Effect of neck posture on the activation of feline neck muscles during voluntary head turns.

1. To determine whether neck posture affects the usage of neck muscles during a specific motor task, we recorded the electromyographic (EMG) patterns of neck muscles in four cats, which made horizontal, head-turning movements to fixate eccentrically placed targets. In some trials, the cervical column was oriented vertically whereas in other trials, the cervical column was oriented more horizontally. 2. During horizontal head movements, five muscles (obliquus capitis inferior, splenius, levator scapulae, complexus, and biventer cervicis) displayed activation patterns that were consistent from cat to cat and did not change when the cats adopted a different neck posture. Most of these muscles are large, superficial muscles that attach to the skull and span many cervical joints. 3. Posturally dependent patterns of activation were observed in five other neck muscles (semispinalis cervicis, longissimus capitis, levator scapulae ventralis, scalenus anterior, and obliquus capitis superior). Most of these muscles lie deeper and more laterally within the neck musculature and generally span fewer cervical joints than the muscles that displayed invariant patterns of activation. 4. These results suggest that the set of invariantly activated muscles may compose part of a basic motor program that is triggered during head movements in the horizontal plane. This motor program appears to be modified by the selective activation of ancillary muscles, which are recruited in a manner related to the neck posture. The deep positioning of the ancillary muscles may permit them to regulate the mobility of the cervical column and to adjust the net muscular force applied across the neck to the skull. Organizing the motor output in this manner might simplify the task of computing the appropriate patterns of neck-muscle activation.