Heart Wall - Tissue Functioning and Remodeling in the Circulatory and Ventilatory Systems - page 307

Tabl e 6. 7. Estimated conduction speeds (m/s) in nodal tissue (AVN: atrioventricular node; SAN:
sinoatrial node).

SAN 0.05

Atrium 0.3-1

AVN 0.02-0.10

His bundle 1-2

Purkinje network 3-5

Ventricle (axial) 0.6-1

Ventricle (transverse) 0.2-0.5

6.2.5.3 Sinoatrial Node

The electrochemical signal starts with automatic self-excitation of cells of the
sinoatrial node (SAN; length 2 mm). This node is located at the

top of the right atrium, more precisely at the junction of the superior vena cava
and right atria, between the orifice of the superior vena cava and the auricle. In the
majority of humans, the sinoatrial node is irrigated by the right coronary artery.
The sinoatrial node is composed of different cell types, stellate and elongated
spindle cells. Stellate myocytes, the primary pacemaker cells, exhibit a faster
functioning rate and longer action potential duration.
In adult mouse hearts, the density of connexin-30 is low in the sinoatrial node,
where it contributes to the regulation of the cardiac frequency, in addition to
hyperpolarization-activated cyclic nucleotide-gated potassium channel HCN4 and
other ion carriers [ 591 ].
The sinoatrial node is endowed by conduction barriers to be functionally
insulated from the surrounding atrial myocardium as well as conduction paths
to transmit action potentials. Anatomical barriers, i.e., connective tissue layers,
adipose tissue, and coronary arteries, as well as functional barriers, which results
from gap junction connexin paucity, protect from hyperpolarizing influence of the
surrounding atrial cells [ 592 ]. Specialized paths — sinoatrial conduction pathways
— represent preferential lines of conduction of action potentials. They transmit
electrochemical impulses from the sinoatrial node to the atrial wall. They consist
of elongated and branching fibers of transitional cells.
In dogs, the sinoatrial node, which is a more realistic animal model for the
human SAN than that of small mammals, is isolated from the surrounding atrium
by 3 branches of the coronary arterial bed and fibrosis [ 592 ].
The sinoatrial node is a heterogeneous structure with many pacemaker com-
partments and conduction pathways, electrophysiological properties of which vary.
Major pacemaker compartments comprise head, center, and tail of the sinoatrial
node [ 592 ]. A pacemaker hierarchy within the SAN exists with the fastest pace-
makers at the head and the slowest at the tail. Electrochemical coupling enables
the synchronization of pacemaker clusters inside the sinoatrial node. Excitation
originates in the quickest pacemaker compartment and slowly propagates through
the SAN tissue. It then runs via different conduction paths and excites atria
(beginning of the P wave on the electrocardiogram).

∼ 8 mm, width ∼