Coordination between head and hindlimb motions during the cat scratch response

Coordination between motions of the head and the hindlimb paw ipsilateral to the stimulated pinna were assessed during the scratch cycle in freely moving cats. Motor patterns were determined by electromyographic (EMG) recordings made from epimysial-patch electrodes surgically implanted on the biventer cervicis (BC), complexus (CM), obliquus capitis inferior (OC), and splenius (SP) muscles and by fine-wire EMG electrodes implanted in two ankle muscles, medial gastrocnemius (MG), and tibialis anterior (TA). To assess head motions during the three phases of the scratch cycle (precontact, contact, postcontact), several responses were filmed, and in some cats an in vivo force transducer was implanted on an ankle extensor muscle (MG or plantaris, PL) to determine the tension profile during the scratch cycle. During the scratch cycle, the head's trajectory was usually characterized by a small oscillation in which the head was pushed away during paw contact (as hindlimb joints extended) and then repositioned during the noncontact phases (as hindlimb joints flexed). Neck muscle activity did not occur during all responses or during all cycles of a single multicycle scratch response, and when it occurred, neck muscle EMG was characterized as phasic (a single burst during the cycle) or tonic (low-level activity during the entire cycle). Neck muscles ipsilateral (i) to the scratching limb exhibited phasic bursts more than contralateral (c) muscles, and phasic activity was most frequently observed in the iBC, iSP, iOC, and cOC muscles. The cOC was reciprocally active with the ipsilateral muscles, and its burst coincided with the postcontact phase and the ankle flexor (TA) burst. The ipsilateral muscles (iOC, iSP, iBC) were active during paw contact, and the termination of all three bursts occurred synchronously just after peak tension of the ankle extensor was reached. The iBC was active before the onset of paw contact and may have been responsible for repositioning the head, along with the cOC, during the precontact phase. The iOC became active after the onset of paw contact (22 ms) and was recruited more often when the peak extensor tendon force was high (10–16 N). The iSP, in contrast, was active during the contact phase of most scratch cycles examined and its recruitment appeared to be unrelated to tendon forces. Our data suggest that phasic neck muscle activity is not obligatory during the cat scratch response, but is related to certain conditions such as a higher than average tendon force of an ankle extensor during contact and the need to reposition the head during the noncontact phases of the cycle. During contact it is possible that active muscle contraction helps to minimize head motion to provide a stable contact surface for the paw, thereby maximizing the possibility of the scratch stimulus being dislodged from the pinna. Possible neural mechanisms, both reflexes and central commands, responsible for coordinating motions of the head and hindlimb during the scratch cycle are discussed.