ACTIVITY OF DESCENDING PROPRIOSPINAL AXONS IN THE TURTLE HINDLIMB ENLARGEMENT DURING 2 FORMS OF FICTIVE SCRATCHING - BROAD TUNING TO REGIONS OF THE BODY-SURFACE

We recorded the activity of descending propriospinal axons at the caud al end of a seven-segment (D3-D9) turtle spinal cord preparation. Thes e seven spinal segments contain sufficient neural circuitry to select and generate fictive rostral scratching or fictive pocket scratching i n response to tactile stimulation in the appropriate region of the bod y surface. Each turtle received two spinal transections, one just caud al to the forelimb enlargement and one in the middle of the hindlimb e nlargement, Descending propriospinal axons were recorded extracellular ly from the hindlimb enlargement on one side of the body, while the ip silateral or contralateral body surface was stimulated. Concurrent rec ordings were made from ipsilateral and contralateral hindlimb muscle n erves to monitor fictive scratch motor patterns. We found that most ta ctilely responsive descending propriospinal axons were excited by stim ulation anywhere within the rostral scratch or pocket scratch receptiv e fields on at least one side of the body, and often on both sides. Th e activity of these neurons was usually rhythmically modulated during fictive rostral scratching and fictive pocket scratching. Many neurons with large excitatory receptive fields generated action potentials at their highest rate during stimulation of a particular region of the b ody surface on one side, and generated action potentials at progressiv ely lower rates during stimulation of sites progressively farther away . Thus, these units were broadly tuned to a region of the body surface . Some were tuned to a region of the rostral scratch receptive field a nd others were tuned to a region of the pocket scratch receptive field . These data suggest that selection of the appropriate form of scratch ing, rostral or pocket, may be mediated by populations of broadly tune d neurons rather than by highly specialized neurons.