compact atrioventricular node of a 54-year old female was selected as best suited for reconstruction and
study. The block of A-V node tissue indicated in figure 1 provided 2100 serial
sections. The compact A-V node in this
specimen extended through 480 sections.
Every fifth section of the compact A-V
node was projected at a magnification of
25 X and traced in different colors. The
plane of section and location of two sections that were used in constructing the
model are indicated (1600 and 1736 in
fig. 1 ) . The histologic appearances of
these two sections are shown in figures 3
and 4. Figure 2 is a reduced tracing of
figure 4. The dotted lines and stippled
areas (fig. 2 ) indicate the portion of this
section that was incorporated into the
definitive model. For proper orientation,
it should be noted that most of the overlying endocardium, atrial muscle fibers
and fat shown in figure 4 were omitted in
the model in order to visualize the endocardial surface of the A-V node.
These tracings were then transferred to
dental wax plates 1 mm in thickness. Connective tissue of the endocardium, annulus, and right fibrous trigone were cut
from blue wax plates, while atrial and
ventricular cardiac muscle were reproduced on red wax plates. The A-V bundle
and intranodal fibers having an oblique
and transverse course within the node were
reproduced in white, whereas longitudinally
oriented intranodal fibers were cut from
orange wax plates. The colored plates were
assembled, like a jig-saw puzzle, for each
level of the reconstructed A-V node. The
completed model represents 4.8 mm of
A-V junction tissue.
Fig. 3 Photomicrograph of serial section number 1600 showing anterior part of A-V
node as indicated in figure 1. Note loose arrangement of fascicles of atrial cardiac muscle
(ACM) between node and endocardium (End.), and within the interatrial septum (IAS).
The muscle fibers of the interventricular septum (IVS) and ventricles have a more compact histologic appearance. Hematoxylin and Eosin. X 12.
Fig. 4 Photomicrograph of serial section number 1736. Plane of section indicated in
figure 1. Note vessels within A-V node (AVN) which is separated from the interventricular
septum (IVS) by the right fibrous trigone (RFT) and annulus fibrosus (AF). The endocardial surface (End.), interatrial septum (IAS) and atrial cardiac muscle (ACM) are
identified for orientation. Hematoxylin and Eosin. X 12.
Two views of the completed model are
shown in figures 5 and 6. There is only
a slight resemblance between the actual
appearance of this important structure and
the many diagrams that have been used to
depict its supposed anatomic configuration. It should be noted that the compact
portion of the human A-V node is an
elongated thin sheet of fibers subdivided
by the intranodal vessels into a superficial
or endocardial portion, and a deeper portion. The fibers within these two layers of
the A-V node do communicate and anastomose with each other freely, but they
usually manifest M e r e n t planes of orientation as indicated by dark and light areas
of figure 6.
The nodal fibers in the superficial portion of the compact A-V node follow a
superior to inferior (cephalocaudal) course,
and appeared as longitudinal fibers in our
microscopic plane of section (i.e. shaded
dark in figs. 5, 6). The most numerous
points of continuity occurred between subendocardial atrial fibers and the longitudinally oriented nodal fibers on the endocardial and superior surfaces of the node,
Such junctional zones between atrial and
nodal muscle fibers account for the irregular, spiked surface appearance of the finished model ( * in figs. 5, 6 ) . The nodal
fibers within the deeper portion of the
A-V node have an oblique or horizontal
course that is almost perpendicular to that
of the longitudinal or more superficial fibers. In our plane of section the deeper
nodal fibers were cut in cross-section, and
are shown as light areas in figures 5 and
6. When the fascicles of the atrioventricular bundle began to penetrate the right
fibrous trigone they originated from the
horizontally oriented fibers of the deeper
node region.
It is evident from histologic sections, as
well as the model, that the inferior portion
of the A-V node is below the level of
origin of the atrioventricular bundle. Thus
the longitudinal nodal fibers on the surface, and those within this inferior part
of the node must ascend to blend with the
deeper nodal fibers that ultimately continue as the fibers of the A-V bundle. In a
similar manner the atrial muscle fibers
that join with such caudally placed nodal
fibers also lie below the level of the A-V
bundle attachment. The communications
between the longitudinal and more horizontal fiber layers of the A-V node are best
observed in figure 6. This view also illustrates the arrangement of the intranodal
fibers in that part of the compact A-V
node that is closest to the ostium of the
coronary sinus (CS in fig. 1 ) . In this
view one can also observe the junction
sites between the fibers of the interatrial
septum and the more deeply placed nodal
fibers that form part of the superior surface of the A-V node.
The morphologic configuration and fiber
lamination within the compact human
A-V node provides an anatomic basis
which may prove helpful in the elucidation of A-V node function. A most revealing feature demonstrated by the four color
wax model reconstruction is the multiple
junctions of atrial and nodal muscle fibers
on the latticed superior, inferior and endocardial surfaces of the A-V node. In fact,
two additional junctional zones exist in
the total fiber complex of the A-V node namely, nodal fiber to nodal fiber at the
juncture of the superficial and deep portions of the node, and at the juncture of
A-V node and the A-V bundle. Although
physiologic evidence cannot be drawn from
the present study, the fiber arrangements
depicted illustrate many direct and indirect routes which could serve as potential
conduction pathways both to and through
the human A-V node. For example, atrial
muscle fibers of the interatrial septum are
continuous with the deep nodal fibers along
the superior surface of the node, and could
thus furnish a direct, short course to the
junction of A-V node and A-V bundle.
On the other hand, atrial muscle fibers
that descend under the endocardium and
have junctures with the superficial nodal
fibers in the more inferior part of the
node (fig. 4) would need to ascend, and
also traverse the thickness of the node,
before reaching the junction of the A-V
node and A-V bundle. This would constitute a longer and more indirect route of
At times some of the largest muscle fibers of the right atrium are found super-
Fig. 5 Drawing of four color reconstruction of human atrioventricular node. Model is viewed
from right side of heart and visualizes the endocardial (superficial) surface of the node. Superficial
portion of node (darker shading) is composed of small nodal fibers directed in a longitudinal plane.
Note numerous points of junction between atrial cardiac muscle fibers and nodal fibers ("). Such
junction sites give the endocardial, superior and inferior borders of the A-V node a latticed or spiked
appearance. IVS indicates the muscular interventricular septum.
Fig. 6
Drawing of four color reconstruction of human A-V node. Model is viewed from left side
of heart and visualize the posterior surface of the node which is nearest to the ostium of the coronary sinus. (CS in fig. 1). Note nodal vessels that subdivide the compact A-V node into superficial
and deep portions. The small nodal fibers of the two portions have numerous points of fiber continuity with each other. Both the superficial and deep parts of the node demonstrated junctional
sites ( * ) between atrial and nodal muscle fibers. Part of the muscular interventricular septum is
identified (IVS) for orientation.
ficial, and adjacent to, the inferior margin of the A-V node as it lies above the
attachment of the posterior leaflet of the
tricuspid valve. Such large atrial fibers
were considered to be Purkinje fibers by
James ('61) and he assumed they might
constitute a rapid by-pass conduction
route for early activation of the atrioventricular bundle. We have observed such
large atrial muscle fibers in several specimens (Truex and Smythe, '64, '66) and
at times have followed them out into the
leaflet of the triscuspid valve. However,
in serial sections we have never seen such
fibers cross the annulus fibrosus independently, or establish continuity with the muscle fibers of the interventricular septum
as observed by James ('61) in two specimens. As noted above, the lowest part of
the A-V node lies below the level of origin
of the A-V bundle. Hence, atrionodal fiber junctions along the inferior margin of
the node would constitute anatomically, a
longer and more indirect route to the A-V
bundle, rather than a shorter route as
suggested by James ('61).
The spiked atrionodal junctions on the
exposed surfaces of the A-V node are
quite similar to the presumed junctions
depicted schematically by Hoffman ('61)
as a result of his physiologic microelectrode studies. Such atrionodal junctions
( * in figs. 5, 6 ) comprise loci of slow conduction of the cardiac impulse within the
atrium. Atrial fiber conduction velocity is
normally 0.8-1.0 meters per second, and
that of the A-V nodal fibers is 0.1 meters
per second. However, fibers at the atrionodal junction conduct at a velocity of approximately 0.05 meters per second (Scher,
'62). Also atriondal junctions are sensitive
to acetylcholine, and are the regions where
first degree A-V nodal block most commonly occurs.
Fiber lamination into superficial and
deep parts of the A-V node were found to
a greater or lesser degree in most human
specimens. In the specimen selected for
this study the two portions were well
demarcated by the nodal artery and small
venules. The significance and alterations
in conduction that are induced by such
nodal laminations must await further physiologic studies using multiple extracellular
and intracellular electrode technics.
Whether such morphologically distinct
laminations are present in the A-V node
region of other mammals (e.g. dog and
rabbit) we are at present unable to say.
The thin layers of atrial muscle fibers between the endocardium and the subajacent compact A-V node (fig. 4 ) do exist
in lower mammals and would need to be
traversed before an endocardia1 exploring
electrode would reach the substance of the
compact A-V node.
It is our feeling, purely from a morphologic viewpoint, that the fascicles of small
nodal fibers which project out into the
atrium and interatrial septum (fig. 1 )
should be looked upon as extensions
from the compact A-V node. Such extensions do approximate the atrial pathways
described by Thorel ('09), Bachman ('16)
and more recently by James ('61). However, they are composed of nodal fibers of
small diameter, and not of the large atrial
muscle fibers as usually described.
Whether these nodal fiber extensions have
atrionodal junctures with the above atrial
pathways we cannot say at this time. The
nodal fiber extensions that traverse the
margins of the coronary sinus may constitute what some authors refer to as the
"coronary sinus node" (Scherf and Cohen,
Some small nodal fibers from the superior and deep portion of the A-V node
sweep dorsally around the base of the
aorta for variable distances before becoming continuous with larger atrial muscle
fibers of the interatrial septum and left
atrium. However, the interatrial septa1 extensions observed in our specimens did not
pierce the fibrous trigone enroute to the
septum and left atrium as described by
Blair and Davies ( ' 3 5 ) .
The authors are indebted to Mr. William
Taylor for his photographic assistance, and
to Miss Marjorie Stodgell, Medical Artist of
the Hahnemann Medical College, who executed the illustrations used in this communication. We express our sincere appreciation to Margaret Jenney Taylor for
her valued technical assistance with both
tissue preparation and the manuscript.
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