Architectural, histochemical, and contractile characteristics of a unique biarticular muscle: the cat semitendinosus

Three-dimensional structure of cat tibialis anterior motor units

The motor unit is the basic unit for force production in a muscle. However, the position and shape of the territory of a motor unit within the muscle have not been defined precisely. The territories of five motor units in the cat tibialis anterior muscle were reconstructed three-dimensionally (3-D) from tracings of the glycogen-depleted fibers belonging to each unit. The motor unit territories did not span the entire length of the muscle and their cross-sectional areas tapered along the proximodistal axis producing a conical shape. In addition, the position of the territory of each unit shifted in an anterior-posterior plane along the longitudinal axis of the muscle, presumably as a consequence of the pinnation of the fibers. The area of the motor unit territory at any given level along the proximodistal axis was highly correlated with the number of fibers within the territory at that level. Connective tissue boundaries (outlining fascicles) appeared to have a strong influence on the shape of the territory, territories showed abrupt changes at connective tissue boundaries as groups of motor unit fibers within a fascicle often terminated together while motor unit fibers in neighboring fascicles did not terminate. It is likely that the mechanical impact of the recruitment of a motor unit is affected by the location and shape of motor units within the same muscle area.(ABSTRACT TRUNCATED AT 250 WORDS)

Lateral rectus EMG and contractile responses elicited by cat abducens motoneurons

Stimulation of 41 single, abducens nucleus motoneurons in the cat evoked electromyographic (EMG) and contractile responses in the ipsilateral lateral rectus muscle. Separate, bipolar, fine wire EMG recording electrodes in the global and orbital muscle layers showed that 22 muscle units were confined to the global layer, 8 to the orbital layer, and 11 units were contained in both ("bilayer") muscle layers. "Bilayer" units demonstrated significantly greater twitch (P < or = 0.002) and maximum tetanic (P < or = 0.001) tensions as well as faster fusion frequencies (P < or = 0.022) than either global or orbital units. "Bilayer" units also showed the lowest average kt values (the slope of the linear relationship between motoneuron stimulation frequency and isometric tetanic tension). "Bilayer" units were predominantly fast fatigable (FF). Global units displayed all muscle unit types including all the nontwitch (NT) units. Orbital units were identified as slow fatigable (SF) and fast fatigue resistant (FR).

Specific tension of elbow flexor and extensor muscles based on magnetic resonance imaging

Series cross-section images of the upper extremity were obtained for four men by magnetic resonance imaging (MRI) and anatomical cross-sectional areas (ACSA) of elbow flexor muscles [biceps brachii (BIC), brachialis (BRA), brachioradialis (BRD)] and extensor muscles [triceps brachii (TRI)] were measured. Physiological cross-sectional area (PCSA) was calculated from the muscle volume and muscle fibre length, the former from the series ACSA and the latter from the muscle length multiplied by previously reported fibre/muscle length ratios. Elbow flexion/extension torque was measured using an isokinetic dynamometer and the force at the tendons was calculated from the torque and moment arms of muscles measured by MRI. Maximal ACSA of TRI was comparable to that of total flexors, while PCSA of TRI was greater by 1.9 times. Within flexors, BRA had the greatest contribution to torque (47%), followed by BIC (34%) and BRD (19%). Specific tension related to the estimated velocity of muscle fibres were similar for elbow flexors and extensors, suggesting that the capacity of tension development is analogous between two muscle groups.

Diverse firing properties of single motor units in the inner and outer portions of the guinea pig anterior digastric muscle

Microwire recordings from the histochemically heterogeneous inner compartment of the guinea pig anterior digastric muscle (ADG) revealed tonic firing of single motor units, which were spontaneously active and could also be recruited following orofacial afferent stimulation and during rhythmic jaw movements (RJM). As units with tonic firing were not observed in the homogeneously fast-twitch outer ADG, the tonic units were classified as slow-twitch motor units. Irregular patterns of motor-unit firing at variable frequencies were observed after orofacial stimulation and during RJM in the outer and inner compartments. The irregular firing pattern of units in the fast-twitch outer compartment was characterized by shorter and less variable bursts than that of units in the heterogeneous inner compartment. A phasic, centrally driven firing pattern was observed during RJM in outer and inner ADG units. The firing frequency of some of these units was modulated during the rhythmical bursts. It is suggested that, as in limb muscles, functionally specialized ADG motor units are recruited in an orderly sequence, starting with spontaneously active, slow-twitch units in the inner compartment, continuing with fast-twitch units recruited upon enhancement of the synaptic drive (as in the case of orofacial stimulation), and ending with massive, rhythmical recruitment of slow- and fast-twitch units during RJM.

Variations in the distribution of motor end-plates in the avian pectoralis

Most avian muscles consist of serially arranged, overlapping fibers that do not extend the length of the muscle. This condition appears to be plesiomorphic with respect to diapsid reptiles. The presence of this serialfibered architecture is evidenced by bands of stained motor end-plates (meps) perpendicular to the columns of fibers and dividing each column into a series of "segments." The avian pectoralis was chosen for a study of variation in the distribution of meps within a single muscle. We report the interspecific variation for 158 specimens in 63 species. We also use additional specimens to examine intraspecific variation. Setting aside hummingbirds, which have an unique and clearly derived condition, the number of mep bands along a column of fibers near the shoulder falls within a remarkably small range. The number of segments is not obviously related to phylogenetic relatedness or to any characteristic of flight or ecology and is only slightly related to size. The largest specimens do average more segments per column, but there are no trends among small to medium-sized species, suggesting that there is an upper limit to fiber length. However, the shape of the sternum and pattern of connective tissue in the pectoralis alleviate the need for additional fibers in many large birds. These findings suggest that the architecture of the avian pectoralis is subject to some as yet unexplained selection that stabilizes the number of myofibers and/or motor neurons. The findings provide few clues as to whether the significant factors are phylogenetic, functional, ontogenetic, or some combination of these. © 1993 Wiley-Liss, Inc.

Architectural design, fiber-type composition, and innervation of the rat rectus abdominis muscle

The rectus abdominis muscle is architecturally compartmentalized by tendinous intersections and is supplied by multiple thoracic nerves. In this study, the rectus abdominis of the rat has been qualitatively and quantitatively examined with regard to muscle dimensions, fiber organization, fiber-type composition, and innervation. The muscle exhibits architectural heterogeneity and different patterns of innervation among its thoracic, epigastric, and hypogastric parts. The epigastric part, adherent to the rectus sheath via tendinous intersections, represents relatively simple design. It is formed by serially arranged compartments with shorter fibers, compared with the other parts. These compartments are segmentally supplied by thoracic nerves. The hypogastric part is more complex, forms an interdigitation of muscular slips, and has segmental distribution of thoracic nerves in mediolateral direction. The thoracic part much differs from the other parts. It has smaller cross-sectional areas, compartments composed of abundant nonspanning fibers with intrafascicular termination, and non-segmental distribution of thoracic nerves. In addition to these craniocaudal specializations among the three parts, the muscle exhibits mediolateral differences in fiber-type composition. Slow-twitch oxidative fibers are more densely distributed in the medial half region than the lateral, whereas fast-twitch glycolytic fibers follow an inverse pattern. The mediolateral differences in fiber-type composition as well as the craniocaudal specializations in architectural design and innervation imply regionally differentiated recruitments of the muscle in various behaviors.

Physiological cross-sectional area of human leg muscles based on magnetic resonance imaging

Magnetic resonance imaging techniques were used to determine the physiological cross-sectional areas (PCSAs) of the major muscles or muscle groups of the lower leg. For 12 healthy subjects, the boundaries of each muscle or muscle group were digitized from images taken at 1-cm intervals along the length of the leg. Muscle volumes were calculated from the summation of each anatomical CSA (ACSA) and the distance between each section. Muscle length was determined as the distance between the most proximal and distal images in which the muscle was visible. The PCSA of each muscle was calculated as muscle volume times the cosine of the angle of fiber pinnation divided by fiber length, where published fiber length:muscle length ratios were used to estimate fiber lengths. The mean volumes of the major plantarflexors were 489, 245, and 140 cm3 for the soleus and medial (MG) and lateral (LG) heads of the gastrocnemius. The mean PCSA of the soleus was 230 cm2, about three and eight times larger than the MG (68 cm2) and LG (28 cm2), respectively. These PCSA values were eight (soleus), four (MG), and three (LG) times larger than their respective maximum ACSA. The major dorsiflexor, the tibialis anterior (TA), had a muscle volume of 143 cm2, a PCSA of 19 cm2, and an ACSA of 9 cm2. With the exception of the soleus, the mean fiber length of all subjects was closely related to muscle volume across muscles. The soleus fibers were unusually short relative to the muscle volume, thus potentiating its force potential.(ABSTRACT TRUNCATED AT 250 WORDS)

Neuromuscular organization of feline anterior sartorius: II. Intramuscular length changes and complex length-tension relationships during stimulation of individual nerve branches

The feline anterior sartorius is a long strap-like muscle composed of short muscle fibers. Nerve branches that enter this muscle contain the axons of motor units whose constituent muscle fibers are distributed asymmetrically within the muscle. In the present study, twitch and tetanic isometric contractions were evoked by stimulating individual nerve branches while muscle force was recorded and intramuscular length changes were monitored optically by the movement of reflective markers on the muscle. Contractions elicited by stimulating the parent nerve produced little change in the positions of the surface markers. Contractions elicited by stimulating the proximally or distally directed nerve branches caused the muscle to shorten at the end closest to the nerve branch and lengthen at the opposite end. Some muscles were supplied by a centrally directed nerve branch whose stimulation produced variable effects: in some cases a portion of the muscle shortened whereas the rest lengthened, but in other cases, the positions of the surface markers showed little change. The intramuscular length changes produced by stimulating single nerve branches were greater during isometric contractions at short whole-muscle lengths than at long whole-muscle lengths. The twitch and tetanic length-tension relationships obtained by stimulating the individual nerve branches were not congruent with the length-tension relationship produced when the parent nerve was stimulated. At short whole-muscle lengths, stimulation of a single nerve branch generated only a small fraction of the force that could be generated by the muscle when the parent nerve was stimulated. As whole-muscle length increased, an increased fraction of total muscle force could be generated by stimulating a single nerve branch. The results suggest that a complex relationship between passive and active elements contributes to the total muscle force and depends on the distribution of active and passive muscle units throughout the muscle.

Quantitative method for comparison of skeletal muscle architectural properties

A numerical method is presented which calculates the architectural difference index between two muscles. This index is based upon the parameters, identified using statistical discriminant analysis, which best characterize the various muscles. This index can be used to reduce multiple architectural properties into a single value and may be useful in selecting donor muscles which are required to perform a substitute function for another muscle which has been lost or injured.

Do muscle fibre size and fibre angulation correlate in pennated human muscles?

Several studies have reported estimations of the total number of fibres in a muscle, e.g. before and after training or before and after inactivity. In those investigations a combination of computed tomographic estimations of muscle size and morphological studies of fibre size has most often been used. There have been doubts about the reliability of those studies on pennate muscles, since changes in muscle fibre size have been said to alter fibre angulation and thus the number of fibres that will cross a section. If such an alteration in fibre angulation takes place with an increase in fibre size, there ought to be some correlation between fibre size and fibre angulation. The present study was designed to test whether repetitive estimations of muscle fibre angulation could be performed in vivo and if any such correlation could be found between fibre size and fibre angulation. A group of 15 women volunteered to take part in the study. Repeated ultrasonographic recordings were made on five subjects on 3 consecutive days to test the repeatability of ultrasonographic measurement of fibre angulation. Both muscle morphological analyses and ultrasonographic measurements of fibre angulation were performed on the other 10 subjects. Ultrasonographic measurement of fibre angulation was found to be reproducible since no variation between measurements made on different days was found. When trying to correlate muscle fibre size to the muscle fibre angulation, measured ultrasonographically, no significant correlation was found.(ABSTRACT TRUNCATED AT 250 WORDS)

Neuromuscular organization of feline anterior sartorius: I. Asymmetric distribution of motor units

The neuromuscular organization of feline anterior sartorius was examined using three experimental approaches. First, the branching pattern of the nerve supplying anterior sartorius was inspected in muscles taken from a large number of feline cadavers. All muscles were found to be supplied by two major nerve branches, one directed proximally and the other directed distally, and most muscles (42/51) had a third distinct branch that entered the muscle centrally. Second, the motoneuronal populations supplying the three nerve branches were investigated by electrophysiological techniques. Motoneurons that supplied axons to the distally-directed branch did not appear to have collaterals in more proximally-located branches. In contrast, other motoneurons supplying the proximally-directed branch also appeared to supply axon collaterals to the centrally-directed branch. This result suggested that the motoneuronal population of the distally-directed branch was largely separate from that supplying the proximally- and centrally-directed branches. Third, the motor unit territories supplied by different nerve branches were mapped using glycogen-depletion methods. Muscle fibers supplied by the distally-directed nerve branch were mostly distributed to the medial portion of anterior sartorius, whereas the fibers supplied by the other two branches were generally found more anteriorly. Further, the muscle fibers supplied by an individual nerve branch were present in greater numbers at the end of the muscle closest to the entry point of that branch. Thus, the motor units supplied by discrete nerve branches were found to be distributed asymmetrically within anterior sartorius, but were arranged neither strictly in-parallel nor strictly in-series.

Functionally complex muscles of the cat hindlimb. I. Patterns of activation across sartorius

The cat sartorius (SA) can be divided functionally into an anterior (SAa), knee extensor portion and a medial (SAm), knee flexor portion; it can be further subdivided anatomically by multiple nerve branches into parallel longitudinal columns that terminate in a distributed insertion at the knee with a continuous range of moment arms. Thus, SA may be controlled by a discrete number of motoneuron task groups reflecting a small number of central command signals or by a continuum of activation patterns associated with a continuum of moment arms. To resolve this question, the activation patterns across the width of the SA were recorded with an electrode array during three kinematically different movements--treadmill locomotion, scratching and paw shaking, in awake, unrestrained cats. Uniformity of activation along the longitudinal axis was also examined because individual muscle fibers do not extend the length of the SA. In addition, the cutaneous reflex responses were recorded throughout all regions of the SA during locomotion. Two fascial surface-patch arrays, each carrying 4-8 pairs of bipolar EMG electrodes, were sutured to the inner surface of the SA, one placed proximally and the other more distally. Each array sampled separate sites across the anterior to medial axis of SA. During locomotion, two basic EMG patterns were observed: the two burst-per-step-cycle pattern typical of SAa and the single burst pattern typical of SAm. There was an abrupt transition in the pattern of activation recorded in the two parts of SA during locomotion, and no continuum in the activation pattern was observed. Stimulation of both sural and saphenous cutaneous nerves during locomotion produced reflex responses that were uniformly distributed throughout SA, in contrast to the regional differences noted during unperturbed walking. Similarly, during scratching and paw shaking all parts of the SA were active simultaneously but with regional differences in EMG amplitude. The abrupt functional border between SAa and SAm coincided with the division of the SA into a knee flexor vs. a knee extensor. In all cases, the quantitative and qualitative differences in SAa and SAm EMGs were uniformly recorded throughout the entire extent of SAa or SAm; i.e., there was no segregation of activity within either SAa or SAm. Furthermore, the time course of EMG from each proximal recording site was nearly identical to the corresponding distal site, indicating no segregation of function along the longitudinal axis of SA. These results indicate that SAa and SAm constitute the smallest functional modules that can be recruited in SA.(ABSTRACT TRUNCATED AT 400 WORDS)

Functionally complex muscles of the cat hindlimb. V. The roles of histochemical fiber-type regionalization and mechanical heterogeneity in differential muscle activation

Several cat hindlimb muscles that exhibit differential activation (activity that is restricted to a specific region of muscle) during natural movements were studied to determine the possible roles of 1) non-uniform distribution of histochemically-identified muscle fiber-types (semitendinosus, ST; tibialis anterior, TA) or 2) mechanical heterogeneity (biceps femoris, BF; tensor fasciae latae, TFL). Using chronic recording techniques, electromyographic (EMG) activity was recorded from multiple sites of each muscle during treadmill locomotion, ear scratch, and paw shake. Standard histochemical analysis was performed on each muscle to determine fiber-type distribution. The histochemically regionalized muscles (ST and TA) were differentially active during slow locomotion; the deep regions (high in type I [SO] fibers) were active, but the superficial regions (high in type IIB [FG] fibers) were inactive. Vigorous movements (fast locomotion, ear scratch, paw shake) produced additional, synchronous activation of the superficial regions. In all movements, ST and TA activation patterns were consistent with the existence of identically timed synaptic inputs to all motoneurons within each motoneuron pool, resulting in an orderly recruitment of each whole pool. The differential activation recorded from ST and TA during slow locomotion was presumably a consequence of the non-uniform distribution of the different muscle fiber types. In contrast, differential activation of the histochemically nonregionalized, mechanically heterogeneous muscles (BF and TFL) resulted from non-synchronous activation of different muscle regions. The selective activation of BF or TFL compartments was indicative of differential synaptic inputs to, and selective recruitment of, subpopulations of the motoneuron pool, with each motoneuron subpopulation exclusively innervating physically separate regions of the muscle consistent with the regions defined by the neuromuscular territories of the major nerve branches supplying each muscle. Individual neuromuscular compartments of BF and TFL differ in their mechanical arrangements to the skeleton and in their contribution to mechanical action(s) at the hip and knee joints. Selective neural activation of mechanically distinct compartments within a mechanically heterogeneous muscle can provide highly advantageous mechanical "options" for animals that perform kinematically diverse movements. With regard to EMG recording techniques, the results of this study emphasize the need for carefully chosen EMG sampling sites and the value of knowing the muscle histochemistry, neuromuscular and musculoskeletal anatomy and possible mechanical functions prior to recording EMG.

Functionally complex muscles of the cat hindlimb. II. Mechanical and architectural heterogenity within the biceps femoris

The goal of this study was to analyze the architecture of the cat biceps femoris (BF), a multifunctional hamstring muscle, and to evaluate the relationships between muscle architecture, limb position, and muscle function during natural movement. The BF muscle consists of three neuromuscular compartments: anterior (BFa), middle (BFm) and posterior (BFp). Each compartment is innervated by a separate nerve branch. Nerve branch stimulation and 2-dimensional surface EMG recordings showed that individual compartment territories were discrete and non-overlapping with well-defined borders. Comparisons of the three compartments revealed consistent differences in architecture, relationship to the skeleton, and function. The BFa crossed only the hip joint and appears to function as a pure hip extensor. The BFm had equal lever arm lengths to the hip and knee joints, appears to function as a hip extensor, and may contribute to knee flexion or femoral rotation. The BFp had a greater lever arm to the knee, functions as a knee flexor, and may contribute to hip extension, femoral rotation or ankle extension. Measurements of individual fascicles from the three compartments revealed a surprising range of lengths, 3.3-12.0 cm. Microdissection of gold-stained tissue showed that fascicles from all compartments were comprised of interdigitated, short fibers (range: 0.6-5.0 cm; average 2.14 cm) arranged in-series in fascicles, running parallel to the origin-insertion axis of each muscle compartment. In regions of fiber interdigitation, the fiber endings were round and tapered (taper lengths: 1-11 mm) although flat, tapering endings like ribbons were occasionally found. As hip and knee joint angles were varied over physiological ranges corresponding to minimal to maximal muscle length, fascicles of the three compartments changed length disproportionately. Long BFa fascicles maximally lengthened 10-18%, consistent with in vivo length measures during treadmill locomotion. However, the long BFp fascicles lengthened 25-45%, and the relatively short fascicles near the BFm/BFp border maximally lengthened 45-53%. How do these unexpectedly large length changes affect sarcomere lengths? Using laser diffraction to measure sarcomeres, static fascicle and sarcomere lengths were compared in muscles that went into rigor mortis after fixing the hip and knee joint angles. Sarcomeres within the short BFm/BFp and long BFp fascicles consistently lengthened proportionately less than the whole fascicle. It remains to be determined how and where the fascicle length changes are dissipated in the connective tissue between the interdigitated muscle fibers and whether such a series-compliance operates during the large excursions over which this muscle normally works.

Regulation of skeletal muscle fiber size, shape and function

There is convincing evidence that the cross-sectional area, the type of myosin expressed, the potential for oxidative phosphorylation and the number of myonuclei of a skeletal muscle fiber are closely interdependent. Each of these variables, as well as the shape of the fiber, has identifiable physiological consequences. Further, it is suggested that the cytoplasmic to myonucleus ratio is a function of the myosin type and the amount and/or rate of protein synthesis and degradation. Although the neuromuscular activity (electromyographic activity) as well as the associated mechanical and metabolic events have significant regulatory influences on protein metabolism, there are other important regulatory factors independent of these activity-related events. Both the activity and non-activity related regulatory mechanisms probably occur via a cascade of cellular events. The specific combinations of cellular responses that occur may define the nature of the modulatory effects on specific proteins. In spite of the complexity of the regulatory mechanisms of protein modulation and how these responses are structurally integrated into or removed from functional fibers, it is suggested that controlled studies of human neuromuscular function can be more accurately defined and interpreted when fiber and muscle size and shape are considered.

Effects of postnatal administration of ethanol on the M. gastrocnemius of the albino mouse

A combined morphometric-histochemical (mATPase) study of the effects of ethanol during postnatal development on the m. gastrocnemius has been performed in the albino mouse. The experimental group received ethanol in the drinking water until sacrifice at the age of 40 days. Based on the fiber composition, three different areas are distinguished in the m. gastrocnemius of the mouse. The typical location of these areas does not change after ethanol administration. However, postnatal administration of ethanol produces a selective atrophy and a decrease of the number of type IIb (fast glycolytic) fibers. Concurrently, the number of type IIa (fast oxidative-glycolytic) fibers increases.

Electromyographic (EMG) amplitude patterns in the proximal and distal compartments of the cat semitendinosus during various motor tasks

The electromyographic (EMG) signals recorded from the proximal (STp) and distal (STd) compartments of the cat semitendinosus muscle (ST) during treadmill running at various speeds, jumping and paw-shaking were quantified to assess the degree of independence of neural control of the two portions of the muscle. Five adult cats were implanted with intramuscular electrodes in the STp and STd. Raw EMG signals were sampled, rectified and a modified form of their running average was used to calculate the mean EMG every 20 ms. EMG amplitudes of each portion of the muscle were plotted and their relative density distributions were generated. The relative density distribution was used to represent a measure of the probability of any two amplitudes occurring simultaneously (i.e. joint probability density distribution). Based on the probability density distributions of the EMG signals from different movements, the patterns of recruitment from the STp and STd were similar. However, during jumping and paw shaking, two relatively vigorous tasks, some deviations in the pattern were apparent. These data, therefore, suggest that the two ends of the ST are subjected to similar, but not identical, control mechanisms during the motor tasks studied.

Skeletal muscle architecture of the rabbit hindlimb: functional implications of muscle design

The muscle-fiber architecture of 29 muscles from six rabbits (Oryctolagus cuniculus) was measured in order to describe the muscular properties of this cursorial animal, which possesses several specific skeletal adaptations. Several muscles were placed into one of four functional groups: hamstrings, quadriceps, dorsiflexors, or plantarflexors, for statistical comparison of properties between groups. Antagonistic groups (i.e., hamstrings vs. quadriceps or dorsiflexors vs. plantarflexors) demonstrated significant differences in fiber length, fiber length/muscle length ratio, muscle mass, pinnation angle, and number of sarcomeres in series (P less than .02). Discriminant analysis permitted characterization of the "typical" muscle belonging to one of the four groups. The quadriceps were characterized by their large pinnation angles and low fiber length/mass ratios, suggesting a design for force production. Conversely, the hamstrings, with small pinnation angles, appeared to be designed to permit large excursions. Similar differences were observed between plantarflexors and dorsiflexors, which have architectural features that suit them for force production and excursion respectively. Although these differences were not absolute, they represented clear morphological distinctions that have functional consequences.

The organization and development of compartmentalized innervation in rat extensor digitorum longus muscle

1. We have examined the innervation of the rat extensor digitorum longus (EDL) muscle by the two extramuscular branches formed from the bifurcation of its muscle nerve. Observations of muscle contractions, recordings of end-plate potentials, and glycogen depletion of young adult muscles show that each branch innervates a separate region or 'compartment' in the muscle. The branch entering the muscle nearer the knee (the K branch) innervates fibres in the anteromedial half of the muscle whereas the branch entering closer to the foot (the F branch) innervates fibres located posterolaterally. Individual EDL motoneurones project either into the K or the F branch and therefore innervate fibres located in one compartment. The boundary between the compartments is usually sharply delineated. No obvious anatomical feature exists within the muscle which would explain the division of the muscle into two distinct regions. 2. The presence of a segmentotopic projection from the spinal cord to the muscle was investigated to evaluate its possible contribution to the compartmental pattern. The most posterior neurones of the EDL motor pool were found to project more frequently to the posterolateral F compartment; similarly, the most anterior neurones most frequently project to the anteromedial K compartment. However, each compartment is innervated by both anteriorly and posteriorly located motoneurones. The segmentotopic projection is too weak to explain the presence of neuromuscular compartments. 3. The post-natal period of synapse elimination appears to play at best a minor role in setting up the compartmentalized innervation. Glycogen depletion and intracellular recording in 1-2-day-old muscles show that each nerve branch innervates fibres in the same region of the muscle as in the adult. Most of the fibres in each compartment are polyneuronally innervated by axons in their own particular nerve branch, although fibres located near the boundary between the two compartments are innervated by axons from both nerve branches. This convergent innervation from the two branches disappears in concert with the elimination of polyneuronal innervation throughout the muscle. A random elimination of these convergent inputs appears adequate to explain the final compartmental pattern. 4. Our findings suggest that the compartmental pattern is primarily the consequence of te segregation of EDL motoneurones into two nerve branches which are directed into separate regions of the muscle.

Organization of segmental input from neck muscles to the external cuneate nucleus of the cat

The musculotopic organisation of projections to the external cuneate nucleus (ECN) from the neck muscles splenius (SP) and biventer cervicis (BC) was examined electrophysiologically. These muscles are divided into a number of serially arranged compartments and are supplied by nerves from different cervical segments. About one-third of ECN neurons receive input from a single nerve. The majority of ECN neurons, however, receive input from more than one nerve in each muscle. ECN neurons are also limited in their ability to follow high frequency nerve stimulation and they frequently exhibit non-linear following. The connections and characteristics of ECN neurons suggest that a minority of neurons in the nucleus have the potential for the faithful transmission of afferent signals, but the majority have the potential to transform incoming patterns of muscle receptor discharge.

Force of knee extensor and flexor muscles and cross-sectional area determined by nuclear magnetic resonance imaging

The maximal strengths of knee extensor (E) and flexor (F) muscles were compared in a group of 6 male subjects aged 24-31 years. Cross-sectional area (CSA) of E and F was evaluated from planimetric measurements of Nuclear Magnetic Resonance (NMR) imaging axial scans, carried out at five levels along the thigh. Maximal CSA for E was found at 2/3 upper femur height and at 1/3 lower femur height for F. Maximum isometric force (MIF) of E was found to be 135% greater than that of F. The maximum CSA of E was found to be 93% larger than CSA of F. The calculated mechanical advantage of the flexors was estimated to be 13.8% higher than that of the knee extensors (0.116 +/- 0.012 and 0.132 +/- 0.005, respectively). However, when MIF of E and F were standardised for their respective CSA, no significant difference was found between their stress: 80.1 +/- 15.5 N.cm-2 for E and 70.5 +/- 7.0 N.cm-2 for F. From the present study, it is concluded that no significant difference exists between the maximum stress of knee extensor and flexor muscles despite large differences in their absolute values of force and CSA and that the NMR imaging technique enables accurate in-vivo determination of the CSA of individual muscles.

Recovery of free muscle grafts in rat: improvement is associated with an increase in cyclic adenosine monophosphate concentration or use of the condition/test paradigm

The muscle fibers in freely grafted skeletal muscles degenerate and are replaced by new fibers which develop within the graft. Myogenesis in regenerating muscle recapitulates, to a large extent, developmental myogenesis and may depend on similar modulating influences. In addition to the generation of new fibers, functional recovery of free muscle grafts also requires reinnervation and revascularization of the new fibers. Recovery of function should be improved by enhancing either myogenesis or reinnervation and revascularization. We have used two procedures, shown previously to stimulate peripheral nerve regeneration, to improve the morphologic and functional recovery of free, orthotopic grafts of rat extensor digitorum longus muscle. Each of the procedures was effective, but had potentially different sites of action. The first procedure, the condition/test paradigm, presumably increases the rate and extent of graft reinnervation. The second procedure, continuous infusion of the adenylate cyclase activator forskolin during the first 21 days after grafting, may influence both myogenesis and nerve regeneration. Each procedure increased regenerating muscle fiber size and functional capacity, and forskolin also significantly increased capillary density and fatigue resistance.

Systematic distribution of muscle fiber types in the medial gastrocnemius of the laboratory mouse: a morphometric analysis

Midbelly cross sections of the medial gastrocnemius muscle of young adult male laboratory mice were subjected to ATPase histochemistry with preincubation at pH 4.6. Through the use of a sampling grid and computer-assisted morphometric analysis, 26 to 35% of the total muscle fibers were sampled and classified as type I, IIa, or IIb. Photomicrographs (16 X 20 in.) of five muscles were divided into octants according to a standardized procedure. Total fiber counts and percent of fibers sampled were determined. Variability of sample size per octant was noted, but when averaged across entire muscles, it was in all instances greater than 33%. Fiber type frequency per octant was tested for goodness of fit to a random model by means of a chi-square statistic for equal expected frequencies. Deviation from random fiber type frequency was significant at the P = 0.001 level for every muscle. More importantly, when these data were pooled and again tested using the same method, the probability estimate was less than P = 0.001. This established that the variations in the fiber type proportions found in each mouse followed a common pattern. The systematic fiber type distribution confirmed by these morphometric and statistical methods supports the impression expressed by many muscle biologists that this muscle displays a consistent and complex intramuscular organization.

Fiber-type composition of selected hindlimb muscles of a primate (cynomolgus monkey)

The distribution of fiber types in selected leg and thigh muscles of three male Cynomolgus monkeys were determined. Almost all fibers could be classified as fast-glycolytic (FG), fast-oxidative glycolytic (FOG), or slow-oxidative (SO) according to the qualitative histochemical staining scheme described by Peter et al. (1972). Most muscles showed regional variations in fiber-type distributions, i.e., the percent SO was higher and the percent FG was lower in the deep, compared to the superficial, regions of the muscle. Exceptions were the soleus and plantaris muscles, which contained similar distributions of fiber types throughout their cross sections. In the extensor compartment of the leg, a layering of fiber types from deep to superficial were evident in the triceps surae and plantaris complex with the deepest muscle, the soleus, having primarily SO fibers. A similar layering arrangement was observed in the extensor compartment of the thigh, with the deepest muscle, the vastus intermedius, having a much larger proportion of SO fibers than the other muscles in the quadriceps complex. These results indicate that Cynomolgus monkey hindlimb muscles, unlike human leg muscles (Saltin and Gollnick: Handbook of Physiology, L.D. Peachey, ed. American Physiological Society, MD, pp. 55-631, 1983) have a regional distribution of fiber types similar to that observed in many subprimate mammals. Further, the presence of compartmentalization of fiber types within the cross section of several of the muscles studied is suggestive of structure-function interrelationships related to motor control.

An anatomical and functional analysis of cat biceps femoris and semitendinosus muscles

The anatomy, architecture, and innervation patterns of the hamstring muscles, biceps femoris, and semitendinosus were examined in adult cats using microdissection and glycogen-depletion techniques. The biceps femoris muscle consists of two heads. The anterior head, which attaches mainly to the femur, is divided into two parts by the extramuscular branches of its nerve. The posterior head is innervated by a single nerve. Semitendinosus is composed of two heads, one proximal and one distal to a tendinosus inscription, each of which is separately innervated. The extramuscular branches of the nerves to these hamstring muscles thus partition them into innervation subvolumes termed parts. The available evidence suggests that each of the parts of these muscles so innervated is not equivalent to the collections of single motor units that have been described for ankle extensors as neuromuscular compartments. It is quite likely that each of the parts of the hamstring muscles may contain more than one neuromuscular compartment. Using chronically implanted EMG electrodes, the activation patterns of different parts of the hamstring muscles were analyzed during locomotion. The anterior and middle parts of biceps femoris are active during the early stance phase, probably producing hip extensor torque. The posterior part of biceps femoris and semi-tendinosus act most consistently as flexors, during the early swing phase, but also may function in synergy with hip, knee, and ankle joint extensors near the time of foot placement. Greater variability is found in the activity patterns of posterior biceps femoris and semitendinosus with respect to the kinematics of the step cycle than is observed for anterior and middle biceps femoris. It is suggested that this variation may reflect a larger role of sensory feedback in shaping the timing of activity in posterior biceps femoris and semitendinosus than in "nonarticular" muscles.