Activity of descending propriospinal axons in the turtle hindlimb enlargement during two forms of fictive scratching: broad tuning to regions of the body surface.

We recorded the activity of descending propriospinal axons at the caudal end of a seven-segment (D3-D9) turtle spinal cord preparation. These seven spinal segments contain sufficient neural circuitry to select and generate fictive rostral scratching or fictive pocket scratching in response to tactile stimulation in the appropriate region of the body surface. Each turtle received two spinal transections, one just caudal to the forelimb enlargement and one in the middle of the hindlimb enlargement. Descending propriospinal axons were recorded extracellularly from the hindlimb enlargement on one side of the body, while the ipsilateral or contralateral body surface was stimulated. Concurrent recordings were made from ipsilateral and contralateral hindlimb muscle nerves to monitor fictive scratch motor patterns. We found that most tactilely responsive descending propriospinal axons were excited by stimulation anywhere within the rostral scratch or pocket scratch receptive fields on at least one side of the body, and often on both sides. The 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 body surface on one side, and generated action potentials at progressively 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 and others were tuned to a region of the pocket scratch receptive field. These data suggest that selection of the appropriate form of scratching, rostral or pocket, may be mediated by populations of broadly tuned neurons rather than by highly specialized neurons.