The specificity of motor innervation of the chick wing does not depend upon the segmental origin of muscles

In vertebrate embryos, motor axons originating from a particular craniocaudal position in the neural tube innervate limb muscles derived from myoblasts of the same segmental level. We have investigated whether this relationship is important for the formation of specific nerve-muscle connections, by altering the segmental origin of muscles and examining their resulting innervation. First, by grafting quail wing somites to a new craniocaudal position opposite the chick wing, we established that the segmental origin of a muscle can be altered: presumptive muscle cells migrated according to their new, rather than their original, somitic level, colonizing a different subset of muscles. However, after reversal of a length of brachial somitic mesoderm along the craniocaudal axis, or exchange or shift of brachial somites, the craniocaudal position of wing muscle motoneurone pools within the spinal cord was undisturbed, despite the new segmental origin of the muscles themselves. While not excluding the possibility that muscles and their motor nerves are labelled segmentally, we conclude that specific motor axon guidance in the wing does not depend upon the existence of such labels.     

Peripheral specification of Ia synaptic input to motoneurons innervating foreign target muscles

Muscle sensory neurons, called Ia afferents, make monosynaptic connections with functionally related sets of motoneurons in the spinal cord. Previous work has suggested that peripheral target muscles play a major role in determining the central connections of Ia afferents with motoneurons. Here, we ask whether motoneurons can also be influenced by their target muscles in terms of the monosynaptic input they receive from Ia afferents, by transplanting thoracic motoneurons into the lumbosacral spinal cord so that they innervate foreign muscles. Three or four segments of thoracic neural tube from stage 14-15 chicken embryos were transplanted to the lumbosacral region of stage 16-17 embryos, and electrophysiological recordings were made from transplanted motoneurons after the embryos had reached stage 38-40. Transplanted thoracic motoneurons innervated limb muscles and received monosynaptic inputs from Ia afferents. These connections were not random: Most of the connections were formed between Ia afferents and motoneurons projecting to the same muscle (homonymous connections). Few aberrant connections were found although the anatomical distribution of afferents in the transplant indicated that they had ample opportunity to contact inappropriate motoneurons. We conclude that although peripheral target cues are not sufficient to respecify an already committed motoneuron (turn a thoracic motoneuron into a lumbosacral motoneuron), they do provide sufficient information for Ia afferent input to be functionally correct.     

Reformation of specific neuromuscular connections during axolotl limb regeneration: evidence that the first contacts are correct

Retrograde neuronal tracing with horseradish peroxidase was used to determine the position in the spinal cord of the motor neurone pools of a proximal (biceps) and a distal (extensor digitorum) limb muscle at various times during axolotl limb regeneration. It was found that from the earliest stages of muscle redifferentiation (as judged by light and electron microscopic analysis) the vast majority of axons innervating the regenerating muscles came from cells within the bounds of the normal motor neurone pool for each muscle. A few incorrect projections were noted in that the regenerating proximal muscle was sometimes innervated by some cells caudal to its normal motor neurone pool. The results are discussed in terms of mechanisms that may be operating in the regenerating limb to ensure that specific neuromuscular connections are made.     

Reformation of the pattern of neuromuscular connections in the regenerated axolotl hindlimb

Retrograde neuronal tracing with horseradish peroxidase (HRP) was used to determine the position in the spinal cord of motor neurone pools innervating muscles in the regenerated axolotl hindlimb. This method allows a detailed analysis of the accuracy of reformation of neuromuscular connections. The results show that regenerated distal limb muscles are reinnervated by motor neurones in the same region of the cord as those that innervate normal control distal limb muscles but that proximal muscles are innervated by a mixture of motor neurones in a normal position and motor neurones in a region of the spinal cord that normally supplies innervation to distal limb muscles. This difference between the reinnervation of proximal and distal limb muscles suggests that axons destined for proximal muscles may not enter distal limb territory during reinnervation of the regenerated limb.     

Facilitation between extensor carpi radialis and pronator teres in humans: A study using a post-stimulus time histogram method

Group I muscle afferents modulate the excitability of motor neurons through excitatory and inhibitory spinal reflexes. Spinal reflex relationships between various muscle pairs are well described in experimental animals but not in the human upper limb, which exhibits a fine control of movement. In the present study, spinal reflexes between the extensor carpi radialis (ECR) and pronator teres (PT) muscles were examined in healthy human subjects using a post-stimulus time histogram method. Electrical stimulation of low-threshold afferents of ECR nerves increased the motor neuron excitability in 31 of 76 PT motor units (MUs) in all eight subjects tested, while stimulation of low-threshold afferents of PT nerves increased the motor neuron excitability in 36 of 102 ECR MUs in all 10 subjects. The estimated central synaptic delay was almost equivalent to that of homonymous facilitation. Mechanical stimulation (MS) of ECR facilitated 16 of 30 PT MUs in all five subjects tested, while MS of PT facilitated 17 of 30 ECR MUs in all six subjects. These results suggest excitatory reflex (facilitation) between PT and ECR. Group I afferents should mediate the facilitation through a monosynaptic path.     

Oro-facial muscles: internal structure,function and ageing

Structure and function are reviewed in the masticatory muscles and in the muscles of the lower face and tongue. The enormous strength of jaw closure is in large part due to the pinnated arrangement of the muscle fibres in the masseter. This muscle, like other masticatory muscles, is unusual in that the cell bodies of the muscle spindle afferents lie in the brain stem rather than in an external ganglion; spindles are absent in the lower facial muscles. Although few data are available, the numbers of motor units in the masticatory muscles, and probably in the lower facial muscles also, appear to he much greater than in limb muscles. The motor units in the facial and tongue muscles are largely composed of histochemical type II (‘fast-twitch’) fibres, but in the masticatory muscles there are substantial numbers of fibres intermediate between type I (‘slow twitch’) and type II, and fibre type grouping is present. In comparison with limb muscles, there is little information on ageing changes in oro-facial muscles. The masticatory muscles do, however, show some atrophy and loss of X-ray density, while motor unit twitches are prolonged. Strength is reduced in the tongue and masticatory muscles. It is known that limb muscle properties are largely governed by their innervation, both through the pattern and amount of impulse activity, and the delivery of trophic messengers; the situation for oro-facial muscles is unclear. The structural and functional differences between the two types of muscle indicate the need for conducting ageing studies on the oro-facial muscles, rather than relying on extrapolations from limb muscles.     

Reflex modulation of motoneurone activity in the cheliped of the crayfish Astacus leptodactylus.

1. The reflex activity elicited by movement of the mero-carpopodite (M-C) joint in the cheliped of the crayfish Astacus leptodactylus is investigated and the role of the different proprioceptors (chordotonal and myochordotonal organs) separately studied. 2. The reflex discharge involves mainly the tonic motoneurones of the extensor (E), the flexor (F) and the accessory flexor (AF) muscles. 3. M-C joint posture is also regulated by the cuticular stress detector (CSD2) afferents: they increase mainly the F discharge and secondarily the AF command. 4. The activity of the motor axons supplying the muscles of the meropodite can be also influenced by a variety of natural stimuli applied to other appendages. The effect usually produced is a general flexion reaction which is characterized by a reciprocity between E and F involving both central and peripheral mechanisms. 5. The AF muscle is innervated by two antagonistic motoneurones, an excitatory neurone functionally linked in its discharge with one of the four excitors supplying F and an inhibitory motoneurone, common with E. The resulting competitive effect between these two neurones has been recorded intracellularly in AF muscle fibres. 6. The role of the myochordotonal organ (MCO) in the crayfish is discussed. In particular the modulation of the AF command in relation to the discharges of the motor nerves to the main muscle E and F is studied.     

A horseradish peroxidase study of motorneuron pools of the forelimb and hindlimb musculature of the axolotl

Motorneuron pools innervating axolotl limb muscles have been investigated by using the retrograde neuronal tracer horseradish peroxidase. Four muscles in the forelimb (biceps, anconeus, flexor digitorum and extensor digitorum) and four functionally equivalent muscles in the hindlimb (puboischiotibialis, iliotibialis, flexor digitorum and extensor digitorum) were studied. Motorneuron pools were characterized by using four criteria: position in the rostrocaudal axis; position of the median in the rostrocaudal axis; number of labelled cells; position of cells in the transverse plane of the spinal cord. Each pool was uniquely defined by the four characteristics, although overlap was found between pools. Two types of motorneuron were found in each pool, distinguished on the basis of size, shape and position in the spinal cord. The first type constituted the majority of cells in a pool, and occupied different positions in the transverse plane for each muscle. The second type was less common and always occupied a characteristic medial ventral position. These data will allow an assay of correct or incorrect innervation in experiments on the regeneration of specific neuromuscular connections in these animals.     

Different fusimotor reflexes from the ipsi- and contralateral hind limbs of the cat assessed in the same primary muscle spindle afferents

Experiments were performed in forty-five cats anaesthetized with alpha-chloralose. The aim of the study was to investigate a sample of primary muscle spindle afferents from triceps muscle with respect to their fusimotor reflex control from ipsi- as well as contralateral hind limb. Primary muscle spindle afferents of the triceps surae muscle were recorded from the mean rate of firing and the modulation of the afferent response to sinusoidal stretching of the triceps surae muscle was determined. Test measurements were made during tonic stretch of the ipsilateral PBSt, contralateral PBSt, contralateral triceps muscle or during extension of the intact contralateral hind limb. Control measurements were made with ipsi- and contralateral PBSt as well as contralateral triceps muscles relaxed and with contralateral hind limb in resting position. The occurrence and types of fusimotor effects were assessed by comparing test to control responses. The main finding of the present investigation was the great variability in type and size of the fusimotor effects evoked by different ipsi- and contralateral reflex stimuli. Both ipsi- and contralateral stimulations gave rise to predominantly dynamic, predominantly static or mixed static and dynamic fusimotor reflexes. In the same preparation, a given reflex stimulus often caused different reflex responses in different triceps surae primary spindle afferents. In the same afferent unit, different reflex stimuli usually produced fusimotor effects which differed from each other in type and/or size. In general, contralateral whole limb extension and stretch of contralateral PBSt muscles were more potent as reflex stimuli than stretch of the ipsilateral PBSt muscle. Stretch of the contralateral triceps surae muscle was, but for a few afferent units, ineffective as reflexogenic stimulus. It is concluded that the individualized receptive profiles of the primary muscle spindle afferents, which have been postulated in earlier investigations where the effects of different stimuli have been investigated on different cell populations, still seems to hold good when the stimuli are tested on the same units. The individuality of the receptive profiles of gamma-motoneurones is discussed in relation to different motor control hypotheses.     

Distribution of motoneurones innervating extraocular muscles in the brain of the marmoset (Callithrix jacchus)

Extraocular muscle motoneurones were localised in the oculomotor nucleus (ON), trochlear nucleus (TN) and abducens nucleus (AN) in the marmoset brain using the horseradish peroxidase (HRP) retrograde labelling technique. HRP pellets injected into individual extraocular muscles revealed one or more groups of labelled neurones occupying discrete loci within these nuclei. Relatively little overlap of motoneurone pools was observed, except in the case of the inferior oblique and superior rectus muscles. Injections of HRP into the medial rectus muscle revealed three separate populations of labelled cells in the ipsilateral ON. Motoneurones innervating the inferior rectus muscle were mainly localised in the lateral somatic cell column of the ipsilateral ON. A second smaller grouping was observed in the medial longitudinal fasciculus. The inferior oblique muscle motoneurones were localised in the ipsilateral medial somatic cell column intermingled with motoneurones supplying the superior rectus muscle of the opposite eye. The superior oblique muscle motoneurones occupied the entire TN and the lateral rectus muscle motoneurones the AN. It was concluded that the organisation of nuclei and subnuclei responsible for controlling the extraocular muscles in the marmoset is broadly similar to that of other primates.     

The distribution of motoneurons supplying hind limb muscles in the clawed toad, Xenopus laevis

The distribution of motoneurons in the lumbar spinal cord (spinal segments 8-10) of the clawed toad, Xenopus laevis, was studied with the horseradish peroxidase technique. In a total of 13 different hind limb muscles this tracer was applied in a slow-release gel. Motoneurons innervating a particular hind limb muscle were clustered in longitudinally arranged motor pools. Motor pools of different muscles did show considerable overlap both in the rostrocaudal and transverse plane. But, the various motor pools clearly show a somatotopic organization of motoneurons even in such a condensed lumbar spinal cord as in Xenopus laevis. Motoneurons innervating more distally positioned muscles are generally found in more caudal segments, while proximal muscles (with the exception of the m. adductor magnus) are supplied by motoneurons more or less throughout the lumbar enlargement. Flexor muscles usually are innervated by motoneurons situated ventrolaterally in the ventral horn, extensor muscles by dorsomedially found motoneurons. This pattern is particularly apparent for proximal (thigh) muscles, less so for more distal (shank and foot) muscles. The present data are in keeping with those obtained with the retrograde cell degeneration technique in ranid frogs and are consistent with observations in other tetrapods, although a more clear separation of motor pools is evident in "higher" vertebrates such as birds and mammals.     

Activity of neck-muscle motoneurones during eye cleaning behaviour in the cricket Gryllus campestris

The activity of neck-muscle motoneurones which control head movements during eye cleaning behaviour was recorded from motor nerves with chronically implanted electrodes in unrestrained crickets. We show that motoneurones of the dorso-ventral muscles displayed strong activity differences between both sides of the neck, with higher discharge frequencies either ipsi- or contralateral to the direction of the head movement. Motoneurones innervating dorsal-longitudinal muscles were equally active on both sides. A single excitatory motoneurone of one dorso-ventral muscle showed a discharge pattern unequivocally related to eye cleaning. Lesions of connectives revealed that this motoneurone is monitored by interneuronal pathways from the suboesophageal ganglion although the primary sensory axons eliciting eye cleaning, project into the prothoracic ganglion.     

Neurotrophin-3 promotes the survival of a limited subpopulation of cutaneous sensory neurons

In the chick embryo, exogenous neurotrophin-3 (NT3) is sufficient to promote the differentiation of proprioceptive afferents even in the absence of limb muscle targets. To determine if NT3 can promote the differentiation of this phenotype in afferents with cutaneous targets, we analyzed the effects of chronic NT3 on cutaneous and muscle sensory neurons that express trkC, a receptor for NT3. In normal embryos, retrograde labeling and immunohistochemistry showed that about 75% of large-diameter muscle afferents express trkC, whereas only about 7% of large-diameter cutaneous afferents express this protein. After chronic treatment with NT3 during the cell death period, both populations of trkC(+) neurons were increased approximately twofold. Because this treatment is known to block cell death in sensory neurons, these results indicate that NT3 can promote the survival of both proprioceptive afferents and cutaneous afferents. To examine the phenotype of the cutaneous afferents rescued by NT3, we analyzed their projections and connections using transganglionic labeling and electrophysiological recording. The results indicate that exogenous NT3 neither altered the pattern of spinal projections nor caused cutaneous afferents to form monosynaptic connections with motor neurons. These results demonstrate that selective cell death does not contribute to the modality-specific pattern of spinal innervation and suggest that proprioceptive afferents may innervate muscle selectively.     

Comparison of global and joint-to-joint methods for estimating the hip joint load and the muscle forces during walking

A three-dimensional musculoskeletal model of the lower limb was developed to study the influence of biarticular muscles on the muscle force distribution and joint loads during walking. A complete walking cycle was recorded for 9 healthy subjects using the standard optoelectronic motion tracking system. Ground contact forces were also measured using a 6-axes force plate. Inverse dynamics was used to compute net joint reactions (forces and torques) in the lower limb. A static optimization method was then used to estimate muscle forces. Two different approaches were used: in the first one named global method, the biarticular muscles exerted a torque on the two joints they spanned at the same time, and in the second one called joint-by-joint method, these biarticular muscles were divided into two mono-articular muscles with geometrical (insertion, origin, via points) and physiological properties remained unchanged. The hip joint load during the gait cycle was then calculated taking into account the effect of muscle contractions. The two approaches resulted in different muscle force repartition: the biarticular muscles were favoured over any set of single-joint muscles with the same physiological function when using the global method. While the two approaches yielded only little difference in the resultant hip load, the examination of muscle power showed that biarticular muscles could produce positive work at one joint and negative work at the other, transferring energy between body segments and thus decreasing the metabolic cost of movement.     

Convergence of forelimb afferent actions on C7-Th1 propriospinal neurones bilaterally projecting to sacral segments of the cat spinal cord

Propriospinal neurones located in the cervical enlargement and projecting bilaterally to sacral segments of the spinal cord were investigated electrophysiologically in eleven deeply anaesthetized cats. Excitatory or inhibitory postsynaptic potentials from forelimb afferents were recorded following stimulation of deep radial (DR), superficial radial (SR), median (Med) and ulnar (Uln) nerves. 26 cells were recorded from C7, 22 from C8 and 3 from Th1 segments. The majority of the cells were located in the Rexed's laminae VIII and the medial part of the lamina VII. In 10 cases no afferent input from the forelimb afferents was found. In the remaining neurones effects were evoked mostly from DR (88%) and Med (63%), less often from SR (46%) and Uln (46%). Inhibitory actions were more frequent than excitatory. The highest number of IPSPs was evoked from high threshold flexor reflex afferents (FRA)--all connections were polysynaptic. However, inhibitory actions were often evoked from group I or II muscle afferents (polysynaptic or disynaptic) and, less frequently, from cutaneous afferents (mostly polysynaptic). Di- or polysynaptic IPSPs often accompanied monosynaptic EPSPs from group I or II muscle afferents. Disynaptic or polysynaptic EPSPs from muscle and cutaneous afferents were also recorded in many neurones, while polysynaptic EPSPs from FRA were observed only exceptionally. Various patterns of convergence in individual neuronal subpopulations indicate that they integrate different types of the afferent input from various muscle and cutaneous receptors of the distal forelimb. They transmit this information to motor centers controlling hind limb muscles, forming a part of the system contributing to the process of coordination of movements of fore--and hind--limbs.     

Traveling-wave pattern generator controls movement and organization of sensory feedback in a spinal cord model

 A traveling wave in a two-dimensional spinal cord model constitutes a stable pattern generator for quadruped gaits. In the context of the somatotopic organization of the spinal cord, this pattern generator is sufficient to generate stable locomotive limb trajectories. The elastic properties of muscles alone, providing linear negative feedback, are sufficient to stabilize stance and locomotion in the presence of perturbative forces. We further show that such a pattern generator is capable of organizing sensory processing in the spinal cord. A single-layer perceptron was trained to associate the sensory feedback from the limb (coding force, length, and change of length for each muscle) with the two-dimensional activity profile of the traveling wave. This resulted in a well-defined spatial organization of the connections within the spinal network along a rostrocaudal axis. The spinal network driven by peripheral afferents alone supported autonomous locomotion in the positive feedback mode, whereas in the negative feedback mode stance was stabilized in response to perturbations. Systematic variation of a parameter representing the effect of gamma-motor neurons on muscle spindle activity in our model led to a corresponding shift of limb position during stance and locomotion, resulting in a systematic displacement alteration of foot positions. Received: 30 July 2001 / Accepted in revised form: 17 April 2002 Correspondence to: A. Kaske (e-mails: alexander.kaske@mtc.ki.se, alexander.kaske@vglab.com)     

Fusimotor reflexes influencing secondary muscle spindle afferents from flexor and extensor muscles in the hind limb of the cat.

The purpose of this study was to investigate secondary muscle spindle afferents from the triceps-plantaris (GS) and posterior biceps and semitendinosus (PBSt) muscles with respect to their fusimotor reflex control from different types of peripheral nerves and receptors. The activity of single secondary muscle spindle afferents was recorded from dissected and cut dorsal root filaments in alpha-chloralose anaesthetized cats. Both single spindle afferents and sets of simultaneously recorded units (2-3) were investigated. The modulation and mean rate of firing of the afferent response to sinusoidal stretching of the GS and PBSts muscle were determined. Control measurements were performed in the absence of any reflex stimulation, while test measurements were made during reflex stimulation. The reflex stimuli consisted of manually performed movements of the contralateral hind limb, muscle stretches, ligament tractions and electrical stimulations of cutaneous afferents. Altogether 21 secondary spindle afferents were investigated and 20 different reflex stimuli were employed. The general responsiveness (i.e. number of significant reflex effects/number of control-test series) was 52.4%, but a considerable variation between different stimuli was found, with the highest (89.9%) for contralateral whole limb extension and the lowest (25.0%) for stretch of the contralateral GS muscle. The size of the response to a given stimulus varied considerably between different afferents, and, in the same afferent, different reflex stimuli produced effects of varying size. Most responses were characterized by an increase in mean rate of discharge combined with a decrease in modulation, indicative of static fusimotor drive (Cussons et al., 1977). Since the secondary muscle spindle afferents are part of a positive feedback loop, projecting back to both static and dynamic fusimotor neurones (Appelberg Et al., 1892 a, 1983 b; Appelberg et al., 1986), it is suggested that the activity in the loop may work like an amplified which, during some circumstances, enhance the effect of other reflex inputs to the system (Johansson et al., 1991 b).     

Stimulation at the cervicomedullary junction in human subjects.

In awake human subjects, corticospinal axons can be activated at the level of the cervicomedullary junction by electrical or magnetic stimulation. Such stimuli evoke single descending volleys which activate motoneurones and elicit responses in muscles of the upper limb. These responses (cervicomedullary motor evoked potentials, CMEPs) have a large monosynaptic component and can be used to test motoneurone excitability in a variety of tasks. CMEPs can be elicited in resting muscle and during all strengths of voluntary contraction. Examination of CMEPs during and after voluntary contractions reveals changes in motoneurone excitability but also suggests activity-dependent changes in the efficacy of the corticospinal pathway. Because they test the same subcortical pathway as transcranial magnetic stimulation, but are unaffected by altered excitability at a cortical level, CMEPs often offer the most appropriate comparison to allow interpretation of changes in motor evoked potentials. The advantages and disadvantages of stimulation at the cervicomedullary junction as a test of motoneurone excitability are reviewed.