ABSTRACT:
A new, non-invasive optical recording technique is employed to study the
spontaneous, long-term contractile activity of strongly-coupled,
self-assembled networks of heart cells. Continuous recording throughout
the network's lifetime reveals a variety of rhythm disorders, which have
never been reported to occur without external stimulation. Here, we focus
on two typical dynamical patterns: sub-harmonic structures with frequent,
irregular, inter-mode transitions, and sudden alternations between two
dominant rates (intermittent dynamics). Both rhythm disorders appear to
occur throughout the field of view, with only slight time lags among
different regions. In both patterns, the degree of synchronization between
different regions is insensitive to the changes in the dynamical pattern.
Neverthe-less, the intermittent pattern exhibits dynamic time lag
alternations that coincide with the transitions between the rates. Further
examination of both patterns sug-gests the emergence of one or more stable
pacemakers in the developed network. Based on these findings, we propose a
model of one ore more nonlinear oscillators. coupled to an excitable
element. This model shows that similar disordered sub-harmonics can be
produced by near threshold excitation of the ecitable element by a single,
slightly variable pacemaker. Furthermore, a simple interference effect
between two, almost identical pacemakers can produce the essential
features of the intermittent rate transitions.