Masseter muscle performance. Significance of structure and metabolism. A morphological and experimental study

The aim of this thesis was to characterize structural factors in masticatory systems relevant to functional evaluations and to elucidate the effect on energy metabolism of electrically induced jaw muscle fatigue. An omnivorous masticatory system (the domestic pig) was compared morphologically with a carnivorous (the dog). Porcine masseter muscles were evaluated by ATP-ase histochemistry as well as with NADH-dehydrogenase and PAS-staining. Contractional characteristics were obtained from the porcine and canine masseters by electrical stimulation. The 133Xenon clearance technique and a flexible oxygen electrode were employed. A bite-force transducer was used. The porcine craniomandibular joint (CMJ) lacked a pronounced mandibular fossa and had anteriorly orientated cylindrical condyles. The dog CMJ comprised a cylindrical condyle orientated at right angles to the satittal plane and medially inclined. The pronounced mandibular fossa and marked tubercle, together with a well-developed retro-articular process, surrounded the condyle. The masticatory muscles were the same in the two species, except for the pig's zygomatico-mandibular muscle. The ATP-ase technique failed to reveal type II:B-fibres in the porcine masseter after acid and alkaline preincubation and it was not possible to separate fibre types by glycogen-staining and NADH-dehydrogenase histochemistry. These findings diverged from the pig soleus histochemical profile (type II:B-fibres 60%). The quantitative evaluation revealed 75% type II:A-fibres in the porcine masseter. No statistically significant difference was found between the various fibre-type diameters in the porcine masseter. The mean fibre Type diameter was larger in the porcine masseter than in the soleus muscle. Type II-fibres were more frequently found on the edge of the fascicles. The bite force recordings showed that the porcine masseter was capable of long endurance performance, in contrast to the easily fatigued canine masseter. Significant reductions of intramuscular substrates and a considerable lactate accumulation were observed. The NADH/NAD-shuttle was oppositely directed in the two species. The blood-flow recordings revealed a marked blood-flow impairment during contraction, followed by a prominent post-exercise hyperaemia. The pO2 recordings were closely related in time as well as in mangnitude to the blood flow. It is thus concluded, based on morphological observations, that the porcine masticatory system bears resemblance to the human situation. In the canine masseter muscle, a relationship was found between metabolism and mechanical bite-force output. This correlation was not so evident in the porcine masseter. Induced jaw muscle hyperactivity may lead to a reduced energy and redox state and, as a late consequence, to fatigue.