ACTIVITY OF DESCENDING PROPRIOSPINAL AXONS IN THE TURTLE HINDLIMB ENLARGEMENT DURING 2 FORMS OF FICTIVE SCRATCHING - PHASE ANALYSES

In the preceding companion article (Berkowitz and Stein, 1994b), we sh owed that many descending propriospinal neurons in the turtle were rhy thmically activated during two different motor patterns, fictive rostr al scratching and fictive pocket scratching. In this article, we prese nt phase analyses of the activity of each such neuron during fictive s cratching. Each neuron's activity was concentrated in a particular pha se of the ipsilateral hip flexor muscle nerve (VP-HP) activity cycle; each had a distinct ''preferred phase.'' Each neuron's preferred phase during fictive rostral scratching was similar to its preferred phase during fictive pocket scratching. This result is consistent with the i dea that some descending propriospinal neurons may contribute to the g eneration of both rostral scratching and pocket scratching. Many desce nding propriospinal neurons were rhythmically activated during fictive scratching evoked on either side of the body. This activity may contr ibute to production of bilateral hindlimb movements during scratching. It is also possible that synaptic interactions between the two sides of the spinal cord may be important in generating the motor patterns f or movement of a single hindlimb. In addition, we present a model whic h illustrates that a population of propriospinal neurons, each of whic h is broadly tuned to a region of the body surface and is rhythmically activated in a constant phase of the hip control cycle, could mediate the selection and generation of rostral scratching and pocket scratch ing. Thus, the selection of an appropriate motor pattern and the produ ction of the required knee-hip synergy may each be distributed over a diverse population of spinal cord neurons. This model requires that ea ch such neuron project to both knee muscle: and hip muscle motoneurons . According to this model, the process of selecting a motor pattern wo uld not be completed until knee muscle motoneurons integrate overlappi ng excitatory and inhibitory inputs.