. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 156 W. H. WATSON ET AL Right. Figure 2. Relationship between putative swim motoneuron activity and swimming movements in Meiihe. Swimming and associated left pedal and right pedal pSMN bursting activity were initiated by dislodging the foot of the animal from the substrate and terminated when the substrate was brought back in contact with the foot. The two up arrows indicate the beginning and end of the time during which the animal was slowly withdrawing from the substate, and the down arrow indicates when swimming stopped a
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 156 W. H. WATSON ET AL Right. Figure 2. Relationship between putative swim motoneuron activity and swimming movements in Meiihe. Swimming and associated left pedal and right pedal pSMN bursting activity were initiated by dislodging the foot of the animal from the substrate and terminated when the substrate was brought back in contact with the foot. The two up arrows indicate the beginning and end of the time during which the animal was slowly withdrawing from the substate, and the down arrow indicates when swimming stopped and the animal reattached to the substrate. The upper trace illustrates the output of a photocell that was used to monitor swimming movements. Output of the photocell was maximal when the animals were flexed to the right. Bursting of the pSMNs occurred before slow bending in the ipsilateral direction. When swimming stopped, the left pSMN remained tonically active, at a frequency that was too low to cause much flexion, while the right SMN stopped firing. foot to be pulled to the ipsilateral side, a movement that is recognized as one of the early events in contralateral bending. Interneurons drive the putative .swim motoneurons The swim pattern appears to be imposed on the pSMNs by the interneurons that generate the pattern (Watson el al., 2001; Thompson and Watson, unpubl. data). When individ- ual pSMNs are either hyperpolarized below threshold or depolarized to elicit continuous spiking, the overall swim- ming pattern is not altered in either intact animals or in isolated brains (Fig. 4). Furthermore, no evidence for syn- aptic interactions between pSMNs was obtained during several different types of experiments. In the first, >70 pairs of pSMNs were impaled in quiescent, non-swimming prep- arations, and action potentials were elicited in one while searching for constant latency PSPs in the other. In the second series of experiments, one cell in a pair was slightly depolarize
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