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PubMed1975

Electromyography and mechanics of mastication in the albino rat.

The masticatory apparatus in the albino rat was studied by means of electromyography and subsequent estimation of muscular forces. The activity patterns of the trigeminal and suprahyoid musculature and the mandibular movements were recorded simultaneously during feeding. The relative forces of the individual muscles in the different stages of chewing cycles and biting were estimated on the basis of their physiological cross sections and their activity levels, as measured from integrated electromyograms. Workinglines and moment arms of these muscles were determined for different jaw positions. In the anteriorly directed masticatory grinding stroke the resultants of the muscle forces at each side are identical; they direct anteriorly, dorsally and slightly lingually and pass along the lateral side of the second molar. Almost the entire muscular resultant force is transmitted to the molars while the temporo-mandibular joint remains unloaded. A small transverse force, produced by the tense symphyseal cruciate ligaments balances the couple of muscle resultant and molar reaction force in the transverse plane. After each grinding stroke the mandible is repositioned for the next stroke by the overlapping actions of three muscle groups: the pterygoids and suprahyoids produce depression and forward shift, the suprahyoids and temporal backward shift and elevation of the mandible while the subsequent co-operation of the temporal and masseter causes final closure of the mouth and starting of the forward grinding movement. All muscles act in a bilaterally symmetrical fashion. The pterygoids contract more strongly, the masseter more weakly during biting than during chewing. The wide gape shifts the resultant of the muscle forces more vertically and moreposteriorly. The joint then becomes strongly loaded because the reaction forces are applied far anteriorly on the incisors. The charateristic angle between the almost horizontal biting force and the surface of the food pellet indicates that the lower incisors produce a chisel-like action. Tooth structure reflects chewing and biting forces. The transverse molar lamellae lie about parallel to the chewing forces whereas perpendicular loading of the occlusal surfaces is achieved by their inclination in the transverse plane. The incisors are loaded approximately parallel to their longitudinal axis, placement that avoids bending forces during biting. It is suggested that a predominantly protrusive musculature favors the effective force transmission to the lower incisors, required for gnawing. By grinding food across transversely oriented molar ridges the protrusive components of the muscles would be utilized best. From the relative weights of the masticatory muscles in their topographical relations with joints, molars and incisors it may be concluded that the masticatory apparatus is a construction adapted to optimal transmission of force from muscles to teeth.

Animals↗
PubMed1984

Movements of the mandibles and tongue during mastication and swallowing in Pteropus giganteus (megachiroptera): a cineradiographical study.

Quantitative lateral and dorsoventral cineradiography shows that the masticatory movements of the mandible, condyles, tongue, and hyoid of Pteropus giganteus (Chiroptera) move along highly regular paths that are characteristic for each of the three food types tested. Mandibular movements are predominantly orthal, although a small forward translation occurs early in opening and small lateral deflections occur in both opening and closing phases. These deflections are related to the existence of active (bolus bearing) and balancing sides of the jaws, chewing being not truly bilateral. The deflections are associated with a shift of both condyles toward one side. In consequence the active condyle is located in a lateral part of the associated fossa, the inactive condyle in a medial part. Food transfer from side to side involves a reversal of the chewing direction during opening. Such reversals are especially frequent near the end of a chewing sequence. The fore, middle, and hind parts of the tongue differ in their movement patterns. Movements of the fore part, and to a lesser extent of the middle part, follow the open-close movements of the lower jaw. The hind part of the tongue moves predominantly dorsally during slow closing and ventrally during fast opening and fast closing. All three parts move forward during slow closing and slow opening, and backward during fast opening and fast closing. Movements of the hyoid are closely synchronized with those of the hind part of the tongue. Furthermore, tongue and hyoid movements are synchronized with jaw movements. All cycles of Pteropus giganteus are transport cycles, and the synchrony appears to reflect the consistency of the food (soft pulp, juices). Food consistency also accounts for the high swallowing rate and the absence of any significant difference between nonswallowing and swallowing cycles.

Adaptation, Physiological↗
PubMed1985

A cineradiographic and electromyographic study of mastication in Tenrec ecaudatus.

Regular chewing was studied in the specialized Malagasy insectivore Tenrec ecaudatus with the aid of precisely correlated electromyography of the main adductors, digastrics, and two hyoid muscles and cineradiography for which metallic markers were placed in the mandibles, tongue, and hyoid bone. During the power stroke the body of the mandible moves dorsally and medially. The medially directed component of movement at this time is greatly increased by simultaneous rotation of the mandible about its longitudinal axis. The highly mobile symphysis, spherical dentary condyle, loss of superficial masseter muscle and zygoma, and the simplified zalamnodont molars all appear to be related to the large amount of mandibular rotation that occurs during occlusion. The balancing side lateral pterygoid muscle (inferior head) apparently shifts the working side mandible laterally during the last part of opening and the first part of closing. The working side temporalis and the superficial masseter muscle are both responsible for the shift back to the midline. The temporalis is usually active to the same extent on the working and balancing sides during the power stroke. The level of activity (amplitude) of the temporalis and duration of the power stroke increase with harder foods. Whenever soft foods are chewed, the superficial masseter is only active on the working side; whenever foods of increasing hardness are chewed, its level of activity on the balancing side increases to approach that of the working side. Mandibular rotation is greatly reduced when hard foods are chewed.

Animals↗
PubMed1996

An analytic probability density for particle size in human mastication.

In previous studies the distribution of particles sizes of chewed food has been described by an empirical function. However, at the beginning of the chewing process, when many unbroken food particles are still present in the mixture, this function failed to give a good description. In the present study, formulae were derived to characterize the distribution of chewed food particles by size as a function of the number of chewing cycles. The reduction of food particle sizes was considered to be the composite result of a selection and a breakage process. Both processes were described by simple functions. The probability density Pn + 1(x) of finding a particle of size x after n + 1 chewing cycles was computed from Pn(x) by selecting a proportion of particles of size y from Pn to be converted to particles of size x < y by a breakage function. Measures of central tendency--average, median and most probable size--were obtained as a function of the number of chewing cycles. The measures of central tendency characterize the degree of food comminution during the chewing process and so can be used to quantify chewing performance. The comminution of food is described in terms of the selection and breakage functions in a convenient, efficient, analytic way, valid for all phases of the chewing process.

Food↗
PubMed1995

Mastication with and without removable partial dentures: an intraindividual study.

The chewing of 22 subjects, with a total of 27 removable partial dentures (RPD) that had been used for more than 6 months, was analyzed by four different methods. The study design was an intraindividual comparison of chewing, with and without the RPDs. Chewing tests were a color mixture of two-colored gums, chewing gum bolus shaping, comminution of Optosil silicone tablets, and number of chewing strokes to first swallow of an almond. The participants were also asked some standardized questions about their eating, biting, and chewing capacity. There was no significant difference in efficiency as measured by chewing gum color mixing, chewing gum shape, and pulverization of Optosil between chewing with and without the RPDs. Significantly more strokes were needed to swallow an almond without than with the RPD in place. With their RPDs in place, all subjects could chew a sandwich and an apple, and all but one could chew whole meat. The study shows that the chewing process should preferably not be analyzed by one method only.

Adult↗