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Abstract
We have used identified neurons from the abdominal ganglion of the mollusc Aplysia
to construct and analyze two circuits in vitro. Each of these circuits was capable
of producing two patterns of persistent activity; that is, they had bistable output
states. The output could be switched between the stable states by a brief, external
input. One circuit consisted of cocultured L10 and left upper quadrant (LUQ) neurons
that formed reciprocal, inhibitory connections. In one stable state L10 was active
and the LUQ was quiescent, whereas in the other stable state L10 was quiescent and
the LUQ was active. A second circuit consisted of co-cultured L7 and L12 neurons that
formed reciprocal, excitatory connections. In this circuit, both cells were quiescent
in one stable state and both cells fired continuously in the other state. Bistable
output in both circuits resulted from the nonlinear firing characteristics of each
neuron and the feedback between the two neurons. We explored how the stability of
the neuronal output could be controlled by the background currents injected into each
neuron. We observed a relatively well-defined range of currents for which bistability
occurred, consistent with the values expected from the measured strengths of the connections
and a simple model. Outside of the range, the output was stable in only a single state.
These results suggest how stable patterns of output are produced by some in vivo circuits
and how command neurons from higher neural centers may control the activity of these
circuits. The criteria that guided us in forming our circuits in culture were derived
from theoretical studies on the properties of certain neuronal network models (e.g.,
Hopfield, J. J. 1984. Proc. Natl. Acad. Sci. USA. 81:3088-3092). Our results show
that circuits consisting of only two co-cultured neurons can exhibit bistable output
states of the form hypothesized to occur in populations of neurons.