Form and distribution of sensory terminals in cat hindlimb muscle spindles

There and back again: 50 years of wandering through terra incognita fusorum

This paper is in two parts: 'There', which is a review of some of the major advances in the study of spindle structure and function during the past 50 years, serving as an introduction to the symposium entitled 'Mechanotransduction, Muscle Spindles and Proprioception' held in Munich in July 2022; and 'And Back Again', presenting new quantitative morphological results on the equatorial nuclei of intrafusal muscle fibres and of the primary sensory ending in relationship to passive stretch of the spindle.

Biophysical model of muscle spindle encoding

Muscle spindles encode mechanosensory information by mechanisms that remain only partially understood. Their complexity is expressed in mounting evidence of various molecular mechanisms that play essential roles in muscle mechanics, mechanotransduction and intrinsic modulation of muscle spindle firing behaviour. Biophysical modelling provides a tractable approach to achieve more comprehensive mechanistic understanding of such complex systems that would be difficult/impossible by more traditional, reductionist means. Our objective here was to construct the first integrative biophysical model of muscle spindle firing. We leveraged current knowledge of muscle spindle neuroanatomy and in vivo electrophysiology to develop and validate a biophysical model that reproduces key in vivo muscle spindle encoding characteristics. Crucially, to our knowledge, this is the first computational model of mammalian muscle spindle that integrates the asymmetric distribution of known voltage-gated ion channels (VGCs) with neuronal architecture to generate realistic firing profiles, both of which seem likely to be of great biophysical importance. Results predict that particular features of neuronal architecture regulate specific characteristics of Ia encoding. Computational simulations also predict that the asymmetric distribution and ratios of VGCs is a complementary and, in some instances, orthogonal means to regulate Ia encoding. These results generate testable hypotheses and highlight the integral role of peripheral neuronal structure and ion channel composition and distribution in somatosensory signalling.

Muscle spindles and their role in maintaining robust locomotion

Muscle spindles, one of the two main classes of proprioceptors together with Golgi tendon organs, are sensory structures that keep the central nervous system updated about the position and movement of body parts. Although they were discovered more than 150 years ago, their function during movement is not yet fully understood. Here, we summarize the morphology and known functions of muscle spindles, with a particular focus on locomotion. Although certain properties such as the sensitivity to dynamic and static muscle stretch are long known, recent advances in molecular biology have allowed the characterization of the molecular mechanisms for signal transduction in muscle spindles. Building upon classic literature showing that a lack of sensory feedback is deleterious to locomotion, we bring to the discussion more recent findings that support a pivotal role of muscle spindles in maintaining murine and human locomotor robustness, defined as the ability to cope with perturbations. Yet, more research is needed to expand the existing mechanistic understanding of how muscle spindles contribute to the production of robust, functional locomotion in real world settings. Future investigations should focus on combining different animal models to identify, in health and disease, those peripheral, spinal and brain proprioceptive structures involved in the fine tuning of motor control when locomotion happens in challenging conditions.

Molecular correlates of muscle spindle and Golgi tendon organ afferents

Proprioceptive feedback mainly derives from groups Ia and II muscle spindle (MS) afferents and group Ib Golgi tendon organ (GTO) afferents, but the molecular correlates of these three afferent subtypes remain unknown. We performed single cell RNA sequencing of genetically identified adult proprioceptors and uncovered five molecularly distinct neuronal clusters. Validation of cluster-specific transcripts in dorsal root ganglia and skeletal muscle demonstrates that two of these clusters correspond to group Ia MS afferents and group Ib GTO afferent proprioceptors, respectively, and suggest that the remaining clusters could represent group II MS afferents. Lineage analysis between proprioceptor transcriptomes at different developmental stages provides evidence that proprioceptor subtype identities emerge late in development. Together, our data provide comprehensive molecular signatures for groups Ia and II MS afferents and group Ib GTO afferents, enabling genetic interrogation of the role of individual proprioceptor subtypes in regulating motor output.

Coordinated movement critically depends on sensory feedback from muscle spindles (MSs) and Golgi tendon organs (GTOs) but the afferents supplying this proprioceptive feedback have remained genetically inseparable. Here the authors use single cell transcriptome analysis to reveal the molecular basis of MS (groups Ia and II) and GTO (group Ib) afferent identities in the mouse.

Information processing in tree networks of excitable elements

We study the collective response of small random tree networks of diffusively coupled excitable elements to stimuli applied to leaf nodes. Such networks model the morphology of certain sensory neurons that possess branched myelinated dendrites with excitable nodes of Ranvier at every branch point and at leaf nodes. Leaf nodes receive random inputs along with a stimulus and initiate action potentials that propagate through the tree. We quantify the collective response registered at the central node using mutual information. We show that in the strong-coupling limit, the statistics of the number of nodes and leaves determines the mutual information. At the same time, the collective response is insensitive to particular node connectivity and distribution of stimulus over leaf nodes. However, for intermediate coupling, the mutual information may strongly depend on the stimulus distribution among leaf nodes. We identify a mechanism behind the competition of leaf nodes that leads to nonmonotonous dependence of mutual information on coupling strength. We show that a localized stimulus given to a tree branch can be occluded by the background firing of unstimulated branches, thus suppressing mutual information. Nonetheless, the mutual information can be enhanced by a proper stimulus localization and tuning of coupling strength.

Generating intrafusal skeletal muscle fibres in vitro: Current state of the art and future challenges

Intrafusal fibres are a specialised cell population in skeletal muscle, found within the muscle spindle. These fibres have a mechano-sensory capacity, forming part of the monosynaptic stretch-reflex arc, a key component responsible for proprioceptive function. Impairment of proprioception and associated dysfunction of the muscle spindle is linked with many neuromuscular diseases. Research to-date has largely been undertaken in vivo or using ex vivo preparations. These studies have provided a foundation for our understanding of muscle spindle physiology, however, the cellular and molecular mechanisms which underpin physiological changes are yet to be fully elucidated. Therefrom, the use of in vitro models has been proposed, whereby intrafusal fibres can be generated de novo. Although there has been progress, it is predominantly a developing and evolving area of research. This narrative review presents the current state of art in this area and proposes the direction of future work, with the aim of providing novel pre-clinical and clinical applications.

The dual role of curcumin and ferulic acid in counteracting chemoresistance and cisplatin-induced ototoxicity

Platinum-based agents, such as cisplatin, form the mainstay of currently used chemotherapeutic regimens for several malignancies; however, the main limitations are chemoresistance and ototoxic side effects. In this study we used two different polyphenols, curcumin and ferulic acid as adjuvant chemotherapeutics evaluating (1) in vivo their antioxidant effects in protecting against cisplatin ototoxicity and (2) in vitro the transcription factors involved in tumor progression and cisplatin resistance. We reported that both polyphenols show antioxidant and oto-protective activity in the cochlea by up-regulating Nrf-2/HO-1 pathway and downregulating p53 phosphorylation. However, only curcumin is able to influence inflammatory pathways counteracting NF-κB activation. In human cancer cells, curcumin converts the anti-oxidant effect into a pro-oxidant and anti-inflammatory one. Curcumin exerts permissive and chemosensitive properties by targeting the cisplatin chemoresistant factors Nrf-2, NF-κB and STAT-3 phosphorylation. Ferulic acid shows a biphasic response: it is pro-oxidant at lower concentrations and anti-oxidant at higher concentrations promoting chemoresistance. Thus, polyphenols, mainly curcumin, targeting ROS-modulated pathways may be a promising tool for cancer therapy. Thanks to their biphasic activity of antioxidant in normal cells undergoing stressful conditions and pro-oxidant in cancer cells, these polyphenols probably engage an interplay among the key factors Nrf-2, NF-κB, STAT-3 and p53.

Emergent stochastic oscillations and signal detection in tree networks of excitable elements

We study the stochastic dynamics of strongly-coupled excitable elements on a tree network. The peripheral nodes receive independent random inputs which may induce large spiking events propagating through the branches of the tree and leading to global coherent oscillations in the network. This scenario may be relevant to action potential generation in certain sensory neurons, which possess myelinated distal dendritic tree-like arbors with excitable nodes of Ranvier at peripheral and branching nodes and exhibit noisy periodic sequences of action potentials. We focus on the spiking statistics of the central node, which fires in response to a noisy input at peripheral nodes. We show that, in the strong coupling regime, relevant to myelinated dendritic trees, the spike train statistics can be predicted from an isolated excitable element with rescaled parameters according to the network topology. Furthermore, we show that by varying the network topology the spike train statistics of the central node can be tuned to have a certain firing rate and variability, or to allow for an optimal discrimination of inputs applied at the peripheral nodes.

Analysis of Proprioceptive Sensory Innervation of the Mouse Soleus: A Whole-Mount Muscle Approach

Muscle proprioceptive afferents provide feedback critical for successful execution of motor tasks via specialized mechanoreceptors housed within skeletal muscles: muscle spindles, supplied by group Ia and group II afferents, and Golgi tendon organs, supplied by group Ib afferents. The morphology of these proprioceptors and their associated afferents has been studied extensively in the cat soleus, and to a lesser degree, in the rat; however, quantitative analyses of proprioceptive innervation in the mouse soleus are comparatively limited. The present study employed genetically-encoded fluorescent reporting systems to label and analyze muscle spindles, Golgi tendon organs, and the proprioceptive sensory neuron subpopulations supplying them within the intact mouse soleus muscle using high magnification confocal microscopy. Total proprioceptive receptors numbered 11.3 ± 0.4 and 5.2 ± 0.2 for muscle spindles and Golgi tendon organs, respectively, and these receptor counts varied independently (n = 27 muscles). Analogous to findings in the rat, muscle spindles analyzed were most frequently supplied by two proprioceptive afferents, and in the majority of instances, both were classified as primary endings using established morphological criteria. Secondary endings were most frequently observed when spindle associated afferents totaled three or more. The mean diameter of primary and secondary afferent axons differed significantly, but the distributions overlap more than previously observed in cat and rat studies.

Distribution of TTX-sensitive voltage-gated sodium channels in primary sensory endings of mammalian muscle spindles

Knowledge of the molecular mechanisms underlying signaling of mechanical stimuli by muscle spindles remains incomplete. In particular, the ionic conductances that sustain tonic firing during static muscle stretch are unknown. We hypothesized that tonic firing by spindle afferents depends on sodium persistent inward current (INaP) and tested for the necessary presence of the appropriate voltage-gated sodium (NaV) channels in primary sensory endings. The NaV_1.6 isoform was selected for both its capacity to produce INaP and for its presence in other mechanosensors that fire tonically. The present study shows that NaV_1.6 immunoreactivity (IR) is concentrated in heminodes, presumably where tonic firing is generated, and we were surprised to find NaV_1.6 IR strongly expressed also in the sensory terminals, where mechanotransduction occurs. This spatial pattern of NaV_1.6 IR distribution was consistent for three mammalian species (rat, cat, and mouse), as was tonic firing by primary spindle afferents. These findings meet some of the conditions needed to establish participation of INaP in tonic firing by primary sensory endings. The study was extended to two additional NaV isoforms, selected for their sensitivity to TTX, excluding TTX-resistant NaV channels, which alone are insufficient to support firing by primary spindle endings. Positive immunoreactivity was found for NaV_1.1, predominantly in sensory terminals together with NaV_1.6 and for NaV_1.7, mainly in preterminal axons. Differential distribution in primary sensory endings suggests specialized roles for these three NaV isoforms in the process of mechanosensory signaling by muscle spindles.NEW & NOTEWORTHY The molecular mechanisms underlying mechanosensory signaling responsible for proprioceptive functions are not completely elucidated. This study provides the first evidence that voltage-gated sodium channels (NaVs) are expressed in the spindle primary sensory ending, where NaVs are found at every site involved in transduction or encoding of muscle stretch. We propose that NaVs contribute to multiple steps in sensory signaling by muscle spindles as it does in other types of slowly adapting sensory neurons.

Emergence and coherence of oscillations in star networks of stochastic excitable elements

We study the emergence and coherence of stochastic oscillations in star networks of excitable elements in which peripheral nodes receive independent random inputs. A biophysical model of a distal branch of sensory neuron in which peripheral nodes of Ranvier are coupled to a central node by myelinated cable segments is used along with a generic model of networked stochastic active rotators. We show that coherent oscillations can emerge due to stochastic synchronization of peripheral nodes and that the degree of coherence can be maximized by tuning the coupling strength and the size of the network. Analytical results are obtained for the strong-coupling regime of the active rotator network. In particular, we show that in the strong-coupling regime, the network dynamics can be described by an effective single active rotator with rescaled parameters and noise.

The innervation of the muscle spindle: a personal history

I present a brief review of current understanding of the innervation of the mammalian muscle spindle, from a personal historical perspective. The review begins with comparative studies on the numbers of spindle afferents and considers how their relative abundance may best be assessed. This is followed by an examination of the distribution and some functional properties of the motor innervation. The primary ending is the subject of the final section, in particular, I look at what can be learned from serial sectioning and volumetric reconstruction, and present new results on a model and simulations concerning sensory terminal deformation during stretch.

Extra forces evoked during electrical stimulation of the muscle or its nerve are generated and modulated by a length-dependent intrinsic property of muscle in humans and cats

Extra forces or torques are defined as forces or torques that are larger than would be expected from the input or stimuli, which can be mediated by properties intrinsic to motoneurons and/or to the muscle. The purpose of this study was to determine whether extra forces/torques evoked during electrical stimulation of the muscle or its nerve with variable frequency stimulation are modulated by muscle length/joint angle. A secondary aim was to determine whether extra forces/torques are generated by an intrinsic neuronal or muscle property. Experiments were conducted in 14 able-bodied human subjects and in eight adult decerebrate cats. Torque and force were measured in human and cat experiments, respectively. Extra forces/torques were evoked by stimulating muscles with surface electrodes (human experiments) or by stimulating the nerve with cuff electrodes (cat experiments). In humans and cats, extra forces/torques were larger at short muscle lengths, indicating that a similar regulatory mechanism is involved. In decerebrate cats, extra forces and length-dependent modulation were unaffected by intrathecal methoxamine injections, despite evidence of increased spinal excitability, and by transecting the sciatic nerve proximal to the nerve stimulations. Anesthetic nerve block experiments in two human subjects also failed to abolish extra torques and the length-dependent modulation. Therefore, these data indicate that extra forces/torques evoked during electrical stimulation of the muscle or nerve are muscle length-dependent and primarily mediated by an intrinsic muscle property.

A comparative analysis of the encapsulated end-organs of mammalian skeletal muscles and of their sensory nerve endings

The encapsulated sensory endings of mammalian skeletal muscles are all mechanoreceptors. At the most basic functional level they serve as length sensors (muscle spindle primary and secondary endings), tension sensors (tendon organs), and pressure or vibration sensors (lamellated corpuscles). At a higher functional level, the differing roles of individual muscles in, for example, postural adjustment and locomotion might be expected to be reflected in characteristic complements of the various end-organs, their sensory endings and afferent nerve fibres. This has previously been demonstrated with regard to the number of muscle-spindle capsules; however, information on the other types of end-organ, as well as the complements of primary and secondary endings of the spindles themselves, is sporadic and inconclusive regarding their comparative provision in different muscles. Our general conclusion that muscle-specific variability in the provision of encapsulated sensory endings does exist demonstrates the necessity for the acquisition of more data of this type if we are to understand the underlying adaptive relationships between motor control and the structure and function of skeletal muscle. The present quantitative and comparative analysis of encapsulated muscle afferents is based on teased, silver-impregnated preparations. We begin with a statistical analysis of the number and distribution of muscle-spindle afferents in hind-limb muscles of the cat, particularly tenuissimus. We show that: (i) taking account of the necessity for at least one primary ending to be present, muscles differ significantly in the mean number of additional afferents per spindle capsule; (ii) the frequency of occurrence of spindles with different sensory complements is consistent with a stochastic, rather than deterministic, developmental process; and (iii) notwithstanding the previous finding, there is a differential distribution of spindles intramuscularly such that the more complex ones tend to be located closer to the main divisions of the nerve. Next, based on a sample of tendon organs from several hind-foot muscles of the cat, we demonstrate the existence in at least a large proportion of tendon organs of a structural substrate to account for multiple spike-initiation sites and pacemaker switching, namely the distribution of sensory terminals supplied by the different first-order branches of the Ib afferent to separate, parallel, tendinous compartments of individual tendon organs. We then show that the numbers of spindles, tendon organs and paciniform corpuscles vary independently in a sample of (mainly) hind-foot muscles of the cat. Grouping muscles by anatomical region in the cat indicated the existence of a gradual proximo-distal decline in the overall average size of the afferent complement of muscle spindles from axial through hind limb to intrinsic foot muscles, but with considerable muscle-specific variability. Finally, we present some comparative data on muscle-spindle afferent complements of rat, rabbit and guinea pig, one particularly notable feature being the high incidence of multiple primary endings in the rat.

The consequences of short-range stiffness and fluctuating muscle activity for proprioception of postural joint rotations: the relevance to human standing

Proprioception comes from muscles and tendons. Tendon compliance, muscle stiffness, and fluctuating activity complicate transduction of joint rotation to a proprioceptive signal. These problems are acute in postural regulation because of tiny joint rotations and substantial short-range muscle stiffness. When studying locomotion or perturbed balance these problems are less applicable. We recently measured short-range stiffness in standing and considered the implications for load stability. Here, using an appropriately simplified model we analyze the conversion of joint rotation to spindle input and tendon tension while considering the effect of short-range stiffness, tendon compliance, fluctuating muscle activity, and fusimotor activity. Basic principles determine that when muscle stiffness and tendon compliance are high, fluctuating muscle activity is the greatest factor confounding registration of postural movements, such as ankle rotations during standing. Passive and isoactive muscle, uncomplicated by active length fluctuations, enable much better registration of joint rotation and require fewer spindles. Short-range muscle stiffness is a degrading factor for spindle input and enhancing factor for Golgi input. Constant fusimotor activity does not enhance spindle registration of postural joint rotations in actively modulated muscle: spindle input remains more strongly associated with muscle activity than joint rotation. A hypothesized rigid alpha-gamma linkage could remove this association with activity but would require large numbers of spindles in active postural muscles. Using microneurography, the existence of a rigid alpha-gamma linkage could be identified from the correlation between spindle output and muscle activity. Basic principles predict a proprioceptive "dead zone" in the active agonist muscle that is related to the short-range muscle stiffness.

Action potential initiation in the peripheral terminals of cold-sensitive neurones innervating the guinea-pig cornea

The site at which action potentials initiate within the terminal region of unmyelinated sensory axons has not been resolved. Combining recordings of nerve terminal impulses (NTIs) and collision analysis, the site of action potential initiation in guinea-pig corneal cold receptors was determined. For most receptors (77%), initiation mapped to a point in the time domain that was closer to the nerve terminal than to the site of electrical stimulation at the back of the eye. Guinea-pig corneal cold receptors are Adelta-neurones that lose their myelin sheath at the point where they enter the cornea, and therefore their axons conduct more slowly within the cornea. Allowing for this inhomogeneity in conduction speed, the resulting spatial estimates of action potential initiation sites correlated with changes in NTI shape predicted by simulation of action potentials initiating within a nerve terminal. In some receptors, more than one NTI shape was observed. Simulations of NTI shape suggest that the origin of differing NTI shapes result from action potentials initiating at different, spatially discrete, locations within the nerve terminal. Importantly, the relative incidence of NTI shapes resulting from action potential initiation close to the nerve termination increased during warming when nerve activity decreased, indicating that the favoured site of action potential initiation shifts toward the nerve terminal when it hyperpolarizes. This finding can be explained by a hyperpolarization-induced relief of Na(+) channel inactivation in the nerve terminal. The results provide direct evidence that the molecular entities responsible for stimulus transduction and action potential initiation reside in parallel with one another in the unmyelinated nerve terminals of cold receptors.

Age-related physiological and morphological changes of muscle spindles in rats

Age-related physiological and morphological changes of muscle spindles were examined in rats (male Fischer 344/DuCrj: young, 4-13 months; middle-aged, 20-22 months; old, 28-31 months). Single afferent discharges of the muscle spindles in gastrocnemius muscles were recorded from a finely split dorsal root during ramp-and-hold (amplitude, 2.0 mm; velocity, 2-20 mm s(-1)) or sinusoidal stretch (amplitude, 0.05-1.0 mm; frequency, 0.5-2 Hz). Respective conduction velocities (CVs) were then measured. After electrophysiological experimentation, the muscles were dissected. The silver-impregnated muscle spindles were teased and then analysed using a light microscope. The CV and dynamic response to ramp-and-hold stretch of many endings were widely overlapped in old rats because of the decreased CV and dynamic response of primary endings. Many units in old rats showed slowing of discharge during the release phase under ramp-and-hold stretch and continuous discharge under sinusoidal stretch, similarly to secondary endings in young and middle-aged rats. Morphological studies revealed that primary endings of aged rat muscle spindles were less spiral or non-spiral in appearance, but secondary endings appeared unchanged. These results suggest first that primary muscle spindles in old rats are indistinguishable from secondary endings when determined solely by previously used physiological criteria. Secondly, these physiological results reflect drastic age-related morphological changes in spindle primary endings.

Response of lumbar paraspinal muscles spindles is greater to spinal manipulative loading compared with slower loading under length control

BACKGROUND CONTEXT

Spinal manipulation (SM) is a form of manual therapy used clinically to treat patients with low back and neck pain. The most common form of this maneuver is characterized as a high-velocity (duration <150 ms), low-amplitude (segmental translation <2 mm, rotation <4 degrees , and applied force 220-889 N) impulse thrust (high-velocity, low-amplitude spinal manipulation [HVLA-SM]). Clinical skill in applying an HVLA-SM lies in the practitioner's ability to control the duration and magnitude of the load (ie, the rate of loading), the direction in which the load is applied, and the contact point at which the load is applied. Control over its mechanical delivery is presumably related to its clinical effects. Biomechanical changes evoked by an HVLA-SM are thought to have physiological consequences caused, at least in part, by changes in sensory signaling from paraspinal tissues.

PURPOSE

If activation of afferent pathways does contribute to the effects of an HVLA-SM, it seems reasonable to anticipate that neural discharge might increase or decrease in a nonlinear fashion as the thrust duration approaches a threshold value. We hypothesized that the relationship between the duration of an impulsive thrust to a vertebra and paraspinal muscle spindle discharge would be nonlinear with an inflection near the duration of an HVLA-SM delivered clinically (<150 ms). In addition, we anticipated that muscle spindle discharge would be more sensitive to larger amplitude thrusts.

STUDY DESIGN/SETTING

A neurophysiological study of spinal manipulation using the lumbar spine of a feline model.

METHODS

Impulse thrusts (duration: 12.5, 25, 50, 100, 200, and 400 ms; amplitude 1 or 2 mm posterior to anterior) were applied to the spinous process of the L6 vertebra of deeply anesthetized cats while recording single unit activity from dorsal root filaments of muscle spindle afferents innervating the lumbar paraspinal muscles. A feedback motor was used in displacement control mode to deliver the impulse thrusts. The motor's drive arm was securely attached to the L6 spinous process via a forceps.

RESULTS

As thrust duration became shorter, the discharge of the lumbar paraspinal muscle spindles increased in a curvilinear fashion. A concave-up inflection occurred near the 100-ms duration eliciting both a higher frequency discharge compared with the longer durations and a substantially faster rate of change as thrust duration was shortened. This pattern was evident in paraspinal afferents with receptive fields both close and far from the midline. Paradoxically, spindle afferents were almost twice as sensitive to the 1-mm compared with the 2-mm amplitude thrust (6.2 vs. 3.3 spikes/s/mm/s). This latter finding may be related to the small versus large signal range properties of muscle spindles.

CONCLUSIONS

The results indicate that the duration and amplitude of a spinal manipulation elicit a pattern of discharge from paraspinal muscle spindles different from slower mechanical inputs. Clinically, these parameters may be important determinants of an HVLA-SM's therapeutic benefit.

Classification of longissimus lumborum muscle spindle afferents in the anaesthetized cat

Recordings have been made from 127 single muscle spindle afferents from the longissimus lumborum muscles of anaesthetized cats. They have been characterized by their responses to passive muscle stretch and the effects of succinylcholine (SCh) and by their sensitivity to vibration. The use of SCh permitted the assessment for each afferent of the influence of bag1 (b1) and bag2 (b2) intrafusal muscle fibres. From this, on the assumption that all afferents were affected by chain (c) fibres, they were classified in four groups: b1b2c (41.9%), b2c (51.4%), b1c (1.3%) and c (5.4%). All the afferents with b1 influence were able to respond one to one to vibration at frequencies above 100 Hz and were considered to belong to primary endings. On the basis of the vibration test, 64% of the b2c type afferents appeared to be primaries and 36% secondaries. Of the units classified as primaries, 41% were designated as b2c and would not therefore be able to respond to dynamic fusimotor activity. The significance of this relatively high proportion of b2c-type spindle primary afferents is discussed in relation to the specialized postural function of the back muscles.

The influence of bag2 and chain intrafusal muscle fibers on secondary spindle afferents in the cat

Static gamma-motor activity is strongly modulated by a particular phase relationship to the cyclic movements of locomotion, and this has a profound effect on the firing patterns of muscle spindle afferents. Whilst primary afferents are affected by both static and dynamic gamma-motor output,secondary afferents are affected significantly only by the static system acting via the intrafusal bag2 and chain fibres. It is therefore important to know how fluctuating patterns of static gamma-motor activity affect secondary afferents and to relate this to the actions of bagt and chain fibres. We have studied the action of single static gamma axons on secondary afferents in cat hindlimb muscles. Various physiological methods were explored to identify which of the intrafusal muscle fibres were being activated in each case, including the use of random stimulation and ramp frequency stimulation. The effects were also recorded of I Hz sinusoidally frequency-modulated gamma-axon stimuli and the amplitude and phase of the resulting afferent modulation related to the involvement of the bag2 and chain fibres. It was found that bag2 fibres are effective in biasing the secondary discharge, but their modulating action is relatively weak and involves a marked phase lag. Chain fibres acting alone cause strong modulation with very little phase lag. Mixed bag2 and chain-fibre action is most effective in modulating afferent discharge and causes intermediate values of phase lag. The results are discussed in relation to the control of natural movements and it is concluded that an important function of the static gamma motor system is to provide a signal to sum algebraically with the length-related signal. The results do not suggest that it could also usefully control stretch sensitivity.

Characterization of spindle afferents in rat soleus muscle using ramp-and-hold and sinusoidal stretches

The discharge properties of 51 afferents were studied in the rat soleus muscle spindles. Under deep anesthesia using a pentobarbital sodium solution (30 mg/kg), a laminectomy was performed and the right L(4) and L(5) dorsal and ventral roots were transected near their entry into the spinal cord. In situ, the minimal (L(min)) muscle length [3 +/- 0.08 (SE) cm] of the soleus was measured at full ankle extension. Unitary potentials from the L(5) dorsal root were recorded in response to ramp-and-hold stretches applied at 3 mm (S3) and 4 mm (S4) amplitudes and four stretch velocities (6, 10, 15, and 30 mm/s), sinusoidal stretches performed at four amplitudes (0.12, 0.25, 0.5, and 1 mm) and six stretch frequencies (0.5, 1, 2, 3, 6, and 10 Hz), and vibrations applied at 50-, 100-, and 150-Hz frequencies. These two kinds of stretches were performed at three different muscle lengths (L(min+10%), L(min+15%), and L(min+20%)), whereas vibrations were applied at L(min+20%) muscle length. Conduction velocity of the fibers was calculated but did not allow to discriminate different fiber types. However, the mean conduction velocity of the first fiber group (43.3 +/- 0.8 m/s) was significantly higher than that of the second fiber group (33.9 +/- 0.9 m/s). Three parameters allowed to differentiate the responses of primary and secondary endings: the dynamic index (DI), the discharge during the stretch release from the ramp-and-hold stretches, and the linear range and the vibration sensitivity from sinusoidal stretches. The slope histogram of the linear regression based on the DI and the stretch velocity was clearly bimodal. Therefore the responses were separated into two groups. During the stretch release at a velocity of 3 mm/s, the first response group (n = 26) exhibited a pause, whereas the second (n = 25) did not. The linear range of the second ending group (0.12-1 mm) was broader than that of the first (0.12-0.25 mm). The first ending group showed a higher sensitivity to high-vibration frequencies of small amplitude than the second. In comparison with the literature, we can assert that the first and the second ending groups corresponded to the primary and secondary endings, respectively. In conclusion, our study showed that in rat soleus muscle spindles, it was possible to immediately classify the discharge of Ia and II fibers by using some parameters measured under ramp-and-hold and sinusoidal stretches.

Effects of the calcium antagonist nifedipine on the afferent impulse activity of isolated cat muscle spindles

The impulse activity of muscle spindles isolated from the cat tenuissimus muscle was investigated under varying concentrations of the L-type calcium channel blocker nifedipine. At a concentration of 25 microM nifedipine impulse activity was clearly diminished in both primary and secondary endings. However, low concentrations of the drug (5-10 microM) exerted unexpected excitatory effects. The dynamic properties of primary endings in particular were augmented; those of secondary endings were also increased, although only slightly. A detailed analysis of the afferent discharge patterns obtained under ramp-and-hold stretches yielded the following effects of 10 microM nifedipine. (1) The initial burst at the beginning of the ramp phase of a stretch was increased in primary endings; (2) the peak dynamic discharge frequency at the end of the ramp phase was considerably increased in most primary endings; (3) the sensitivity of the peak dynamic discharge value to varying amplitudes and velocities of stretch was significantly enhanced in primary endings, and also increased, although only slightly, in secondary endings; (4) the rise in the discharge frequency during the ramp phase of a stretch was augmented in both types of ending, the effect being again stronger in primary endings; (5) the fast adaptive decay of the impulse frequency following the ramp phase of a ramp-and-hold stretch was significantly increased in primary endings, but remained unaffected in secondary endings. The enhanced dynamic properties of primary endings were also observed under small sinusoidal stretch stimuli (10 microm, 40 Hz), where nifedipine induced a significant shift in the position of the 1:1 driven action potentials toward smaller phase values. In view of an increase in tension in the isolated muscle spindle and an increased initial burst in primary endings in the presence of nifedipine, it is suggested that the drug facilitates the attachment of cross-bridges in the poles of the intrafusal muscle fibers. The increase in the dynamic properties of primary endings points to the possibility that the drug preferentially affects the nuclear bag(1) fiber. The inhibitory effect on the afferent discharge rate at high doses of the drug is interpreted as the consequence of a calcium channel block in the membranes of the sensory endings. The membrane potential of sensory endings appears to be highly dependent on sustained Ca(2+) conductance.

Expression of myosin heavy chain isoforms along intrafusal fibers of rat soleus muscle spindles after 14 days of hindlimb unloading

Morphological, contractile, histochemical, and electrophoretical characteristics of slow postural muscles are altered after hindlimb unloading (HU). However, very few data on intrafusal fibers (IFs) are available. Our aim was to determine the effects of 14 days of hindlimb unloading on the morphological and immunohistochemical characteristics of IF in rat soleus muscle. Thirty-three control and 32 unloaded spindles were analyzed. The number and distribution of muscle spindles did not appear to be affected after unloading. There was no significant difference in number, cross-sectional area, and histochemical properties of IF between the two groups. However, after unloading, a significant decrease in slow type 1 MHC isoform and a slight increase in slow-tonic MHC expression were observed in the B and C regions of the bag1 fibers. The alpha-cardiac MHC expression was significantly decreased along the entire length of the bag2 fibers and in the B and C regions of the bag1 fibers. In 12 muscle spindles, the chain fibers expressed the slow type 1 and alpha-cardiac MHC isoforms over a short distance of the A region, although these isoforms are not normally expressed. The most striking finding of the study was the relative resistance of muscle spindles to perturbation induced by HU.

Static gamma-motoneurones couple group Ia and II afferents of single muscle spindles in anaesthetised and decerebrate cats

Ideas about the functions of static gamma-motoneurones are based on the responses of primary and secondary endings to electrical stimulation of single static gamma-axons, usually at high frequencies. We compared these effects with the actions of spontaneously active gamma-motoneurones. In anaesthetised cats, afferents and efferents were recorded in intramuscular nerve branches to single muscle spindles. The occurrence of gamma-spikes, identified by a spike shape recognition system, was linked to video-taped contractions of type-identified intrafusal fibres in the dissected muscle spindles. When some static gamma-motoneurones were active at low frequency (< 15 Hz) they coupled the firing of group Ia and II afferents. Activity of other static gamma-motoneurones which tensed the intrafusal fibres appeared to enhance this effect. Under these conditions the secondary ending responded at shorter latency than the primary ending. In another series of experiments on decerebrate cats, responses of primary and secondary endings of single muscle spindles to activation of gamma-motoneurones by natural stimuli were compared with their responses to electrical stimulation of single gamma-axons supplying the same spindle. Electrical stimulation mimicked the natural actions of gamma-motoneurones on either the primary or the secondary ending, but not on both together. However, gamma-activity evoked by natural stimuli coupled the firing of afferents with the muscle at constant length, and also when it was stretched. Analysis showed that the timing and tightness of this coupling determined the degree of summation of excitatory postsynaptic potentials (EPSPs) evoked by each afferent in alpha-motoneurones and interneurones contacted by terminals of both endings, and thus the degree of facilitation of reflex actions of group II afferents.

Patterns of fusimotor activity during locomotion in the decerebrate cat deduced from recordings from hindlimb muscle spindles

1. Recordings have been made from multiple single muscle spindle afferents from medial gastrocnemius (MG) and tibialis anterior (TA) muscles of one hindlimb in decerebrate cats, together with ankle rotation and EMG signals, during treadmill locomotion. Whilst the other three limbs walked freely, the experimental limb was denervated except for the nerves to MG and TA and secured so that it could rotate only at the ankle joint, without any external load. Each afferent was characterised by succinylcholine testing with regard to its intrafusal fibre contacts. Active movements were recorded and then replayed through a servo mechanism to reproduce the muscle length changes passively after using a barbiturate to suppress gamma-motor firing. 2. The difference in secondary afferent firing obtained by subtracting the discharge during passive movements from that during active movements was taken to represent the profile of static fusimotor activity. This indicated an increase before the onset of movement followed by a strongly modulated discharge in parallel with muscle shortening during locomotion. The pattern of static firing matched the pattern of unloaded muscle shortening very closely in the case of TA and with some phase advance in the case of MG. The same effects were observed in primary afferents. 3. Primary afferents with bag1 (b1) contacts in addition showed higher firing frequencies during muscle lengthening in active than in passive movements. This indicated increased dynamic fusimotor firing during active locomotion. There was no evidence as to whether this fluctuated during the movement cycles. 4. When the mean active minus passive difference profile of firing in bag2-chain (b2c) type primary afferents was subtracted from that for b1b2c afferents, the difference was dominated by a peak centred on the moment of maximum lengthening velocity (v). 5. The component of the active minus passive difference firing due to b1 fibre contacts could be modelled by f(t) = av (where a is a constant) during lengthening and by f(t) = 0.2 av during shortening. The remainder of the difference signal matched the predictions of the static fusimotor signal derived from secondary afferents. 6. The findings are discussed in relation to the concept that the modulated static fusimotor pattern may represent a 'temporal template' of the expected movement, though the relationship of the results to locomotion in the intact animal will require further investigation. The analysis of the data indicates that the combined action of muscle length changes and static and dynamic fusimotor activity to determine primary afferent firing can be understood in terms of the interaction between the b1 and b2c impulse initiation sites.

The role of muscle receptors in the detection of movements

This review discusses the role of muscle receptors, in particular, that of muscle spindles, in the detection of movements, both passive and active. Emphasis is placed on the importance of conditioning the muscles acting at a joint before making measurements of thresholds to passive movements, to take into account muscle's thixotropic property. The detection threshold:movement velocity relation is discussed and described for a number of different joints. Implications for muscle spindles are considered from the generalisation that, when expressed in terms of proportion of muscle fascicle length change, detection thresholds are about the same at different joints. It is concluded that the available data supports the view that muscle spindles lie in parallel with only a portion of a muscle fascicle and not the whole fascicle. At the elbow joint, where it has been tested, movement detection threshold is lower during passive movements than during contraction of elbow muscles. Both peripheral mechanisms and mechanisms operating within the central nervous system may be responsible for the rise in threshold. The signalling of movements by spindles during a contraction raises the question of how the central nervous system is able to extract the length signal under such circumstances, given that there is likely to be co-activation of alpha and gamma motoneurones. The evidence for a central subtraction of fusimotor-evoked impulses and some recent experiments relevant to this idea are described. In conclusion, a number of points of uncertainly have been revealed in this area and these should be the subject of future experiments.

Correlated histological and physiological observations on a case of common sensory output and motor input of the bag1 fibre and a chain fibre in a cat tenuissimus spindle

In muscle spindles of the cat, independent control of dynamic and static components of the response of the primary sensory ending to stretch is provided by separate motor inputs to the various kinds of intrafusal muscle fibre: dynamic axons (gamma or beta) to the bag1 fibres and static axons to the bag2 (typically gamma only) and chain (gamma or beta) fibres. Nonlinear summation of separately evoked effects during combined stimulation of dynamic and static motor axons appears to be due to mutual resetting by antidromic invasion of separate encoding sites, leading to partial occlusion of the momentarily lesser response by the greater. The encoding sites are thought to be located within the primary ending's preterminal branches which from first-order level are normally segregated to the bag1 fibre and to the bag2 and chain fibres. Here we describe the analysis of a special case that arose in a histophysiological study which had shown that the degree of occlusion was related to the minimum number of nodes between the putative encoding sites. Three-dimensional reconstruction of the primary ending revealed that the terminals of one chain fibre were derived entirely from the first-order branch that supplied the bag1 fibre, including one terminal that was shared directly with the bag1 (sensory cross-terminal). The other first-order branch supplied the bag2 and remaining chain fibres as normal. The degree of occlusion seen during simultaneous stimulation of a dynamic beta axon and a static gamma axon indicated that the encoding sites were separated by both first-order branches. Schematic reconstruction of the motor innervation revealed that the static gamma axon was most unlikely to have supplied the chain fibre which shared sensory terminals with the bag1, but that these fibres also shared a motor input with histological characteristics of beta type. Ramp-frequency stimulation of the dynamic beta axon at constant length evoked a driving effect which persisted after fatiguing the extrafusal component and was therefore explicable on the basis of the observed pattern of motor innervation, though the identity of the axon could not be conclusively proved. Individually, instances of shared sensory terminals and motor input of bag1 and chain fibres are rare in the cat; their combination in a single spindle with correlated physiology is described here for the first time. The observation is considered in relation to the importance of dynamic and static segregation in motor control, since it may imply that there is a lower limit to the degree of segregation that the developmental programme can provide.

Pacemaker activity in a sensory ending with multiple encoding sites: the cat muscle spindle primary ending

1. A combined physiological, histological and computer modelling study was carried out on muscle spindles of the cat tenuissimus muscle to examine whether there was any correlation between the functional interaction of putative encoding sites, operated separately by static and dynamic fusimotor neurones, and the topological structure of the preterminal branches of the primary sensory ending. 2. Spindles, whose I a responses to stretch and separate and combined static and dynamic fusimotor stimulation were recorded in physiological experiments, were located in situ. Subsequently the ramifications of the sensory ending were reconstructed histologically, and the topology of the branch tree was used in computer simulations of I a responses to examine the effect of the electronic separation of encoding sites on the static-dynamic interaction pattern. 3. Interactions between separate static and dynamic inputs, manifest in responses to combineed stimulation, were quantified by a coefficient of interaction (Ci) which, by definition, was 1 for strictly linear summation of separate inputs and zero for maximum occlusion between inputs. 4. For the majority of spindles static-dynamic interactions were characterized by pronounced occlusion (C1 < 0.35). In these spindles putative encoding sites (the peripheral heminodes of the branches supplying the intrafusal fibres activated by individual fusimotor efferents) were separated by a minimum conduction path of between three and ten myelinated segments (2-9 nodes of Ranvier). In contrast, significant summation (C1, approximately 0.7) was found in only one spindle. In this case putative encoding sites were separated by a single node. 5. Occlusion was not due to encoder saturation and it could not be accounted for by any other known physiological mechanisms (intrafusal fatigue or unloading). It is therefore attributed to competitive pacemaker interaction between encoding sites which are largely selectively operated by static and dynamic fusimotor efferents. 6. Model simulations of real preterminal-branch tree structures confirmed that short conduction paths between encoding sites were associated with manifest summation, whereas longer minimum conduction paths favoured pronounced occlusion. 7. In the extreme, occlusion could be so pronounced as to give rise to negative values of C1 during critical segments of response cycles. This was associated with lower discharge rates during combined static and dynamic stimulation than the higher of the individual stimulation effects. This phenomenon is referred to as hyperocclusion. Computer simulations demonstrated that hyperocclusion could be accounted for by a slow ionic adaptation process. e.g. by a very slowly activating K+ conductance.

Repetitive activity of a branched Hodgkin-Huxley axon with multiple encoding sites

Afferent activity in a receptor afferent fiber with several encoding sites is generally believed to represent the activity of the fastest pacemaker that resets all more slowly encoding sites. Alternatively, some impulse mixing as well as some nonlinear summation of receptor current to a single encoder have been considered. In this article the repetitive firing activity of a Hodgkin-Huxley axon consisting of two branches that join into a single stem axon was investigated. The model axon was stimulated by constant-current injection into either the right or the left or both branches. It was found that the model axon generated an (infinite) train of action potentials if the input current was large enough. The discharge frequency found was constant, and on combined stimulation of both branches with different current, the site of impulse initiation was always in the branch receiving the higher input current, excluding a simple impulse mixing. On the other hand, the combined stimulation of both branches evoked repetitive firing with a higher frequency than expected by the pacemaker-resetting hypothesis. Moreover, a stimulus that is subthreshold for repetitive firing if injected into one branch yields repetitive firing when it is injected into both branches, a behavior inconsistent with impulse mixing and pacemaker resetting. On the other hand, current injection into one branch allowed repetitive activity only within a rather limited range of firing frequencies. Using distributed current injection into both branches, however, allowed many more different firing frequencies. Such behavior is inconsistent with both pacemaker resetting and (nonlinear) input current summation. Consequently, the repetitive firing behavior of a branched Hodgkin-Huxley axon with multiple encoding sites appears to be more complex than postulated in the simple hypotheses.

Projection of cat jaw muscle spindle afferents related to intrafusal fibre influence

1. A method of classification of muscle spindle afferents using succinylcholine (SCh) and ramp stretches has recently been described, which appears to estimate separately the strength of influence of bag1 (b1) and of bag2 (b2) intrafusal fibres. Increase in dynamic difference (delta DD) indicates b1 influence whilst increase in initial frequency (delta IF) indicates b2 influence. The significance of this classification has now been examined by correlation with the strength of synaptic projection of jaw muscle spindle afferents to the fifth motor nucleus (MotV) and the supratrigeminal region (STR) in anaesthetized cats. 2. Projection strength was estimated by computing the extracellular focal synaptic potential (FSP) from spike-triggered averages of 1024 sweeps at 100 microns intervals along tracks through STR and MotV. Trigger pulses were derived from spindle afferent cell bodies of the jaw-closer muscles recorded in the mesencephalic trigeminal nucleus, and characterized by the effect of SCh on their responses to ramp-and-hold stretches. 3. The maximum size of FSPs in tracks traversing STR and MotV ranged from 2.08 to 36.99 microV with a mean of 7.55 microV. The amplitudes were bimodally distributed into roughly equal-sized groups with high and low amplitude FSPs. 4. Mean values of delta IF were significantly greater for the group with large FSPs than for those with small FSPs. There were no significant differences in delta DD. FSP amplitude was significantly positively correlated with delta IF, but not with delta DD. 5. Spindle afferents with high values of FSP amplitude in MotV had a wide range of values of delta DD (b1b2c and b2c groups), while units with large FSPs in STR were all in the b2c category. Some evidence is presented to indicate that this reflects a preferential projection of secondary afferents to the STR. 6. For those units with projection to both STR and to MotV, there was a significant positive correlation between FSP amplitude in the two nuclei. 7. These results indicate that the extent of the b2 influence on spindle afferents predicts the central projection strength better than does the b1 influence. This finding is discussed from the viewpoint of possible developmental and functional issues.

Subdivision of primary afferents from passive cat muscle spindles based on a single slow-adaptation parameter

38 primary afferents originating from de-efferented cat tibialis anterior muscle spindles were investigated. Ramp-and-hold stretches of the host muscle were performed with a varying amount of muscle pre-stretch while recording the primary afferent discharges. From the discharge responses an interspike interval function was estimated. This revealed a slow adaptation during the hold phase of stretch which could be approximated quite well by a power function. The slow-adaptation power function exponent (SAE) was found to be rather independent of the amount of pre-stretch applied to the host muscle and grouped around a value characteristic for each afferent. These 'characteristic SAEs' showed a clearly bimodal distribution within the population of primaries studied. Moreover, the distribution around both modes was narrow enough to justify the subdivision of the primary afferents according to their characteristic SAE as either high-SAE (10 afferents; 26%) or low-SAE (28 afferents; 74%) afferents. The most likely explanation for this bimodality of slow-adaptation behavior in primary afferents is given by the assumption that the afferent discharge of the passive spindle is mainly provided from a branch innervating either the bag1 (for high-SAE units) or the bag2 and chain (for low-SAE units) intrafusal fibers.

The classification of afferents from muscle spindles of the jaw-closing muscles of the cat

1. The effects of the muscle-depolarizing drug succinylcholine (SCh) on the stretch responses of jaw-closer muscle spindle afferents were studied in the anaesthetized cat. Using ramp and hold stretches repeated every 6 s the basic measurements made were: initial frequency (IF), peak frequency (PF) and static index (SI), the frequency 0.5 s after the end of the ramp of stretch. Derived from these were: dynamic difference (DD) = PF-IF, dynamic index (DI) = PF-SI and static difference (SD) = SI-IF. Increases in these measures caused by a single I.V. dose of SCh (200 micrograms kg-1) are symbolized by the prefix delta. 2. In a population of 234 units, delta DD and delta IF were each distributed bimodally, but were uncorrelated, thus defining four subgroups. 3. delta DD was argued to be an index of the effect of bag1 intrafusal fibre contraction and delta IF to be an index of the effect of bag2 fibre contraction. On this basis it is proposed that units can be divided into four groups according to the predominant influences of the bag1, bag2 and chain fibres as b1c (6.8%), b1b2c (22.2%), b2c (54.3%) or c (16.7%). 4. Testing with sine wave stretches at 1 Hz showed that changes in mean frequency and amplitude of response caused by SCh correlated with delta IF and delta DD respectively, but separated groups of units much less effectively than did ramp and hold testing. 5. Evidence is presented to indicate that the control value of DD in passive spindles does not relate to the potential strength of bag1 fibre effects in fully activated spindles. The bag1 fibre appears to contribute little to responses of spindle afferents in the passive state. DD is superior to DI as a measure of bag1 effects. 6. Conduction velocity was unimodally distributed in masseter spindle afferents and was not correlated with delta DD or delta IF and was therefore of no value in classifying them. 7. Neither the threshold of afferents to quick transient stretch nor the coefficient of variation of interspike intervals provided any significant additional help in classification. 8. The unexpectedly high proportion of units of b2c type is thought to include primaries lacking appreciable bag1 fibre contacts and secondaries with more or less substantial bag2 contracts.

The effect of succinylcholine on cat gastrocnemius muscle spindle afferents of different types

1. A population of 269 gastrocnemius muscle spindle afferents have been studied in anaesthetized cats for the effects of succinylcholine (SCh) on their response to ramp and hold stretches repeated every 6 s. The effectiveness and reliability of the SCh test was improved by prior stimulation of the muscle at 10 Hz for 30 s to increase the blood flow. 2. Responses have been assessed from averaged cycle histograms before and after a single I.V. dose of SCh of 200 micrograms kg-1. As for previous studies of jaw muscle spindles the basic measurements were initial frequency (IF), peak frequency (PF) and static index (SI), the frequency 0.5 s after the end of the ramp of stretch. Dynamic difference (DD = PF-IF), dynamic index (DI = PF-SI) and static difference (SD = SI-IF) were derived from these and increases caused by SCh indicated by the prefix delta. 3. delta DD and delta IF were each distributed bimodally and since they were uncorrelated formed the basis for a four-way classification. Since delta DD can be attributed to activation of bag1 (b1) intrafusal fibres and delta IF to bag2 (b2) fibres, while all afferents receive input from chain (c) fibres it is proposed as with the jaw spindles that the classes correspond to predominant influence from b1c, b1b2c, b2c and c intrafusal fibres. 4. The proportion of units in the different groups were similar to those in the jaw muscles except for there being very few b1c type in gastrocnemius. 5. Conduction velocity was bimodally distributed with the best dividing line at 63.2 m s-1. The b1b2c units were all, save one, in the fast group, while the b2c units were equally divided between fast and slow. 6. Mean control values for DD did not differ between the b1b2c and the b2c groups, which is taken to indicate that the b1 fibre does not contribute significantly to the dynamic stretch response of spindles with no intrafusal contraction. 7. The results emphasize the importance of recognizing that some apparently primary afferents lack b1 fibre influence, while many secondaries have marked b2 fibre influence. 8. The importance of the SCh classification is discussed in relation to the identification of fusimotor effects on spindle discharge and in relation to studies of central connectivity.

The 'initial burst' of muscle spindle afferents with or without terminals on the bag1 intrafusal muscle fibre

The initial burst has been re-examined following conditioning of the muscle to investigate its intrafusal origins. The burst was present in the response to stretch of spindle afferents that innervated the bag1 fibre and those that did not. There was, however, a difference in amplitude of the burst. These findings are discussed in terms of intrafusal mechanisms.

Development of the sensory innervation of muscle spindles in the kitten

This is a report of the changes observed in the pattern of sensory innervation of muscle spindles in hindlimb muscles of kittens during the first four weeks of life. The structural analysis, made on teased, silver-stained preparations, was complemented by a series of recordings of afferent responses of kitten spindles during ramp-and-hold stretches of the muscle. The primary endings of spindles from newborn animals showed a large degree of variability in their branching pattern and branches formed a network across the intrafusal fibres. In older animals there was less variability and lateral branches of stem axons began to encircle the intrafusal fibres. The process of maturation was characterized by a more uniform shape of the endings and more complete, evenly spaced, annulospiral terminals. Recordings of the responses of primary endings of spindles during muscle stretch showed that several features of the adult response were already present in the newborn, although the overall rate of discharge was very much lower. It was concluded that the changes observed in the structure of the sensory endings of kitten spindles did not have clearly identifiable physiological correlates. It appears that an annulospiral shape of the sensory terminals is not a necessary prerequisite for the generation of stretch responses. The predominant factor which appears to determine the responses of spindles to stretch is the maturity of the intrafusal fibres, in particular, the bag2 fibre.

Muscle history dependence of responses to stretch of primary and secondary endings of cat soleus muscle spindles

1. Responses were recorded from both primary and secondary endings of soleus muscle spindles in the anaesthetized cat during slow stretches of the muscle after conditioning contractions at different lengths. 2. After a 5 mm length step and a fusimotor-strength contraction given at the longer length, on return to the initial length the response to a slow test stretch (0.5 mm s-1) showed a change in slope midway through the stretch which was attributed to taking up of slack in intrafusal fibres. 3. The onset of the change in slope during the test stretch depended on the size of the conditioning step. With no conditioning length change, just a fusimotor-strength contraction, there was no slope change and the response consisted of an initial burst followed by a maintained high rate of discharge. 4. Following a conditioning length step, the point of onset of the slope change during the test stretch could be altered by stimulating single identified fusimotor fibres to the spindle. Stimulating some static axons produced large changes in the stretch response while other static axons and dynamic axons had only small effects. 5. Many secondary endings showed a delay in onset of their response to a test stretch, dependent on the size of the preceding conditioning step, signalling the presence of slack in much the same way as the primary endings. Other secondary endings, however, appeared to have stretch responses that were largely independent of muscle conditioning. 6. Muscle history-independent responses of secondary endings were associated with low axonal conduction velocities. It is proposed that secondary endings which remain unaffected by muscle conditioning lie on more distal regions of nuclear chain fibres in the S2-S5 position. Here they are stimulated during both the take-up of slack and the subsequent direct stretch of the intrafusal fibres.

The distribution of static gamma-axons in the tenuissimus muscle of the cat

1. The distribution of static gamma-axons within and among muscle spindles of the tenuissimus muscle has been studied in the anaesthetized cat, on the basis of the effects on the responses of primary endings when bag2 or chain fibres or both are activated by static gamma-stimulation. 2. Locations of spindles were marked for subsequent histological analysis using teased, silver-impregnated preparations. 3. Static effects were classified into: (i) biassing; (ii) driving; or (iii) indeterminate categories. 4. Critical correlations established that the biassing type was produced by bag2 activity, either alone or in combination with chain fibres, whereas the driving type was produced by chain fibres active alone. Indirect evidence suggested that indeterminate effects were produced by bag2 and chain fibres active together. 5. The static gamma-axons showed some differential distribution according to their conduction velocities: faster-conducting axons were likely to be more widely distributed among spindles but less likely to innervate chain fibres alone than were more slowly conducting axons. 6. The results are discussed in terms of their possible functional and developmental significance.

Muscle spindle ultrastructure revealed by conventional and high-resolution scanning electron microscopy

Muscle spindles in the tenuissimus muscle of mature golden Syrian hamsters were examined by conventional and high-resolution scanning electron microscopy (HRSEM). For conventional SEM, entire muscles were first fixed in 2.5% buffered glutaraldehyde. Spindles were then isolated with a dissecting microscope under darkfield illumination and postfixed in 1.0% OsO4. Some spindles were treated with 8 N HCl at 60 degrees C to clearly expose intrafusal fiber surfaces once the outer capsular sheath was mechanically disrupted. Preparation for HRSEM included aldehyde/osmium fixation and freeze-cleavage in liquid N2. The cytosol and certain cellular elements were also selectively extracted by immersion in 0.1% OsO4 for varying time intervals. In these preparations, the capsular sleeve showed a multilayered pattern of vesicle-laden cells with variant surface topography in different regions, including filopodia and small bristle-like surface-projections. An interlacing three-dimensional network of collagen fibrils intervened between the capsular lamellae. Within the spindles, sensory and fusimotor nerve endings closely adhered to the outer surfaces of intrafusal fibers. Sensory nerve terminals were enveloped by a prominent external lamina, and those that were cleaved open contained a plethora of elongated mitochondria that ran parallel with the longitudinal axis, along with vesicles, axoplasmic filaments, and lysosomes. Multiple adhesion sites between the sensory nerve membrane and the underlying sarcolemma of the intrafusal fiber were also observed in select regions. Fusimotor nerve endings were covered externally by processes of Schwann cells and their axoplasm was filled with a multitude of cellular organelles and synaptic vesicles.(ABSTRACT TRUNCATED AT 250 WORDS)

Responses of Ia afferent axons from muscle spindles lacking a bag1 intrafusal muscle fibre

The Ia afferent axon innervating the muscle spindle is characterised by the dynamic nature of its response to stretch. Traditionally this response has been attributed to the properties of the bag1 intrafusal muscle fibre, a view supported recently by reports of stretch-activation of the bag1 fibre. Some spindle capsules lack a bag1 fibre and the Ia afferent axon terminates on the bag2 and chain fibres only. Such axons may be identified by their lack of an enhanced dynamic response to infusions of succinylcholine. In these experiments the responses to passive stretch of these axons have been investigated and found to be indistinguishable from those of the main Ia population. This implies that the bag1 fibre is not essential to the generation of the dynamic response of primary endings.

Responses of cat muscle spindles which lack a dynamic fusimotor supply

1. The experiments reported here support the view that some spindles in the peroneus tertius muscle of the anaesthetized cat lack a nuclear bag1 intrafusal fibre. 2. The bag1 fibre is characterized by the fact that it is innervated exclusively by dynamic fusimotor axons. A method was devised to test each spindle in peroneus tertius for a dynamic fusimotor innervation. The ventral roots containing the muscle's motor supply were subdivided into five portions, approximately equal in terms of the tension they generated, and each piece was stimulated in turn, repetitively, at fusimotor strength, during ramp stretch of the muscle, to look for a large increase in dynamic response. 3. The method allowed confirmation that the majority of spindles in peroneus tertius had a dynamic fusimotor innervation. However, where the dynamic effect was weak and accompanied by a strong static fusimotor action and extrafusal unloading, it risked being overlooked. 4. The confirmatory test for the presence of a bag1 fibre was whether or not the spindle showed a large increase in dynamic response in the presence of the drug succinyl choline (SCh) injected arterially close to the muscle in which the spindle is located. SCh is known to induce a contracture in the bag1 fibre and therefore mimics tonic dynamic fusimotor stimulation. 5. In five experiments, of a total of forty-two spindles with afferents conducting within the group I range, five examples were encountered where there was no increase in dynamic response, either with ventral root stimulation or perfusion with SCh. It was concluded that these were spindles which lacked a bag1 fibre. 6. Passive stretch of such spindles revealed no feature in the response which allowed them to be distinguished from spindles in which the bag1 fibre was present. This conclusion posed the question, what contribution, if any, does the bag1 fibre make to the stretch response? 7. It was possible to show that under some conditions the bag1 fibre did contribute to the response to stretch. If the spindle was conditioned by repetitive nerve stimulation, at fusimotor strength, at a length longer than that at which a test stretch was applied, the response to the stretch was delayed, and it began at a lower rate than after conditioning stimulation at the test length. The delayed response was attributed to the presence of slack in intrafusal fibres.(ABSTRACT TRUNCATED AT 400 WORDS)

Neural organization of spindles in three hindlimb muscles of the rat

The neuroanatomical organization of the dynamic (bag1) and static (bag2 and chain) intrafusal systems was compared by light and electron microscopy of serial sections among 71 poles of muscle spindle in soleus (SOL), extensor digitorum longus (EDL), and lumbrical (LUM) muscles in the rat. Eighty-four percent of 195 fusimotor (gamma) axons to the spindles innervated either the dynamic bag1 fiber or the static bag2 and/or chain fibers. Sixteen percent of the gamma axons coinnervated the dynamic and static intrafusal fibers. Some of these nonselective axons were branches of effernts that also gave rise to axons selective to either the dynamic or static types of intrafusal fibers in one or more spindles. Thus activation of individual stem gamma efferents might not have a purely dynamic or purely static effect on the integrated afferent outflow from spindles of a hindlimb muscles in the rat. In addition, primary afferents in all muscles had terminations that cross-innervated the dynamic bag1 and static bag1 and/or chain intrafusal fibers in individual spindles, an arrangement that may enhance the mixed dynamic/static behavior of afferents when different intrafusal fibers are activated concurrent. Spindles of the slow SOL and fast EDL muscles had similar features, whereas differences were observed in the organization of the proximal (SOL and EDL) and distal (LUM) muscles. Spindles in LUM muscles had fewer static intrafusal fibers, a higher ratio of dynamic to static gamma axons, and a higher incidence of skeletofusimotor (beta) innervation to intrafusal fibers than spindles in the SOL or EDL muscles. Thus, the relative contribution of dynamic and static systems to muscle afferent outflow may differ among spindles located in different segments of the rat hindlimb. However, the dynamic and static intrafusal systems of spindle were less sharply demarcated in each of the three hindlimb rat muscles than in the cat tenuissimus muscle.

Morphology and morphometry of motor endings on macaque intrafusal fibers

The ultrastructural studies have shown three types of motor endings in the macaque intrafusal fibers: 1) unindented axon terminals with smooth or shallowly folded postsynaptic membrane; 2) indented terminals with few postsynaptic folds; and 3) indented terminals with heavily folded postsynaptic membrane. The terminals on bag 1 and chain fibers were generally more indented than those on the bag 2 fibers. Deeply indented terminals with highly folded postsynaptic membranes were noticed on the bag 1 and chain endings in spindles from lumbrical but not the biceps muscle. In the individual intrafusal fibers from the biceps and lumbrical spindles, the degree of indentation did not correlate with the extent of postsynaptic folding (P greater than .01). Endings on bag 1 and chain fibers in the lumbrical spindles showed a positive correlation between indentation of terminals and their distance from the primary sensory endings (P less than .01), whereas the lumbrical bag 2 endings and the biceps intrafusal endings did not (P greater than .01). The shape of the intrafusal motor endings thus is independent of their location but dependent on the type of intrafusal fibers.

Regeneration and recovery of cat muscle spindles after devascularization

1. We have assessed the sensory reinnervation and recovery of regenerated muscle spindles in extensor digitorum longus (EDL) 6, 8 and 13 weeks after the muscle, with its nerve left intact, had been devascularized. Recordings were made from the dorsal roots of the responses of single afferent fibres to ramp-and-hold stretch of the regenerated spindles whose sensory reinnervation was subsequently examined in teased, silver preparations. 2. The spindle population in four normal EDL muscles ranged from 53 to 83 (mean 69); analysis of the afferent innervation of 166 normal b1b2c spindles showed that 23% had primary endings supplied by two Ia afferents. Regenerated spindles were identified as belonging to one of four groups in which afferents establish sensory endings on intrafusal muscle fibres in groups 1-3, but not in group 4. Sensory reinnervation was complete after 6 weeks recovery and similar proportions of group 1-3 spindles occurred after each recovery period, i.e. 58% after 6 weeks, 65% after 8 weeks and 62% after 13 weeks. We estimate that about half the original spindle population was lost owing to persistent ischaemic necrosis; that 30% regenerated and acquired functional afferent connections (group 1-3 spindles); and that the total loss of spindle afferents was over 60%. 3. The conduction velocities of the regenerated spindle afferents were very similar to those of normal EDL spindle afferents. The proportions that responded normally to ramp-and-hold stretch at the end of each recovery period increased from 58% after 6 weeks to 61% after 8 and 88% after 13. Other responsive spindle afferents were either predominantly phasic or only responded to supramaximal stretch. The proportions of these decreased as recovery progressed reducing from 19% after 6 weeks to 9% after 13 weeks in the case of those giving predominantly phasic responses and from 23% after 6 weeks to 3% after 13 weeks in the case of those unresponsive to physiological stretch. 4. The mean peak and held firing rates of regenerated spindle afferents responsive to 10 mms-1 ramp-and-hold stretches were all significantly lower than normal. There was no marked trend towards higher firing rates after longer periods of recovery through, considered separately, the mean peak firing rates of the normally responding afferents did show a distinct improvement. The mean dynamic and velocity indexes were not significantly different from normal. 5. The ratio of Ia:spindle II afferents, as identified by their responses to stretch, was 1:1.22 in the control animals and 1:1.26 in the experimental series.(ABSTRACT TRUNCATED AT 400 WORDS)

Classification of muscle spindle afferents in the peroneus brevis muscle of the cat

Muscle-spindle afferents are commonly classified according to their conduction velocity. Under certain conditions such classifications may not be feasible and another form of identification is required. In this study 5 tests, comprising either quantitative or qualitative criteria, have been evaluated as a means of classifying spindle afferents. The choice of these tests was made on the basis of predicted physiological differences arising from the structural variations in the endings. Prior conditioning of the spindles was found to enhance the distinction between the two types of afferent. All the tests generated similar identifications with a maximum of 10% of afferents being classified differently by any two tests.

Random-sequence stimulation of the G1 hair afferent unit

Impulse trains were recorded from the parent axon of the cat G1 hair afferent unit. Separate random (Poisson-like) trains of mechanical stimuli were applied to two coinnervated receptive field hairs individually or concurrently. The objective was to determine whether the parent axonal impulse train elicited by dual-hair stimulation was due to a temporal combining ("mixing"; Fukami, 1980) of the impulse trains elicited in the parent axons by the same stimulation to each hair alone. Both impulse rates and patterns were assessed. During single-hair random stimulation, impulse trains differed from stimulus trains, having lower mean rates and short-interval doublets. During dual-hair random stimulation, mean impulse frequencies were on average 36% less than those predicted for mixing. There were no correlations between stimulus amplitude and departures from mixing. As a further test of the mixing hypothesis, the two single-hair-elicited impulse trains were temporally merged (i.e., superimposed to form one impulse train). Such merged impulse trains were compared with the corresponding dual-hair-elicited impulse train. Dual-hair-elicited frequencies were typically less than those of the merged trains, despite the use of an absolute-refractory-period criterion during merging. The impulse patterns elicited by dual-hair stimulation usually differed from the merged-train patterns. Temporal coupling between stimuli and impulses was either variable or absent during single-hair random stimulation; such coupling was altered during dual-hair random stimulation. In summary, this work showed that the dual-hair responses could not be predicted from the single-hair responses. Limitations of the mixing hypothesis and possible biophysical mechanisms in the axonal arborization are discussed. The results are consistent with a general hypothesis of analog processing within the arborization of the parent axon.

Re-afferent effects of individual static and dynamic gamma-stimuli during maintained fusimotor stimulation

The ability of maintained dynamic and static fusimotor stimulation to modulate the primary afferent response of the muscle spindle in the rhythm of gamma-stimulation was investigated using a highly sensitive method for modulation detection. The effect of 41 gamma-fibers (13 dynamic; 28 static) on 38 primary afferents obtained from the tibialis anterior muscle of the cat was studied. It was found that maintained stimulation of 10 out of the 13 dynamic (77%) and of 25 out of the 28 static (89%) gamma-fibers could evoke significant modulations of the primary afferent response in the rhythm of fusimotor stimulation at a minimum of one stimulation rate. Moreover, both static and dynamic gamma-stimulations could evoke significant primary afferent modulations almost over the entire range of stimulation rates studied (30-300 stimuli per second). These results show that both gamma-systems can modulate the primary afferent response in the rhythm of fusimotor stimulation over a wide range of stimulation rates; thus the central nervous system may be provided with re-afferent information about the effect of each individual gamma-motoneuron discharge. Some hypotheses for the internal spindle mechanism responsible for the afferent modulations are discussed.