The neural circuitry generating lamprey undulatory swimming
is among the most accessible and best known of the vertebrate
neuronal locomotor systems. It therefore serves as an experimental
model for such systems. Modeling and computer simulation of
this system was initiated at a point when a significant part
of the network had been identified, although much detail was
still lacking. The model has been further developed over 10
years in close interaction with experiments. The local burst
generating circuitry is formed by ipsilateral excitatory neurons
and crossed reciprocal inhibitory neurons. Early models also
incorporated an off-switch lateral interneuron (L), the connectivity
of which suggested it could contribute to burst termination
at moderate to high bursting frequencies. Later examination
of this model suggested, however, that the L interneuron was
not of primary importance for burst termination, and this was
later verified experimentally. Further, early models explained
the effects of 5-HT on bursting frequency, spike frequency,
and burst duration as being due to its modulatory action on
the spike frequency adaptation of lamprey premotor interneurons.
In current network models, accumulated adaptation is in addition
the main burst terminating factor. Drive-related modulation
of adaptation is explored as a mechanism for control of burst
duration. This produces an adequate burst frequency range and
a constant burst proportion within each cycle. It further allows
for hemisegmental bursting, which has been observed experimentally.
The local burst generator forms the basis of a network model
of the distributed pattern generator that extends along the
spinal cord. Phase constancy and flexibility of intersegmental
coordination has been studied in such a simulated network. Current
modeling work focuses on neuromodulator circuitry and action,
network responses to input transients, how to model the intact
versus an isolated piece of spinal cord, as well as on improving
an earlier neuromechanical model of lamprey swimming.
