Organization Of Motor Units Research Articles

Motor unit organization in developing muscle

Etude des changements intervenant dans les nerfs moteurs innervant les muscles stries au cours du developpement precoce chez le rat. Analyse chronologique de ces changements et correlation avec la physiologie et le developpement des muscles

Motor unit territories supplied by primary branches of the phrenic nerve.

Recent studies have demonstrated that, under certain circumstances, the diaphragm does not contract as a homogeneous unit. These observations suggest that motor units may not be randomly distributed throughout the muscle but confined to localized subvolumes. In the present study, electromyographic (EMG) and glycogen depletion methods were combined to investigate the organization of motor units supplied by the primary branches of the phrenic nerve in the cat. Four primary branches are generally present, one branch to the crus and three branches to the sternocostal region. The gross motor-unit territory of each of the four phrenic primary branches was determined by stimulating each nerve separately, while recording from nine EMG electrodes distributed over the hemidiaphragm. Stimulation of the crural branch evoked activity in the ipsilateral crus, whereas stimulation of each of the remaining branches evoked activity in discrete but overlapping areas of the sternocostal diaphragm. A more precise analysis of the distribution and borders of the motor territories was obtained by mapping regions depleted of muscle glycogen due to stimulation of each primary branch for 90 min. Glycogen depletion results closely matched the EMG findings of a localized distribution of motor units served by single primary branches. Stimulation of the crural branch typically caused depletion of the ipsilateral crus, whereas the sternocostal branches each served a striplike compartment. In the majority of cases, the borders of the sternocostal compartments were relatively abrupt and consisted of a 1- to 2-mm transition zone of depleted and nondepleted fibers. These studies demonstrate that motor unit territories of the primary branches of the phrenic nerve are highly delineated. This compartmentalization provides the central nervous system with the potential for a more precise regional motor control of costal and crural diaphragm than previously suspected.

The motor-unit population of the cat tenuissimus muscle

1. We studied the organization of motor units in the tenuissimus (TEN) muscle of pentobarbital-anesthetized cats. The cat TEN is a long, delicate straplike muscle that spans hip and knee, which has a very flat length-tension curve through 22 mm of length change. 2. The TEN motor nucleus, labeled by retrograde transport of several forms of horseradish peroxidase, was composed of 8-31 cells in different cats, of which about half were, on average, in the size range of alpha-motoneurons. TEN motoneurons were scattered through the ventrolateral portion of lamina IX, over a rostrocaudal distance of up to 6.5 mm, making it relatively easy to isolate individual TEN motor axons for single motor-unit stimulation. 3. Individual TEN muscle units were classified into four groups [fast-twitch, fatigable (FF), intermediate, fatigue-resistant (Fint), fast-twitch, fatigue-resistant (FR), and slow-twitch, fatigue resistant (S)] on the basis of "sag" and fatigue index mechanical properties, as in other cat hindlimb muscles. There was a relatively large proportion of Fint units (28%) in the TEN sample, and the range of tetanic tension (approximately 19-fold) was much smaller than found in other cat hindlimb muscles. 4. A majority of TEN muscle fibers could be classified into the three major histochemical types (IIB, IIA, and I) found in other cat muscles, but a substantial minority remained "unclassified." A single type Fint muscle unit was successfully depleted of glycogen for histochemical study. It exhibited a typical type IIB histochemical profile. 5. Despite its unusual morphology, the cat TEN contains the same types of motor units found in larger, more "typical" limb muscles.

Properties of motor units in the transversus abdominis muscle of the garter snake.

1. The organization of motor units in the single-fibre-thick transversus abdominis muscle of the garter snake has been studied. This small segmental muscle (60-100 fibres) contains three distinct fibre types (faster twitch, F; slower twitch, S; and tonic, T) which are predominantly arranged in the repeating pattern F, T, S, T, F, T, etc. 2. Motor-unit maps were obtained by activating an individual motor axon and identifying all of the muscle fibres innervated by that axon, using either the activity-induced uptake of extracellular marker molecules to label presynaptic terminals of the stimulated axon, or systematic intracellular recording to identify muscle fibres activated by the axon. 3. Each muscle contained three types of motor units (F, T and S) that corresponded to the three types of fibres. All of the muscle fibres in a motor unit were of the same type. Each segmental muscle contained approximately the same number of motor units: one to two faster twitch, three to four slower twitch, and three tonic. 4. Each motor unit was dispersed widely throughout the muscle. Fibres within a motor unit were neither clustered nor anticlustered. This suggests that despite the fact that axons are constrained to innervate fibres of the appropriate type, the distribution of each motor unit does not generate the alternating pattern of fibre types. 5. In several experiments, all of the twitch motor units in one segmental component of the muscle were mapped. The projection of any one axon appeared random not only with respect to the muscle's fibre type pattern, but also with respect to the innervation supplied by other axons. 6. Twitch motor units were arranged according to a hierarchy of sizes. In each muscle examined, the largest motor unit was faster twitch; a single faster twitch motor axon usually innervated all of the faster twitch fibres in the muscle (fourteen to twenty-four fibres). This was followed by three to four slower twitch motor units which varied in size from eight to ten fibres to very small motor units containing only four to five fibres. 7. Each tonic motor axon innervated an average of ninety-three end-plates per segmental muscle. The relatively large size of tonic motor units compared to twitch motor units is related to the ability of tonic muscle fibres to retain polyneuronal innervation into adulthood, both by providing five to seven end-plate sites per fibre, and by allowing terminal boutons from different tonic motor axons to co-innervate the same end-plate.(ABSTRACT TRUNCATED AT 400 WORDS)

Organization of motor units following cross-reinnervation of antagonistic muscles in the cat hind limb.

Peripheral reorganization of nerve and muscle properties was studied following misdirection of regenerating axons to foreign muscles. The tibial nerve, which innervates all of the distal extensor muscles, was cross-united with the common peroneal nerve, which innervates all of the distal flexor muscles, in one hind limb of seven 2-6 month old cats. After 18-24 months the properties of the motor units in the reinnervated triceps surae muscles were studied by dissection and stimulation of the ventral root filaments. The normal size relationships were re-established in reinnervated medial gastrocnemius (m.g.) and lateral gastrocnemius-soleus (l.g.s.) muscles in so far as motor unit force was directly correlated with the extracellularly recorded amplitude of the axon potential and with contractile speed. The range of sizes of motor units in reinnervated m.g. and l.g.s. muscles was similar to normal, both in terms of the muscle unit size (determined by measuring tetanic tension) and axon size (determined from the amplitude of the extracellularly recorded action potential). The amplitude of the axon potentials in the crossed nerves was weakly correlated with calculated conduction velocities because of the variation in the conduction delays across the suture line. As axon potential amplitude is a direct function of axon size at the recording site, it provided a reliable measure of regenerating axon size. Motor units were classified as slow (s.), fast fatigue resistant (f.r.), fast intermediate (f.i.) or fast fatigable (f.f.) by their contractile speed and fatigue properties. Tetanic tensions generally increased from slow to fast units with s. less than f.r. less than f.i. less than f.f. in reinnervated and normally innervated muscles, but the overlap between different unit types was considerably greater in reinnervated muscles. Thus, peripheral reorganization of nerve and muscle properties according to size can occur in reinnervated muscles, irrespective of the source of the regenerating nerves.

Activity of forelimb motor units and corticomotoneuronal cells during ramp-and-hold torque responses: comparisons with oculomotor cells

Publisher Summary To compare the functional organization of the somatomotor and oculomotor systems, it may be helpful to begin by comparing the response properties of their motoneurons and monosynaptically connected premotor neurons under behavioral conditions, which are as comparable as possible. In the somatomotor system, the activity of single forelimb motor units and input cells from precentral motor cortex, during ramp-and-hold torque responses has been sufficiently characterized to provide some basis for comparison. Such ramp-and-hold responses, alternating between flexion and extension target zones, were chosen to reveal the neural activity related to changes in force and to maintain static force; for purposes of discussion, these ramp-and-hold wrist responses may be considered analogous to saccadic eye movements followed by fixation. This chapter discusses the response properties and organization of somatic motor units and corticomotoneuronal (CM) cells, with brief comments on some interesting analogies with comparable oculomotor neurons. More information on the correlational linkages of the observed cells with their inputs and their targets would be helpful for understanding the source and consequences of their response properties.

Limbsvs. Jaws: Can They Be Compared?

Current experimental research on mammalian limb muscle structure and function is compared to that on mammalian jaw muscles. Two major areas of comparison are stressed: structural and functional. Comparisons of limbs and jaws are made from the point of view of the impact of recent studies on simple mechanical models of limb/jaw muscle function. Limb muscle structure is compared to jaw muscles at the level of muscle architecture, muscle histochemical and motor unit properties, and the organization of motor units into neuromuscular compartments. Such comparisons reveal that although limb muscles and jaw muscles might be organized in similar ways, fundamental differences exist, both in terms of muscle structure and the functional conclusions which have been based on studies of muscle structure. The comparisons also demonstrate that much recent evidence from structural studies have had little direct impact on simple models of muscle function but a much larger influence on the assumptions of the models. Comparisons of limb/jaw muscle function from kinematic and EMG studies, indicate that many masticatory strategies are used by different mammals but the basic problems of posture and locomotion have been met with essentially similar solutions, even among diverse mammalian groups. The results of such comparisons demonstrate that both limb and jaw muscle function are sufficiently complex that new or re-vitalized models are needed if the relationship between structure and function are ever to be understood.

I-cell disease (mucolipidosis II): Differential expression in satellite cells and mature muscle fibers

A well documented case of I-cell disease is presented. Light- and electronmicroscopic studies of muscle revealed marked accumulation of characteristic I-cell inclusions in satellite cells and only scattered autophagic vacuoles in muscle fibers. Correlation with previous tissue culture studies indicated an amelioration of structural abnormalities with differentiation from satellite cell to mature muscle fiber. Histochemically, the muscle demonstrated paucity of type I fibers without evidence of denervation thus suggesting a developmental disturbance in motor unit organization. Selective type I fiber dysfunction and reduced satellite cell regenerative capacity may be related factors in the neuromuscular disability of patients with I-cell disease.

The motor unit. Anatomy and physiology.

The physiological and anatomical properties of mammalian motor units are discussed, and the results of human and animal studies are compared. A physiological organization of motor units based on the mechanical properties of their associated muscle units is examined. It is concluded that such an organizing principle has broad universal application in both animal and human studies. An anatomical organization of muscle units by their association with their physiological properties and the histochemical profile of their muscle fibers is also considered. The anatomical organizing principle also has broad-ranging applicability. The organization of muscle units according to muscle architecture and innervation patterns is described and its potential applicability to considerations of muscle structure and function is discussed. It is concluded that a number of gaps in our knowledge of muscle unit organization have been identified, especially among human studies, but the potential to fill these gaps rapidly is great.

Motor-unit organization in flexor digitorum longus muscle of the cat.

Motor-unit organization in flexor digitorum longus muscle of the cat.R P Dum, R E Burke, M J O'Donovan, J Toop, and J A HodgsonR P Dum, R E Burke, M J O'Donovan, J Toop, and J A HodgsonPublished Online:01 Jun 1982https://doi.org/10.1152/jn.1982.47.6.1108MoreSectionsPDF (3 MB)Download PDF ToolsExport citationAdd to favoritesGet permissionsTrack citations ShareShare onFacebookTwitterLinkedInWeChat Previous Back to Top Next Download PDF FiguresReferencesRelatedInformation Cited ByTonic and Phasic Discharge Patterns in Toe Flexor γ-Motoneurons During Locomotion in the Decerebrate CatP. R. Murphy1 January 2002 | Journal of Neurophysiology, Vol. 87, No. 1Force-Frequency and Fatigue Properties of Motor Units in Muscles That Control Digits of the Human HandAndrew J. Fuglevand, Vaughan G. Macefield, and Brenda Bigland-Ritchie1 April 1999 | Journal of Neurophysiology, Vol. 81, No. 4Organization of Recurrent Inhibition and Facilitation in Motor Nuclei Innervating Ankle Muscles of the CatVladimir V. Turkin, Katrina S. Monroe, and Thomas M. Hamm1 February 1998 | Journal of Neurophysiology, Vol. 79, No. 2Anatomical and physiological study of interneurons in an oligosynaptic cutaneous reflex pathway in the cat hindlimbBrain Research, Vol. 586, No. 2Postnatal development of peroneal motoneurons in the kittenDevelopmental Brain Research, Vol. 54, No. 2Neuronal and muscle unit properties at different rostro-caudal levels of cat's motoneurone poolBrain Research, Vol. 335, No. 1Somatotopic relations between the motor nucleus and its innervated muscle fibers in the cat tibialis anteriorExperimental Neurology, Vol. 86, No. 2Relation between location of a motor unit in the human biceps brachii and its critical firing levels for different tasksExperimental Neurology, Vol. 85, No. 3The development of the spinal motor column in relation to the myotomal muscle fibers in the zebrafish (Brachydanio rerio)Anatomy and Embryology, Vol. 167, No. 1 More from this issue > Volume 47Issue 6June 1982Pages 1108-25 https://doi.org/10.1152/jn.1982.47.6.1108PubMed7108574History Published online 1 June 1982 Published in print 1 June 1982 Metrics

Joint mechanics as a determinant of motor unit organization in man

To support a given weight a muscle which inserts close to the axis of a single joint must develop more force than a muscle which inserts further from the axis. Using available anatomical and histochemical data we have shown an association between the mechanical leverage of a muscle at a joint and the proportion of type II (fast twitch, large tension) muscle fibres. We suggest that the mechanical leverage at joints determines the proportion of fast and slow motor units within a muscle in a teleologically appropriate manner. This determination probably occurs before the differentiation of motorneurones into fast and slow types.

Motor unit organization of human medial gastrocnemius.

1. The properties of fifty-seven motor units in human medial gastrocnemius have been studied using controlled intramuscular microstimulation, glycogen depletion and muscle biopsy. 2. Motor units could be divided into three classes on the basis of their mechanical properties. Type S units were slow, small and fatigue resistant. Type FR units were fast, intermediate in size, and fatigue resistant. Type FF units were fast, large and fatigable. 3. Glycogen depletion of a number of type S and FF units revealed them to be composed of type 1 and type 2b muscle fibres respectively. 4. The results suggest that during slowly increasing voluntary contractions where units are recruited in order of size, type 1 and 2a muscle fibres would be employed at low force levels followed by type 2b muscle fibres in stronger contractions.

Contractile properties and fiber type compositions of flexors and extensors of elbow joint in cat: implications for motor control.

ArticlesContractile properties and fiber type compositions of flexors and extensors of elbow joint in cat: implications for motor controlT. C. Collatos, V. R. Edgerton, J. L. Smith, and B. R. BottermanT. C. Collatos, V. R. Edgerton, J. L. Smith, and B. R. BottermanPublished Online:01 Nov 1977https://doi.org/10.1152/jn.1977.40.6.1292MoreSectionsPDF (2 MB)Download PDF ToolsExport citationAdd to favoritesGet permissionsTrack citations ShareShare onFacebookTwitterLinkedInWeChat Previous Back to Top Next Download PDF FiguresReferencesRelatedInformation Cited ByFiber type composition of epaxial muscles is geared toward facilitating rapid spinal extension in the leaper Galago senegalensis10 January 2018 | American Journal of Physical Anthropology, Vol. 166, No. 1Morphology and Myofiber Composition of Skeletal Musculature of the Forelimb in Young and Aged Wild Type and Myostatin Null MiceRejuvenation Research, Vol. 12, No. 4Comparison of the exercise pressor reflex between forelimb and hindlimb muscles in catsNaoyuki Hayashi, Shawn G. Hayes, and Marc P. Kaufman1 October 2001 | American Journal of Physiology-Regulatory, Integrative and Comparative Physiology, Vol. 281, No. 4Electromyographic activity in fast and slow elbow extensors during a visually guided forelimb movement in catsBrain Research, Vol. 259, No. 1Arm trajectory formation in monkeys3 September 2013 | Experimental Brain Research, Vol. 46, No. 1Mechanical properties of muscles: Implications for motor controlTrends in Neurosciences, Vol. 5Joint mechanics as a determinant of motor unit organization in manMedical Hypotheses, Vol. 6, No. 5Period-amplitude analysis of EMG from slow and fast extensors of cat during locomotion and jumpingElectroencephalography and Clinical Neurophysiology, Vol. 47, No. 5Rhythmic properties of motoneurones innervating muscle fibres of different speed in m. gastrocnemius medialis of the catBrain Research, Vol. 160, No. 1 More from this issue > Volume 40Issue 6November 1977Pages 1292-1300 Copyright & PermissionsCopyright © 1977 the American Physiological Societyhttps://doi.org/10.1152/jn.1977.40.6.1292PubMed925731History Published online 1 November 1977 Published in print 1 November 1977 Metrics

Isometric contractions of motor units in self-reinnervated fast and slow twitch muscles of the cat.

1. Isometric contractions of motor units have been studied in self-reinnervated soleus and flexor digitorum longus muscles, the motor nerves of which had been transected 6 months earlier.2. The distributions of motor unit tetanic tensions in both muscles were skewed towards large values and showed greater variance than normal: there were motor units which produced more tension and others which developed less tension than in control muscles. Mean tensions of motor units are compared with those of the controls.3. The variance of times to peak of motor units in both muscles was significantly smaller than normal.4. Conduction velocities of axons were slower than normal above the point of nerve section. A number of axons did not functionally cross this point and those that did conducted slowly below the neuroma.5. Many of the correlations seen in normal muscle between axon conduction velocity, motor unit tetanic tension, twitch time to peak and ratio of twitch to tetanic tension were demonstrated in reinnervated muscle.6. We suggest that the pattern of motor unit organization in self-reinnervated muscle can in part be due to random regrowth of axons but is largely produced by specific processes related to axon size which are not identical in fast and slow muscle.

Motor innervation, motor unit organization and afferent innervation of m. extensor digitorum communis of the baboon's forearm

1. One hundred and fifty efferent axons innervating m. extensor digitorum communis (EDC) were isolated in filaments of C7 and C8 ventral roots of baboons. Conduction velocities were measured antidromically by stimulating the muscle nerve and recording from the filaments, and fell into two groups: a fast (49-84 m/sec) and a slow (22-41m/sec), presumably fusimotor group. The threshold for these latter axons exceeded the strength needed to elicit the maximal motor twitch.2. Stimulation of ventral root filaments containing slow axons produced no contractile tension in EDC.3. Stimulation of ventral root filaments containing fast-group axons elicited all-or-nothing twitches of motor units of EDC. The twitch tensions of 66.3% of the units were < 2.0 g wt.; only 8.7% were > 5.0 g wt. Tetanus-twitch ratios were 1.4-4.7 in a sample of 14 units. Contraction times were between 15 and 35 msec in 97% of the units. There was no correlation between contractile properties and axonal conduction velocity.4. Afferent volleys from the stimulated EDC nerve were recorded from C6 or C7 dorsal roots. The threshold was below the threshold for a just-detectable motor twitch in ten out of eleven baboons. Conduction velocity of the earliest component of the muscle afferent volley was 67-83 m/sec.5. The conduction velocities of twenty-eight spindle afferents, identified by their responses to linear stretches of EDC and by their unloading by maximal twitches, were all < 70 m/sec. Higher dynamic sensitivity tended to be associated with higher conduction velocity.