Rhythmicity of spinal neurons activated during each form of fictive scratching in spinal turtles

Are behaviors that rely on common muscles and motoneurons generated by sepa rate or overlapping groups of pattern-generating neurons? This question was investigated for the three forms of scratching in immobilized, spinal turt les. Individual neurons were recorded extracellularly from the gray matter through most of the spinal cord hindlimb enlargement gray matter, but were avoided in the region of motoneuron cell bodies. Each form of fictive scrat ching was elicited by mechanical stimulation of the body surface. The rhyth mic modulation of spinal neurons was assessed using phase histograms and ci rcular statistics. The degree of rhythmic modulation and the phase preferen ce of each rhythmically active neuron were measured with respect to the act ivity cycle of the ipsilateral hip flexor nerve. The action potentials of r hythmic neurons tended to be concentrated in a particular phase of the ipsi lateral hip flexor activity cycle no matter which form of fictive scratchin g was elicited. This consistent phase preference suggests that some of thes e neurons may contribute to generation of the hip rhythm for all three form s of scratching, strengthening the case that vertebrate pattern-generating circuitry for distinct behaviors can be overlapping. The degree of rhythmic modulation of each unit during fictive scratching was consistently correla ted with the dorsoventral location of the recording, but not with the medio lateral or rostrocaudal location; neurons located more ventrally tended to be more rhythmic. The phase preferences of units were related to the region of the body surface to which each neuron responded maximally (i.e., the re gion to which each unit was broadly tuned). Units tuned to the rostral scra tch or pocket scratch region tended to have a phase preference during ipsil ateral hip flexor activity, whereas units tuned to the caudal scratch regio n did not. This suggests the hypothesis that the hip flexes further during rostral and pocket scratching, and extends further during caudal scratching , due to the net effects of a population of spinal interneurons that are bo th broadly tuned and rhythmically active.