The most extensive set of behavioral experiments addressing this topic were performed by Erich Schlüter (1933), who reported that leeches with the SubEG but not the SupraEG intact increased the number of swims and decreased cycle periods as compared to intact leeches. 2006) however, because the SupraEG was also disconnected from the midbody nerve cord in these experiments, individual functions of the two structures could not be confirmed. One study concluded that removal of the SubEG reduced activity levels in nearly-intact animals ( Cornford et al. The head brain in the leech has usually been treated as a single entity, with relatively little effort directed toward understanding the respective roles of its ganglia in the control of locomotion. Removal of the entire brain prevents coordinated crawling ( Puhl and Mesce 2010) but enhances rhythmic swimming swims are easier to initiate and have longer durations ( Brodfuehrer and Friesen 1986a Brodfuehrer et al. In the leech, the “head brain” comprises both the SubEG and SupraEG. Finally, removing multiple anterior ganglia from the grasshopper results in continuous rhythmic oviposition digging ( Thompson 1986a, b da Silva and Lange 2011). In cockroaches, these two ganglia had different effects on locomotion depending on whether walking or flying was being examined ( Ridgel and Ritzmann 2005 Gal and Libersat 2006). In contrast, whereas praying mantises with the SubEG destroyed were nearly motionless, those with just the supraesophageal ganglion (SupraEG) removed initiated movement more readily and walked for longer periods of time than control animals ( Roeder 1937). In the locust, removal of just the brain as well as the brain plus the subesophageal ganglia (SubEG) resulted in a decrease in the step frequency and duration of the walking bouts ( Kien 1983). The direction of the change in duration and frequency of the locomotor response following such manipulations is mixed. For example, the praying mantis and stick insect are both able to walk after isolation of the ventral nerve cord from the brain with only minor changes in their gait ( Roeder 1937 Graham 1979), while locusts lacking descending outputs are capable of flight ( Wilson 1961). These animals have thus been utilized to investigate the role of cephalic ganglia, together with sensory feedback, in controlling rhythmic behaviors. 2005 Kagaya and Takahata 2010 Puhl and Mesce 2010 Mullins et al., 2011b). It has been well documented that many invertebrate and some vertebrates can generate rhythmic behaviors in the absence of cephalic neural structures or “brains,” with the dynamics of these behaviors modified only slightly from those of the intact animal ( Kien 1983 Brodfuehrer and Friesen 1986a Thompson 1986a, b Chrachri and Clarac 1990 Cohen 1992 Facciponte and Lange 1992 Marder et al. These results suggest that the supraesophageal ganglion is the primary structure that constrains leech swimming however, the control of swim duration in the leech is complex, especially in the intact animal. Experiments on the nearly intact leeches show that, in these preparations, the subesophageal ganglion acts to decrease cycle period but, unexpectedly, also decreases swim duration. The prolonged swim durations observed with the anterior-most ganglion removed were abolished by removal of the tail ganglion. We found that, in isolated preparations, swim episode duration and swim burst frequency are greatly increased when the supraesophageal ganglion is removed, but the subesophageal ganglion is intact. Here we describe the influence of these two structures and that of the tail brain on rhythmic swimming in isolated nerve cord preparations and in nearly-intact leeches suspended in an aqueous, “swim-enhancing” environment. In the leech, removing the entire head brain enhances swimming, but the individual roles of its components, the supra- and subesophageal ganglia, in the control of locomotion are unknown. In many species, dedicated brain regions initiate and maintain behavior and set the duration and frequency of the locomotor episode. Locomotor systems are often controlled by specialized cephalic neurons and undergo modulation by sensory inputs.
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