Effects of temperature on the discharges of muscle spindles and tendon organs

1. In anaesthetized cats the effects of temperature on the nervous outflow from skeletal muscle via thick myelinated afferent fibres were studied. Single unit recordings were made from afferents of muscle spindles and tendon organs during slow and fast temperature changes of the medial gastrocnemius muscle which was deefferented by ventral root section and prestretched to a tension of 100 p.
2. Group I afferent units from muscle spindles were activated by warming and depressed by cooling, the effect of warming being much more pronounced than that of cooling. Afferents from secondary spindle endings with a high background discharge behaved similar to Ia fibres, whereas those with a low initial discharge rate showed an activation by cooling and a depression (mostly to cessation of firing) by warming. The discharges of group I afferents from tendon organs varied; an activation by warming was the most frequently observed reaction.
3. Some of the afferents from muscle spindles and tendon organs showed signs of a dynamic sensitivity to thermal stimulation, but in general the dynamic component in the responses to temperature changes was only small.
4. The results suggest that the afferent outflow via thick myelinated fibres from a resting, moderately prestretched muscle strongly depends on temperature. At raised intramuscular temperatures (about 42°C) the nervous outflow is characterized by an increased activity in all of the I a and many of the I b afferents, while the majority of group II spindle afferents will be depressed. In contrast, in a cold muscle (about 29°C) the nervous outflow via afferents from primary spindle endings will be reduced, while the net activity from secondary spindle endings will be increased and no marked changes are expected to occur in the discharges of I b fibres.

     

Characteristic curves of the dynamic response of primary muscle spindle endings with and without gamma stimulation

1. The characteristic curves of the dynamic response of primary muscle spindle endings (i.e. the dependence of dynamic response on the stretch volocity) have been recorded with and without gamma stimulation in 75 muscle spindles of the cats' M. tibialis anterior. They can be described by a power function DR=a · v _r. ⁿ
2. Under dynamic fusimotor stimulation the prefactor ‘a’ increases and the exponent ‘n’ decreases; under static gamma stimulation vice versa.
3. The characteristic curves with and without gamma stimulations of a single spindle are intersecting, because of the change of the exponent ‘n’.
4. The dyamic response under dynamic gamma stimulation usually increases, but we observed a decrease, too. Under static gamma stimulation the dynamic response usually decreases.
5. In the following we looked on the gamma stimulations statistically. We had 47 dynamic gamma stimulations. Here we observed a separation into an upper and a lower group of the characteristic curves, which proved to be statistically significant, according to their effects on the course of the characteristic curves and the velocity sensitivity.
6. The lower group of the dynamic gamma stimulations and all 24 static gamma stimulations do not differ significantly i) from the group of characteristic curves without gamma stimulation and ii) in the absolute course of the characteristic curves and in the velocity sensitivity under gamma stimulation.
7. The upper group of dynamic gamma stimulations is the only one to cause significantly strong effects on the course of the characteristic curves and velocity sensitivity.
8. The upper group of dynamic gamma fibre stimulations extends the measurable velocity range to very small stretch velocities, which otherwise do not cause a measurable dynamic response.

     

Reflex activation of muscle spindles in human pretibial muscles during standing

1. Experiments were performed in standing subjects to determine whether low-threshold cutaneous and muscle afferents from mechanoreceptors in the human foot reflexly influence fusimotor neurons innervating pretibial flexor muscles. Recordings were made from 30 identified muscle-spindle afferents, four tendon-organ afferents, and one alpha-motor axon innervating the pretibial flexor muscles. The subjects stood without support or vision on a force platform while trains of electrical stimuli (5 stimuli, 300 Hz) were delivered at nonpainful intensities to the sural nerve or to the posterior tibial nerve at the ankle. 2. Seventeen of the 30 spindle endings had no background discharge, and none was activated by the sural or posterior tibial stimuli. Five silent afferents were given a background discharge by sustained pressure on the relevant tendon, but with two the discharge was dominated by a tremor rhythm obscuring any reflex response to the stimuli. Based on peristimulus time histograms (PSTHs), the sural stimuli then produced increases in discharge of two of the remaining three endings at latencies of 84 and 90 ms. These effects could not be explained by muscle stretch and are presumed to have been fusimotor mediated. 3. When the subjects stood freely without support or vision, 13 muscle-spindle endings had a background discharge, but with three endings tremor developed at the ankle and dominated the spindle discharge. Sural stimuli affected the discharge of five of nine endings unaffected by tremor. With three of these endings, there were changes in discharge that could be explained by muscle stretch.(ABSTRACT TRUNCATED AT 250 WORDS)     

Post-contraction changes in human muscle spindle resting discharge and stretch sensitivity

Summary The activities of human muscle spindle primary endings were recorded in the lateral peroneal nerve using the microneurographic method. The aim of the study was to test whether voluntary isometric contraction causes any after-effects, first in the resting discharge of muscle spindle primary endings and secondly in their responses to a slow ramp stretch. To investigate the latter point, the initial angular position of the ankle was passively adjusted until the unit fell silent, in order to introduce a delay in the responses to muscle stretch. The results were as follows: (1) most of the units did not exhibit the post-contraction sensory discharge reported to occur in numerous animal experiments; this means that the muscle spindle resting discharge was essentially the same before and after isometric voluntary contraction. (2) Isometric voluntary contraction led to changes in muscle spindle stretch sensitivity which resulted in a reduction in the stretch threshold and a decrease in the muscle spindle dynamic sensitivity. These data suggest that the after-effects observed may have been triggered by static fusimotor neurones. The results are discussed with reference to the theory according to which the processing by the CNS of muscular proprioceptive messages deals mainly with signals arising from muscles stretched during movement, and it is concluded that the coactivation of and y motoneurones during the contraction facilitates the coding of the parameters of forthcoming stretching movements, by the muscle spindles.     

The effects of muscle cooling and stretch on muscle spindle secondary endings in the cat.

1. The objectives of the investigation were to identify the muscle spindle endings which respond to cooling of the relaxed muscle and to study their response to stretch. 2. The discharge of single afferents from 162 de-efferented muscle spindles in the relaxed medial gastrocnemius muscle of the anaesthetized cat was studied in vivo during cooling of the muscle from 37 to 24 degrees C. Temperature measurements were made at the inner surface of the muscle, while cooling (never below 15 degrees C) was applied at the skin over the muscle. 3. The endings were classified as primary or secondary endings on the basis of their conduction velocity, the dividing line being set at 70 m/sec. A response to cooling was obtained only from endings with afferents conducting at velocities of 20-70 m/sec. These fifty-six endings (CR) represented 65% of the secondary endings studied; the remaining secondary endings (NCR) and the primary endings showed no activity during cooling of the relaxed muscle. 4. During maintained stretches of 4-12 mm, activity of the NCR and primary endings decreased when the muscle was cooled. Cooling affected the CR endings in the same way, but only if the muscle was stretched 6 mm or more. During a smaller maintained muscle stretch, cooling caused an increase in CR activity, superimposed on the response to stretch. 5. The response to a 10 mm stretch at velocities of 10-70 mm/sec was studied in twenty-six CR, eleven NCR and twenty-one primary endings. 6. The dynamic responses of CR endings were intermediate between those of the primary endings and NCR endings. For any velocity of stretch the mean dynamic index of the CR endings was significantly greater than that of the NCR endings but significantly less than that of the primary endings. 7. The mean static responses of the CR and primary endings, measured 0-5 sec after the end of ramp stretch, were the same and significantly greater than that of the NCR endings. 8. The results indicate that cooling of the relaxed mammalian muscle may be used to differentiate between primary endings and about two-thirds of the secondary endings. The remaining secondary endings can be recognized by their small dynamic and static response to stretch.     

The response to stretch of muscle spindle afferents of baboon''s tibialis anticus and the effect of fusimotor stimulation

1. Systematic studies of the response of baboon's tibialis anticus muscle spindles to stretch were undertaken. Most of the spindle afferents studied had conduction velocities between 72 and 78 m/sec with a range from 39 to 93 m/sec. There was no clear bimodality in the histogram of the conduction velocity. Measurements were made of the axon diameters of the nerve to tibialis anticus. The largest number of the larger axons had diameters from 11 to 13 mu. The largest axon diameter measured 17 mu and there were very few of these.2. The dynamic index for any given afferent tended to be greater for the more rapidly conducting afferents and lower for the more slowly conducting afferents. However, a statistically significant regression line of this relationship could only be drawn for a stretch velocity of 64 mm/sec.3. The spindle afferent response to stretch was studied from different muscle lengths. It was found that the velocity sensitive portion of the response to phasic stretch decreased when the stretch extended up to or beyond the maximum physiological length of the muscle.4. Dynamic and static fusimotor fibres were isolated. The response of the spindle afferent to stretch, while the dynamic fusimotor fibre was being stimulated, was the same as that reported for the cat by P. B. C. Matthews (1962). During static fusimotor stimulation the response of the spindle afferent to stretch was usually like that reported for the cat. In a single case, however, static fusimotor stimulation resulted in a lowering of the peak frequency of the response of the spindle afferent to the phasic portion of stretch.     

Mechanical influence of the extrafusal muscle on the static behaviour of deefferented primary muscle spindle endings in cat

Summary In 16 anesthetized cats 24 afferents of deefferented primary muscle spindle endings from the extensor digitorum longus muscle (EDL) were functionally isolated and their sensitivity to static stretches was tested. Each spindle was carefully located within the EDL using direct focal stimulation. Muscle spindle endings with poor static length sensitivity of stretch were always found in the most peripheral parts of the muscle fibre bundles, near to the aponeurotic and tendineous insertions. Endings having good static sensitivity to stretch were exclusively found in more central portions of the muscle fibre bundles. Using a photographic method, the alterations in the distance between designated points on the muscle surface ware measured during maintained muscle stretch. Only medial portions were markedly extended, whereas little or no increases in length occurred in peripheral parts of the muscle fibre bundles.Quantitatively, a direct relationship was found between the lengthening of those muscle portions, in which the muscle spindles were localized, and the receptors' sensitivity to maintained stretch.Preliminary reports of some of the results were given at the 1. Wissenschaftliche Tagung der Deutschen Gesellschaft für medizinische Physik (Rieboldet al., 1970; Robrechtet al., 1970) and at the Muscle-Spindle Symposium of the Anatomical Society of Great Britain and Ireland, April 4–6, 1974, at the University of Durham (Henatschet al., 1974). The results were presented in more detail in Meyer-Lohmann's Habilitationsschrift (1972).Supported by a grant from the Deutsche Forschungsgemeinschaft (Schwer-punktprogramm Rezeptorphysiologie).     

The effects on primary and secondary spindle afferents of fusimotor reflex during ramp and sinusoidal muscle stretch

Summary Tetanic stimulation of ipsilateral Triceps surae afferents leads to a reflex increase in the initial burst from the primary spindle afferents during ramp stretch in both intercollicular decerebrate and spinal preparations. The dynamic index does not change significantly.Slow, low-amplitude sinusoidal muscle stretches (1.6 Hz, 0.55 mm peak-peak) disclose an increased dynamic sensitivity of primary afferents during reflex activation. The same activation in the statically adapted muscle results in a great many cases in little or no increase in spindle discharge rate.In the frequency-stretch relation, the discharge rates of primary afferents in the spinal cat and of secondaries in decerebrate animals are greatly increased by combined fusimotor activation and low-amplitude sinusoidal stretch.We conclude that both static and dynamic motoneurons are implied in this reflex.     

The independence of corticomotoneuronal and fusimotor pathways in the production of muscle contraction by motor cortex stimulation

1. The response of thirty spindle afferents was studied during cortical stimulation of the ;best point' for eliciting a contraction of tibialis anticus in baboons anaesthetized with N(2)O/O(2) mixtures supplemented by small doses of barbiturates.2. No evidence of fusimotor activity before a contraction of the muscle was found. Evidence of fusimotor activity during the contraction and after the cortical stimulus was found. The various problems associated with interpretation of this activity are discussed.3. Depression of the spindle afferent response by tetanic cortical stimulation, which produced no detectable tension change, was found for four spindle afferents. This depression was particularly well documented for one spindle afferent. The depression could be explained by a decrease of static fusimotor activity.4. The spindle afferent response to muscle stretch was studied during light anaesthesia. The effect of barbiturates on this response suggested a depression of static fusimotor activity.     

Reflex control of the retractor bulbi muscle in the cat

The effects of vestibular and trigeminal stimulation on reflex responses of each slip of the retractor bulbi muscle were tested by recording the electromyogram.

1. In “encéphale isolé” cat, phasic electrical stimulation of the horizontal canal induced no response in the RB slips. Repetitive vestibular stimulation did not produce nystagmus in the RB muscle while strong muscular discharges were observed in the nystagmus lateral rectus muscle.
2. In anaesthetized cats, three trigeminal inputs elicited strong reflex responses in each slip of the RB muscle. Electrical stimulation of the vibrissae or the infra-orbital nerve evoked a two component reflex response (latencies: 5ms±0.5 and 14ms±2). Electrical stimulation of the supraorbital nerve elicited a single component reflex response (latency: 6ms±0.5). Electrical stimulation of the long ciliary nerves evoked a complex response with four components (latencies: 7.5ms±0.5, 10ms±2,15ms±2,20ms±2)
3. Pentobarbital and morphine produced lasting depression of the reflex responses of the RB muscle. The depressive effect of morphine was reversed by naloxone.

     

The dependence of cardiac membrane excitation and contractile ability on active muscle shortening (Cat papillary muscle)

1. A quick release of an isometrically contracting cat papillary muscle results in a depression of the ability to redevelop tension (deactivation) and an increase in the duration of the accompanying action potential (prolonged depolarization). The nature of the mechanical perturbation influencing both phenomena was investigated.
2. The prolongation of the action potential depends on the amplitude of the release and the time it is applied and, provided quick release-quick restretch cycles of less than 50 ms are used, on the duration of the cycle.
3. No change in action potential duration is observed, if initial muscle length, or the velocity of shortening is altered, or if the muscle is stretched at any time during contraction.
4. Although stretches and releases both have a “deactivating” effect on the muscle the effect is more pronounced with releases. This difference in “deactivation” is related to the prolongation of the action potential in so far as it is also controlled by the time and extent of release and release-restretch cycle duration, and is independent of shortening velocity.
5. Caffeine (8 mmol/l) in the bathing solution prolongs isometric tension development whilst the duration of the action potential is relatively unchanged. Under these conditions release-restretch cycles applied at times when the membrane has apparently repolarized, produce a deactivation and an after depolarization which can reach threshold to elicite an action potential.
6. If the membrane is partially depolarized by increasing extracellular potassium to 20 mmol/l, release-restretch cycles still induce deactivation but no change in the action potential.
7. The results are in keeping with the hypothesis that shortening during contraction partly contributes to the deactivating effect by reducing the concentration of internal free ionic calcium. This change in [Ca]_i decreases the outward potassium current to produce a prolongation of depolarization which can take the form of an increase in action potential duration or an afterdepolarization wave.

     

Acetylcholine-sensitivity and local regenerative activity in denervated frog slow muscle fibres

1. The distribution of acetylcholine (ACh)-sensitive membrane areas was determined in 11–75 days denervated slow muscle fibres ofRana temporaria by iontophoretic application of acetylcholine. The fibres were also stimulated directly, and their electrical activity was recorded with an intra- and an extracellular electrode.
2. During the first two weeks following denervation the size of the ACh-sensitive fibre surface was similar to that of normal slow fibres, but a spreading out of ACh-sensitivity occurred between the 13th and 20th day.
3. The slow fibre membrane did not become homogeneously ACh-sensitive; even after long periods of denervation large local sensitivity gradients could be observed. Throughout the denervation period maximum values of ACh-sensitivity were in the same range as in normal slow fibres.
4. Action potentials were fully developed when ACh-sensitivity started to spread out. Extracellularly recorded inward currents varied in amplitude along the fibre surface, and either one or two peaks were observed in individual fibres.
5. The spatial relationship between inward current peaks and peaks of ACh-sensitivity was investigated in 12 fibres. Fifteen inward current peaks were located at distances of 30–640 μm from points of maximum ACh-sensitivity; only once did the centers of ACh-sensitivity and excitability coincide.
6. It is concluded that Na channels are incorporated into nonjunctional membrane areas of denervated slow fibres; this process preceds the incorporation of ACh-receptors by approximately one week.

     

Long-lasting discharge of postganglionic neurones to skin and muscle of the cat's hindlimb after repetitive activation of preganglionic axons in the lumbar sympathetic trunk

1. Postganglionic neurones to hairy skin and to skeletal muscle of the cat's hindlimb, probably vasoconstrictor in function, have been investigated for their response characteristics on repetitive stimulation of the preganglionic axons in the lumbar sympathetic trunk in chloralose anaesthetized cats.
2. Repetitive stimulation of the preganglionic axons with 50 stimuli at 25 to 50 Hz and stimulus strengths which excited all classes of preganglionic axons generated eartly short latency responses and late low frequency responses in postganglionic neurones. These late responses occurred predominantly in postganglionic neurones supplying skeletal muscle and in only 30% of postganglionic neurones supplying hairy skin. In most neurones the late discharges could only be elicited by trains of more than 5–10 preganglionic stimuli at frequencies of more than 5–10 Hz.
3. The threshold stimulus strength applied to the preganglionic axons to generate the late discharge was on average 4 times higher than for the early discharge. This finding implies that the late discharge requires the activation of thin preganglionic axons.
4. In most postganglionic neurones, the late discharges could be blocked by atropine, indicating that they were generated by muscarinic action of acetylcholine. The late discharges in a few cutaneous postganglionic neurones were unaffected by atropine. The early discharges were blocked by hexamethonium.
5. The results argue that the transmission of impulses from pre- to postganglionic neurones may be accomplished by two separate pathways acting via nicotinic and muscarinic receptors respectively.

     

Inotropic effects of potassium rich solutions on frog cardiac muscle

1. Inotropic effects of potassium rich solutions on frog cardiac muscle have been studied both in current clamp and voltage clamp conditions, using a double sucrose gap apparatus.
2. Potassium rich solutions cause either a positive or a negative inotropic effect, together with an increase or a decrease in the duration of the action potential, according to the preparation.
3. The phasic phase of contraction and the slow inward current are decreased in amplitude; the reversal potential of the slow inward current is shifted towards more negative values.
4. The tonic phase of contraction is first decreased, then increased; the effects are correlated with modifications of the background current, initially in the inward, then in an outward direction.
5. The tension level obtained in contracture experiments is increased or decreased, according to the direction of the changes in the background current.
6. The effects of potassium rich solutions are still observed in the presence of ouabain, suggesting that they are independent of any effect on the sodium-potassium pump.
7. The effects of potassium rich solutions are still observed when external sodium is replaced by sucrose; they disappear (except the effect on the background current) when external sodium is replaced by lithium.
8. The results, which indicate that potassium ions play a role in the regulation of the intracellular concentration of calcium ions, are discussed in relation to a possible K/Ca exchange mechanism, to the Na/Ca exchange and to the role of intracellular calcium stores.

     

Intercellular coupling in frog heart muscle

1. Passive electrical parameters of bullfrog atrial trabeculae were measured in a single gap arrangement. Attention was focussed on the resistance of internal longitudinal pathway. The influence of external Ca²⁺ depletion was tested using EGTA as chelating agent.
2. Morphometry of trabeculae, fine structure of junctional complexes, and distribution of membrane-bound Ca were investigated by light and electron microscopic methods.
3. The specific internal resistance to longitudinal current flow was 523 Ωcm with normal Ringer as perfusing fluid and 1140 Ωcm in EGTA-containing solution. These values are considered to represent the sum of myoplasmic and junctional resistivity.
4. Morphometrical studies indicated an interstitial space of 12%, a mean cell length of 358 μm, and a mean cell diameter of 3.2 μm.
5. In freeze-fractured preparations junctional structures were observed in the form of “atypical gap junctions” consisting of 10 nm particles arranged in a circular or linear array. The number of gap junctions was estimated to range between 20 and 50/cell which is equivalent to a junctional area of 0.01 or 0.03% of total surface area. A mean number of 55 particles/gap junction was calculated. After 20 min of exposure to EGTA the majority of junctional complexes were converted to clusters; the number of particles/gap junction was not significantly altered.
6. The fluorescent dye CTC was used as a probe for membrane-bound Ca of isolated living cells. In normal Ringer a strong fluorescence was seen at the cell surface and in different intracellular compartments. With EGTA both superficial and internal fluorescence disappeared completely.
7. From a combination of electrical and morphometrical data the resistance of intercellular junctions was calculated. Under normal conditions the specific resistance of junctional membrane amounted to 0.4 Ωcm² and the resistance of an individual connexon was of the order of 10¹¹ Ω. With EGTA, the respective values were increased by about 230%. The mechanism underlying this depression of junctional conductance is not clear. It seems not related to a rise of cytoplasmic free Ca²⁺. The EGTA-induced increase in internal resistance was reflected by a decrease of the length constant of a bundle.
8. The nature of “atypical gap junctions” and their relation to tight junctions are discussed. It is concluded that the junctions observed in frog atrial muscle are analogous to gap junctions of insect or mammalian cells in spite of the different size and arrangement of the particles.
9. A theoretical model is presented for the electrical behaviour of a bundle in a single gap arrangement. The bundle is simulated by a partially isolated cable of finite length. Voltage distribution along the cable in response to a d.c. pulse applied externally is described as a function of distance and time.

     

Properties of cat neck muscle spindles and their excitation by succinylcholine

Summary The sensitivity to sinusoidal stretching and small-amplitude vibration, and the variability of the resting discharge rate of de-efferented muscle spindles in the neck extensor muscle biventer cervicis (b.c.) of the cat have been studied. The effects of intra-arterial infusion of succinylcholine (SCh) on the response of the receptors to ramp stretches of the muscle were also determined. When activated by SCh, one group of afferents showed only a slow facilitation of their discharge rate, similar to that of spindle secondary sensory endings in hind-limb muscles. A second group of afferents developed a large dynamic response and a marked increase in their static discharge rate and were presumed to originate in normal b₁b₂c spindles in the b.c. muscle. A third group of afferents developed only a marked increase in static discharge, without potentiation of the dynamic response, suggesting an origin in the b₂c units of tandem spindles which are relatively common in the neck muscles. On the basis of their passive characteristics alone, afferents from b₁b₂c units could not be readily distinguished from those from b₂c units. The characteristics of these receptors, and their differences from the well-studied hind-limb spindle afferents, are discussed.     

Increased intra-articular concentration of bradykinin in the temporomandibular joint changes the sensitivity of muscle spindles in dorsal neck muscles in the cat

The aim of the present study was to investigate to what extent activation of bradykinin-sensitive nerve endings of the temporomandibular joint (TMJ) might induce changes in the muscle spindles output from neck muscles through reflex effects on cervical fusimotor neurones. To this end, 26 muscle spindle afferents (MSAs) emanating from the trapezius and splenius muscles of the anaesthetised cat (alpha-chloralose, initial dosage 60 mg/kg) were recorded during injection of Bradykinin (BK) (12.5-50 microg/ml) in the ipsilateral TMJ. Fifteen (58%) MSAs exhibited statistically significant fusimotor effects to injection of BK into the TMJ. Of the 15, ten MSAs showed a response related to activation of static fusimotor neurones, three MSAs showed a response related to an activation of both static and dynamic fusimotor neurones and two MSAs showed a inhibition of both static and dynamic fusimotor neurones. The control experiments suggests that the effects observed were due to activation of BK sensitive nerve endings in the TMJ. It seems possible that the reflex connections between TMJ nociceptors and the fusimotor-muscle spindle system of dorsal neck muscles might be involved in the pathophysiological mechanisms responsible for the sensory-motor disturbances in the neck region often found on patients with temporomandibular disorders.     

Effects of fusimotor stimulation on dynamic and position sensitivities of spindle afferents in the primate.

The effects of stimulation of single static and dynamic fusimotor fibers on the dynamic sensitivity and position sensitivity of primary and secondary spindle afferents have been studied in the soleus muscle of the baboon. Static fusimotor fibers decreased the mean dynamic sensitivity of primary afferents at all rates of stretch and stimulation. The magnitude of the decrease in dynamic sensitivity increased as the rate of fusimotor stimulation was increased. Qualitatively similar effects were observed in secondary afferents. Static fusimotor stimulation had a strong excitatory effect on spindle afferent resting discharge and greatly increased the mean position sensitivity of both primary and secondary afferents. Dynamic fusimotor fibers increased the mean dynamic index of primary afferents at all rates of stretch and stimulation. The effect of dynamic fusimotor fibers on the mean dynamic sensitivity, however, was dependent on the rate of muscle stretch; at rates below 15 mm/s the dynamic sensitivity was substantially increased, whereas at rates greater than 15 mm/s it was either unchanged or decreased. Dynamic fusimotor fibers slightly decreased the mean position sensitivity of primary afferents.     

Effects upon spindle discharges of electrical stimulation of static fusimotor fibers with concomitant application of muscle vibration.

Discharges of single afferent fibers from the primary endings of the soleus muscle spindles were recorded from thin dorsal root filaments in cats anesthetized with urethane and chloralose. The distal cut end of the ventral root was split into fine filaments to obtain functionally single fusimotor fibers. The fusimotor fibers obtained in this study were of the static type. The soleus muscle was sinusoidally stretched at 70 Hz with various amplitude concurrently with 100 Hz electric stimulation of fusimotor fiber. The spindle afferent discharges were analysed by compiling inter-spike interval histograms and cross-correlograms between the afferent spikes and the stimulus pulses applied to the fusimotor fiber. The same analysis was also made between the afferent spikes and peak extensions of muscle yielded by vibratory stimulation. One-third of the fusimotor fibers were capable of driving the spindle afferents. The driving of fusimotor stimulation was replaced by driving by muscle dibration of more than 10 mum amplitude applied concurrently with fusimotor stimulation. The remaining two-thirds of the fusimotor fibers could not drive the spindle afferents. In this case, the driving by muscle vibration was obtained when vibration of more than 5 mum amplitude was applied concurrently with fusimotor stimulation. It was suggested that fusimotor fibers which produced driving of the spindle afferents would terminate on nuclear chain fibers and those not producing driving on nuclear bag fibers, or the latter would terminate relatively distant from the primary ending as compared with the former.     

Analysis of the dynamic responses of deefferented primary muscle spindle endings to ramp stretch

Summary A detailed analysis of the complex dynamic response of 8 deefferented primary muscle spindle endings to ramp stretches of the extensor digitorum longus muscle (EDL) was made in anaesthetized cats. The analysis was based on Lennerstand's linear muscle spindle model, in which the dynamic peak, i.e. the peak frequency at the end of the dynamic phase of a ramp stretch, is assumed to consist of three components: a position response, a slow velocity response, and a quick velocity response. The components of the dynamic peaks analysed were fairly well correlated with the static behaviour of the primary muscle spindle endings, the latter, in turn, being regarded as an indicator of the spindle's location in the non-homogeneous extrafusal muscle. The results thus provide a satisfactory explanation of the high correlations between the static and gross dynamic behaviour of deefferented primary muscle spindle endings of the EDL previously reported from our laboratory.A preliminary report of some of the results (Henatsch et al., 1974) was presented at the Symposium on Muscle Spindles held by the Anatomical Society of Great Britain and Ireland at the University of Durham, England April 4–6, 1974.Supported by grants from the Deutsche Forschungsgemeinschaft (Schwerpunktprogramm Rezeptorphysiologieand SFB 33: Nervensystetm und biologische Information, Göttingen).