Regional differences in hyoid muscle activity and length dynamics during mammalian head shaking

The sternohyoid (SH) and geniohyoid (GH) are antagonist strap muscles that are active during a number of different behaviors, including sucking, intraoral transport, swallowing, breathing, and extension/flexion of the neck. Because these muscles have served different functions through the evolutionary history of vertebrates, it is quite likely they will have complex patterns of electrical activity and muscle fiber contraction. Different regions of the SH exhibit different contraction and activity patterns during a swallow. We examined the dynamics of the SH and GH muscles during an unrestrained, and vigorous head shaking behavior in an animal model of human head, neck, and hyolingual movement. A gentle touch to infant pig ears elicited a head shake of several revolutions. Using sonomicrometry and intramuscular EMG, we measured regional (within) muscle strain and activity in SH and GH. We found that EMG was consistent across three regions (anterior, belly, and posterior) of each muscle. Changes in muscle length, however, were more complex. In the SH, mid-belly length-change occurred out-of-phase with the anterior and posterior end regions, but with a zero lag timing; the anterior region shortened before the posterior. In the GH, the anterior region shortened before and out-of-phase with the mid-belly and posterior regions. Head shaking is a relatively simple reflex behavior, yet the underlying patterns of muscle length dynamics and EMG activity are not. The regional complexity in SH and GH, similar to regionalization of SH during swallowing, suggests that these anatomically simple hyoid strap muscles have more complex function than textbooks often suggest.