Actions on gamma-motoneurones elicited by electrical stimulation of group I muscle afferent fibres in the hind limb of the cat

Electromyography of the muscle spindle

In needle electromyography, there are two spontaneous waveforms, miniature end plate potentials and “end plate spikes”, appearing usually together. Miniature end plate potentials are local, non-propagating postsynaptic waves, caused by spontaneous exocytosis of acetylcholine in the neuromuscular junction. The prevailing hypothesis states that “end plate spikes” are propagated postsynaptic action potentials of muscle fibers, caused by presynaptic irritation of the motor nerve or nerve terminal. Using several small concentric needle electrodes in parallel with the muscle fibers, most “end plate spikes” are strictly local or propagating for 2–4 mm. At the end plate zone, there are miniature end plate potentials without “end plate spikes”. Local “end plate spikes” are junctional potentials of intrafusal gamma neuromuscular junctions of the nuclear bag fibers, and propagated “end plate spikes” are potentials of nuclear chain muscle fibers of muscle spindles. Miniature end plate potentials without “end plate spikes” at the end plate zone derive from alpha neuromuscular junctions. These findings contrast with the prevailing hypothesis. The history of observations and different hypotheses of the origin of end plate spikes are described.

Secondary endings of muscle spindles: Structure, reflex action, role in motor control and proprioception

NEW FINDINGS

What is the topic of this review? We describe the structure and function of secondary sensory endings of muscle spindles, their reflex action and role in motor control and proprioception. What advances does it highlight? In most mammalian skeletal muscles, secondary endings of spindles are more or much more numerous than primary endings but are much less well studied. By focusing on secondary endings in this review, we aim to redress the balance, draw attention to what is not known and stimulate future research.

ABSTRACT

Kinaesthesia and the control of bodily movement rely heavily on the sensory input from muscle spindles. Hundreds of these sensory structures are embedded in mammalian muscles. Each spindle has one or more sensory endings and its own complement of small muscle fibres that are activated by the CNS via fusimotor neurons, providing efferent control of sensory responses. Exactly how the CNS wields this influence remains the subject of much fascination and debate. There are two types of sensory endings, primary and secondary, with differing development, morphology, distribution and responsiveness. Spindle primary endings have received more attention than secondaries, although the latter usually outnumber them. This review focuses on the secondary endings. Their location within the spindle, their response properties, the projection of their afferents within the CNS and their reflex actions all suggest that secondaries have certain separate roles from the primaries in proprioception and motor control. Specifically, spindle secondaries seem more adapted than primaries to signalling slow and maintained changes in the relative position of bodily segments, thereby contributing to position sense, postural control and static limb positioning. By highlighting, in this way, the roles of secondary endings, a final aim of the review is to broaden understanding of muscle spindles more generally and of the important contributions they make to both sensory and motor mechanisms.

Motoneuronal Spinal Circuits in Degenerative Motoneuron Disease

The most evident phenotype of degenerative motoneuron disease is the loss of motor function which accompanies motoneuron death. In both amyotrophic lateral sclerosis (ALS) and spinal muscular atrophy (SMA), it is now clear that dysfunction is not restricted to motoneurons but is manifest in the spinal circuits in which motoneurons are embedded. As mounting evidence shows that motoneurons possess more elaborate and extensive connections within the spinal cord than previously realized, it is necessary to consider the role of this circuitry and its dysfunction in the disease process. In this review article, we ask if the selective vulnerability of the different motoneuron types and the relative disease resistance of distinct motoneuron groups can be understood in terms of their intraspinal connections.

Escape from homeostasis: spinal microcircuits and progression of amyotrophic lateral sclerosis

In amyotrophic lateral sclerosis (ALS), loss of motoneuron function leads to weakness and, ultimately, respiratory failure and death. Regardless of the initial pathogenic factors, motoneuron loss follows a specific pattern: the largest α-motoneurons die before smaller α-motoneurons, and γ-motoneurons are spared. In this article, we examine how homeostatic responses to this orderly progression could lead to local microcircuit dysfunction that in turn propagates motoneuron dysfunction and death. We first review motoneuron diversity and the principle of α-γ coactivation and then discuss two specific spinal motoneuron microcircuits: those involving proprioceptive afferents and those involving Renshaw cells. Next, we propose that the overall homeostatic response of the nervous system is aimed at maintaining force output. Thus motoneuron degeneration would lead to an increase in inputs to motoneurons, and, because of the pattern of neuronal degeneration, would result in an imbalance in local microcircuit activity that would overwhelm initial homeostatic responses. We suggest that this activity would ultimately lead to excitotoxicity of motoneurons, which would hasten the progression of disease. Finally, we propose that should this be the case, new therapies targeted toward microcircuit dysfunction could slow the course of ALS.

Postural Stability after Vestibular Schwannoma Surgery

Factors related to postural stability and the course of recovery after vestibular schwannoma surgery were analyzed in a retrospective study of 177 patients and in a prospective study of 44 patients. Before surgery, 7 of the 44 patients in the prospective group complained of postural instability. The body sway velocity was measured. In the retrospective study, the sway velocity was abnormal under nonvisual control in 63% of the patients and under visual control in 34%. The sway velocity correlated with tumor size, smooth pursuit deficit, and postoperative work history. We compared patients with abnormal postural control to those with normal postural control, and found that the former relied to a greater extent on visual information and the latter on proprioceptive information. In the prospective study, the sway velocity increased after the operation, and 1 year after surgery it had not returned to preoperative values. In the logistic regression analysis, a translabyrinthine approach, rehabilitation, the patient's age, and preserved function of the facial nerve predicted a good outcome of postural stability. The retrosigmoid approach and depression were the most significant risk factors for postoperative gait difficulties. Vestibular rehabilitation increased the reliance on proprioception for maintenance of postural control and improved postural stability.

Functional coupling between motor and sensory nerves through contraction of sphincters in the pudendal area of the female cat

The question of whether skin receptors might help in the perception of muscle contraction and body movement has not been settled. The present study gives direct evidence of skin receptor firing in close coincidence with the contraction of the vaginal and anal sphincters. The distal stump of the sectioned motor pudendal nerve was stimulated. Single shocks induced a wavelike increase in the lumen pressure of the distal vagina and the anal canal, as well as constriction of the vaginal introitus and the anus. The constriction pulls on and moves the surrounding skin, which was initially detected visually. In the present experiments, a thin strain gauge that pressed on the skin surface detected its displacement. Single shocks to the motor nerve induced a wave of skin movement with maximal amplitude at 5 mm from the anus and propagated with decrement beyond 35 mm. The peripheral terminals of the sensory pudendal nerve and the posterior femoral nerve supply the skin that moves. Sensory axons from both nerves fired in response to both tactile stimulation and the skin movement produced by the constriction of the orifices (motor-sensory coupling). In cats with all nerves intact, a single shock to the sensory nerves induced reflex waves of skin movement and lumen pressure (sensory-motor coupling). Both couplings provide evidence for a feedforward action that might help to maintain the female posture during mating and to the perception of muscle contraction.

Gamma→Alpha Linkage and Persistent Firing of Ia Fibers by Pudendal Nerve Stimulation in the Decerebrate Cat

The sensory pudendal nerve (SPN) was stimulated in decerebrate female cats. Spikes of single Ia muscle spindle afferents from the medial gastrocnemius (MG) muscle were recorded in dorsal root filaments. Electroneurography (ENG) was recorded in a cut nerve filament to the MG muscle; MG electromyography (EMG) was also recorded. Single shock to SPN induced discharges of small ENG spikes (SS) with similar amplitude to that of gamma spikes elicited by ventral root stimulation. Thus SS were identified as gamma spikes. The latency of the gamma discharge was ∼15 ms. As expected, the onset of the gamma discharge preceded a discharge of Ia spikes; the time difference between both discharges was ∼5 ms. After the initial bursts, the Ia and the gamma activities paused during 20–30 ms but later increased again to last ∼1 s. After the shock, the EMG activity was depressed during ∼50 ms; later, motor-unit spikes may show transient activation. Thus the onset of the gamma activation preceded the activation of motor units (gamma→alpha link). Trains of shocks (1 or 100 Hz) to SPN induced a sustained increase in the frequency of gamma spikes, Ia spikes, and motor units that outlasted the train by 20–120 s. The sustained firing of Ia fibers might trigger or help to trigger and maintain the response of alpha-motoneurons.

The sensorimotor system, part I: the physiologic basis of functional joint stability

OBJECTIVE

To define the nomenclature and physiologic mechanisms responsible for functional joint stability.

DATA SOURCES

Information was drawn from an extensive MEDLINE search of the scientific literature conducted in the areas of proprioception, neuromuscular control, and mechanisms of functional joint stability for the years 1970 through 1999. An emphasis was placed on defining pertinent nomenclature based on the original references.

DATA SYNTHESIS

Afferent proprioceptive input is conveyed to all levels of the central nervous system. They serve fundamental roles in optimal motor control and sensorimotor control over functional joint stability.

CONCLUSIONS/APPLICATIONS

Sensorimotor control over the dynamic restraints is a complex process that involves components traditionally associated with motor control. Recognizing and understanding the complexities involved will facilitate the continued development and institution of management strategies based on scientific rationales.

The role of muscle weakness in the pathogenesis of osteoarthritis

To date, very few studies have investigated the role of muscle dysfunction in the pathogenesis of osteoarthritis (OA). Using largely indirect evidence, this article hypothesizes that motor and sensory dysfunction of muscle may be important factors in the pathogenesis of articular damage and are not simply a consequence of joint damage. A new paradigm is constructed to better describe the complex interrelationship between muscle sensorimotor dysfunction, joint damage, and disability in OA. If the hypothesis is correct, because muscle is a relatively plastic tissue, maintaining well-conditioned muscles may delay or prevent the onset of OA, and rehabilitation exercise therapy that reverses muscle sensorimotor dysfunction may ameliorate the effects of OA.

New observations on coupling between group II muscle afferents and feline gamma-motoneurones

1. Extra- or intracellular recordings were made from seventy-six gamma-motoneurones of hindlimb muscles in chloralose anaesthetized cats to re-assess the coupling between secondary muscle spindle afferents (group II muscle afferents) and these neurones. The latencies of a number of responses evoked by group II muscle afferents in gamma-motoneurones were shorter than minimal latencies of responses induced disynaptically in other spinal neurones. These latencies are therefore compatible with monosynaptic coupling between muscle spindle secondaries and gamma-motoneurones. 2. Responses fulfilling criteria for monosynaptically evoked responses were seen in about one third of gamma-motoneurones with input from the group II muscle afferents tested (in 6 of 18 motoneurones recorded intracellularly and in 26 of 74 motoneurones recorded extracellularly). They were usually evoked from only one of the stimulated nerves, stimulation of group II afferents of other nerves being followed by responses at longer latencies. 3. Most gamma-motoneurones were excited by group II afferents from several muscles, both flexors and extensors. However, a comparison of group II input to gamma-motoneurones innervating medial gastrocnemius and four other hindlimb muscles revealed differences in both incidence and sources. 4. This study extends results of previous studies by providing evidence that some synaptic actions of group II afferents, including afferents from the same muscle, are evoked monosynaptically, and may assist in sustaining the activation of gamma-motoneurones by positive feedback.

Changes in fusimotor activity during repetitive lengthening muscle contractions in decerebrate cats

Responses of fusimotor neurons to lengthening vs isometric contractions have been studied in decerebrate cats. Spike discharges of fusimotor neurons to the medial gastrocnemius muscle were recorded from this muscle nerve filament during sequences of contractions and/or stretches of the lateral gastrocnemius and soleus muscles. The sequences lasted for 250-450s (duty cycle 4:2 s). Isometric contractions were elicited by electrical stimulation (40 Hz, 1.3 times motor threshold) of the muscle nerves. Lengthening contractions were elicited in the same way while the muscles were stretched by 4 mm at a velocity of 1 mm/s. Of 25 fusimotor neurons studied, 23 responded to muscle contractions with an increase in firing rate, subsiding towards the end of the sequence. The increase was either modulated with each subsequent contraction or smooth throughout the sequence. Approximately 64% of fusimotor neurons, responding to muscle contractions, responded in a similar way to the sequences of muscle stretches, applied alone. Responses to sequences of the lengthening contractions were significantly larger, on average, than those to the isometric ones, but smaller than the sum of the responses to the contractions and stretches applied separately. On the other hand, they were also larger in fusimotor units, showing no overt responses to muscle stretches alone.

Sensitivity of H-reflexes and stretch reflexes to presynaptic inhibition in humans

The sensitivity of soleus H-reflexes, T-reflexes, and short-latency stretch reflexes (M1) to presynaptic inhibition evoked by a weak tap applied to the biceps femoris tendon or stimulation of the common peroneal nerve (CPN) was compared in 17 healthy human subjects. The H-reflex was strongly depressed for a period lasting up to 300-400 ms (depression to 48 +/- 23%, mean +/- SD, of control at a conditioning test interval of 70 ms) by the biceps femoris tendon tap. In contrast, the short-latency soleus stretch reflex elicited by a quick passive dorsiflexion of the ankle joint was not depressed. The soleus T-reflex elicited by an Achilles tendon tap was only weakly depressed (92 +/- 8%). The H-reflex was also significantly more depressed than the T-reflex at long intervals (>15 ms) after stimulation of CPN (H-reflex 63 +/- 14%, T-reflex 91 +/- 13%; P < 0. 01). However, the short-latency (2 ms) disynaptic reciprocal Ia inhibition evoked by stimulation of CPN was equally strong for H- and T-reflexes (H-reflex 72 +/- 10%, T-reflex 67 +/- 13%; P = 0.07). Peaks in the poststimulus time histogram (PSTH) of the discharge probability of single soleus motor units (n = 53) elicited by an Achilles tendon tap had a longer duration than peaks evoked by electrical stimulation of the tibial nerve (on average 5.0 ms as compared with 2.7 ms). All parts of the electrically evoked peaks were depressed by the conditioning biceps femoris tendon tap (average depression to 55 +/- 27% of control; P < 0.001). A similar depression was observed for the initial 2 ms of the peaks evoked by the Achilles tendon tap (69 +/- 48%; P < 0.001), but the last 2 ms were not depressed. Conditioning stimulation of the CPN at long intervals (>15 ms) also depressed all parts of the electrically evoked PSTH peaks (n = 34; average 65%; P < 0.001) but had only a significant effect on the initial 2 ms of the peaks evoked by the Achilles tendon tap (85%; P < 0.001). We suggest that the different sensitivity of mechanically and electrically evoked reflexes to presynaptic inhibition is caused by a difference in the shape and composition of the excitatory postsynaptic potentials underlying the two reflexes. This difference may be explained by a different composition and/or temporal dispersion of the afferent volleys evoked by electrical and mechanical stimuli. We conclude that it is not straightforward to predict the modulation of stretch reflexes based on observations of H-reflex modulation.

Experimental muscle pain increases the human stretch reflex

In this study we investigated the effect of human experimental muscle pain on H- and stretch reflexes as indicators of changes in muscle spindle sensitivity. Fourteen healthy, male volunteers participated in the study. Muscle pain was produced by infusion of 5% hypertonic saline over a period of 10-15 min in m. soleus and in m. tibialis anterior. Reflexes were elicited in the relaxed and active soleus muscle (10-15 Nm ankle torque) before, during and after muscle pain. Control measurements were made with infusions of 0.9% isotonic saline. Surface electromyograms (EMG) were measured from the soleus muscle, and torque was measured from the ankle joint. With pain in the soleus muscle the mechanical stretch reflex response (ankle torque) increased significantly (P = 0.0007) as compared to before pain. With pain in the tibialis anterior muscle both the mechanical and EMG responses increased significantly (P = 0.001; P = 0.0003) as compared to before pain. The H-reflex showed no significant changes during the infusions in either muscles. This study has demonstrated a muscle pain-related increase in the amplitude of the stretch reflex without a corresponding increase in the H-reflex amplitude. One explanation could be an increased dynamic sensitivity of the muscle spindles during muscle pain caused by an increased firing rate in the dynamic gamma-motoneurones. However, the data could not support the vicious cycle model because the excitability of the alpha-motoneurone pool was unchanged.

Short-latency synaptic patterns among cat peroneal motoneurons

Motoneurons innervating peroneal muscles in the cat leg (PB, PT and PL, respectively, for peroneus brevis, tertius and longus) were examined for their connections with afferents from these and other leg muscles and with cutaneous afferents. The aim was to investigate (1) whether inputs from nearby muscles and cutaneous areas are likely to assist or oppose the excitation elicited in peroneal motoneurons by PB contractions, and (2) whether reflex connectivity might allow distinction of alpha (i.e. motoneurons innervating skeletal muscle fibres) and beta (i.e. motoneurons innervating both skeletal and intrafusal muscle fibres) subgroups among PB and PT motoneurons. In the three peroneal pools, every motoneuron had excitatory monosynaptic connections with Ia afferents from each of the three peroneal muscles, and nearly every motoneuron received di- or trisynaptic excitation from low-threshold cutaneous afferents in sural or superficial peroneal nerves. Inputs from these sources might facilitate the contraction-induced positive feedback. In contrast, the patterns of short-latency synaptic connections with group I afferents from pretibial flexor and post-tibial extensor muscles were heterogeneous among peroneal motoneurons but did not point to any specific beta pattern.

Sensorimotor changes and functional performance in patients with knee osteoarthritis

OBJECTIVE

Muscles are essential components of our sensorimotor system that help maintain balance and perform a smooth gait, but it is unclear whether arthritic damage adversely affects muscle sensorimotor function. Quadriceps sensorimotor function in patients with knee osteoarthritis (OA) was investigated, and whether these changes were associated with impairment of functional performance.

METHODS

Quadriceps strength, voluntary activation, and proprioceptive acuity (joint position sense acuity) were assessed in 103 patients with knee OA and compared with 25 healthy control subjects. In addition, their postural stability, objective functional performance (the aggregate time for four activities of daily living), and disabilities (lequesne index) were also investigated.

RESULTS

Compared with the control subjects, the patients with knee OA had weaker quadriceps (differences between group mean 100N, CI 136, 63N), poorer voluntary activation (20% CI 13, 25%) that was associated with quadriceps weakness, and impaired acuity of knee joint position sense (1.28 degrees, CI 0.84, 1.73 degrees). As a group the patients were more unstable (p = 0.0017), disabled (10, CI 7, 11), and had poorer functional performance (19.6 seconds, CI 14.3, 24.9 seconds). The most important predictors of disability were objective functional performance and quadriceps strength.

CONCLUSIONS

In patients with knee OA, articular damage may reduce quadriceps motoneurone excitability, which decreases voluntary quadriceps activation thus contributing to quadriceps weakness, and diminishes proprioceptive acuity. The arthrogenic impairment in quadriceps sensorimotor function and decreased postural stability was associated with reduced functional performance of the patients.

On the role of recurrent inhibitory feedback in motor control

This article reviews presumed roles of recurrent inhibition in motor control, that have been proposed over the past five decades. The discussion is structured in an order of increasing complexity. It starts out with the simplest and earliest circuit, that is recurrent self-inhibition of skeleto-motoneurons, and related functions. It soon becomes clear that in order to understand recurrent inhibition, we must look beyond the simple self-inhibitory CNS circuit. First, recurrent inhibition must be seen in the context of other neural circuits. Second, some quantitative features appear to be correlated with features of the neuromusculo-skeletal periphery. Third, the aspect of lateral inhibition between different members of a motoneuron pool as well as between different motoneuron pools points to the essential multiple input-multiple output structure of recurrent inhibition that again can be understood only by correlating it with features of the neuromusculo-skeletal periphery. Another extension results from the discovery that recurrent inhibition affects not only skeleto-motoneurons, but also gamma-motoneurons, Ia inhibitory interneurons mediating reciprocal inhibition between antagonist motoneurons, other Renshaw cells and cells of origin of the ventral spinocerebellar tract (VSCT). Then the view broadens again, investigating the potential role that recurrent inhibition plays in two far-ranging theories of motor control, the inverse-dynamics approach and the equilibrium-point hypothesis. Finally, the present author tries to formulate, in broad strokes, a personal functional interpretation of recurrent inhibition. All the functional considerations, right or wrong, should yield ideas for new experiments, and this then is the last objective of this review.

Fusimotor outflow to pretibial flexors during fatiguing contractions of the triceps surae in decerebrate cats

Changes in discharge rate of fusimotor neurones to pretibial flexor muscles were recorded during and after long-lasting fatiguing isometric and/or isotonic contractions of triceps surae in decerebrate cats. The contractions were elicited by electrical stimulation of the nerves to triceps. Fusimotor spikes were recorded from nerve filaments dissected free from the peroneal nerve. Responses of the fusimotor neurones were diverse. In isometric regime, 22 out of 40 units recorded exhibited an initial increase at the onset of muscle contraction, different in amplitude and duration among the units. In seven of these units an additional brisk burst of spike discharges, of different duration, occurred at the end of the contraction. In 15 fusimotor neurones (14 units with the initial response and an additional unit without it) a slow increase in discharge rate developed, starting during the contraction and outlasting it. In another eleven units the initial response was a decrease in discharge rate, lasting in six of them throughout the contraction. Another six units exhibited a sustained increase in discharge rate throughout the contraction, as well as, at a lower level, but still above the spontaneous one, thereafter. Similar patterns of changes in discharge rate, recorded in 31 of the units, were encountered during isotonic triceps contractions. It should be mentioned that many (about 20) additional silent neurones, responding to manipulating the skin and paw and/or stroking the fur, but not to triceps contractions were encountered. The majority of changes in discharge rate of fusimotor neurones to pretibial flexors differed markedly from those found previously in fusimotor neurones to triceps and hamstring muscles.(ABSTRACT TRUNCATED AT 250 WORDS)

Changes in fusimotor discharge rate provoked by isotonic fatiguing muscle contractions in decerebrate cats

Changes in discharge rate of 26 fusimotor neurones to medial gastrocnemius muscle were studied during isotonic fatiguing contractions of lateral gastrocnemius and soleus muscles in decerebrate cats. Muscle contractions were elicited by either continuous or repetitive electrical stimulation of the muscle nerves. The muscles were considered to be fatigued when, against a load equal to one third of the tension developed at the onset of an isometric contraction, i.e. to the tension indicating isometrically-induced fatigue, they returned to the length at which the isometric contraction was elicited. At the onset of muscle contraction an increase in discharge rate, lasting for 5-220 s, occurred in all except one of the neurones. In 73% of the units a late increase developed in addition in parallel with muscle fatigue outlasting the contraction for 5-180 s. All but one of the remaining neurones exhibited a short lasting burst of spike discharges coincident with the end of contraction. Enhancement of the late increase by muscle ischaemia indicates contribution of chemosensitive small-diameter muscle afferents, while the short lasting burst is supposed to be elicited rather by the mechanosensitive units sensitized by metabolic products liberated during contraction and/or fatigue. Differences of the fusimotor reflex responses to isotonic vs. isometric contraction and/or fatigue and their possible functional role are discussed.

Fusimotor-induced changes in muscle spindle outflow and responsiveness in muscle fatigue in decerebrate cats

Changes in discharge rate and responsiveness of muscle spindle afferents from triceps surae muscles were studied during long-lasting fatigue isometric contractions of either medial gastrocnemius or lateral gastrocnemius and soleus muscles in decerebrate cats. The rest of the hind limb was either denervated or its innervation was preserved. In denervated preparations a long-lasting post-contraction increase in discharge rate developed in the majority of primary (15 of 18) and in all (20) secondary afferents. This increase was abolished, while the decrease during muscle contraction was enhanced after application of procaine to the corresponding muscle nerve, to block either the small-diameter afferents from the contracting muscle or the fusimotor axons to the spindle of origin of the afferent recorded. In innervated preparations the long-lasting increase was replaced in the majority of primary endings (14 of 22) by a sharp burst at the end of muscle contraction, while in secondary afferents it was either absent or shorter-lasting than in denervated preparations. Changes in responsiveness to sinusoidal muscle length changes, indicating influences of both static and dynamic fusimotor neurons, were, however, similar in innervated and denervated preparations. The results obtained provide evidence that changes in muscle spindle outflow and responsiveness are elicited by the reflex increase in fusimotor activity developing in response to the fatigue-induced afferent discharges from the contracting muscle. Concomitant afferent inflow of another origin to fusimotor neurons affects the changes in spindle outflow, but not in responsiveness. In this way an appropriate increase in support to skeletomotor activity as well as in information on the fatigued muscle of higher motor centres, initiated by the fatigue itself, could be achieved through the gamma loop.

Fusimotor responses to fatiguing muscle contractions in non-denervated hindlimb of decerebrate cats

Changes in discharge rate of 21 fusimotor neurons to medial gastrocnemius muscle during long-lasting fatiguing contractions of lateral gastrocnemius and soleus muscles were recorded in decerebrate cats with innervation of the same hindlimb preserved. Both the spontaneous activity and reflex responses of fusimotor neurons differed from those found previously in preparations with denervated hindlimb. Higher proportion of units fired at rest at rates above 20 impulses/s, the initial increase in discharge rate at the onset of muscle contraction was markedly prolonged, lasting in the majority of units throughout the muscle contraction, while the late increase in discharge rate developing with muscle fatigue was either absent or short-lasting. It is suggested that the increase in spontaneous firing rate occurs in dynamic fusimotor neurons being supported by afferent inflow from secondary muscle spindle endings from non-contracting muscles, the enhancement of the early responses to be primarily due to recurrent disinhibition and the differences in changes of early and late responses to reflect their partly different origin. The possibility is raised that the late reflex responses are lacking more often in static than in dynamic fusimotor neurons. From the functional point of view the differences in fusimotor reflex responses in innervated versus denervated hindlimb may indicate their susceptibility to modifications by changes in afferent inflow according, supposedly, to the current demands of motor control of the active and/or fatigued muscle.

Neurophysiological basis of functional recovery in the neonatal spinalized rat

It had been shown previously that, following spinalization of the neonatal rat on postnatal day 7 (PN7), at the middle thoracic level, there was spontaneous recovery of coordinated stepping in the hindlimbs, enabling the animal to execute quadrupedal locomotion, with an ataxic gait. No significant recovery occurred in rats that were similarly spinalized on PN14. Despite the functional recovery in the PN7 group, their hindlimbs were paralyzed when not in contact with a surface. In the present experiments, at 16-18 weeks after spinalization, muscle spindle GpIa and cutaneous afferents were tested for functional connectivity to the alpha motoneurons (a-MNs) that innervate the right triceps surae (TS) muscles. The Hoffmann reflex (H-reflex), the tonic stretch reflex (TSR), and cutaneous reflexes were recorded from the right TS muscles in the nonanesthetized, intercollicular decerebrate preparation. The H-reflex and the TSR were readily elicited from the PN7 animals, but not from the PN14 animals. The PN14 preparations were characterized by prolonged (> 18 h in 3/8 cases), spontaneous discharge of motor units, and prominent M responses. There was widespread, bilateral convergence of cutaneous afferents from the hindquarters to the a-MNs of the TS muscles in both the PN7 and the PN14 preparations. In the nonspinalized, control preparation, only ipsilateral, cutaneous afferents activated the right TS a-MNs. These results demonstrate that in the chronic (> 3 months after spinalization), spinalized PN7 rat, but not in the PN14 rat, there is a tight functional connectivity between the hindlimb GpIa afferents and their homonymous a-MNs.

Fusimotor effects of midbrain stimulation on jaw muscle spindles of the anaesthetized cat

The effects of electrical stimulation within the midbrain on fusimotor output to the jaw elevator muscles were studied in anaesthetized cats. Muscle spindle afferents recorded in the mesencephalic trigeminal nucleus were categorised as primary or secondary by their responses to succinylcholine during sinusoidal or ramp-and-hold stretches. Changes in their stretch responses during midbrain stimulation were then assessed by changes in bias and in dynamic sensitivity. Problems were encountered in interpreting changes in sine wave stretch responses of primary afferents, in some of which a very small change in firing pattern produced large changes in estimates of the response amplitude. Sine wave testing also sometimes over-estimated static effects and under-estimated dynamic effects relative to ramp responses. On other occasions a small amount of static fusimotor activity caused a marked increase in sine response amplitude, which could be wrongly interpreted as a dynamic effect. Consequently, ramp responses only were used for diagnosing fusimotor changes. The most effective region for producing pure dynamic fusimotor excitation was directly rostral to the red nucleus, extending dorsally and ventrally approximately in the course of the retroflex bundle. Stimulation of regions caudal and dorso-caudal to the red nucleus, previously designated as the mesencephalic area for dynamic fusimotor control of leg muscles, gave static or mixed static and dynamic effects on jaw spindles. The use of midbrain stimulation to identify fusimotor neurones of jaw muscles as static or dynamic would be most reliable with stimulation just rostral to the red nucleus and would require spindle afferent behaviour to be monitored at the same time with ramp stretches.

The size and dendritic structure of HRP-labeled gamma motoneurons in the cat spinal cord

We report quantitative data obtained from 60 fully reconstructed dendritic trees belonging to eight gamma-motoneurons (gamma-MNs) and six additional gamma-MNs that were not completely reconstructed. The cells were labeled intracellularly with horseradish peroxidase (HRP). These data are compared to measurements from 79 reconstructed dendrites belonging to seven documented alpha-motoneurons (alpha-MNs), supplemented by a larger sample of alpha-MNs labeled intracellularly or by retrograde transport with HRP. As expected from earlier studies, the soma dimensions and total membrane area of gamma-MNs were smaller than those of alpha-MNs. Although gamma-MN dendrites were, on average, slightly but significantly longer than those of alpha-MNs, the former had, on average, smaller diameter stem dendrites, less membrane area, and less profuse branching, and they tended to branch closer to the soma and to terminate farther from the soma. These differences were evident even when subsets of dendrites with similar stem diameters were compared. Some of the anatomical distinctions suggest that gamma-MNs are qualitatively as well as quantitatively different from alpha-MNs, even though the distributions of many of the morphological variables examined showed no abrupt discontinuities between the two motoneuron groups.

Discharge behaviour of feline gamma-motoneurones following induction of an artificial myositis

The study was undertaken to test the widely held hypothesis that a painful lesion of the skeleto-motor system leads to an increase in the neuromuscular component of muscle tone by activating gamma-motoneurones in the affected region. In chloralose-anaesthetized cats, artificial myositis was induced in the lateral gastrocnemius-soleus (LGS) muscle and several hours later the impulse activity was recorded from single gamma-motoaxons supplying the medial gastrocnemius (MG) muscle. Under the conditions of the study, the majority of the fusimotor neurones had a resting activity and could be readily excited by natural stimuli. In contrast to the assumptions of the working hypothesis, the gamma-motoneurones in the myositis animals were not activated but showed a strong inhibition; both resting activity and excitability by electrical and natural stimuli were decreased. Additional recordings from fusimotor neurones of a flexor muscle (tibialis anterior, TA) demonstrated that in the preparation used, the behaviour of the flexor gamma-motoneurones was different from extensor ones in that the former usually had no resting activity and did not respond to natural stimuli. The only discernible effect of a myositis of the LGS muscle on the TA gamma-motoneurones was a decrease in their electrical reflex threshold. The results of the study do not support the assumption that a painful muscle lesion is followed by an activation of the gamma-loop that leads to an increase in muscle tone. Instead, the data may offer an explanation for the weakness and--in chronic cases--the reflex atrophy of lesioned muscles.

Pathophysiological mechanisms involved in genesis and spread of muscular tension in occupational muscle pain and in chronic musculoskeletal pain syndromes: a hypothesis

This paper introduces a pathophysiological model for the cause of muscular tension and pain in occupational pain syndromes and chronic muskuloskeletal pain syndromes, which also might clarify why these conditions have a tendency to perpetuate themselves and spread from one muscle to another. The model can briefly be described as follows. Metabolites produced by (static) muscle contractions stimulate group III and IV muscle afferents, which activate gamma-motoneurones projecting to both homonymous and heteronymous muscles. The gamma-motoneurones influence the stretch sensitivity and discharges of secondary and primary spindle afferents. Increased activity in the primary muscle spindle afferents enhances the muscle stiffness, which leads to further production of metabolites in both homo- and heteronymous muscles. Increased activity in secondary spindle afferents, which project back to the gamma system, constitutes a 'built in' second positive feedback loop which may perpetuate the condition with less 'support' from activity in group III and IV muscle afferents.

Excitatory responses of the dorsal root discharge to stimulation of cutaneous and muscle afferents in the cat

Excitatory responses of the dorsal root discharge (DRD) consisting in transient increase in its frequency have been studied in non-anaesthetized low spinal cats. They were evoked by stimulation of cutaneous nerves (superficial peroneal and posterior tibial) and muscle nerves (gastrocnemius-soleus and posterior biceps-semitendinosus), recorded from the central cut ends of single fibres of L7 dorsal roots. The excitatory responses were elicited by single volleys in low threshold cutaneous and group II muscle afferents. Group III afferents of gastrocnemius-soleus nerve were also effective. Increase in strength of stimulation of posterior biceps-semitendinosus nerve from 4T to 40T which activated group III muscle afferents significantly decreased the excitatory effects of the DRD. Incidence of excitatory responses of the DRD to volleys both in low and high threshold cutaneous afferents, was 100%. Frequency of occurrence of excitatory effects to volleys in group II muscle afferents ranged from 23% to 43% (for responses to posterior biceps-semitendinosus and gastrocnemius-soleus volleys, respectively). It was increased to 47% and 72% when excitatory effects were elicited by group III afferent volleys. These findings indicate that only some types of afferent fibres evoke excitatory responses of the DRD. Variable incidence of the excitatory responses to stimulation of cutaneous and muscle afferents suggests important difference in effectiveness of connections between both types of fibres and interneurones generating the DRD.

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.

Reflex responses of fusimotoneurons to sinusoidal muscle stretching

Reflex response of fusimotoneurons to sinusoidal muscle stretching were investigated in decerebrated cats. Nerve impulses of single fusimotoneurons were recorded from thin filaments dissected from otherwise intact nerves to triceps surae muscles. Amplitude of the sinusoidal stretching of these muscles was 3 mm peak-to-peak and the frequencies 0.1-10 Hz. Electric muscle activity was also recorded in some experiments. Fusimotor responses were similar to those of the skeletomotoneurons in that both were advanced in phase with respect to muscle length changes, while their amplitude increased with increase in stretching frequency. Modulation at the frequency corresponding to the second harmonic of the input signal was predominant in fusimotor responses. It is supposed to appear mainly due to the convergence to fusimotoneurons of afferent impulses from different muscle receptors arriving after different delays. Its functional role is discussed.

Effects of electrical and natural stimulation of skin afferents on the gamma-spindle system of the triceps surae muscle

The aim of the present study was to investigate the extent to which skin receptors might influence the responses of primary muscle spindle afferents via reflex actions on the fusimotor system. The experiments were performed on 43 cats anaesthetized with alpha-chloralose. The alterations in fusimotor activity were assessed from changes in the responses of the muscle spindle afferents to sinusoidal stretching of their parent muscles (triceps surae and plantaris). The mean rate of firing and the modulation of the afferent response were determined. Control measurements were made in absence of any cutaneous stimulation. Tests were made (a) during physiological stimulation of skin afferents of the ipsilateral pad or of the contralateral hindlimb, or (b) during repetitive electrical stimulation of the sural nerve in the ipsilateral hindlimb, or of sural or superficial peroneal nerve in the contralateral hindlimb. Of the total number of 113 units tested with repetitive electrical stimulation of the ipsilateral sural nerve (at 20 Hz), 24.8% exhibited predominantly dynamic fusimotor reflexes, 5.3% mixed or predominantly static fusimotor reflexes. One unit studied in a preparation with intact spinal cord exhibited static reflexes at low stimulation intensities and dynamic ones at higher stimulation strengths. The remaining units (69%) were uninfluenced. When the receptor-bearing muscle was held at constant length and a train of stimuli (at 20 Hz) was applied to the ipsilateral sural nerve, the action potentials in the primary muscle spindle afferent could be stimulus-locked to the 3rd or 4th pulse in the train (and to the pulses following thereafter), with a latency of about 24 ms from the effective pulse. This 1:1 pattern of driving seemed to be mediated via static and/or dynamic fusimotor neurons. Natural stimulation influenced comparatively few units (3 of 65 units tested from the ipsilateral pad and 10 of 98 tested from the contralateral hindlimb), but when the effects were present they were quite large. The results are discussed in relation to previous studies on reflex control of fusimotor neurones from cutaneous afferents. It is suggested that the wide range of fusimotor effects from cutaneous afferent fibres observed in this study (from complete absence of any effect, via moderate excitatory and inhibitory effects, to the 'driving pattern', i.e. pulse-to-pulse response) may reflect that different gamma-motoneurones have individualized reflex profiles, and it may also indicate that groups of fusimotor neurones and spindle afferents play specific roles in different motor acts.

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.

Rubrospinal and rubrobulbospinal influences on dynamic and static gamma-motoneurones

In a series of experiments made on alpha-chloralose-anaesthetized cats, the effects evoked both by electrical stimulation of the rubrospinal (RS) and robrobulbospinal (RBSP) paths and by graded electrical stimulation of muscle, skin and joint nerves were investigated in the same sample of intra- and extracellularly recorded lumbar gamma-motoneurones (n = 120). The cells were classified as dynamic or static by stimulation of the RBSP, which is known to influence dynamic gamma-motoneurones selectively. The majority of classified cells, which responded to peripheral stimuli, were also responsive to NR stimulation. Multivariate analysis of the cell response patterns in gastrocnemius et soleus gamma-motoneurones indicated that in general the types of effects evoked via RS and RBSP could not be predited from the effects elicited from the periphery. Yet, at least for some cells, covariation between excitatory RS effects and excitatory responses from the superficial peroneal nerve and between inhibitory RS effects and inhibitory effects from group III muscle afferents was indicated. Hence it is tentatively suggested that there may exist functional subpopulations of gamma-cells with coupling between effects from the RS path and different peripheral afferent systems and that, if the RS path coactivates alpha- and gamma-motoneurones, then movements with simultaneous activation of certain afferent systems may favour alpha-gamma-linkage.

Fusimotor reflexes to antagonistic muscles simultaneously assessed by multi-afferent recordings from muscle spindle afferents

Several single agonist/antagonist primary muscle spindle afferents were simultaneously recorded in chloralose anaesthetized cats. It was shown that their dynamic and static sensitivity to sinusoidal muscle stretches could be increased or decreased via the fusimotor system by extension and flexion of the contralateral hind limb as well as by stretch of ipsilateral muscles and stimulation of ipsilateral skin nerves. The results seem to support the hypothesis that the primary muscle spindle afferents convey complex multisensory messages to the central nervous system (CNS).

Contralateral intramuscular acupuncture-like electrical stimulation differentially changes the short-latency responses to muscle stretch

Measurements were made from the human first dorsal interosseous and extensor digitorum communis muscles of the surface electromyographic activity reflexly produced by brief stretch of the muscle. For the first dorsal interosseous muscle, reflex EMG activity was also produced by electrical stimulation of the ulnar nerve at the wrist. The procedures were carried out before, during, and after 25 min of nonspecific, low-frequency electrical stimulation to the contralateral arm delivered through intramuscular electrodes. Control stimulation was delivered subcutaneously. The EMG recorded during a maintained contraction was rectified, filtered, and averaged. Two reflex components (M1 and M2) of the EMG response to muscle stretch or ulnar nerve stimulation were investigated. During nonspecific intramuscular stimulation to the contralateral arm, M1 responses of the extensor digitorum communis were depressed, initially by 37%. The effect began to fade during stimulation but extended beyond it. Reflex responses were elicited alternately by brief stretch of the first dorsal interosseus muscle and by electrical stimulation of the ulnar nerve in the same experiment. Nonspecific intramuscular stimulation to the contralateral arm depressed the M1 response to stretch, but had no effect on the M1 response to electrical stimulation. It is concluded that nonspecific intramuscular electrical stimulation reduces the amplitude of the M1 component of the response to brief stretch of contralateral muscle, either through depression of fusimotor activity or inhibition of oligosynaptic pathways that contribute to the early reflex response.

Efferent and afferent activity in a gastrocnemius nerve branch during locomotion in the thalamic cat

The firing patterns of alpha and gamma efferent fibres and of group I and group II afferent fibres innervating the gastrocnemius muscle were observed during spontaneous locomotor movements in the thalamic cat. Multi-unit discharges of each kind of fibre were obtained by electronic sorting of the action potentials from the whole activity of a thin branch of gastrocnemius lateralis or medialis nerve. The main results were: During the locomotor cycle the activity of the afferent and efferent populations was highly modulated. alpha- and gamma-motoneurones were co-activated within the locomotor cycle during ankle plantar-flexion. The gamma discharge began to rise earlier and to fall later than did the alpha discharge. The amplitude of the gamma discharge, unlike that of the alpha discharge, was largely independent of the vigour of walking. Between the cyclic discharges, most of gamma populations were tonically active whereas alpha populations were silent. Subgroups of the alpha and gamma populations were not usually activated according to the cell-size principle, but, the activation of the latest gamma subgroup always preceded that of the earliest alpha subgroup. Modulation of the group I and II afferent discharges was closely related to the cyclic length changes of the parent muscle. Fusimotor activation during the active shortening of gastrocnemius muscle prevented the afferent discharges from pausing. The pattern of afferent and efferent activity during selective curarisation of the extrafusal junctions indicated that the discharge of static gamma-motoneurones is modulated during the locomotor cycle. After curarisation of both extrafusal and intrafusal junctions, an efferent-discharge pattern of central origin persisted alternately in extensor- and flexor-muscle nerves (fictive locomotion). The durations of the fictive locomotor cycle and of the cyclic discharge in the sartorius nerve were increased as a consequence of the suppression of phasic afferent inputs to the C.N.S. Maintained ankle dorsi-flexion slowed the fictive locomotor rhythm and elicited opposite effects, respectively excitation and depression, on the magnitude of the alpha and gamma discharges. Maintained ankle plantar-flexion scarcely perturbed the duration of the fictive locomotor cycle, but the duration of the sartorius-nerve discharge lengthened at the expense of that of the gastrocnemius discharge. Both gastrocnemius alpha- and gamma-motoneurones were depressed, the former considerably more than the latter. The roles of the gastrocnemius afferents and gamma-efferents during the locomotor cycle are discussed in the light of these results.

The degree of short-term synchrony between alpha- and gamma-motoneurones coactivated during the flexion reflex in the cat

Cross-correlation analysis of unitary neuronal discharges has been used to study the linkage between alpha- and gamma-motoneurones coactivated during the flexion reflex of the semitendinosus muscle in the decerebrated spinal cat. A flexion reflex was elicited by firm grip or squeeze of the ipsilateral heel, shank or foot. The stimulus excited the discharges of both alpha- and gamma-motoneurones and increased the frequency of discharge of those gamma-motoneurones that had shown a background discharge prior to intentional stimulation. Short-term synchrony was present between a high proportion of semitendinosus gamma-motoneurones both for background discharges (sixteen out of nineteen pairs) and during the flexion reflex (thirteen out of fifteen pairs). All nineteen pairs of alpha-motoneurones examined during the flexion reflex showed short-term synchrony of discharge. Few alpha-motoneurones displayed background discharges but synchrony was observed in the two instances studied. The degree of synchrony was measured as the ratio (kappa) of the total counts contributing to the peak of the correlogram over the number expected by chance alone. The ratio was higher when the average frequency of motoneurone discharge was low. Kappa was generally higher for alpha-motoneurone pairs than for gamma-motoneurone pairs. The higher degree of synchrony for alpha-motoneurones reflected their lower discharge rates. During the flexion reflex the degree of synchrony between gamma-motoneurones was greater than expected for that same discharge rate in the absence of intentional stimulation. Only twenty-seven out of forty pairings of an alpha- with a gamma-motoneurone showed a significant degree of synchrony of discharge. On average, the degree of synchrony for alpha/gamma pairs was lower than that for either alpha/alpha or gamma/gamma pairings at the equivalent discharge rate. The results support the conclusion that coactivation of alpha- and gamma-motoneurones during the flexion reflex occurs largely through independent sets of interneurones. The possibility is discussed that those alpha-motoneurones which showed short-term synchrony with gamma-motoneurones were skeleto-fusimotor (beta-motoneurones) in nature.

Efferent discharges recorded from single skeletomotor and fusimotor fibres in man

Experiments were performed on awake human subjects in which single nerve fibre activity was recorded in the lateral peroneal nerve using tungsten micro-electrodes as described by Hagbarth & Vallbo (1967, 1968a). The discharge of twelve single efferent fibres innervating the tibialis anterior muscle (t.a.) or the extensor digitorum longus muscle (e.d.l.) was recorded. On the basis of their functional activity, six fibres were identified as skeletomotor and six as fusimotor fibres. Skeletomotor fibres, which were completely silent in relaxed subjects, discharged when subjects performed voluntary isometric or isotonic contractions, they also fired during Jendrassik's manoeuvre and tonic vibration reflex (t.v.r.) induced by mechanical vibration applied to the distal muscle tendon. Units considered as fusimotor fibres were generally spontaneously active with some fluctuation in the discharge frequency. Various tests used to identify afferent fibres elicited no response of these fibres (nor of the skeletomotor fibres). Efferent fibres were considered as fusimotor because their discharges were uncorrelated with any activation of extrafusal muscle fibres. Several means were used to detect activation of extrafusal fibres: surface electromyogram (e.m.g.) electrodes, tungsten electrodes deeply implanted in the muscle and especially the use of a high-sensitivity tension transducer (0 X 1 mN) placed on muscle tendons. The activity in fusimotor fibres could be either elicited or modulated under the following conditions: clenching of the fists, pinna twisting, mental computation, voluntary isometric contraction, passive phasic stretch of the muscle, environmental disturbances, subject laughing, the sound of hand clapping, and subject listening to manoeuvre instructions. Moreover, during spontaneous fusimotor fibre activity the subject was able to voluntarily stop the unit discharge. The results are compared to those obtained in animal studies and discussed with reference to the notion of alpha-gamma linkage, static and dynamic gamma-motoneurone activities, and to other available data concerning the effects of various stimulations on muscle spindle afferent activities in man.

Actions on gamma-motoneurones elicited by electrical stimulation of joint afferent fibres in the hind limb of the cat

Effects on seventy-one single lumbar gamma-motoneurones, evoked by graded electrical stimulation of fibres running in the posterior articular nerve of the ipsilateral knee joint (p.a.n.), were studied by micro-electrode recording in twenty-one cats anaesthetized with alpha-chloralose. Sixty-seven of the gamma-cells were classified indirectly as dynamic (thirty-seven) or static (thirty) using the method of mesencephalic stimulation (cf. Appelberg, Hulliger, Johansson & Sojka, 1982). A high general responsiveness (i.e. number of cells with effect/number of cells tested) was found for the whole sample of gamma-cells (91.9% for dynamic and 93.3% for static cells). The thresholds for the effects were related to the stimulation intensity at which the early negative cord dorsum potential appeared (T). For all subpopulations of gamma-cells (dynamic and static, flexor and extensor cells) excitatory as well as inhibitory effects were observed at 0.9-1.1 T, probably corresponding to 1.1-1.4 times the threshold for evoking a compound action potential in p.a.n. (cf. Discussion). In addition, a considerable number of high-threshold effects were found. Some cells were influenced only from low-threshold joint afferents, some only from high-threshold joint afferents and some cells were influenced from both low- and high-threshold joint afferents. No statistically significant differences in thresholds were found between dynamic and static cells. Among flexor gamma-cells excitatory effects were found to predominate, while for extensor gamma-cells excitation and inhibition occurred with about equal frequency. The shortest latencies for excitatory effects on dynamic gamma-motoneurones were compatible with a trisynaptic pathway, while the routes for excitation of static units and for inhibition of both types of gamma-cells seemed to be longer. The possible functional significance of the findings is discussed. The findings seem to support the idea, as suggested by Freeman & Wyke (1967b), that the joint receptors may contribute to the 'co-ordination of muscle tone in posture and movement' via the gamma-loop. It is furthermore suggested that the latter mechanism may serve to regulate joint stiffness and joint stability.

Fusimotor reflexes in triceps surae muscle elicited by stretch of muscles in the contralateral hind limb of the cat

Experiments were performed on twenty-one cats anaesthetized with alpha-chloralose. The aim of this study was to investigate the reflex effects on triceps surae and plantaris fusimotor neurones elicited by tonic stretch of the contralateral posterior biceps and semitendinosus (p.b.s.t.) and the contralateral triceps surae and plantaris muscles, to compare these effects with the effects evoked by flexion or extension of the intact contralateral hind limb (Appelberg, Hulliger, Johansson & Sojka, 1984) and to clarify the interactions between the reflexes from contralateral and ipsilateral muscles. Activity in fusimotor neurones was studied indirectly by recording from primary muscle spindle afferents of the triceps surae and plantaris muscle. The mean rate of firing and the modulation of the afferent response to sinusoidal extension of the triceps surae and plantaris muscles was determined. Control measurements were made with the ipsilateral p.b.s.t., the contralateral p.b.s.t. and the contralateral triceps and plantaris muscles relaxed. Tests were made with tonic stretch of one of these muscles alone or with two of them simultaneously. With stretch of the contralateral p.b.s.t. ten out of eighty-four primary afferents (11.9%) showed predominantly dynamic reflexes (six out of forty-one in spinalized preparations: 14.6%), twenty-two (26.2%) showed mixed or predominantly static effects (one spinalized: 2.4%) and fifty-two units (61.9%) showed no effect (thirty-four spinalized: 83.0%). The reflex effects could be reproduced by electrical stimulation of the cut contralateral p.b.s.t. nerve either at group II or at group III strength. With stretch of the contralateral triceps and plantaris muscles seventy out of seventy-six (92.1%) primary muscle spindle afferents showed no effect and six (7.9%) mixed or predominantly static reflex effects. In general, the reflex effects were not accompanied by detectable electromyographic (e.m.g.) activity in the ipsilateral triceps and plantaris (recorded with surface or needle electrodes), indicating that the reflexes mainly involved gamma-motoneurones. The difference in efficacy between contralateral flexor (p.b.s.t.) and extensor (triceps and plantaris) muscles seems to be in accordance with the response pattern found with extension or flexion of the intact contralateral hind limb (Appelberg et al. 1984).(ABSTRACT TRUNCATED AT 400 WORDS)

Actions on gamma-motoneurones elicited by electrical stimulation of cutaneous afferent fibres in the hind limb of the cat

The reflex actions elicited by graded electrical stimulation of hind-limb cutaneous (sural, superficial peroneal and tibial) nerves were investigated with intra- and extracellular micro-electrode recordings in gamma-motoneurones projecting to hind-limb muscles in twenty-four cats anaesthetized with alpha-chloralose. In total, reflex responses of 100 gamma-motoneurones were analysed. 82 of the gamma-cells were classified as dynamic (43) or static (39) using the method of mesencephalic stimulation (Appelberg, Hulliger, Johansson & Sojka, 1982). The general responsiveness (i.e. number of input nerves with effect/number of input nerves tested) of the whole sample of gamma-cells to stimulation of skin nerves was extremely high (94.8%). All negative observations were encountered among static and non-classified gamma-cells. Generally, the stimulation strengths needed for evoking effects in the gamma-cells were very low. A majority of the excitatory effects in the dynamic cells appeared with stimulation intensities below 1.5 threshold (T), while most static cells were excited with stimulation strengths between 1.5 and 2 T. Also a statistical comparison of the populations of stimulation strength thresholds for the excitatory effects revealed a significant difference (P less than 0.0009) between dynamic and static gamma-cells. By contrast, the thresholds for inhibitory effects in dynamic cells were slightly higher than for excitatory effects (P less than 0.0009). As regards excitation of static cells, inhibition of dynamic cells and inhibition of static cells, no statistically significant threshold differences were found. A strong dominance of excitation over inhibition was found in both dynamic and static flexor (posterior biceps and semitendinosus) gamma-motoneurones from all input nerves. In comparison to flexor gamma-motoneurones, there was a much higher incidence of inhibitory and mixed (excitatory and inhibitory) responses in extensor (triceps) gamma-motoneurones, from all nerves tested. For dynamic cells there was an about even balance between excitation and inhibition, while for static cells inhibition seemed to prevail. The latencies for excitatory effects in dynamic gamma-cells differed from those in static gamma-cells (P less than 0.027). The shortest latencies of excitatory effects found for dynamic gamma-cells indicate a disynaptic coupling, while for static cells the shortest route seemed to involve at least three synapses.(ABSTRACT TRUNCATED AT 400 WORDS)

Fusimotor reflexes in triceps surae muscle elicited by extension of the contralateral hind limb in the cat

Experiments were performed in thirty-two cats anaesthetized with chloralose. The aim of the study was to investigate the reflex effects of flexion or extension of the contralateral hind limb on ipsilateral fusimotor neurones, to compare these effects with the effects elicited by stretch of the ipsilateral posterior biceps and semitendinosus (p.b.s.t.) muscles (Appelberg, Hulliger, Johansson & Sojka, 1982) and to clarify the interactions between the reflexes elicited from the ipsilateral and the contralateral side. Activity in fusimotor neurones was studied indirectly by recording from primary and secondary muscle spindle afferents of the triceps surae muscle. The mean rate of firing and the modulation of the afferent response to sinusoidal extension of the triceps surae was determined. Control measurements were made with the ipsilateral p.b.s.t. muscles relaxed and the contralateral hind limb in resting position. Tests were made with stretch of the ipsilateral p.b.s.t. and/or extension/flexion of the contralateral hind limb. With extension of the contralateral hind limb 64 out of 210 primary afferents (30.5%) showed predominantly dynamic reflexes (41 out of 134 in spinalized preparations: 30.6%), 25 (11.9%) showed mixed or predominantly static effects (1 spinalized: 0.7%), 121 (57.6%) showed no effect (92 spinalized: 68.7%). Flexion of the limb gave, with only two exceptions, no observable effect. Thirty-three secondary afferents were investigated. Five responded to extension of the contralateral hind limb with excitatory reflex effects. Flexion did not influence the secondary afferents. Mostly the reflex effects were not accompanied by detectable electromyogram (e.m.g.) activity in the ipsilateral triceps (surface recordings), indicating that the reflexes mainly involved gamma-motoneurones. A comparison was made between the reflexes elicited by stretch of the ipsilateral p.b.s.t. and extension of the contralateral hind limb. The percentage of responsive units was higher for the contralateral stimulus. Spinalization almost abolished the statis reflex responses to both ipsi- and contralateral stimulation, and it increased the number of dynamic responses to ipsilateral stimulation. The ipsilaterally elicited reflexes also seemed more dependent upon background activity. Ipsilateral stimulus could facilitate or reduce a contralaterally evoked response, even when the ipsilateral stimulus alone gave no effect. The reflexes could also summate. Quite often combined stimuli changed the character of the reflex from dynamic to static or vice versa.(ABSTRACT TRUNCATED AT 400 WORDS)

Cortical projection of afferent information from tendon organs in the cat

In cats anaesthetized with chloralose, evidence has been sought for the projection of information from tendon organs to the sensory receiving areas of the cerebral cortex. Selective stimulation of afferent fibres from tendon organs has been achieved by raising the threshold to electrical stimulation of the fibres from primary endings of muscle spindles. The method uses longitudinal vibration at 200-250 Hz to elicit, over a period of 20 min, one impulse for each excursion of the vibrator from all of the spindles in the test muscle, soleus or medial gastrocnemius. The accumulated post-spike positivities following passage of the impulses are thought to be responsible for the rise in threshold. Segmental monosynaptic reflex testing after a bout of vibration was used to confirm that the residual Group I volley no longer contained impulses from muscle spindles. The volley in response to stimulating the nerve of the test muscle was timed to facilitate the monosynaptic reflex of a synergist. Before vibration 5- to 10-fold facilitation of reflex amplitude could be produced; however, after vibration, if all the spindle primary endings had been effectively engaged by the stimulus, no detectable facilitation remained. This test was found to be sensitive and reproducible. An afferent volley containing only activity of tendon organ afferents evoked small-amplitude potentials from the post-sigmoid gyrus of the contralateral pericruciate cortex. The field was highly localized and lay caudal to the main receiving area for activity from the sural nerve and from afferents of hip flexor muscles. Recordings with tungsten micro-electrodes revealed that the surface-evoked activity took origin in cellular discharges in the internal pyramidal layer of area 3a. Recent psychophysical experiments have provided evidence for a sense of muscle tension, as distinct from a sense of effort, and the tendon organ has been suggested as the likely receptor of origin. Our electrophysiological observations now provide a firm experimental basis for such a proposal.

Changes in fusimotor outflow during vibration-induced contraction of triceps surae muscles in decerebrate cats

Changes in fusimotor outflow in medial gastrocnemius and lateral gastrocnemius plus soleus nerves were investigated during vibration-induced contraction of triceps surae muscles in decerebrate cats. A sustained increase in spike occurrence appeared in 33 of 58 investigated neurons, and a decrease in 13. Transient changes of an opposite sign occurred in 19 excited and 6 inhibited fusimotor neurons during the rising phase of reflex muscle contraction. The later changes appeared coincidently with silence in the electromyogram. Thus the discharge of one-third of the examined fusimotor population directed to the triceps surae muscles was modulated parallelly with that of the skeletomotor neurons. It is supposed that the changes in fusimotor spike occurrence during the rising phase of reflex muscle tension are induced mainly by changes in afferent input from muscle spindle primary endings and/or Golgi tendon organs. We could not ascertain whether the different responses (excitation vs inhibition) are related to fusimotor type or to changes in transmission through the reflex pathways. The possible influence of the changes in fusimotor outflow on muscle spindle pause and the silent period in skeletomotor discharge at the beginning of reflex muscle contraction induced by vibration is considered.