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Earlier simulation studies of the lamprey spinal neuronal network have shown that the currently known circuitry can account for swimming at different speeds and for lateral (yaw) turns (Ekeberg 1993).
We have now suggested a possible neuronal mechanism that also incorporates pitch and roll turns (Ekeberg, Grillner, Lansner 1995). The model utilizes two loosely coupled oscillator circuits with separate outputs to dorsally and ventrally located muscles. The oscillators are crossed so that the one driving the dorsal muscles on one side drives the ventral ones on the other. By varying the tonic excitatory drive to different parts of the network, this system is capable of generating the muscle activation patterns necessary for pitch and roll. Pitch turns are produced through an asymmetry in the activation of the dorsal and the ventral muscles. Roll turns are generated when the two oscillators are running out-of-phase, giving rise to a spiral shape of the activation pattern.
By extending the earlier neuronal network model to incorporate the separate dorsal and ventral outputs we have shown through simulation experiments that the resulting system is indeed capable of generating activation patterns for yaw, pitch, and roll controlled only by means of asymmetries in the tonic excitatory input.
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