During mammalian ontogeny, there is a transition from suckling to the chewing of food. The question was asked: Is suckling a neuromuscular precursor to chewing, or are suckling and chewing independent systems? Electromyograms (EMGs) were recorded in rat pups of ages 6, 9, 12, 15, 18, and 21 days from the superficial masseter, anterior digastric, sternohyoideus, and genioglossus muscles during suckling and chewing. The EMG patterns of the 3 components of suckling behavior (nipple attachment, rhythmic sucking and the stretch response) are distinctive from one another and reflect the musculoskeletal biomechanics of suckling. Chewing EMGs are present by 12 days of age and attain the adult pattern by 18-21 days of age. During nipple attachment, pups exhibit a motor pattern that is similar to that of adult chewing, but other aspects of suckling differ from chewing in some EMG features. Comparison of EMGs between behaviors and between ages allowed interpretation of the degree of contunity of muscular activity across the suckling-to-chewing transition.

Ontogeny of feeding motor patterns in infant rats: an electromyographic analysis of suckling and chewing.

Green, Jordan R., Christopher A. Moore, Jacki L. Ruark, Paula R. Rodda, Wendy T. Morvee, and Marcus J. VanWitzenburg. Development of chewing in children from 12 to 48 months: longitudinal study of ...

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Development of Chewing in Children From 12 to 48 Months: Longitudinal Study of EMG Patterns

Movements of the oral apparatus begin during the fetal period and develop in a consistent order. Jaw openingappears first, followed by active jaw closure and tongue movements, lip movements, sucking, and finally masticatory movements. The later developing movements appear prenatally in precocious mammals such as guinea pigs and sheep, but are postnatal in altricial mammals such as rats, hamsters and rabbits. The orderly development of oral behavior is probably related to the progressive maturation of the nervous system and neuromuscular connections. Most newborn mammals feed exclusively bysuckling, a combination of the tongue working against the nipple and negative pressure at the back of the oral cavity. Thetransition from suckling to mastication is gradual and involves considerable learning. In at least one species, the domestic pig, infant animals chew using a somewhat different muscular contraction pattern from that of adults. Age changes in muscle action lines are the most likely explanation for this difference. After being established in infancy, the process of mastication undergoes only minor changes in rate and relative muscle activity during the juvenile period. Throughout ontogeny there is a reciprocal relation between morphology and behavior. While masticatory performance depends on structure at any given stage, it also has profound effects on further musculoskeletal growth and differentiation.

The Ontogeny of Mammalian Mastication

Epigenetics: Linking Genotype and Phenotype in Development and Evolution 1. Introduction: Emergent Properties and the Phenotype Hallgrimsson and Hall Historical and Philosophical Foundations 2. Historical Foundations Brian K. Hall 3. Heuristic Reductionism and the Relative Significance of Inheritance James Grisemer Approaches to Epigenetics 4. Genomic imprinting Vett Lloyd 5. Methylation Mapping in Humans Christoph Grunau, Montpellier 6. Asexuality and Epigenetic Variation Root Gorelick, Manfred Laubichler and Rachel Massicotte 7. Preformation and the Humpty Dumpty Problem Ellie Larsen 8. A Principle of Developmental Inertia Allesandro Minelli Epigenetics of Vertebrate Organ Development 9. Nervous System Development Carol Schuurmans 10. Morphogenesis of pigment patterns Lennart Olsson 11. Interactions of the Cardiac Neural Crest Margaret Kirby 12. Bone and Cartilage Development Tamara Franz-Odendaal 13. Muscle-Bone Interactions and the Development of Skeletal Phenotype Sue Herring 14. Apical Ectoderm in the Developing Vertebrate Limb Cooper, L.N, Armfield, B.A., J.G.M. Thewissen 15. Role of Skeletal Muscle in the Shaping of Organs, Tissues and Cell Fate Choices Boris Kablar Epigenetics in Evolution and Disease 16. Integration, Complexity and Evolvability of the Head Dan Lieberman 17. Epigenetic interactions: The developmental route to functional integration Miriam Leah Zelditcha and Donald L. Swiderski 18. Epigenetic Contributions to Adaptive Radiation Susan Foster 19. Learning, Developmental Plasticity, and the Rate of Morphological Evolution Rich Palmer and Chris Neufeld 20. Epigenetics: Adaptation or Contingency Thomas Hansen 21. Dysmorphology Joan Richtsmeier 22. Human Disease Peter Gluckman 23. Epigenetics: The Context of Development Hallgrimsson and Hall

Epigenetics: Linking Genotype and Phenotype in Development and Evolution

To investigate the biomechanical effects of juvenile growth changes in the rabbit masticatory apparatus a comparison was made of mastication in just-weaned and adult animals. Mandibular movements in two planes were registered by cineradiography. Masticatory muscle activity was recorded by fine-wire electromyography. The same pattern of unilateral mastication was present in the two ages. The most important changes in the jaw movements are 1) a decrease of jaw opening speed and chewing frequency and an increase in jaw opening time, 2) a decrease in maximum gape in soft food and an unaltered gape in small-particle hard food, and 3) an increase in lateral jaw excursion, mainly due to a more pronounced movement of the jaw to the balancing side (lingual phase). The contraction patterns were basically similar in the two ages. The higher chewing frequency in young animals was attained by a larger degree of overlap between opening and closing muscle activities. Young animals used relatively more EMG activity to chew hay, the hardest food. The changes in opening speed, gape, and chewing frequency are consistent with earlier predictions from the morphological changes, and so is the extra activity needed to chew hard food. The increase in lateral excursion was not predicted. It is suggested to be caused by cheek teeth wear, making possible smooth occlusal guidance of the jaw at the balancing side. Some of the changes in juvenile morphology can be viewed as adaptations to a changing diet.

Jaw movements and muscle activity during mastication in growing rabbits.

The oral apparatus of neonatal and juvenile golden hamsters was investigated by clearing and staining of whole crania, videotaping of behavior, and electromyography of several jaw muscles. Chewing developed during the first postnatal week and matured in the second; however, suckling was still the primary mode of feeding. Micromovements of the jaws occurred early when the osseous skeleton and joints developed. Macromovements correlated well with EMG records and were limited to jaw opening at birth. Muscles of the oral floor generated large bursts of activity during jaw opening and tongue protrusion from 0 days postnatal (dpn), when simple and stereotyped gaping was induced, until 14 dpn, when movements were spontaneous and not stereotyped nor inducible. However, adductor muscle activity was brief, low in amplitude, and primarily involved with jaw stabilization until 4 dpn, when these muscles became active during closing the jaws; closing activity increased in frequency and amplitude until the end of the second week. Development of frequent, coordinated macromovements of chewing was associated with the refinement of joint structure and dental occlusion and with the growth of the craniofacial skeleton. Jaw movements and associated EMG's correlated better with available data on development of neural circuitry than with that for musculoskeletal development.

Thomas C. Lakars

Papers

Ontogeny of oral function in hamsters (Mesocricetus auratus)