Muscle fibre types in the suprahyoid muscles of the rat

Five muscle fibre types (I, IIc, IIa, IIx and IIb) were found in the suprahyoid muscles (mylohyoid, geniohyoid, and the anterior and posterior bellies of the digastric) of the rat using immuno and enzyme histochemical techniques. More than 90% of fibres in the muscles examined were fast contracting fibres (types IIa, IIx and IIb). The geniohyoid and the anterior belly of the digastric had the greatest number of IIb fibres, whilst the mylohyoid was almost exclusively formed by aerobic fibres. The posterior belly of the digastric contained a greater percentage of aerobic fibres (83.4%) than the anterior belly (67.8%). With the exception of the geniohyoid, the percentage of type I and IIc fibres, which have slow myosin heavy chain (MHCβ), was relatively high and greater than has been previously reported in the jaw‐closing muscles of the rat, such as the superficial masseter. The geniohyoid and mylohyoid exhibited a mosaic fibre type distribution, without any apparent regionalisation, although in the later MHCβ‐containing fibres (types I and IIc) were primarily located in the rostral 2/3 region. In contrast, the anterior and posterior bellies of the digastric revealed a clear regionalisation. In the anterior belly of the digastric 2 regions were observed: both a central region, which was almost exclusively formed by aerobic fibres and where all of the type I and IIc fibres were located, and a peripheral region, where type IIb fibres predominated. The posterior belly of the digastric showed a deep aerobic region which was greater in size and where type I and IIc fibres were confined, and a superficial region, where primarily type IIx and IIb fibres were observed.     

Observation of anomalus triplication of unilateral anterior digastric muscle

We describe a rare anomaly in the submental region of a single male cadaver specimen. The anterior belly of the right digastric muscle was observed to have three separate insertions. Most medial of these crossed the midline and inserted to the digastric fossa with the opposite digastric muscle. These muscle bands were united in a common tendon as they continued with the posterior belly. This is an anatomical variation in the mylohyoid digastric muscle group in the floor of the mouth. Consideration of such variations of the anterior bellies of the digastric muscles should be noted during the evaluation of the floor of the mouth in the CT examination or MR imaging. © 1993 Wiley-Liss, Inc.     

Bilateral anomaly of anterior bellies of digastric muscles

During dissection of the submental region, the anterior bellies of the right and left digastric muscles were found to have two separate portions, inserting into different locations in the submental region. The lateral portions of the anterior bellies of the digastric muscles originated from the digastric fossa and inserted into the hyoid bone. The medial parts of the anterior bellies of the right and left digastric muscles ran medially and inserted into the mylohyoid raphe on both sides, forming accessory digastric muscles. This anatomic abnormality of the muscle could be significant in surgical procedures involving the submental region.     

Bilateral anatomical anomaly of anterior bellies of digastric muscles.

During the gross anatomy dissection of the submandibular region, both anterior bellies of the digastric muscles, especially the left one, were found to be enlarged. They were arranged as two parallel asymmetric bands extending from the hyoid bone to the chin. The posterior bellies of the digastric muscles were normal. The suprahyoid muscles showed no abnormalities. This anomaly represents an anatomical variation in the mylohyoid digastric muscle group in the floor of the mouth.     

Prenatal development of the muscles in the floor of the mouth in human embryos and fetuses from 6.9 to 76 mm CRL

The development of the muscles in the floor of the mouth is described in 10 human embryos and fetuses ranging from 6.9 to 76 mm CRL by means of computer-aided graphical 3D-reconstructions. All primordia of the muscles in the floor of the mouth could be identified from the 15.6 mm CRL stage on. The proportions and insertion lines of the early muscles were found to be different from adult anatomy. Each muscle first inserted in the medial surface of Meckels cartilage, but during the developmental period between 19 and 68 mm CRL the insertion lines were gradually transposed to the bony ridges of the mandible which progrediently embraced Meckels cartilage. The fibers of the mylohyoid muscles left the anterior region near the symphysis mentalis free during all stages of this study. The digastric muscle revealed only one belly with a constriction of its continuous fibers where it passed the hyoid bone primordium. There was no attachment of digastric muscle fibers to the hyoid; only geniohyoid and mylohyoid fibers. Geniohyoid and genioglossus muscles basically correspond to their definite arrangement, but they underwent proportional changes. Individual specimens embodied irregularities such as accessory geniohyoid and hyoid portions and muscle fibers separate from the mylohyoide muscle.     

Bilateral abnormal anterior bellies of digastric muscles

During dissection of the submental region of a male cadaver, we encountered an abnormal digastric muscle on both sides. Two muscle bundles, both of which fused with mylohyoid muscle close to the midline, were observed on the right side. The anterior one originated from the digastric fossa and its length and width were 27 and 9 mm, respectively. The posterior accessory belly originated from the right intermediate tendon and it was 32 mm in length and 11 mm wide. On the left side, there was a single accessory bundle that originated from the left intermediate tendon and inserted into the mylohyoid raphe at the fusion point of the right accessory bundles. The length and width of this belly were 29 and 9 mm, respectively. The remaining suprahyoid muscles of both sides were normal. Anatomical variations of digastric muscle have to be considered in the imaging procedures of the soft tissue masses in the submental region.     

Median accessory digastric muscle: radiological and surgical correlation

A median accessory digastric muscle was revealed during a dissection of the submental region. The muscle was located between the anterior bellies of the digastrics, external to the mylohyoid and deep to the platysma. It appeared as a flat quadrilateral sheet. Its base arose from the front of the body of the hyoid bone near its upper border. Lower portion of the fibers, at the right base of the muscle, initially traveled perpendicular to the lower fibers of the right mylohyoid. The rest of the muscle had a median course, with the cranial end inserting into the mandible between the digastric fossae. The muscle elevated the hyoid bone and depressed the mandible when appropriate stress was applied. No other morphologic abnormalities were found in this region. Aberrant anterior bellies of the digastric muscles are uncommon and occur bilaterally or unilaterally. This observation of a median accessory digastric muscle has not previously been reported. Knowledge of this variant will help to avoid confusion with pathological conditions of the floor of the mouth and the submental region. It is relevant both for the interpretation of radiological images and during surgical procedures such as dissection of the anterior belly of the digastric for a malignant disease and graft positioning.     

The human digastric muscle: Patterns and variations with clinical and surgical correlations

The digastric muscle is located in the suprahyoid region on each side and frequently exhibits two muscular bellies (anterior and posterior) linked by an intermediate tendon. The paired digastric muscles act together either depressing the mandible or elevating the hyoid bone; therefore acting as a single muscle with important physiological roles. In the present study, the digastric muscle has been analyzed bilaterally in 74 adult human cadavers. A computerized morphometrical investigation of the digastric muscles has been performed (Image Pro Plus software package, Media Cybernetics, USA) and the resulting quantitative data have been statistically assessed (SPSS 11.0 for Windows, USA). We hereby propose an original morphological classification that encompasses five types (I-V) for the anterior belly (AB); three types (I-III) for the intermediate tendon (IT); and two types (I-II) for the posterior belly (PB) of the human digastric muscle. In each digastric muscle, the aforementioned anatomical types have been characterized according to the muscular bellies and intermediate tendon. Consequently, as a result of the combinations of those diverse types, individual digastric muscles have been considered as pertaining to distinctive morphological patterns (named from A to J). Cases with absence of either AB or PB have been included in patterns K and L and would be more appropriately defined as monogastric muscles. This innovative classification provides clear-cut anatomical parameters for interpreting morphological variants of the digastric muscle with relevant clinical and surgical correlations.     

Effects of running exercise with increasing loads on tibialis anterior muscle fibres in mice

Cross-sectional areas and succinate dehydrogenase (SDH) activities of type identified fibres in the deep, middle and superficial regions of the tibialis anterior muscle in mice were examined after 4 weeks of voluntary running exercise with increasing loads. Nineteen-week-old male mice were assigned randomly to either a control or exercise group. The mean cross-sectional areas of all types (IIa, IIx and IIb) of fibres in the superficial region of the muscle were greater in the exercise group than in the control group. The mean SDH activities of type IIx and type IIb fibres in the middle region and of all types (IIa, IIx and IIb) of fibres in the superficial region of the muscle were greater in the exercise group than in the control group. These results suggest that voluntary running exercise with increasing loads causes hypertrophy and/or an increase in the SDH activity of fibres in the specific muscle region where fibres with a high threshold and a low-oxidative enzyme activity are distributed, and these fibres are recruited to adapt to changes in exercise conditions.     

Morphology of diaphragm neuromuscular junctions on different fibre types

Summary We hypothesize that the morphology of the neuromuscular junction on different muscle fibre types varies, reflecting differences in activation history. In the rat diaphragm muscle, we used a three-colour fluorescent immunocytochemical technique to simultaneously visualize (1) innervating axons and presynaptic nerve terminals, (2) motor endplates and (3) myosin heavy chain isoform expression (muscle fibre type). Laser-scanning confocal microscopy was then used to optically section the triple-labelled muscle fibres, and create three-dimensional views of the neuromuscular junction. Type I fibres were innervated by the smallest axons, and type IIa, IIx and IIb fibres by progressively larger axons. Absolute planar areas of nerve terminals and endplates progressively increased from type I, IIa, IIx to IIb fibres. When normalized for fibre diameter planar areas of nerve terminals were largest on type I fibres, with no difference among type II fibres. The normalized planar area of endplates were larger for type I and IIb fibres, compared to type IIa and IIx fibres. The three-dimensional surface area of endplates was largest on type I fibres, with no differences across type II fibres. When normalized for fibre diameter, endplate surface areas increase progressively from type I, IIa, IIx to IIb fibres. The branches increased progressively from type I, IIa, IIx to IIb fibres. Conversely, individual branch length was longest on type I fibres, and shortest on type IIb fibres. The extent of overlap of pre- and postsynaptic elements of the neuromuscular junction decreased progressively on type I, IIa, IIx and IIb fibres. We conclude that these morphological differences at the neuromuscular function of different fibre types reflect differences in activation history and may underlie phenotypic differences in neuromuscular transmission.     

Abnormal digastric muscle with unilateral quadrification of the anterior belly

During dissection of the submental region we observed that the anterior belly of the left digastric muscle had four separate insertions. These muscle bands united in a common tendon as they continued with the posterior belly. This is an anatomical variation in the mylohyoid digastric muscle group in the floor of the mouth. When an asymmetry in the floor of the mouth is detected during diagnostic procedures, such as radiologic studies, anomalies of the anterior belly of the digastric muscle should be considered besides other reasons of asymmetry. Additionally, possible occurrence of such anomalies should be remembered during surgical procedures involving the submental region. This unique variation has not been reported in the literature.     

Double innervation of the anterior belly of the digastric muscle

We came across a very rare case in which the anterior belly of the digastric muscle was innervated by the twigs of the facial nerve in addition to those of the mylohyoid nerve. The anomaly was discovered in the cadaver of an 84-year-old Japanese male bequeathed for a training seminar in gross anatomy at Kumamoto University in 2003. One twig issued from the marginal mandibular branch of the facial nerve and entered the central region of the anterior belly of the digastric muscle on the lower surface. The other twig issued from the stylohyoid branch of the facial nerve, descended along the lateral margin of the stylohyoid muscle and entered the anterior belly of the digastric muscle on the lower surface near the intermediate tendon. The twig from the marginal mandibular branch was distributed to the shallow (lower) and central region near the medial margin of the anterior belly. The twig from the stylohyoid branch was distributed to the shallow and lateral region of the anterior belly. These two twigs communicated with the mylohyoid nerve at several peripheral parts. Textbooks on general anatomy make mention of only one nerve, the mylohyoid, supplying the anterior belly of the digastric muscle. However, the present case manifests that the anterior belly receiving twigs from the mylohyoid and facial nerves is formed with the second brachial component as well as the first.     

Abnormal anterior belly of the digastric muscle: a proposal for the classification of abnormalities

We recognized an abnormal anterior belly of the digastric muscle in an 83-year-old male cadaver. Three muscle bundles were observed on the left anterior belly: (i) attached to the left digastric fossa; (ii) attached to the right digastric fossa; and (ii) attached to the raphe of the mylohyoid muscle. Four muscle bundles were recognized on the right anterior belly: (i) attached to raphe of the mylohyoid muscle; (ii, iii) attached to the exterior surface on the base of the mandible from the raphe of the mylohyoid muscle; and (iv) attached to the interior surface on the base of the mandible from the raphe of the mylohyoid muscle. The raphe of the mylohyoid muscle was curved significantly to right and the four abnormal bundles found on the right anterior belly (see above) were attached to its curved point.     

Anatomical study of the human omohyoid muscle: regarding intermediate morphologies between normal and anomalous morphologies of the superior belly

Intermediate morphologies between normal and anomalous morphologies of the superior belly of the omohyoid muscle (Om) were macroscopically and stereomicroscopically observed in 34 cadavers (24 males and 10 females aged between 51 and 97 years; average age 71.0 years) for anatomical practice, which had been preserved in the Department of Morphological Biology, Ohu University School of Dentistry. The intermediate morphologies were classified into four types on the basis of the developmental degree of the muscle fibers and the number and origin of the belly as follows: type 1, the anterior margin of the belly was unclear owing to poor myofiber development; type 2, the superior belly was composed of a posterior large belly and an anterior small belly; type 3, composed of three to five bellies, with the bellies arranged in a roof tile-like morphology; and type 4, the belly was composed of two bellies arranged anterior-posteriorly parallel to each other (the anterior belly was found to be the inferior belly that had developed and reached the superior belly area). For the intermediate morphologies of the Om superior belly observed in the present study, although type 4 was due to the development of an inferior belly, the other three types were considered to be caused by the poor development of the myofibers in the formation process and by the division of the superior belly into two muscles, or secondary lamellar division of the belly with growth.     

Electromyography of the oral stage of swallowing in man

The geniohyoid, anterior belly of digastric, mylohyoid, and genioglossus muscles of 20 human subjects were studied electromyographically to determine the temporal relationships of their activities during the act of swallowing. Although the firing order of the four muscles varied within the same subject, the best estimate of the “true” firing sequence was established for each of the 18 subjects who provided statistically significant data. However, no definite universal pattern could be established for the four muscles because there was great inter-subject variability in both the duration and the sequence of activity. Therefore, at least with respect to these four muscles, each individual has his own swallowing pattern, but different people may swallow quite differently. The type of bolus (saliva vs water) may influence the duration of the muscles' activity. On the other hand, posture (semi-reclined vs sitting) did not seem to have any influence. There was no evidence to indicate that posture and/or the type of bolus are correlated with the sequence of muscular activity. The anterior belly of the digastric muscle was not active in one-quarter of the swallows studied. When active during deglutition, all muscles had a general electromyographic pattern of one to many summations of activity separated by relatively quiet periods before and after each swallow.     

Derivation of the anterior belly of the digastric muscle receiving twigs from the mylohyoid and facial nerves

The anterior belly of the digastric muscle is usually supplied by the mylohyoid nerve, and in general anatomy textbooks, the anterior belly is invariably described as receiving no other nerve except the mylohyoid nerve. In fact, however, it is sometimes supplied by a branch of the facial nerve in addition to the mylohyoid nerve. Such cases were found in 8 bodies or 9 head sides among 539 bodies or 1078 head sides of Japanese subjects. Those nine cases were investigated in detail and it was clarified that they had the following three characteristics in common: (1) the twig originating from the facial nerve appears as the twig of the stylohyoid branch in most cases, (2) the twig from the facial nerve enters the anterior belly on its lower (shallow) surface and the twig of the mylohyoid nerve on its upper (deep) surface, (3) the twig of the mylohyoid nerve is distributed to the deep region and the twig of the stylohyoid branch is distributed to the shallow region of the anterior belly. From these results, it was concluded that the anterior belly, receiving the twigs of the mylohyoid and facial nerves, had been formed by secondarily combining the most ventral and rostral part of the primordium of the stylohyoid muscle in the second branchial arch with the caudal part of the primordium of the anterior belly in the first branchial arch.     

Quantification of fibre type regionalisation: an analysis of lower hindlimb muscles in the rat

Newly developed concepts and methods for the quantification of fibre type regionalisation were used for comparison between all muscles traversing the ankle of the rat lower hindlimb (n = 12). For each muscle, cross‐sections from the proximodistal midlevel were stained for myofibrillar ATPase and classified as type I (‘slow’) or II (‘fast’). For the 11 ‘fast’ muscles (i.e. all except soleus), the muscle outline and the position of each type I fibre were digitised for further computer processing. Two potentially independent aspects of type I fibre regionalisation were evaluated quantitatively: (1) the degree to which type I fibres were restricted to a limited portion of the total cross‐sectional area (‘area‐regionalisation’) ; (2) the extent and direction of the difference (if any) between the centre of the muscle cross‐section and the calculated centre for the type I fibre cluster (‘vector regionalisation’). Statistical analysis showed that type I fibres were vector regionalised in practically all investigated muscles and area regionalised within most of them, the only consistent exceptions being peroneus brevis and peroneus digitorum 4, 5. In muscles with a high degree of area regionalisation the population of type I fibres also had a markedly eccentric intramuscular position (i.e. high vector regionalisation). A significant relationship was observed between the relative position of a muscle within the hindlimb (transverse plane) and the direction and degree of its type I fibre eccentricity. On average, the degree of type I fibre eccentricity was greater for muscles remote from the limb centre than for those situated more centrally. In addition, the intramuscular concentration of type I fibres was typically greatest towards the centre of the limb, the most striking exception being tibialis posterior. For the slow soleus muscle, which is centrally placed within the limb, our analysis concerned the type II fibres, which were found to be weakly vector regionalised but not significantly area regionalised. It is concluded that, within muscles of the rat's lower hindlimb, fibre type regionalisation is a general and graded phenomenon which may reflect differentiating (embryological?) mechanisms of a transmuscular significance. Furthermore, the analysis demonstrated the usefulness of our new methods and concepts for the quantification of fibre type regionalisation.     

Supernumerary muscle bundles in the submental triangle: their positional relationships according to innervation

The positional relationships between the supernumerary muscle bundles within the submental triangle and their innervating branches from the mylohyoid nerve were investigated. Ten heads of Japanese cadavers that showed aberrant muscle bundles within the submental triangle were examined. Three additional heads without such aberrant bundles were used for comparison. All cadavers were fixed with 8% formalin and preserved in 30% ethanol. After the examination of the origin and insertion of the muscles, the bony elements were removed, and then their innervating branches from the mylohyoid nerve were examined in detail under a binocular microscope. In 11 head-halves of six cadavers unilateral supernumerary bundles were found. Right and left mylohyoid nerves gave off branches that crossed the inner surface of the bundles of each respective side. Supernumerary bundles ran across the median line in two heads. In one head, the twigs from the mylohyoid nerve of the same side as the mandibular origin entered the inner surface of the bundles. The other head received double innervation from right and left nerves. Three heads showed supernumerary bundles that attached to the mandible or the hyoid bone at one end and joined the mylohyoid muscle at the other end. The branches from the mylohyoid nerve of the digastric side entered the inner surface of the bundles, and those of the mylohyoid side entered their outer surface. After giving off branches to the muscles, the mylohyoid nerve continued as a cutaneous nerve of the submental region. Based on the innervation patterns of the aberrant bundles within the submental triangle, it was suggested that these bundles result from the combination of the remnants of the primordia of the mylohyoid muscle and the anterior belly of the digastric.     

Fibre type regionalisation in lower hindlimb muscles of rabbit, rat and mouse: a comparative study

The topographical distribution of different fibre types in muscles of the lower hindlimb in rabbits and mice was quantitatively determined. The results were compared to those previously obtained, using the same new quantification methods, in homologous muscles of the rat. Type I fibres ('slow') were identified using myofibrillar ATPase histochemistry and mapped out at the mid proximo-distal level for 11 'fast' muscles in the rabbit and 7 'fast' muscles in the mouse. For the slow soleus muscle the procedure was undertaken for the type II fibres. Furthermore, for 5 of the 'fast' muscles from each animal species (extensor digitorum longus; flexor digitorum and hallucis longus; gastrocnemius medialis; peroneus longus; tibialis anterior), several more proximal and distal cross-sectional levels were also analysed. All the investigated 'fast' muscles showed a significant degree of topographical eccentricity in the midlevel distribution of type I fibres. For most muscles, the direction of this 'vector regionalisation' of type I fibres was similar between the three animal species. For homologous muscles, the degree of vector regionalisation was significantly different: mouse > rat > rabbit. The relative area of the region containing the type I fibres, inversely related to the degree of 'area regionalisation', was also significantly different: mouse < rat < rabbit. Also within each animal species, muscles with a marked degree of vector regionalisation tended to show a marked area regionalisation. Proximo-distal differences in type I fibre density were observed in all the three species of animals; also these patterns showed marked inter-species differences. The findings demonstrate the general occurrence of, and systematic relationships between, different aspects of type I fibre regionalisation. The observed interspecies differences suggest that the expression of this phenomenon is adapted to differing functional needs.