Expression of sarcoplasmic-endoplasmic reticulum Ca-ATPase isoforms in masticatory muscles.

The aim of this study was to characterize the sarcoplasmic-endoplasmic reticulum Ca-ATPase (SERCA) isoforms in rabbit masticatory muscles compared with those in fast-twitch muscle. It was hypothesized that combined expression of the SERCA isoforms in fast- and slow-twitch muscles accounts for lower Ca-ATPase activity. SERCA was isolated by differential centrifugation, the isoforms were determined by ELISA, and the activity of each isoform was measured using a colorimetric method. Activity was tested for significance by anova, and the distribution of isoforms was assessed using the chi-square test (P < 0.05) and correlated to SERCA activity using Spearman's rank correlation. SERCA1 was predominant (90.5%) in fast-twitch muscle, whereas a mixture of SERCA isoforms was found in masticatory muscles: 62-78% was SERCA2, 20-37% was SERCA1, and the SERCA3 content was negligible. Depressor muscles showed a significantly higher content (77.8%) of SERCA2, and elevator muscles showed a higher content (35.4%) of SERCA1. Elevator muscles showed higher expression of SERCA2a (58%), and depressor muscles showed higher expression of SERCA2b (20%). The SERCA1 content was mainly SERCA1a and significantly higher for elevator muscles (33%), whereas depressor muscles showed a higher content of SERCA1b (4%). The SERCA1 content of fast-twitch muscle was mainly SERCA1a (88.5%). It is concluded that the mixture of different SERCA isoforms, along with a substantial content of SERCA2b, in masticatory muscles would support lower Ca-ATPase activity and calcium transport.

Histological and histochemical effects after occlusion alteration in suprahyoid muscles.

This study verified the effect of unilateral teeth extraction on the suprahyoid muscles in gerbils (Meriones unguiculatus). Ten adult male gerbils weighing about 50g had induced occlusal alterations by upper molar teeth extraction on the left side while the other ten animals were only subjected to surgical stress, control group. After 60 days, animals of both groups, experimental and control had the suprahyoid muscles removed and processed for histological and histochemical (adenosine triphosphatase (ATPase), nicotine adenine dinucleotide tetrazolium reductase (NADH-TR) and succinate dehydrogenase (SDH)) purposes. The fiber type area was estimated in % according to Weibel method (point-counting method) using a test-system. The myosinic ATPase pH 4.7 activity in the control group of the digastric, milohyoid and geniohyoid muscles presented a small area of type I fiber and a larger area of type IIa fibers; in the experimental group, significant contractile capacity alteration was not observed. Samples of the digastric, milohyoid and geniohyoid muscles, after SDH activity, showed a small area with high metabolic activity fibers, and a large area with intermediary and low metabolic activity fibers in the control group. The milohyoid muscle of the experimental group presented low metabolic fibers in a reduced area, in both sides, however without significant difference. In the experimental group, high metabolic fibers were observed on the left side in a reduced area in the geniohyoid muscle, but without statistical significance. Thus, the geniohyoid muscle did not change the metabolic activity after occlusal alteration. In conclusion, 60 days of unilateral malocclusion induced was able to alter the fibers oxidative activity of the suprahyoid muscles, however, it does not affect the contractile property of the fibers. The digastric muscle has adequate fibers to produce fast contraction and able to resist to fatigue in intermediate degrees, but became more fatigable after unilateral exodontia.

Fiber type composition of the muscle responsible for throat fan extension in green anole lizards.

Throat fan (dewlap) extension is sexually dimorphic in green anole lizards (Anolis carolinensis). Males have larger dewlaps which they display more frequently than females. Correlated with the behavior, sexual dimorphisms occur in the skeletal, muscular and neural structures responsible for dewlap extension in green anoles. We used histochemical techniques to stain for myosin ATPase and succinate dehydrogenase (SDH) to determine whether sex differences also exist in fiber type composition of the ceratohyoideus, the muscle that extends the dewlap. Based on the staining pattern for the two enzymes, four fiber types were identified: fast-oxidative-glycolytic (FOG), fast-glycolytic (FG), slow-oxidative (SO), and tonic. In the ceratohyoideus of both sexes, the predominate fiber types were FOG (approximately 43%) and FG (approximately 34%). Also in both males and females, the FOG and FG fibers had approximately twice the cross-sectional area of the SO and tonic fibers. No sex differences occurred in the percentages of FOG and FG fibers. However, males had a greater percentage of tonic fibers than females, whereas females had a greater percentage of SO fibers than males. The high proportion of FOG fibers in the anole ceratohyoideus makes it similar to other relatively fatigue-resistant muscles used in movements of moderate speed and duration. Although the precise role of tonic fibers in dewlap extension is not known, the greater percentage of these fibers in the male ceratohyoideus might be required to stabilize or maintain extension of the large dewlap apparatus in males.

Efeito do etanol sobre o músculo digástrico de ratos (Rattus norvegicus): uma avaliação histoenzimológica / Effect of ethanol in digastricus mucle in rats (Rattus norvegicus): an histoenzimologic evaluation

Estima-se que entre 1 milhão de adultos do hemisfério ocidental, 20.000 tenham anomalias nos músculos esqueléticos como conseqüência da ingestão de álcool. Estudos revelam que nestes casos ocorre uma fraqueza progressiva afetando predominantemente os músculos da coxa, ombro e quadris, e uma artrofia das fibras do tipo II ou de contração rápida. Na literatura consultada não foi observado relato do que acontece com os músculos da mastigação, por isso este estudo foi desenvolvido com o intuito de observar se as fibras de contração rápida do músculo digástrico apresentam alterações semelhantes às que ocorrem nos músculos do tronco e membros. Para tanto, amostras dos músculos digástrico e reto do abdome foram retiradas de cinco animais de cada grupo Controle (C), Alcoolizado (A) e Isocalórico (I). As amostras foram submetidas às reações de m-ATPase (com pré-incubações ácida e alcalina) e NADH-TR, para a classificação das fibras de acordo com os resultados obtidos nestas reações em FG (fast-twitch-glycolitic, FOG (fast-oxidative-glycolitic) e SO (slow-oxidative). Os resultados mostraram uma diferença significativa, na área das fibras dos tipos FG e FOG, entre os grupos estudados no músculo digástrico e na área das fibras do tipo FG no músculo reto do abdome; uma diferença sigificativa na freqüência das fibras dos tipos FG e FOG no músculo reto do abdome. Baseando-se nestes resultados, é possivel concluir que o etanol produz uma atrofia nas fibras da contração rápida dos músculos da mastigação, semelhante à dos músculos do tronco, mas com um coeficiente de atrofia diferente...

Morphology, histochemistry, and function of epaxial cervical musculature in the horse (Equus caballus).

The semispinalis capitis and splenius muscles of the horse were analyzed for gross morphology, microarchitecture, fiber length, and fiber type. Although these two muscles are similar in size and anatomical position, they are very different from one another in structural design and histochemistry, implying diverse functional roles in the animal's behavior. The histochemical staining profile was limited to two fiber types: slow oxidative and fast glycolytic. The splenius muscle has simple architecture, long fibers, and a 60/40 ratio of SO to FG cross-sectional area. The semispinalis capitis has complex architecture with short-fibered, concentric compartments dorsal to its central tendon and longer-fibered compartments ventrally. The entire dorsal region has an increasing gradient of slow oxidative fiber percentage from caudal to cranial (58-71% SO). In contrast, the ventral region has a decreasing gradient of slow oxidative fibers from caudal to cranial (48-67% FG). These patterns can be interpreted within the context of the cervical musculature during locomotion and posture to indicate the functional advantages of this organization.

Comparative histochemical composition of muscle fibres in a pre- and a postvertebral muscle of the cervical spine.

References to histochemistry are extensive for human limb muscles but occur less frequently in relation to vertebral muscle. Most vertebral muscle literature has been concerned with muscle fibre characteristics in the lumbar and thoracic spine, due in large part to the incidence of low back pain and idiopathic scoliosis. However few studies have investigated the histochemical composition of neck muscles in humans: and, to our knowledge, no previous study has examined the antagonistic longus colli and multifidus muscle pair. In addition, while age-related segmental degeneration is most prominent between C5 and C7, it is not known whether these osteoligamentous changes are paralleled by changes in muscle fibre ratio. Tissue blocks comprising muscle and bone from C5-C7 segments were harvested at autopsy from 16 subjects with ages ranging from 4 to 77 years. The prevertebral longus colli and postvertebral multifidus muscle pairs were randomly selected from one or other side in each subject. The tissue was frozen, sectioned and histochemically stained for myofibrillar adenosine triphosphatase. Analysis of muscle fibre types was performed by light microscopy. Wilcoxon paired t-tests were used to ascertain whether intramuscular and intermuscular differences in fibre composition were significant. In addition, correlation and regression analyses were used to determine whether fibre type proportions changed in either muscle with increasing age. The present study has revealed histochemical differences between longus colli and multifidus at the level of the C5-C7 vertebral segments. Multifidus comprises a significantly greater proportion of type I than type II fibres. Longus colli comprises a significantly greater proportion of type II fibres than multifidus. Further there were no changes in fibre type proportion in either muscle with increasing age. These observations suggest that longus colli responds equally to postural and phasic demands, whereas multifidus is predominantly postural. Also it would appear that age-related structural alterations in lower cervical segments are not paralleled by changes in muscle fibre ratio.

Neuromuscular specializations of the pharyngeal dilator muscles: II. Compartmentalization of the canine genioglossus muscle.

The genioglossus (GG) muscle is divided into horizontal and oblique compartments that are the main protrusor and depressor muscles of the tongue, respectively. In humans the GG plays an important role in speech articulation, swallowing, and inspiratory dilation of the pharynx. At present, little is known about the neuromuscular specializations of the GG in any mammal. This study examined the specializations of these compartments in the canine tongue using a variety of anatomical and histochemical techniques. Six canine GG muscles were sectioned and stained for myofibrillar ATPase to study muscle fiber types; five whole-mount GG muscles were stained for acetylcholinesterase (AChE) to study the distribution of motor endplates; and eight whole mount GG muscles were processed with Sihler's stain to study the entire nerve supply pattern. In addition, the arrangement of muscle fibers of the GG within the tongue was also determined (N = 3). The most notable difference between the compartments of the GG was their proportions of fast and slow twitch muscle fibers: the horizontal compartment contained 64% slow twitch muscle fibers compared to 41% in the oblique compartment. In addition, although the oblique compartment appeared to be grossly homogeneous, it could be divided into thirds by significant differences in the percentages of slow twitch fibers: posterior (23%), middle (15%), and anterior (56%; P < 0.05). The muscle fibers of the oblique GG within the tongue were found to be divided into medial and lateral layers that run vertically and transversely, respectively. The nerve supply to each third of the oblique GG formed a plexus with the anterior third being the densest. The innervation pattern of the oblique GG was also notable as terminal nerve branches coursing parallel to the muscle fascicles gave off perpendicular secondary branches along each motor endplate band. These secondary nerve branches connected the primary nerves and formed a regularly spaced grid throughout the compartment. Evidently, the two compartments of the GG exhibited different anatomical specializations. The horizontal had a slow muscle fiber profile and simple innervation pattern; these qualities are possibly related to its single force vector and respiratory related activity. The oblique compartment had a relatively fast muscle fiber profile with evidence for three separate functional subdivisions. The most anterior part was noticeably different, and was presumably specialized for fine motor control of the tip of the tongue. The vertically oriented fibers of the oblique GG within the tongue body may function as a midline depressor of the tongue, whereas its transversely oriented fibers could play a role in narrowing the tongue during other motor tasks.

Diverse changes in fibre type composition of the human lateral pterygoid and digastric muscles during aging.

The fibre type composition of the superior and inferior portions of the human lateral pterygoid and the anterior and posterior bellies of the digastric muscles of five elderly subjects (mean age 73 years) was studied by morphological and enzyme-histochemical methods. Both muscles showed significant age-related changes in fibre type composition as compared with previous data for young adults. In the lateral pterygoid we observed a large proportion of type IIA fibres, which are rare or absent in young adults, and muscle fibre atrophy and an increased variability in fibre diameter. The digastric muscle of elderly showed a decrease in the proportion of type IIB fibres. The only difference in age-related changes between muscle portions was found in the lateral pterygoid with fibre atrophy in its inferior portion. Both the lateral pterygoid and digastric muscles are known to be active in mandibular depression (jaw opening) and horizontal positioning of the mandible. The present results and previous data from young adults show that the lateral pterygoid and digastric muscles differ not only in fibre type composition, but also in muscular changes following aging. This suggests that, even if they are simultaneously active, they fulfill different, specific tasks in natural jaw function. The differences in age-related changes in fibre type composition between these two muscles indicate that mechanisms underlying their alterations during aging are muscle specific. The results indicate that, although nerve supply and developmental history are essential for fibre composition of skeletal muscles, functional tasks and demands are of major importance.

Neuromuscular specializations of the pharyngeal dilator muscles: I. Compartments of the canine geniohyoid muscle.

BACKGROUND: Little is known about the structure and innervation of the geniohyoid muscle (GH), which is an important pharyngeal dilator muscle activated in swallowing and respiration. METHODS: The neuromuscular specializations of the canine GH were studied in detail by using a combination of histological, histochemical, and anatomical techniques. First, hematoxylin and eosin staining, Gomori's rapid one-step trichrome stain, and silver impregnation were used to determine the terminations of muscle fibers and existence of fibrous septa within the muscle (n = 8). Second, myofibrillar ATPase staining was employed to document the muscle fiber type distribution (n = 8). Finally, Sihler's stain (n = 10) and wholemount acetylcholinesterase staining (n = 8) were used to examine the distribution of the nerve supply within the muscle. RESULTS: The canine GH is divided into rostral and caudal compartments, which are arranged in series and separated by a transverse fibrous septum. Each compartment receives its own primary nerve branch, which supplies a separate motor endplate zone. The rostral compartment is innervated bilaterally, whereas the caudal compartment is innervated ipsilaterally. The rostral compartment was composed of significantly more type I (slow twitch) muscle fibers (56%) than the caudal compartment (25%). CONCLUSIONS: The canine GH is composed of two in-series neuromuscular compartments rather than a single muscle as traditionally believed. This anatomical finding suggests that these two compartments may function independently under different physiological conditions.

Duration of pain and muscular adaptations in patients with dysfunction of the cervical spine.

Biopsies of the sternocleidomastoid and omohyoid muscle were taken from 24 patients who underwent arthrodesis for cervical dysfunction of different etiologies. The two muscles, which are involved differently in movements of the head and cervical spine, were investigated histochemically. Muscle fibers were classified as type I, IIA, IIB, or IIC (transitional fibers) according to the pH lability of myofibrillar ATPase and calculated relative distribution. In both muscles, fiber transformations (as evidenced by an increase in the relative amount of type-IIC fibers) were regularly observed within the first 2 years after the onset of the symptoms. The occurrence of the transformation processes was independent of the patient's age and sex and was the same for the different etiologies. Since the overall fiber composition of the muscles remained essentially unchanged, the fiber transformations must occur alternatingly in both directions (from "slow" to "fast" and the reverse). Muscles of patients with a long case history showed no greater signs of fiber transformation. Therefore, fiber transformations in response to cervical dysfunction occur in the initial stage of the disease and involve different types of muscles. The muscles then return to a "stable" condition, independent of the continuation of the dysfunction and the chronic neck pain.

Intravascular and intramyofibre growth of pericranially invading meningiomas.

In meningiomas extending into pericranial structures, phenomena of invasive growth are observed that have been little known up to now in this onkotype but are of differential diagnostic importance in these locations. 6 out of 10 meningiomas invading the ethmoidal sinus and/or skeletal muscles showed intravasal extension of tumour cells in small vessels of the neighbouring tissue. In 5 out of 7 cases with muscle invasion next to interstitial growth, the spreading of meningioma cells within isolated myofibers could be observed. As shown by enzyme- and immunohistochemical methods, the latter survived for a longer time in spite of active mitotic proliferation and axial spreading of the tumour cells within the sarcoplasm tube. A relation to peculiarities of histology, growth activity and clinical course does not seem to exist.

Metabolic transitions in rat jaw muscles during postnatal development.

The program of acquisition of adult metabolic phenotypes was studied in three jaw muscles in order to determine the link between muscle metabolism and functional development. During early postnatal stages, there were similar transitions in the masseter, anterior digastric, and internal pterygoid muscles with respect to fiber growth, fiber type composition, and whole muscle energy metabolism. Oxidative capacity, as judged by the activities of the enzymes succinate dehydrogenase (SDH), malate dehydrogenase (MDH), and beta-hydroxyacyl CoA dehydrogenase (beta OAC), rose sharply after birth to reach near maximal levels by 3 weeks. The capacities for glycolytic metabolism represented by lactate dehydrogenase (LDH), and for high-energy phosphate metabolism represented by adenylokinase (AK) and creatine kinase (CK) activities, rose gradually, not reaching peak values until 6 weeks or later. Thus, the maturation of oxidative metabolism preceded that of glycolytic metabolism in the developing jaw muscles. This was documented for individual fibers in the masseter muscle. Differential metabolic maturation among the jaw muscles was evident beyond 3 weeks. All three jaw muscles attained their specific adult fiber-type profile by about 6 weeks. This maturation program differed from that of hindlimb muscles [Nemeth et al., J Neurosci 9:2336-2343, 1989] and diaphragm muscle [Kelly et al., J Neurosci 11:1231-1242, 1991], reflecting their differential energy demands for contractile performance.

Quantitative histochemical studies on the cat infrahyoid muscles.

The intrinsic laryngeal muscles have been reported to be composed of muscle fibers that have unusual physiologic, morphologic, and biochemical characteristics, and it has been suggested that the relatively unique differentiation of these muscle fibers is the result of their specific activity patterns. Because the infrahyoid muscles are recruited for some of the same laryngeal functions, it was of interest to determine if they also included unusual fiber types. In order to examine this possibility, microdensitometry was carried out on type I, type IIA, and type IIB fibers in the cat sternohyoid, sternothyroid, and thyrohyoid muscles using histochemical techniques for a variety of enzymatic markers of oxidative and glycolytic capacity and for fiber size. It was found that the infrahyoid muscles were composed of muscle fibers having enzyme profiles generally similar to those of fiber types in the limb muscles.

Structure and function of masticatory muscles in a case of muscular dystrophy.

Histologic examination of muscle biopsies and functional examination comprising electromyography and force measurements in a 19-yr-old boy with muscular dystrophy showed different wasting patterns of mandibular elevator and depressor muscles. Pronounced histopathologic changes were present in the masseter muscle, whereas pathologic findings in the anterior digastric muscle were limited to increased number of cells in slightly enlarged interfiber connective tissue. The masticatory pattern was distorted, and strength of mandibular elevator muscles was less than one third of the norm, whereas depressor strength corresponded more to reference values. This difference of muscular wasting might be caused by protective enzymes in the digastric muscle and/or functionally induced damage of the masseter. As affection from muscular dystrophy may vary greatly between the masticatory muscles, structural and functional examination should be used routinely to clarify prognosis before initiation of treatment procedures.

Actomyosin adenosine triphosphatase activities of the cat infrahyoid muscles.

By use of actomyosin ATPase histochemistry, it was found that there were large differences among the three cat infrahyoid muscles (sternohyoid, sternothyroid, and thyrohyoid) with respect to their percentages of different muscle fiber types. It has been established that the individual activity patterns of the component motor units in each muscle drive the biochemical and physiologic differentiation of the muscle fibers associated with each motor unit. Therefore, the data obtained in the present investigation provide an indication of the characteristics of long-term use of each of the various types of motor units, as well as the associated differences in the physiologic capacities of the different motor unit types composing each of these infrahyoid muscles.

Histochemical classification of neck and limb muscle fibers in a turtle, Pseudemys scripta: a study using microphotometry and cluster analysis techniques.

We have attempted to develop an objective, semiquantitative classification of fiber types in turtle neck and limb muscle using microphotometry and multivariate statistical techniques. We first stained serial sections for myosin adenosine triphosphatase (ATPase) (with acid and alkaline preincubation and without preincubation), NADH-diaphorase, and two glycolysis-associated markers, alpha-glycerophosphate dehydrogenase (alpha-GPDH) and glycogen phosphorylase A (GPA). This allowed us to characterize individual muscle fibers in terms of their contraction speed and metabolic properties. Next we used microphotometry to measure the optical density of the reaction product in each fiber, and we subjected the resulting optical density matrix to cluster and discriminant function analyses in order to assign fibers to groups (fiber types) and to determine which stains contribute most to the distinction between groups. As a control, we processed a well characterized mammalian muscle (rat sternomastoid) simultaneously. Our results suggest that both neck and limb muscle in Pseudemys can best be described as falling into three groups: 1) slow oxidative (SO) fibers; 2) fast oxidative glycolytic (FOG) fibers, with relatively high oxidative and glycolytic capacities; and 3) fast glycolytic (Fg) fibers, with low oxidative, low/intermediate alpha-GPDH, and high GPA activities. These three fiber types differ from like-named types in rat muscle both in the pH lability of their myosins and in their metabolic profiles.

Fiber architecture and histochemistry in the cat neck muscle, biventer cervicis.

1. Biventer cervicis (BC) is an anatomically complex muscle that is divided by tendinous inscriptions into five in-series compartments of motor units. We have analyzed the fiber architecture and fiber-type composition of these different compartments using microdissection and histochemical methods. 2. BC narrows as it runs rostrally, but its in-series compartments have similar cross-sectional areas. The tapered shape of BC comes about because tendinous inscriptions and the tendon of insertion are oriented obliquely and muscle fibers attach in a progressively offset fashion from the medial to the lateral muscle edge. 3. Individual compartments of BC differ from one another in their architecture. The rostral two compartments (1 and 2) contain fibers of similar length that run between two plates of tendinous tissue. Compartments 3 and 4 are divided into two or three in-parallel subvolumes whose fiber bundles differ in their lengths and sites of attachment. Compartment 5 is the most variable in its structure. In some cats it is separated from compartment 4 by a tendinous inscription, but in other cats, it blends with a dorsomedial part of compartment 4 to form a single subvolume. 4. The relative lengths of fibers in different compartments were analyzed when the head and neck were held in different postures. Fibers in rostromedial regions were stretched more effectively when the head was flexed at suboccipital joints, and appeared to be less sensitive to movements at lower cervical joints. Movements across lower cervical joints produced substantial length changes in caudolateral parts of BC. 5. Muscle fibers of different histochemical types were not distributed evenly within each muscle compartment. Slow, oxidative (SO) fibers accounted for the majority of fibers near the nuchal midline but for only 30%-45% of fibers in lateral muscle regions. Proportions of fast, glycolytic (FG) fibers were greatest in lateral regions. Fast, oxidative-glycolytic (FOG) fibers were distributed quite uniformly throughout each compartment. 6. The specialized architecture of BC may shape its physiological capabilities. The complex internal structures of different compartments may alter the length-tension properties of BC.(ABSTRACT TRUNCATED AT 400 WORDS)

Quantitative analysis of cervical musculature in rats: histochemical composition and motor pool organization. I. Muscles of the spinal accessory complex.

In this paper we characterize the architecture and segmental innervation, histochemical composition, muscle spindle populations, and motor pool organization of rat spinal accessory (SA) muscles: sternomastoid (SM), cleidomastoid (CM), cleidotrapezius (CT), and acromiotrapezius (AT). We also consider whether individual rat neck muscles are supplied by more than one population of motor neurons as they are in turtles and cats and whether in SA muscles motor neuron size scales with target muscle fiber type. SM, CM, and CT are ventral, parallel strap muscles. Each can be divided into grossly visible white and red compartments. AT is a dorsolateral sheetlike muscle that shows no gross compartmentalization. All four muscles are dominated by fast-twitch glycolytic (FG) and fast oxidative glycolytic (FOG) fibers, and FG fibers are significantly more numerous than the FOG type in three out of four muscles. Thus SA muscles in rats appear to be specialized for rapid, phasic head movements. Topographical analyses revealed that there is a striking compartmentalization of fiber types in the ventral muscles that corresponds to the red and white segments seen grossly. Spindles are found only in regions containing slow-twitch oxidative (SO) fibers. Cross-muscle comparisons indicate that there are significant differences between SA muscles in their fiber type composition. The motor pools of SA muscles form a single column from lower medulla to C5. Rostral cells lie dorsomedially in the ventral horn and, at the C1/C2 junction, the column shifts ventrolaterally. Within this column, each motor pool occupies a characteristic rostrocaudal position in the order SM:CM:CT:AT. Thus SM and (in part) CM motor neurons lie more medially than cells supplying the trapezius complex, suggesting that they may be under different patterns of synaptic drive. We saw no evidence that rat SA muscles are supplied by more than one population of motor neurons. Direct comparisons between the soma sizes of motor neurons that supply muscles or parts of muscles with significantly different histochemical compositions indicate that these size differences are in the direction predicted from their histochemical profiles, thus suggesting that in these muscles motor neuron soma size may scale with muscle fiber type.