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Swimming in vertebrates like eel and lamprey involves the coordination of alternating left and right activity in each segment. Forward swimming is achieved by a lag between the onset of activity in consecutive segments rostrocaudally along the spinal cord. The intersegmental phase lag is approximately 1 percent of the cycle duration per segment independent of the swimming frequency. Since the lamprey has approximately one hundred spinal segments, at any given time one wave of activity propagates along the body.
Previous simulations of intersegmental coordination in the lamprey
have treated the cord as a chain of coupled oscillators or well
defined segments. This work describes a continuous network model of
the lamprey spinal cord, incorporating Hodgkin-Huxley type neurons.
It is based upon known segmental and intersegmental connectivity,
where each cell is modelled with intermediate complexity. Cell sizes,
synaptic conductances and delays are normally distributed around a
mean. Simulations were run using the SWIM simulator
.
Simulations using a 60 segment continuous network have produced phase lags over a wide range of frequencies (1 to 11 Hz). The simulated network favors forward swimming when the level of excitatory drive along the cord is equal. Local increases in excitatory drive in the simulated network are likewise sufficient to produce motor patterns as in backward and narrow swimming. Phase lag values remained constant over the length of the cord, with the exception of the ends where variations were caused by differences in synaptic input. Phase lags increased with frequency independent of synaptic strengths or the extent of the synaptic connectivity. Due to the large size of the network and its subsequent parameter space, reduced networks are presently being used to study this problem (Wadden et. al. 1994).
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