Scratch-swim hybrids in the spinal turtle: Blending of rostral scratch andforward swim

Turtles with a complete transection of the spinal cord just posterior to th e forelimb enlargement at the D2-D3 segmental border produced coordinated r hythmic hindlimb movements. Ipsilateral stimulation of cutaneous afferents in the mid-body shell bridge evoked a rostral scratch. Electrical stimulati on of the contralateral dorsolateral funiculus (DLF) at the anterior cut fa ce of the D3 segment activated a forward swim. Simultaneous stimulation of the ipsilateral shell bridge and the contralateral DLF elicited a scratch-s wim hybrid: a behavior that blended features of both rostral scratch and fo rward swim into each cycle of rhythmic movement. This is the first demonstr ation of a scratch-locomotion hybrid in a spinal vertebrate. The rostral sc ratch and the forward swim shared some characteristics: alternating hip fle xion and extension, similar timing of knee extensor activity within the hip cycle, and a behavioral event during which force was exerted against a sub strate. During each cycle, each behavior exhibited three sequential stages, preevent, event, and postevent. The rostral scratch event was a rub of the fool against the stimulated shell site. The forward swim event was a power stroke, a hip extension movement with the foot held in a vertical position with toes and webbing spread. The two behaviors differed with respect to se veral features: amount of hip flexion and extension, electromyogram (EMG) a mplitudes, and EMG duty cycles. Scratch-swim hybrids displayed two events, the scratch rub and the swim powerstroke, within each cycle. Hybrid hip fle xion excursion, knee extensor EMGs, and hip flexor EMGs were similar to tho se of the scratch; hybrid hip extension excursion and hip extensor EMGs wer e similar to those of the swim. The hybrid also had three sequential stages during each cycle: I) a combined scratch prerub and swim postpowerstroke, 2) a scratch rub that also served as a swim prepowerstroke, and 3) a swim p owerstroke that also served as a scratch postrub. Merging of the rostral sc ratch with the forward swim was possible because of similarities between th e sequential stages of the two forms, making them biomechanically compatibl e for hybrid formation. Kinematic and myographic similarities between the r ostral scratch and the forward swim support the hypothesis that the two beh aviors share common neural circuitry. The common features of the sequential stages of each behavior and che production of scratch-swim hybrids provide additional support for the hypothesis of a shared core of spinal cord neur ons common to both rostral scratch and forward swim.