Sharing of sensory terminals between the dynamic bag1 and static bag2 fibers in the rat muscle spindle

The nuclear bag1 intrafusal fiber mediates the dynamic (velocity) sensitivity, whereas the nuclear bag2 and nuclear chain fibers mediate the static (length) sensitivity of muscle spindles to stretch. The pattern of branching of primary and secondary afferents, the distribution of their terminals to the 3 types of intrafusal fibers, and the incidence of sensory cross-terminals were determined by reconstruction of 4 spindles from serial 1-micron and ultrathin transverse sections of rat extensor digitorum longus muscles. A single primary afferent supplied each spindle, and secondary afferents innervated intrafusal fibers in 3 spindles. Only static intrafusal fibers shared cross-terminals of the secondary afferents. In contrast, the dynamic bag1 and static bag2 fibers of each spindle shared at least one terminal of the primary afferent. Cross-terminals shared by the dynamic bag1 and static bag2 fiber parallel the presence of fusimotor (gamma) axons which coinnervate these types of intrafusal fibers in muscle spindles of rats. Consequently, the greater degree of overlap of elements comprising the dynamic and static systems of spindles of the rat relative to that of the cat reduces the probability of generating a purely dynamic or purely static response to an applied stretch.     

The period of latency before a muscle receptor generates an action potential as a response to a muscle stretch

Six primary (Ia) and seven secondary (II) muscle spindle afferents and eight Golgi tendon organ afferents (Ib) from the tibial anterior muscle of the cat, recorded at the dorsal roots, were subjected to a sinusoidal stretch of the host muscle, the frequency of which increased linearly from 2 to 80 Hz over four different lengths of time. Both the amplitude of the sinusoidal stretch and the prestretch of the muscle were varied. The phase of the action potentials was determined. The phase of the action potential, driven 1:1, increased linearly with frequency. From the gradient of the phase of this action potential the muscle-muscle receptor latency was determined, i.e., the period of latency between the stretch of the muscle and the occurrence of the action potential at the muscle nerve where it enters the muscle. The muscle-muscle receptor latency had values lying between 3 and 8 ms: it was dependent on the experimental parameters and became shorter as the conduction velocity of the afferent fiber increased. In three experiments the muscle latency was determined, i.e., the period of latency before the stretch was transferred from the tendon of the muscle to the proximal third of the muscle belly. The muscle was stretched sinusoidally under the same varying parameters as given above. The length changes occurring in the proximal third of the muscle were measured with a piezo element. The muscle latency was determined from the slope of the phase of the zero points of the sinusoidal piezo length changes; the phase increases linearly with frequency. The muscle latency had values lying between 6 and 15 ms: it was dependent on the experimental parameters. The muscle spindle latency, i.e., the period of latency between the stretch of the polar parts of the intrafusal muscle fibers and the recording of the action potentials from the spindle nerve near the spindle capsule, was determined from 5 Ia fibers and 1 II fiber of isolated muscle spindles. The isolated muscle spindle was stretched under the same varying parameters as given above. The muscle spindle latency was determined from the slope of the phase of the phase-locked action potential. The muscle spindle latency as measured by our method proved to be 0 ms. The latencies of the three elements and their dependence on the experimental parameters are discussed in the light of the transfer properties of the muscle and the muscle receptors.     

Inputs to spinocerebellar tract neurones located in stilling's nucleus in the sacral segments of the rat spinal cord

Spinocerebellar neurones located in the sacral segments of the rat spinal cord have been identified electrophysiologically. The neurones studied were located 0.7-1.1 mm deep to the cord dorsum, lateral and dorsal to the central canal in the medial part of lamina VII. Neurones were identified as spinocerebellar by antidromic activation from the cerebellar surface, the lowest threshold stimulation sites being near the midline on the posterior lobe vermis (lobule VIII). Estimates of conduction velocities of the axons ranged from 15-32 m/s (mean 22.8 m/s) and are directly comparable to velocities of presumed ventral spinocerebellar tract neurones recorded in the same animals. In intact animals, activity was most strongly influenced by passive movement of the tail. Activation by proprioceptors was confirmed with nerve stimulation: all of the neurones studied were discharged by stimulation of nerves which innervate ipsilateral tail muscles. In many cases responses appeared close to the threshold of the nerve, indicating that the largest, fastest conducting afferents (group Ia muscle spindle primary afferents) were responsible for them. Latencies of EPSPs or spikes were brief and in many cases indicative of a monosynaptic connection. We conclude that this group of neurones is powerfully and monosynaptically excited by group I muscle afferents and thus resemble the cells of Clarke's column and cells of the central cervical nucleus, both of which occupy a similar location in the grey matter of more rostral segments.     

Spindle gain increase during muscle unit fatigue

In anaesthetized cats, medial gastrocnemius motor units (MUs) were stimulated with random sequences (mean rates between 6 and 12 pps) of electrical pulses delivered to their axons in small ventral root filaments. Muscle tension was recorded under isometric conditions, and spike trains of muscle spindle afferents were recorded from small dorsal root filaments during prolonged MU activation. Time-domain (PSTH) and frequency-domain (gain) computations were performed to study the effects of fatiguing muscle unit contractions on the signal transmission from skeletomotor efferents to spindle afferents. In the course of muscle unit fatigue, during which the gain of the force-producing sub-system decreased, the gain of the sub-system transforming force to afferent discharge increased so that the overall gain between skeletomotor efferents and spindle afferents remained relatively high. This could be a mechanism that preserves a high quality of afferent information on MU contractions.     

Pain from excitation of identified muscle nociceptors in humans

The technique of intraneural microstimulation (INMS) combined with microneurography was used to excite and to record impulse activity in identified afferent peroneal nerve fibers from skeletal muscle of human volunteers. Microelectrode position was minutely adjusted within the impaled nerve fascicle until a reproducible sensation of deep pain projected to the limb was obtained during INMS. During INMS trains of 5–10 s in duration and at threshold for sensation, volunteers perceived a well defined area of deep pain projected to muscle. Psychophysical judgements of the magnitude of pain increased with increasing rates of INMS between 5 and 25 Hz. Also, the area of the painful projected field (PF) evoked during trains of INMS of various duration but constant intensity and rate typically expanded with duration of INMS. The intraneural microelectrode was alternatively used to record neural activity originating from primary muscle afferents. Eight slowly adapting units with moderate to high mechanical threshold were identified by applying pressure within or adjacent to the painful PF. Conduction velocities ranged from 0.9 to 6.0 m/s, and fibers were classed as Group III or Group IV. Capsaicin (0.01%) injected into the RF of two slowly conducting muscle afferents (one Group III and one Group IV) produced spontaneous discharge of each fiber and caused intense cramping pain, suggesting that the units recorded were nociceptive. Our results endorse the concept that the primary sensory apparatus that encodes the sensation of cramping muscle pain in humans is served by mechanical nociceptors with slowly conducting nerve fibers. Results also reveal that muscle pain can be precisely localized, although the human cortical function of locognosia for muscle pain becomes blunted as a function of duration of the stimulus.     

Expression of c-fos-like immunoreactivity in the feline brainstem in response to isometric muscle contraction and baroreceptor reflex changes in arterial pressure

This study compared whether activation of muscle ergoreceptor afferents caused by isometric muscle contraction, activation of baroreceptor afferents induced by i.v. infusion of phenylephrine, or baroreceptor afferent inactivation, caused by carotid artery occlusion, elicit similar patterns of c-Fos induction in brainstem areas. Adult cats were anesthetized with alpha-chloralose, and in each case, the experimental intervention caused an increase in the arterial blood pressure. There were two sets of control experiments: in both, animals underwent the same surgical procedures but then either remained at rest for the entire study, or the tibial nerve was stimulated, as in the contraction group, following muscle paralysis with tubocurarine. Following the procedures, animals rested for 90 min to allow neuronal expression of c-Fos. Control cats showed very little c-Fos immunoreactivity (c-Fos-ir) in the brainstem. Muscle contraction induced c-Fos-ir expression mainly in the nucleus tractus solitarius, lateral reticular nucleus, lateral tegmental field, vestibular nucleus, subretrofacial nucleus, spinal trigeminal tract and in a lateral region of the periaqueductal grey (P 0.5-1.0). The majority of the c-Fos-ir was found in brainstem areas contralateral to the contracted muscle. In addition, muscle contraction induced c-Fos-ir in the dorsal horns of spinal segments L6-S1 on the ipsilateral side of the spinal cord. Phenylephrine infusion caused c-Fos-ir expression in the nucleus tractus solitarius, spinal trigeminal tract, solitary tract, and dorsal motor nucleus of the vagus. No c-Fos-ir was apparent in the periaqueductal grey. Carotid occlusions induced c-Fos-ir expression in the area postrema, nucleus tractus solitarius, solitary tract, and spinal trigeminal tract. Expression was bilateral. Areas that exhibited c-Fos-ir correspond to sites previously reported to release various neuropeptides in response to muscle contraction or carotid occlusions. These results indicate that the exercise pressor reflex and baroreflex activate similar, but not completely identical, sites in the brainstem.     

Effect of catechol on the discharge of muscle spindle afferents from the hind limb of the rat

Previous work had shown that some of the effects of catechol could be via the fusimotor system. In order to determine the extent of fusimotor involvement, recordings have been made from muscle spindle afferents in split dorsal root filaments of anaesthetised rats. Catechol failed to excite de-efferented muscle spindles therefore eliminating many possible non-fusimotor effects. Over 80% of spindle afferents with intact efferents showed increased discharge frequency 1 min after injection, this increase often following a biphasic pattern with a pronounced pre-myoclonic burst and decline followed by a more sustained period of activity during the myoclonic phase. Analysis of spontaneous twitches or twitches evoked by ipsilateral auditory stimulation showed in addition a phasic increase in discharge suggesting alpha-gamma co-activation. Both primary and secondary afferents from many muscle groups around ankle and toes had their discharge frequencies elevated. Elevation of discharge frequencies of secondaries implies increased gamma-s activity confirmed by a decrease in dynamic index. Both gamma-d and gamma-s involvement in catechol action on primaries is suggested by dynamic index measurements. Perhaps a more continuous form of testing such as sinusoidal stretches would reveal any rapid switching between activities in the two systems.     

Inputs from identified jaw-muscle spindle afferents to trigeminothalamic neurons in the rat: A double-labeling study using retrograde HRP and intracellular biotinamide

Projections from physiologically identified jaw-muscle spindle afferents onto trigeminothalamic neurons were studied in the rat. Trigeminothalamic neurons were identified by means of retrograde transport of horseradish peroxidase from the ventroposteromedial nucleus of the thalamus. Labeled neurons were found contralaterally in the supratrigeminal region (Vsup), the trigeminal principal sensory nucleus, the ventrolateral part of the trigeminal subnucleus oralis, the spinal trigeminal subnuclei interpolaris and caudalis, the reticular formation, and an area ventral to the trigeminal motor nucleus (Vmo) and medial to the trigeminal principal sensory nucleus (AVM). Jaw-muscle spindle afferents were physiologically identified by their increased firing during stretehing of the jaw muscles and intracellularly injected with biotinamide. Axon collaterals and boutons from jaw-muscle spindle afferents were found in Vmo; Vsup; the dorsomedial part of the trigeminal principal sensory nucleus (Vpdm); the dorsomedial part of the spinal trigeminal subnuclei oralis, interpolaris (Vidm) and caudalis; the parvicellular reticular formation (PCRt); and the mesencephalic trigeminal nucleus. Trigeminothalamic neurons in Vsup, Vpdm, Vidm, PCRt, and AVM were associated with axon collaterals and boutons from intracellularly stained jaw-muscle spindle afferents. Trigeminothalamic neurons in Vsup, Vpdm, Vidm, and PCRt were closely apposed by one to 14 intracellularly labeled boutons from jaw-muscle spindle afferents, suggesting a powerful input to some trigeminothalamic neurons. These data demonstrate that muscle length and velocity feedback from jaw-muscle spindle afferents is projected to the contralateral thalamus via multiple regions of the trigeminal system and implicates these pathways in the projection of trigeminal proprioceptive information to the cerebral cortex. © 1995 Wiley-Liss, Inc.     

Fusimotor neurones can be reflexly influenced by activity in receptor afferents from the posterior cruciate ligament

Recordings were made simultaneously from 2-4 primary muscle spindle afferents from triceps surae and/or posterior biceps and semitendinosus muscles in cats anaesthetized with alpha-chloralose. It was demonstrated that stretch of the posterior cruciate ligament of the ipsilateral knee could cause changes in dynamic and/or static sensitivity of these afferents to sinusoidal stretching. The changes were due to reflex actions of stretch/tension-sensitive receptors in the cruciate ligaments onto fusimotor neurones. It is concluded that the cruciate ligaments may play an important 'sensory' role and that they may participate, via reflex actions on the gamma-motor-muscle spindle system, in the regulation of muscular stiffness of the knee joint, and thereby of the knee joint stability.     

Differences in monosynaptic EPSPs elicited by masseter and temporalis spindle afferents in anaesthetised rats

We have used the intracellular variant of the spike triggered averaging method to examine the monosynaptic connexions of masseter and temporalis spindle afferents on jaw-elevator motoneurones. Temporalis spindle afferents elicited larger averaged EPSPs in motoneurones than masseter spindle afferents, in part because transmission at synapses of temporalis afferents was associated with lower incidences of failures. We conclude that EPSP amplitude in this motor system is governed, at least in part, by the presynaptic neurone rather than the identity of the postsynaptic neurone.     

Re-afferent effects of individual static and dynamic γ-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 γ-stimulation was investigated using a highly sensitive method for modulation detection. The effect of 41 γ-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%) γ-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 γ-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 γ-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 γ-motoneuron discharge. Some hypotheses for the internal spindle mechanism responsible for the afferent modulations are discussed.     

Two types of jaw-muscle spindle afferents in the cat as demonstrated by intra-axonal staining with HRP

Intra-axonal records and horseradish peroxidase (HRP) injection techniques were employed to define the response properties of the jaw-closing muscle spindle afferents in the trigeminal mesencephalic nucleus (Vmes) and their morphological characteristics. The axonal trajectories of 9 spindle afferents from the masseter and 4 afferents from the temporalis were recovered for detailed analyses. Of 13 afferents, 6 cell bodies were stained and they were located at the rostrocaudal mid-levels of the Vmes. The central courses of the stem fibers were organized in a similar manner to the Vmes periodontal afferent nerves with the exception that peripheral (P) fibers of all spindle afferents passed through the trigeminal motor tract and root. On the basis of collateral terminal arborizations, the Vmes spindle afferents could be classified into two types: type I (n = 6) and type II (n = 7). Type I afferents sent their collaterals into the trigeminal motor nucleus (Vmo), intertrigeminal region (Vint) and juxtatrigeminal region (Vjux), but collaterals from the two neurons also projected to Vmes and the nucleus oralis (Vo). The collaterals from type II afferents formed their terminal arbors in the supratrigeminal nucleus (Vsup) in addition to the Vmo, Vint and Vjux, but collaterals from one neuron also projected to the Vo. In type I afferents, terminal arbors encompassed the whole Vmo including jaw-closing motoneurons. In contrast, boutons from type II afferents were restricted to a few small portions within the Vmo in proximity to its lateral and dorsal boundaries. The diameters of the united (U), central (C) and peripheral (P), fibers were larger in type I than type II afferents; those of the U fibers were statistically significant. Any differences between the two distinct types were not found in the response pattern to the sustained jaw opening. These results suggest that the difference of primary and secondary muscle-spindle afferent nerves is reflected in a distinctive morphology in the terminal arborizations and in the diameters of united fibers rather than the response patterns in deeply anesthetized cats.     

The information carried by spindle afferents on motor unit activity as revealed by spectral analysis

Spectral analysis was used to study the effects of motor unit activity on the discharge patterns of muscle spindle endings. Spindle afferents of hind-limb muscles of the cat were recorded during electrical stimulation of one or more motor units, and, for comparison, while the receptors discharged in the absence of induced extrafusal activity ('background discharge'). The stimulus sequences used were random, but had characteristic frequency components representing an underlying rhythm, similar to those of trains in real alpha-motoneuron output. The computed afferent spectra and coherences between stimulus and afferent trains indicate that the discharge patterns of muscle spindles carry information on the activity of particular subsets of motor units. The spectra also demonstrate a complex interaction of internal spindle (pacemaker) mechanisms and external (modulating) processes which determine the discharge patterns of primary and secondary endings. In addition, they reveal interesting differences between primaries and secondaries, possibly indicative of a particular role for each type of ending in motor control.     

The structural and functional development of muscle spindles and their connections in fetal sheep

In this paper we have studied the structural and functional development of hindlimb muscle receptors and the connections of their afferent fibres in fetal sheep (n = 26) from 67–143 days of gestation (term = 146 days). By recording extracellular discharges in dorsal root ganglia (L7, S1) we have shown that muscle spindle afferents first respond to a ramp-and-hold stretch at mid-gestation ( 75 days). Silver-stained preparations of muscle spindles revealed that afferent fibres are just beginning to form annulospiral windings at this age. It therefore appears that the annulospiral formation is not a necessary requirement for the generation of the response. By 87–92 days some receptors had developed a discharge at resting muscle length. Discharges were generally more robust and easier to elicit and static and dynamic components could be identified in the response to stretch. Although static sensitivity was generally low it was more evident than dynamic sensitivity. By 107–115 days it was possible to clearly distinguish between muscle and tendon afferents and to tentatively classify muscle responses as originating from primary or secondary afferent spindle endings. With increasing gestational age there was a progressive increase in the length and complexity of the spindle innervation in parallel with the maturation of functional activity. Biocytin injections into the dorsal root ganglia revealed afferent projections to the motoneuron pools by 67 days. Silver-staining of muscles showed that innervation of extrafusal fibres was also present by this age. We therefore conclude that the neural pathways necessary for reflex activity involving muscle spindles are present and functional from early in gestation and could contribute to early fetal movements.     

Response of human muscle spindle afferents to abrupt load perturbations

Response patterns of single muscle spindle primary afferents to abrupt load perturbations inflicted upon the contracting finger flexors were investigated in awake human subjects using the technique of microneurography. Units showed a transient high-frequency discharge in the rising phase of the stretch. No repeated burst activity attributable to irregularities of the stretch velocity or to mechanical oscillations of the muscle could be detected.     

Do spindle afferents monitor joint position in man? A study with active position holding

The activity of human spindle afferents from finger extensor muscles has been studied during voluntary position holding at two finger positions, to disclose any explicit monitoring of muscle length. The results indicate that, taken as groups, neither primary nor secondary afferents show explicit position responses during active position tracking. Thus any contribution from muscle spindle to position sense must be derived from other components in spindle afferent discharge.     

Effects of extensor muscle afferents on the timing of locomotor activity during walking in adult rats

The influence of hind leg extensor muscle afferents on the timing of locomotor phase transitions was examined in adult, decerebrate rats, walking on a treadwheel. Walking occurred either spontaneously or was induced by stimulation of the mesencephalic locomotor region. Large diameter muscle afferents innervating the lateral or medial gastrocnemius were electrically stimulated during walking. A stimulus was delivered either at the onset of extensor muscle activity, or randomly during the step cycle. Stimulation with a train duration of 300 ms at the onset of extension increased the duration of the extensor bursts. The subsequent flexion phase was delayed. Stimulation with a shorter stimulus train (150 ms) early in extension had little effect on the extension phase duration. However when delivered at the end of extension the same stimulus significantly increased the duration of the extension phase and decreased the duration of the following flexion phase. Stimulating near the end of the flexion phase delayed onset and decreased duration of the subsequent extension phase. The effects of stimulating extensor afferents during the extension phase were weaker but qualitatively similar, to those in cats, suggesting similar mechanisms. The results of this study also show major differences in the integration of extensor muscle afferents between adult and neonatal rats.     

Stimulation-dependent uptake of glutamic acid by hippocampal slices

The activity of human spindle afferents from finger extensor muscles has been studied during voluntary position holding at two finger positions, to disclose any explicit monitoring of muscle length. The results indicate that, taken as groups, neither primary nor secondary afferents show explicit position responses during active position tracking. Thus any contribution from muscle spindle to position sense must be derived from other components in spindle afferent discharge.     

Functional organization of the spinal reflex pathways from forelimb afferents to hindlimb motoneurons in the cat. II. Conditions of the interneuronal connections

The interneuronal conditionsof the descending pathways from forelimb afferents to hindlimb motoneurones were investigated by testing spatial interactions in these pathways and between these pathways and segmental lumbar reflex pathways. In high spinal unanaesthetized cats hindlimb motoneuroneswere intracellularly recorded and spatial interactions were tested between effects evoked by stimulation of pairs of ipsi- and contralateral forelimb nerves or pairs of a forelimb and an ipsilateral hindlimb nerve. The excitatory and late inhibitory pathways from forelimb afferents projecting to most of the hindlimb motoneurone pools, showed an interactive pattern which was distinctly different to the fast inhibitory pathway projecting specifically for ipsilateral forelimb afferents to flexor digitorum and hallucis longus (FDHL) motoneurones. Stimulation of homonymous or heteronymous pairs of two forelimb nerves of both sides evoked generally a distinct spatial facilitation of the excitatory and late inhibitory effects, while the specific early IPSPs to FDHL motoneurones were not facilitated. Paired stimulation of two forelimb nerves of one side only produced spatial facilitation of EPSPs or late IPSPs if low strength stimuli were used, using higher strength which induced larger effects, generally caused occlusion instead. In case of large IPSPs this may be due to the vicinity to the equilibrium potential. Except for an inhibition of cutaneous reflex pathways, the spatial interaction of the excitatory and late inhibitory pathways onto segmental lumbar reflex pathways was weak and variable. The fast inhibitory pathway to FDHL motoneurone showed a partial spatial facilitatory interaction with lumbar reflex pathways from cutaneous and group II muscle afferents. The second IPSP wave evoked by this pathway was inhibited by antidromic stimulation of the ventral root L7S1 and of the α-efferents of the antagonistic peroneal nerve. From the results conclusions are drawn on the interneuronal organization of the descending pathways from forelimb afferents to hindlimb motoneurones.     

Facilitatory interaction between cutaneous afferents from low threshold mechanoreceptors and nociceptors in segmental reflex pathways to α-motoneurons

In spinal cats weak stroking of the fur of the foot facilitated PSPs from group II muscle afferents in α-motoneurons. This spatial facilitation indicates that afferents from low threshold cutaneous mechanoreceptors and group II muscle afferents may converge onto common interneurons in segmental reflex pathways. The results confirm the contribution of group II muscle afferents to a multisensorial spinal reflex system for movement control.