Jaw stretch reflexes in children

Abstract

The substantial morphological transformations that occur during human development present the nervous system with a considerable challenge in terms of motor control. Variability of skilled motor performance is a hallmark of a developing system. In adults, the jaw stretch reflex contributes to the functional stability of the jaw. We have investigated the response properties of the jaw stretch reflex in two groups of young children and a group of young adults. Response latencies increased with development, and all age groups produced stimulus-magnitude-dependent increases in reflex gain and resulting biting force. Reflex gain was largest for the older children (9–10 years), yet net increases in resulting biting force were comparable across age groups. These data and earlier experiments suggest that oral sensorimotor pathways mature throughout childhood in concert with the continued acquisition of complex motor skills.

Appendix

Adjustment for structure size differences

To ensure that measured data were representative of comparable levels of muscle stretch across the three age groups, three two-dimensional trigonometric models of the mandible/masseter complex were developed on the basis of average measurements from Bhatia and Leighton (1993) (Fig.1). The angle of the masseter muscle was established at 74.4° relative to the palatal plane for all three models (Hsu et al. 2001). These models were used to adjust measured data to compensate for differences in mandibular size and morphology between the three age groups.

Calculated increases in masseter length due to inter-incisal displacements of 0.079, 0.26, and 0.46 mm were compared for the three models. Because of morphological differences in mandibular geometry, the mandibular model for 5–6-year-olds yielded masseter “stretches” that were 98.18% those of the young adult model, and calculated masseter stretches for the model for 9–10-year-olds were 93.82% those of the young adult model. Therefore, an 0.26 mm inter-incisal displacement that resulted in an increase in masseter length of 0.0612 mm for the young adult model yielded masseter stretches of 0.0601 mm and 0.0574 mm for the 5–6-year-old and 9–10-year-old models, respectively.

To adjust for the differences in absolute masseter stretch due to mandibular morphology across the three age groups, second-order polynomial functions were fit to the averaged reflex (latency and EMG amplitude) and resulting force data across the three stimulus levels for each participant in the 5–6 and 9–10-year-old age groups. Therefore, unique polynomial functions were generated for each participant’s reflex and force data sets, based on their own measured data. The resulting polynomial fits were used to estimate reflex onset latency, reflex amplitude, and resulting force data for the 5–6 and 9–10-year-old groups based on the respective percentage of masseter stretch relative to the young adult group. These adjusted data were used in all statistical calculations.

On average, the adjusted data for the 5–6-year-old group were 1.6% greater in terms of reflex amplitude, 0.13% shorter in terms of reflex onset latency, and 2.2% greater in terms of resulting force compared with the measured data. For the 9–10-year-old group, the adjusted data were (on average) 7.4% greater in terms of reflex amplitude, 0.17% shorter in terms of reflex onset latency, and 8.8% greater in terms of resultant force compared with the measured data.