SCRATCH RESPONSES IN NORMAL CATS - HINDLIMB KINEMATICS AND MUSCLE SYNERGIES

1. Scratch responses evoked by a tactile stimulus applied to the outer ear canal were characterized in nine adult cats. Chronic electromyographic (EMG) electrodes were surgically implanted in selected flexor and extensor muscles of the hip, knee, and ankle joints to determine patterns of muscle activity during scratching. In some trials EMG records were synchronized with kinematic data obtained by digitizing high-speed cine film, and in one cat, medial gastrocnemius (MG) tendon forces were recorded along with EMG. For analysis the response was divided into three components: the approach, cyclic, and return periods. Usually scratch responses were initiated with the cat in a sitting position, but in some trials the animal initiated the response from a standing or lying posture. 2. During the approach period the hindlimb ipsilateral to the stimulated ear was lifted diagonally toward the head by a combination of hip and ankle flexion with knee extension. Hindlimb motions during the approach period were associated with sustained EMG activity in hip-flexor, knee-extensor (occasionally), and ankle-flexor muscles. Initial hindlimb motions were typically preceded by head movements toward the hindpaw, and at the end of the approach period, the head was tilted downward with the stimulated pinna lower than the contralateral ear. During the return period movements were basically the reverse of the approach period, with the hindpaw returning to the ground and the head moving away from the hindlimb. 3. During the cyclic period the number of cycles per response varied widely from 1 to 60 cycles with an average of 13 cycles, and cycle frequency ranged from 4 to 8 cycles/s, with a mean of 5.6 cycles/s. During each cycle the paw trajectory followed a fairly circular path, and the cycle was defined by three phases: precontact, contact, and postcontact. On average the contact phase occupied approximately 50% of the cycle and was characterized by extensor muscle activity and extension at the hip, knee, and ankle joints. The hindpaw contacted the pinna or neck at the base of the pinna throughout the contact phase, and paw contact typically resulted in a rostral motion of the head as the hindlimb extended. 4. The postcontact phase constituted approximately 24% of scratch cycle and was usually initiated by the onset of knee flexion. Ankle and then hip flexion followed knee flexion, and flexor muscles were active during the postcontact phase as the paw was withdrawn from the head. The precontact phase constituted approximately 26% of scratch cycle and was initiated by knee joint extension and knee-extensor activity. Ankle-extensor activity and ankle extension followed closely, and the paw moved towards the stimulated ear. During the precontact phase the hip continued to flex, and hip-extensor activity was initiated at or immediately after paw contact. 5. Muscle synergies for scratching were generally characterized by alternate flexor [tibialis anterior (TA), extensor digitorum longus (EDL), and iliopsoas (IP)[ and extensor ]lateral gastrocnemius (LG), soleus (SOL), MG, vastus lateralis (VL), anterior biceps femoris (ABF), and gluteus medius (GM)[ activity; however, within each synergy, recruitment of different muscles was not synchronous. Onset of extensor activity, for example, was sequential: ankle-, knee-, and hip-extensor activity; this recruitment matched the order of joint extension during the precontact phase. On average, extensor muscles were active for approximately 40% of the cycle, whereas flexor activity occupied approximately 50% of the cycle, and increases in TA and LG burst durations were related to cycle-period increases. Activity of the semitendinosus (ST), a bifunctional muscle, spanned the extensor-flexor transition and was not classified as part of either synergy. 6. Brief extensor bursts and low extensor-tendon forces occurred when paw contact was light; conversely, extensor bursts were prolonged and tendon forces increased when firm contact occurred. These data suggest that feedback during the initial phase of contact may prolong extensor activity and delay the onset of flexor activity. Similarity, comparisons of our records with those from no-contact, air-scratch cycles-typical of decerebrate cats studied by others-revealed marked differences in the relative timing of flexor and extensor activity. In normal scratching flexor and extensor muscles have bursts of similar duration, but in air scratching flexor bursts are approximately 8-10 times longer than extensor bursts. Taken together, these data suggest that the relative timing of extensor-flexor activity is modulated by motion- and posture-related feedback from limb and head receptors, or by input from descending pathways controlled by supraspinal centers.