Heart Wall - Tissue Functioning and Remodeling in the Circulatory and Ventilatory Systems - page 306

Tabl e 6. 6. Auto-excitation frequency (Hz) in nodal tissue (AVN: atrioventricular node; SAN:
sinoatrial node). SAN frequen cy is decreased by the vagal influence (cranial nerve X).

X
→ SAN 2.0-2 . 3 1 . 2-1.3

AVN 0.5-1

His bundle 0.3-0.6

Purkinje fibers 0.2-0.5

In the absence of neural and hormonal control, nodal cells naturally create action
potentials, i.e., electrochemical signals at a given frequency that varies along the
nodal tissue. When an upstream element becomes dysfunctional, a group of cells in
a downstream part of the nodal tissue takes the relay, but at a lower frequency.
Tight junctions (Vol. 1 - Chap. 7. Plasma Membrane) facilitate action-potential
propagation. Gap junctions also transmit electrochemical signals in the excitable
myocardium, in which response speed and tissue activity synchronization are crucial
for ion transfer.
Nodal myocytes are sparse with dispersed gap junctions that have

connexin-45. 53 high-conductance connexin-40 and low-conductance Nodal

Purkinje cells have abundant gap junctions with connexin-40 and -45 [ 333 ].

6.2.5.2 Architecture of the Nodal Tissue

Nodal cell automaticity relies on spontaneous, rhythmic, local depolarization asso-
ciated with ionic fluxes. The sinoatrial node (SAN) that has the highest emission
frequency of action potentials (Table 6.6 ) constitutes the “natural pacemaker”.
The nodal tissue permits electrochemical conduction through the heart from the
sinoatrial node, both atria, to the atrioventricular node. Electrochemical signals then
propagate across the atrioventricular septum and ventricles using the His bundle and
its 2 branches — left and right bundle branches — that ramify into Purkinje fibers
organized in many fascicles. The latter transmits the depolarization wave to the inner
layers of cardiomyocytes. Afterward, the electrochemical wave spreads through the
surface and the width of the ventricular myocardium in a given time.
An automatism hierarchy exists between different regions of nodal tissue, accord-
ing to the emission frequency of the action potential (Table 6.6 ). The propagation
speed of the electrochemical signal also varies with the heart region (Table 6.7 ).

53 Many types of connexins aggregate to form gap junctions between adjacent cells. They form
homo- or heteromeric hemichannels and homotypic, heterotypic, or heteromeric channels with
different conductance, permeability, and gating properties. Connexin type expression varies
according to the heart site. Cells that express human connexin Cx31.9 exhibit much faster transport
than cells expressing other connexin types [ 590 ].