The contractile patterns, anatomy and physiology of the hyoid musculature change longitudinally through infancy

Abstract

All mammalian infants suckle, a fundamentally different process than drinking in adults. Infant mammal oropharyngeal anatomy is also anteroposteriorly compressed and becomes more elongate postnatally. While suckling and drinking require different patterns of muscle use and kinematics, little insight exists into how the neuromotor and anatomical systems change through the time that infants suckle. We measured the orientation, activity and contractile patterns of five muscles active during infant feeding from early infancy until weaning using a pig model. Muscles not aligned with the long axis of the body became less mediolaterally orientated with age. However, the timing of activation and the contractile patterns of those muscles exhibited little change, although variation was larger in younger infants than older infants. At both ages, there were differences in contractile patterns within muscles active during both sucking and swallowing, as well as variation among muscles during swallowing. The changes in anatomy, coupled with less variation closer to weaning and little change in muscle firing and shortening patterns suggest that the neuromotor system may be optimized to transition to solid foods. The lesser consequences of aspiration during feeding on an all-liquid diet may not necessitate the evolution of variation in neuromotor function through infancy.

Keywords: EMG; feeding; mammal; ontogeny; swallowing.

Figure 1.

Muscle orientations for newborn (a) and older, pre-weaning (b) pigs in lateral (a,b) and ventral (c) views relative to a newborn skull. In (c), the newborn musculature is shown on the top half of the image, and the musculature of the older pig is on the bottom. White circles in (c) indicate location of bead placement for fluoromicrometry. White bolt symbols indicate location of electrodes for electromyography, and the medial ends of the hyoid bone are indicated in brown. Green, geniohyoid; blue, digastric; yellow, stylohyoid; red, omohyoid; purple, thyrohyoid. (Online version in colour.)

Figure 2.

Muscle activity relative to the frame of the beginning of the swallow identified by X-ray video (time = 0, grey dashed line) for 7-day-old (red) and 21-day-old (blue) pigs when swallowing. Grey bars indicate ± standard deviation. There is very little variation in timing of activity between seven and 21 days, although 7-day-old pigs are more variable in their muscle firing timing than 21-day-old pigs for thyrohyoid and digastric. (Online version in colour.)

Figure 3.

Interpolated muscle length changes during activity while swallowing (orange) and sucking (blue) in 7-day-old (left) and 21-day-old (right) pigs. Muscle length changes are generally consistent through ontogeny, but differ between sucking and swallowing. Mean standard deviation through the cycle per pig is provided in blue (sucking) or orange (swallowing), highlighting that for most muscles, standard deviation of length changes is less in older pigs than in younger pigs. (Online version in colour.)