A. W. Crompton

Coordination of mastication and swallowing.

The coordination of mastication, oral transport, and swallowing was examined during intake of solids and liquids in four normal subjects. Videofluorography (VFG) and electromyography (EMG) were recorded simultaneously while subjects consumed barium-impregnated foods. Intramuscular electrodes were inserted in the masseter, suprahyoid, and infrahyoid muscles. Ninety-four swallows were analyzed frame-by-frame for timing of bolus transport, swallowing, and phases of the masticatory gape cycle. Barium entered the pharynx a mean of 1.1 s (range −0.3 to 6.4 s) before swallow onset. This interval varied significantly among foods and was shortest for liquids. A bolus of food reached the valleculae prior to swallow onset in 37% of sequences, but most of the food was in the oral cavity at the onset of swallowing. Nearly all swallows started during the intercuspal (minimum gape) phase of the masticatory cycle. Selected sequences were analyzed further by computer, using an analog-to-digital convertor (for EMG) and frame grabber (for VFG). When subjects chewed solid food, there were loosely linked cycles of jaw and hyoid motion. A preswallow bolus of chewed food was transported from the oral cavity to the oropharynx by protraction (movement forward and upward) of the tongue and hyoid bone. The tongue compressed the food against the palate and squeezed a portion into the pharynx one or more cycles prior to swallowing. This protraction was produced by contraction of the geniohyoid and anterior digastric muscles, and occurred during the intercuspal (minimum gape) and opening phases of the masticatory cycle. The mechanism of preswallow transport was highly similar to the oral phase of swallowing. Alternation of jaw adductor and abductor activity during mastication provided a framework for integration of chewing, transport, and swallowing.

Optimization of bone growth and remodeling in response to loading in tapered mammalian limbs

SUMMARY How bones respond dynamically to mechanical loading through changes in shape and structure is poorly understood, particularly with respect to variations between bones. Structurally, cortical bones adapt in vivo to their mechanical environments primarily by modulating two processes, modeling and Haversian remodeling. Modeling, defined here as the addition of new bone, may occur in response to mechanical stimuli by altering bone shape or size through growth. Haversian remodeling is thought to be an adaptation to repair microcracks or prevent microcrack propagation. Here, we examine whether cortical bone in sheep limbs modulates periosteal modeling and Haversian remodeling to optimize strength relative to mass in hind-limb midshafts in response to moderate levels of exercise at different growth stages. Histomorphometry was used to compare rates of periosteal growth and Haversian remodeling in exercised and sedentary treatment groups of juvenile, subadult and young adult sheep. In vivo strain data were also collected for the tibia and metatarsal midshafts of juvenile sheep. The results suggest that limb bones initially optimize responses to loading according to the varying power requirements associated with adding mass at different locations. In juveniles, exercise induces higher rates of periosteal modeling in proximal midshafts and higher rates of Haversian remodeling in distal midshafts. Consequently, distal element midshafts experience higher strains and, presumably, have lower safety factors. As animals age, periosteal modeling rates decline and Haversian remodeling rates increase, but moderate levels of mechanical loading stimulate neither process significantly.

Relationships of the Liassic Mammals Sinoconodon, Morganucodon oehleri, and Dinnetherium

A lack of morphological information makes it difficult to interpret the relationships of advanced cynodonts and early mammals. To address this problem, we studied new skulls of Sinoconodon and Morganucodon oehleri from the Liassic of Yunnan, China, and Dinnetherium from the Kayenta Formation of Arizona.

Why Fuse the Mandibular Symphysis? A Comparative Analysis

Fused symphyses, which evolved independently in several mammalian taxa, including anthropoids, are stiffer and stronger than unfused symphyses. This paper tests the hypothesis that orientations of tooth movements during occlusion are the primary basis for variations in symphyseal fusion. Mammals whose teeth have primarily dorsally oriented occlusal trajectories and/or rotate their mandibles during occlusion will not benefit from symphyseal fusion because it prevents independent mandibular movements and because unfused symphyses transfer dorsally oriented forces with equal efficiency; mammals with predominantly transverse power strokes are predicted to benefit from symphyseal fusion or greatly restricted mediolateral movement at the symphysis in order to increase force transfer efficiency across the symphysis in the transverse plane. These hypotheses are tested with comparative data on symphyseal and occlusal morphology in several mammals, and with kinematic and EMG analyses of mastication in opossums (Didelphis virginiana) and goats (Capra hircus) that are compared with published data on chewing in primates. Among mammals, symphyseal fusion or a morphology that greatly restricts movement correlates significantly with occlusal orientation: species with more transversely oriented occlusal planes tend to have fused symphyses. The ratio of working- to balancing-side adductor muscle force in goats and opossums is close to 1:1, as in macaques, but goats and opossums have mandibles that rotate independently during occlusion, and have predominantly vertically oriented tooth movements during the power stroke. Symphyseal fusion is therefore most likely an adaptation for increasing the efficiency of transfer of transversely oriented occlusal forces in mammals whose mandibles do not rotate independently during the power stroke.

Jaw movements and patterns of mandibular bone strain during mastication in the monkey Macaca fascicularis.

Small amalgam fillings were placed in maxillary and mandibular second molar and canine teeth for cine-radiographic analysis. The rosette strain gauges were bonded bilaterally to mandibular cortical bone below the second or third molars. The monkeys were placed in a restraining chair that did not restrict normal head, neck or jaw movements; they were fed various foods and the bone-strain data recorded. Simultaneous jaw movements were recorded with cine-radiographic apparatus synchronized with the bone-strain recordings. During vigorous mastication, the transition between fast close and the power stroke was correlated with a sharp increase in masticatory force. In most instances, the jaws were maximally-loaded prior to maximum intercuspation, i.e. during the buccal phase (phase I) of occlusion. The macaques swallowed frequently throughout a chewing sequence and these swallows were intercalated into the chewing cycle toward the end of the power stroke. Such swallows had little effect on the magnitude or direction of peak principal strains during the power stroke. Bone-strain data suggested that unloading patterns during the power stroke of mastication were largely a function of the relaxation time of the jaw adductors. The period from 100 per cent peak strain to 50 per cent peak strain during unloading closely approximated to the half-relaxation time of the whole adductor jaw muscles.

Transition From Suckling to Drinking at Weaning: A Kinematic and Electromyographic Study in Miniature Pigs

The movements of the tongue, hyoid, and jaw were recorded cineradiographically in preweaning pigs as they suckled bariumized milk from a veterinary teat or drank it from a bowl. The movements were quantified by measuring the X, Y coordinates of radioopaque markers embedded in the tongue and attached to both jaws and to the hyoid. EMG activity in masseter, anterior digastric, geniohyoid, genioglossus, hyoglossus, sternohyoid, stylohyoid, and omohyoid muscles was recorded synchronously with cineradiography at 100 frames/sec. In both suckling and drinking, the movements were characterized by minimal movements of the jaw and hyoid but extensive movements of the tongue. In suckling, the movements were largely confined to the midposterior part of the tongue. A seal was formed between the posterior tongue and soft palate while a depression formed in the mid-tongue; this was associated with fluid moving into the depression probably because of a reduced intraoral pressure. The depression was associated with increased EMG activity in the genioglossus muscle and overlapping activity in digastric, geniohyoid, hyoglossus, and sternohyoid muscles. In drinking cycles, significant movement occurred in all parts of the tongue; milk ingestion was associated with tongue movements that combined elements characteristic both of suckling (mid-tongue depression with a posterior seal) and of lapping (extensive anteroposterior movements within the tongue itself). In drinking, compared to suckling, there was a major reduction in EMG activity in masseter, digastric, geniohyoid, and sternohyoid muscles. After milk had accumulated in the valleculae, swallows usually occurred in every other cycle during suckling and in every third or fourth cycle during drinking. The emptying of the valleculae was an event that was embedded in the early jaw-opening phase of an otherwise normal suckling or drinking cycle. Emptying of the valleculae was associated with posteriorly directed movement of the back of the tongue and increased EMG activity in hyoglossus, styloglossus, and omohyoid muscles. No differences were noted in the kinematics associated with swallowing in the two activities, but, in the normalized and averaged EMG data, there were significant differences in the timing of genioglossus activity and in the relative balance of hyoglossal and stylohyoid activity.

TRANSFORMATION OF THE QUADRATE (INCUS) THROUGH THE TRANSITION FROM NON-MAMMALIAN CYNODONTS TO MAMMALS

ABSTRACT The quadrate (incus) bone underwent important evolutionary transformations through the cynodont-mammal transition. The following character transformations played crucial roles in modifying the cynodont quadrate into the mammalian incus: 1) progressively greater rotation of the dorsal plate relative to the trochlea; 2) the contact facet of the dorsal plate becomes concave; 3) development of a constricted neck between the dorsal plate and the trochlea; 4) simplification of the quadrate-cranium joint, resulting in better mobility of the joint; and 5) introduction of a stapedial process (crus longum). The dorsal plate rotation, the concave contact facet, the constricted neck, the mobile joint of the quadrate and the cranium are also present in some advanced non-mammalian cynodonts. Broad phylogenetic distributions of these features suggest that the major features of the incus of early mammals, as represented by Morganucodon, originated much earlier in phylogenetic history among non-mammalian cynodont...

The Southern African Liassic prosauropod Massospondylus discovered in North America

ABSTRACT Skull material of the prosauropod dinosaur Massospondylus, previously known only from Africa, has been found in the Kayenta Formation (Glen Canyon Group) of northeastern Arizona. A reconstruction of the skull, corrected for distortion, is compared with that of Plateosaurus. The dentition provides evidence for an herbivorous diet in prosauropods; the presence of small palatal teeth is reported for the first time in any dinosaur. The associated fauna is related to faunas from the Elliot and Clarens sandstone formations in southern Africa, and an Early Jurassic (rather than Late Triassic) age is accepted for these formations as well as for the Kayenta Formation.

Integration of the Reflex Pharyngeal Swallow Into Rhythmic Oral Activity in a Neurologically Intact Pig Model

Mammalian swallowing involves the coordinated and sequential activity of many oropharyngeal muscles. Using synchronous electromyography (EMG) and videofluorography, we recorded the pattern of EMG activity for 12 muscles during swallowing in neurologically intact suckling pigs. We tested the hypothesis that this EMG pattern corresponded to the established pattern of activity for the isolated, reflexive pharyngeal swallow of the decerebrate infant pig. The EMG activity associated with the normal swallow of the intact animal had two components: a staggered pattern of single EMG bursts that were prominent in the stylohyoid, thyrohyoid, cricothyroid, and omohyoid muscles and double bursts of activity in some muscles, including geniohyoid and genioglossus, with the same underlying periodicity as suckling. Most of the staggered activity pattern, a linear sequence of progressively delayed activities in different muscles, was not statistically different from that previously found in the reflexive pharyngeal swallow of the decerebrate. However, not all components of the linear sequence of the reflexive swallow were inserted unchanged into the intact swallow. Some components appeared to be delayed or advanced, bringing them into phase with the underlying rhythmic activity. The difference between swallows of intact and of decerebrate animals was not solely due to the presence of rhythmic activity in the former. The timing of some EMG activities in intact animals also differed from the same activities in the few decerebrates that exhibited rhythmic tongue and jaw activity. These results suggest cerebral function influences the EMG pattern of the pharyngeal swallow, which has traditionally been considered a purely reflex pattern.

Evolutionary Origins of The Mammalian Promontorium and Cochlea

ABSTRACT Mammals have two apomorphies in the ear region: an elongated cochlear canal and an eminence on the tympanic side of the cochlear housing, known as the petrosal promontorium. In nonmammalian cynodonts, the cochlear recess is shorter, smaller, and oriented more medially than the mammalian cochlear canal; their cochlear housing is formed by the prootic, opisthotic, and basisphenoid. New fossil materials of two early mammals, Adelobasileus and Sinoconodon, reveal evidence on the evolutionary transformation of the mammalian cochlear canal and bony cochlear housing. Adelobasileus is more derived than any known non-mammalian cynodont in possessing an incipient promontorium, but more primitive than other mammals in retaining a vestigial basisphenoid wing. Sinoconodon has a fully developed petrosal promontorium but a short cochlea that extends only one third of the promontorium length. The cochlear canal of Morganucodon is twice as long as that of Sinoconodon, relative to both skull and promontorium. More...