Arrangement of smooth muscle cells and intramuscular septa in the taenia coli

Summary Bands of electron-dense material beneath the cell membrane of smooth muscle cells of the guinea-pig taenia coli provide attachment to thin myofilaments and to intermediate (10 nm) filaments; about 50% of the cell membrane is occupied by dense bands in muscle cells transversely sectioned at the level of their nucleus, and between 50 and 100% in smaller cell profiles nearer the cell's ends. In addition to the known cell-to-cell junctions (intermediate contacts), more complex apparatuses anchor muscle cells together, either end-to-end or end-to-side or side-to-side. They consist of elaborate folds, invaginations and protrusions accompanied by large amounts of basal lamina material. In the end-to-end anchoring apparatuses numerous finger-like and laminar processes from the two cells interdigitate. Other muscle cells have a star-shaped profile in the last few microns of their length, or show longitudinal invaginations occupied by a thickened basal lamina and occasionally by collagen fibrils. The septa of connective tissue extend only for a few hundred microns along the length of the taenia. In taeniae fixed in condition of mild stretch the muscle cells form an angle of about 5° with the septa. In muscles fixed during isotonic contraction the angle increases to about 20–22°, and in longitudinal sections the muscle cells appear arranged in a herring-bone pattern. The collagen concentration in the taenia coli is 4–6 times greater that in skeletal and cardiac muscles. These various structures are discussed in terms of their possible role in the mechanism of force transmission.I thank Mr. S.J. Sarsfield and Miss E.M. Franke for expert technical assistance, and Dr. Adam Yamey for much help in the experiments on collagen content. This work is supported by grants from the Medical Research Council     

Architecture of the hind limb muscles of cats: Functional significance

Force, velocity, and displacement properties of a muscle are determined in large part by its architectural design. The relative effect of muscle architecture on these physiological variables was studied by determining muscle weight, fiber length, average sarcomere length, and approximate angle of pinnation for 24 cat hind limb muscles. Muscle lengths ranged from 28.3 to 144 mm, whereas fiber lengths ranged from 8.4 to 105.5 mm. Generally, fiber to muscle length ratios were similar throughout a muscle. Estimated angles of pinnation of muscle fibers varied from 0 to 21° with most having an angle of less than 10°. The cross-sectional area of the knee extensors was similar to the knee flexors (16.43 vs. 16.83 cm²) whereas the cross-sectional area of the ankle extensors was more than six times greater than the ankle flexors (18.59 vs. 2.83 cm²). There was a 6.7-fold difference in the maximal force between muscles, when normalized to a constant weight, that could be attributed to architectural features. Rations of wet weight to predicted maximal tetanic tension for each muscle and group were calculated to compare the relative priority of muscle force versus muscle length-velocity for a given mass of muscle. These ratios varied from 0.4 to 4.84. The ratios suggest that velocity and/or displacement is a priority for the hamstrings, whereas force is a priority for the quadriceps and lower leg muscles. As much as a 12.6-fold difference in maximal velocity between muscles can be attributed to differences in fiber lengths. This can be compared to approximately a 2.5-fold difference in maximal velocity reported to occur as a result of biochemical (intrinsic) differences.     

江豚鼻道肌的解剖和构筑研究

江豚的鼻部肌共分为后外肌、前外肌、后内肌、前内肌和深肌5层,无间肌和大小内肌较退化,无对角膜肌。通过测定各肌的肌重、平均肌纤维长、平均肌小节长以及肌纤维角度,计算了各肌的生理横截面积,估计最大强直张力和肌鲜重对估计最大强直张力之比值等指标。鼻部肌各肌的相对肌纤维长度相似。各鼻部肌的肌纤维角度均为零。前部肌比后部肌具有较大的收缩速度和收缩位移优势,后部肌则具有较强的张力产生能力。着于额隆和唇部吻肌的张力产生能力很强。     

Neural control of bumblebee fibrillar muscles during shivering

Summary Muscle potentials were recorded extracellularly from the fibrillar flight muscles ofBombus sonorus andB. fervidus during shivering and during flight. During some bouts of shivering motor units of the same muscles, of synergistic muscles, and of antagonistic muscles were excited with relatively synchronous bursts of impulses. These bursts were separated from each other by varying intervals. The latency between the beginning of bursts in different units of antagonistic muscles was usually less than 12 msec. However, during other bouts of less vigorous shivering the synchrony was less precise. During any one portion of flight the interspike intervals of any one muscle unit were relatively constant, and all possible phase relationships were observed between the different muscles. The results show that control of fibrillar muscle involves 1) the average frequency of activation of individual muscles and 2) timing of the activation of muscles with respect to each other.We thank Dr. Robin Thorpe and Dennis Briggs for providing theBombus sonorus. Supported, in part, by NSF grant GB-31542 to the junior author.     

Hypertrophic smooth muscle

The smooth muscle cells of the circular musculature of the guinea pig ileum are connected by gap junctions (nexuses) which occupy about 0.21% of the cell surface. When the muscle hypertrophies in the portions of the ileum oral to an experimental stenosis, the muscle cells increase in size and number. Gap junctions become markedly larger than in control muscles and occupy 0.49% of the cell surface. While the cells double their surface area, the number of nexuses per unit surface remains unchanged (47--48 per 1000 microns2). The packing density of intramembrane particles (or pits) in the nexuses of hypertrophic muscle cells is 6700 . microns-2, which is slightly less than in control muscle cells (7200 . microns-2). A characteristic grouping of the particles (or the pits) within the nexus is often observed. Nexuses between two processes originating from the same cell are common. Nexuses do not occur in the longitudinal muscle.     

Urinary bladder of rat: fine structure of normal and hypertrophic musculature

Summary The fine structure of the muscle of the urinary bladder in female rats is similar to that of other visceral muscles, although it is arranged in bundles of variable length, cross-section and orientation, forming a meshwork. When distended, the musculature is 100–120 m thick, with some variation and occasional discontinuity. Extended areas of cell-to-cell apposition with uniform intercellular space occur between muscle cells, whereas attachment plaques for mechanical coupling are less common than in other visceral muscles. There are no gap junctions between muscle cells. Many bundles of microfilaments and small elastic fibres run between the muscle cells. After chronic partial obstruction of the urethra, the bladder enlarges and is about 15 times heavier, but has the same shape as in controls; the growth is mainly accounted for by muscle hypertrophy. The outer surface of the hypertrophic bladder is increased 6-fold over the controls; the muscle is increased 3-fold in thickness, and is more compact. Mitoses are not found, but there is a massive increase in muscle cell size. There is a modest decrease in percentage volume of mitochondria, an increase in sarcoplasmic reticulum, and no appreciable change in the pattern of myofilaments. Gap junctions between hypertrophic muscle cells are virtually absent. Intramuscular nerve fibres and vesicle-containing varicosities appear as common in the hypertrophic muscle as in controls. There is no infiltration of the muscle by connective tissue and no significant occurrence of muscle cell death.     

Controlled differentiation of myoblast cells into fast and slow muscle fibers

Skeletal muscles are classified into fast and slow muscles, which are characterized by the expression of fast-type myosin heavy chains (fMyHCs) or slow-type myosin heavy chains (sMyHCs), respectively. However, the mechanism of subtype determination during muscle fiber regeneration is unclear. We have analyzed whether the type of muscle is determined in the myoblast cells or is controlled by the environment in which the muscle fibers are formed from myoblast cells. When myoblast cells from 7-day-old chick embryo were cultured and formed into muscle fibers, more than half of the fibers produced only fMyHCs, and the remaining fibers produced both fMyHCs and sMyHCs. However, when myoblast cells were cultured in medium supplemented with a small amount of slow muscle extract, the expression of sMyHCs in muscle fibers increased, whereas the expression of fMyHCs increased in the group supplemented with fast muscle extract compared with the control group. The same results were obtained when cloned mouse myoblast cells (C₂C₁₂ cells) were cultured and formed into muscle fibers. The data presented here thus show that the subtype differentiation of muscle fiber is controlled by the environment in which the muscle fiber forms. This work was funded by the Sasakawa Scientific Research Grant of the Japan Science Society.     

OBLIQUELY STRIATED MUSCLE : IV. Sarcoplasmic Reticulum, Contractile Apparatus, and Endomysium of the Body Muscle of a Polychaete, Glycera, in Relation to Its Speed

Body muscle cells of the bloodworm Glycera, a polychaete annelid, were studied by electron microscopy and compared with muscle cells of the more slowly acting nematode Ascaris, which have been described previously. Both muscles are obliquely striated. The predominant type of bloodworm fiber is characterized by a prominent transversely oriented sarcoplasmic reticulum with numerous dyads at the surface of each cell. Thick myofilaments are ~3 µ long and overlap along ~60% of their length in extended fibers and ~80% in shortened fibers. There is virtually no endomysium and very little intracellular skeleton, and the cells are attached by desmosomes to one another rather than to connective tissue. Dense bodies are absent from the fibers and in their place are Z lines, which are truly linear rather than planar. Scattered among the predominant fibers are others, less orderly in arrangement, in which the SR is much less prominent and in which the thick filaments are thicker and longer and overlap to an even smaller degree. It is suggested that physiological differences between bloodworm and Ascaris muscles derive from differences in the proportion of series to parallel linkages between the contractile elements, differences in the amount and disposition of the SR, and differences in the impedance to shear within the myofibrils.     

Parasternal and external intercostal muscle shortening during eupneic breathing

The interosseous external intercostal (EI) muscles of the upper rib cage are electrically active during inspiration, but the mechanical consequence of their activation is unclear. In 16 anesthetized dogs, we simultaneously measured EI (3rd and 4th interspaces) and parasternal intercostal (PA) (3rd interspace) electromyogram and length. Muscle length was measured by sonomicrometry and expressed as a percentage of resting length (%LR). During resting breathing, each muscle was electrically active and shortened to a similar extent. Sequential EI muscle denervation (3rd and 4th interspaces) followed by PA denervation (3rd interspace) demonstrated significant reductions in the degree of inspiratory shortening for each muscle. Mean EI muscle shortening of the third and fourth interspaces decreased from -3.4 +/- 0.5 and -3.0 +/- 0.4% LR (SE) under control conditions to -0.2 +/- 0.2 and -0.8 +/- 0.3% LR, respectively, after selective denervation of each of these muscles (P less than 0.001 for each). After selective denervation of the PA muscle, its shortening decreased from -3.5 +/- 0.3 to +0.6% LR (SE) (P less than 0.001). PA muscle denervation also caused the EI muscle in the third interspace to change from inspiratory shortening of -0.2% to inspiratory lengthening of +0.2% +/- 0.2 (P less than 0.05). We conclude that during eupneic breathing 1) the EI muscles of the upper rib cage, like the PA muscles, are inspiratory agonists and actively contribute to rib cage expansion and 2) PA muscle contraction contributes to EI muscle shortening.     

Mechanical function of hyoid muscles during spontaneous breathing in cats

We assessed the mechanical behavior of the geniohyoid and sternohyoid muscles during spontaneous breathing using sonomicrometry in anesthetized cats. When the animals breathed O2, the hyoid muscles either became longer or did not change length (but never shortened) during inspiration. During progressive hyperoxic hypercapnia, transient increases in geniohyoid muscle inspiratory lengthening occurred in many animals; however, at high PCO2 the geniohyoid invariably shortened during inspiration (mean 4.9% of resting length at the end of CO2 rebreathing; P less than 0.001). The PCO2 at which geniohyoid inspiratory lengthening changed to inspiratory shortening was significantly higher than the CO2 threshold for the onset of geniohyoid electrical activity (P less than 0.01). For the sternohyoid muscle, hypercapnia caused inspiratory lengthening in 13 of 17 cats and inspiratory shortening in 4 of 17 cats; on average the sternohyoid lengthened by 1.6% of resting length at the end of CO2 rebreathing (P less than 0.01). Sternohyoid lengthening occurred in spite of this muscle being electrically active. These results suggest that the relationship between hyoid muscle electrical activity and respiratory changes in length is very complex, so that the presence of hyoid muscle electrical activity does not necessarily indicate muscle shortening, and among the geniohyoid and sternohyoid muscles, the geniohyoid has a primary role as a hypopharyngeal dilator in the spontaneously breathing cat, with the sternohyoid muscle acting in an accessory capacity.     

The number of sarcomeres and architecture of the M. gastrocnemius of the rat

The aim of this study was to investigate the number of sarcomeres of different regions (proximal, intermediate and distal third) of the M. gastrocnemius of the rat and compare them with in vivo measurements of the length of the most proximal and distal muscle bundles. These lengths were measured with the aid of dividers at the muscle resting length. The number of sarcomeres was calculated from the length of fibres (measured at 20 times enlargement) tested from HNO3-treated muscle and the average sarcomere length (determined from 80 microns samples taken along the fibres every 800 microns). Ten fibres were isolated from each of three regions of six muscles. All muscles showed the smallest number of sarcomeres in the proximal region of the muscle and increasingly higher numbers in the intermediate and distal parts. The number of sarcomeres in the proximal region is significantly (p less than 0.01) smaller than that of the distal region. These results agree with the results of in vivo length measurements of the most proximal and distal bundles (resp. 31 and 36% of the muscle resting length), the former being significantly (p less than .01) smaller. As there is no significant difference (p less than 0.01) in the length of the treated fibres of the three regions it is concluded that HNO3 treatment does affect the fibres of the muscle in the different regions in a non uniform fashion.     

Fine structural study of the abdominal muscle receptor organs of the crayfish (Procambarus clarkii). Fast and slow receptor muscles

Receptor muscles of the abdominal muscle receptor organs of the crayfish, Procambarus clarkii, were examined by electron microscopy. Both the fast and the slow receptor strand comprises a single muscle fibre which is divided by invagination of the cell membrane into numerous cytoplasmic processes in its intermediate region (the so-called intercalated tendon). Most of these myofibrillar processes insert in this region, but some of them pass through the intermediate region without interruption and join the other portion of the fibre. Thus the receptor muscles, whilst maintaining cytoplasmic continuity throughout their whole length, are modified in their intermediate regions, becoming fasciculated and providing spaces which are occupied by the connective tissue and the dendrites of the sensory neurone. Clear-cut differences in fine structure are shown between the muscle of the two types of receptor unit. The fast receptor muscle shows the typical features of arthropod fast muscles, including short sarcomere length (on average 3.3 μm), cylindrical myofibrils, well-developed sarcoplasmic reticulum, and regular hexagonal array of the myofilaments. By contrast, the slow receptor muscle fibre is characterized by long sarcomeres (average 6.5 μm) and unique organization of the myofilaments, with very thick ‘thick’ filaments having diameters in the range of 25–36 nm surrounded by about 12 thin filaments.     

Force deficit during the onset of muscle hypertrophy

The purpose was to study selected structural changes associated with the deficit in maximum specific force (N/cm2) during the early development of skeletal muscle hypertrophy. Ablation of gastrocnemius and plantaris muscles was performed bilaterally in 35-day-old rats (n = 41), and the soleus muscle was studied from days 1 to 30 thereafter. Compared with control muscles from age-matched unoperated rats (n = 48), muscle mass and cross-sectional area increased in parallel from 28 to 52% over the 30-day postoperative period. Specific force of hypertrophied muscle was depressed 38% at days 1 and 3, and by 28% from days 5 to 30 after synergistic muscle ablation compared with age-matched control values. Interstitial space was 38% greater than the control value of 20.4 +/- 1 microliters/100 mg at day 1 only. Protein concentration was depressed 15% for 7 days after the ablation operation, and connective tissue protein concentration was unchanged. The relative magnitude of increased mean fiber cross-sectional area was less than that of muscle mass until day 7 after ablation. Mononuclear cell infiltration in interfascicular spaces occurred from days 3 to 30 without light microscopic evidence of muscle fiber injury. Initial functional deficits are explained in part by an enlarged interstitial space and decreased protein concentration; later deficits are likely accounted for by intracellular changes.     

Effect of collagenase on mechanical activity and fine structure of an intestinal smooth muscle

Summary The effect of the enzyme collagenase (40–200 units · ml^-1) on the spontaneous mechanical activity in vitro and on the fine structure of the taenia coli of the guinea-pig was investigated. Initially, the spontaneous activity of the taenia was enhanced both in the isometric and isotonic recordings; after several minutes the muscles became slack or elongated to up to twice their resting lengths. The structural changes were dramatic but a number of muscle cells remained apparently unaltered even with the highest concentration and the longest incubation time (120 minutes). The large variety of structural changes were tentatively grouped into two separate sequences. One sequence involved swelling of the muscle cell, dispersion of the filaments and breaking up of the cell membrane: the thick myofilaments increased considerably in size and became heterogeneous in size and shape, but were still recognizable after disruption of the cell membrane. The other disruptive sequence involved separation of the superficial part of the muscle cell, which became electron-lucent, from the core of the cell where filaments were very densely packed. Few or no changes were observed in non-muscle cells.Supported by grants from the Medical Research Council and the Central Research Fund of the University of LondonFinancial support from the F.W.G.O. (Grant n 20.487) is gratefully acknowledged     

A new genus and species of heteronemertean from the Changjiang (Yangtze) River Estuary

A new genus and species of heteronemertean, Yinia pratensis gen. nov. and sp. nov., collected from low salinity waters (salinity 0.2–0.4 ‰) at Changjiang River Estuary, is described and illustrated. The species possesses a proboscis with an outer circular and an inner longitudinal muscle layer, and is placed in family Lineidae sensu Gibson. The following combination of morphological features distinguishes the new species from any other genera in this family: proboscis with two muscle crosses; dermis without connective tissue layer between gland cells and body wall outer longitudinal muscle layer; rhynchocoel wall circular muscles not interweaving with adjacent body wall longitudinal muscles; foregut with circular somatic muscles and subepithelial gland cell layer; neurochord cells present in central nervous system; caudal cirrus missing; blood system developed into alimentary plexus extending almost the full length of the body. Another significant character is that the lobular excretory cells are extremely well developed which may represent adaptation to water of low salinity. This revised version was published online in July 2006 with corrections to the Cover Date.     

Changes in the X-ray diffraction pattern from rigor muscles by application of external length changes

The effect of external force on the X-ray pattern from frog muscles in rigor was studied by a time-resolved diffraction technique. When sinusoidal length changes (1.5–3% of the muscle length, 5Hz) were applied to the muscle, the 14.3 nm intensity decreased during the releasing phase and increased during the stretching phase. The intensity ratio of the equatorial 1,0 and 1,1 reflections did not change, nor were there any appreciable intensity changes in the 5.9 nm and 5.1 nm reflections during the length change. Experiments were also done with the relaxed muscles and no change was seen in any reflection, indicating that the rigor linkages are needed to produce the 14.3 nm intensity change. Thus the distinct effect of the length change was detected only on the 14.3 nm reflection. These results suggest no large conformational changes are induced in both the distal part of the myosin head attached to actin and the actin filament during the oscillation. It is therefore most probable that the proximal portion of myosin heads including S-2 contributes to the intensity change in response to the length change (see, also ref.21). When the muscle was stretched beyond the filament overlap, the 14.3 nm intensity change was suppressed to less than 50% of that of the slack length. It was also found that the tension change delayed the intensity change during the length oscillation. However, this delay of the tension change as observed in the muscle at the slack length was lacking in the overstretched muscle, indicating that the 14.3 nm intensity change may arise partly from a portion other than the crossbridges.     

Cell proliferation in skeletal muscle following denervation or tenotomy

Summary Autoradiographic experiments using ³H-thymidine were designed to analyse cell proliferation which occurs in skeletal muscle after denervation and after tenotomy. In mouse tibialis anterior and tongue muscles during the first 24 h after denervation or tenotomy labelling levels were low and did not differ significantly from sham operated control muscles. By 48 h after denervation and tenotomy of tibialis anterior muscles, increased levels of labelling occurred in both muscle and connective tissue nuclei. Daily pulse labelling for 7 days after denervation produced a labelling level which was 8 times that of sham operated controls, 25–30% of the total nuclear population being labelled. Denervated muscles had twice the level of labelling compared to tenotomised muscles. These results provide conclusive evidence that both denervation and tenotomy stimulate cell proliferation in skeletal muscle and it is suggested that the increased numbers of labelled muscle nuclei are likely to be the result of mitotic activity in muscle satellite cells.     

Musculature and innervation of the neck of the desert locust,Schistocerca gregaria (Forskål)

The innervation of each of the muscles involved in mediating head movement in the desert locust Schistocerca gregaria is described in detail. The number of motor neurones to each muscle and the neutral pathway and ganglion of origin of each are deduced from both histological and electrophysiological evidence. Only two of the muscles are, on histological evidence, innervated by as few as four different neurones, while several receive more than ten, and one at least 13. Individual muscles are shown physiologically to receive, in a few cases, as many as six different motor neurones. At least six muscles are innervated by motor neurones originating in more than one ganglion. One group of four muscles consisting in total of less than 100 muscle fibres receives more than 20 different motor neurones from three different ganglia through three or four different nerve roots. In these muscles, many single muscle fibres receive innervation from at least two different ganglia. It is concluded that the segmental nature of an insect muscle can not be deduced solely from a knowledge of the ganglion of origin of the motor innervation to that muscle. The innervation patterns that exist today must reflect past evolutionary development, but changes in the peripheral distribution of motor neurones, or migration of motor neurone cell bodies from one ganglion to another, or the development of additional motor neurones, or several of these factors together, must have formed a part of that development.     

The fine structure of the somatic muscles and their attachment to the cuticle in two species of Pycnogonida

The fine structure of the somatic muscles and their attachment to the cuticle in the pyenogonids Nymphon (Chaetonymphon) macronyx G. O. Sars and Boreonymphon cf. abyssorum (Norman) is described. The muscles possess characteristics which are typical of arthropod slow muscle fibers: relatively long sarcomeres, a mean A-band length of about 6 μm and a ratio of thin to thick contractile filaments of 4:1. The sarcotubular system consists of distinct t-tubules, an irregular SR part and randomly distributed dyads and triads, the muscles are attached to the cuticle by specialized epidermal cells containing microtubules extending from the cuticular to the muscular side. The myoepidermal and epidermal-cuticular junctions are described.     

Satellite cell proliferation is reduced in muscles of obese Zucker rats but restored with loading

The obese Zucker rat (OZR) is a model of metabolic syndrome, which has lower skeletal muscle size than the lean Zucker rat (LZR). Because satellite cells are essential for postnatal muscle growth, this study was designed to determine whether reduced satellite cell proliferation contributes to reduced skeletal mass in OZR vs. LZR. Satellite cell proliferation was determined by a constant-release 5-bromo-2-deoxyuridine (BrdU) pellet that was placed subcutaneously in each animal. Satellite cell proliferation, as determined by BrdU incorporation, was significantly attenuated in control soleus and plantaris muscles of the OZR compared with that shown in the LZR. To determine whether this attenuation of satellite cell activity could be rescued in OZR muscles, soleus and gastrocnemius muscles were denervated, placing a compensatory load on the plantaris muscle. In the LZR and the OZR after 21 days of loading, increases of approximately 25% and approximately 30%, respectively, were shown in plantaris muscle wet weight compared with that shown in the contralateral control muscle. The number of BrdU-positive nuclei increased similarly in loaded plantaris muscles from LZR and OZR. Myogenin, MyoD, and Akt protein expressions were lower in control muscles of OZR than in those of the LZR, but they were all elevated to similar levels in the loaded plantaris muscles of OZR and LZR. These data indicate that metabolic syndrome may reduce satellite cell proliferation, and this may be a factor that contributes to the reduced mass in control muscles of OZR; however, satellite cell proliferation can be restored with compensatory loading in OZR.