The proprioceptive activation of stretch tension in pretibial flexor muscles induced by stimulation of antagonistic muscle afferents.

1. The effect of tetanic stimulation of ipsilateral group I afferents from the GS muscle on a synchronous stretch of the flexor EDL/TA muscles has been investigated in precollicular decerebrate cats. 2. The stretch-induced tension of the EDL/TA muscles increases remarkably during simultaneous stimulation of the GS nerve with maximal intensities corresponding to 1.5 times the threshold for the group 1 afferents. This increas appears above all in the dynamic part of stretch. 3. Under our experimental conditions there is no activation fo flexor-alpha-motoneurones during tetanic stimulation of the GS afferents without muscle stretch, as measured by the resting tension of the EDL/TA muscles. 4. Desptie an increase in the stretch-induced tension during fusimotor stimulation of antagonistic group I afferents, a transmission loss in the excitation via the psi-loop to the flexor-alpha-motoneurones occurs. This could be demonstrated by the ratio: increase in the tension T /increase in the number of Ia spikes. This is explained by snychronous convergence of the discharges of Ia inhibitory interneurones to the flexor-alpha-motoneurones. 5. The system studied demonstrates an input-output relation of the stretch reflex during conditions in which both reciprocal inhibition and autogenetic excitation via the psi-loop occur. It appears however, that the reciprocal inhibition is partly overwhelmed by the autogenetic excitation which results from the increase in the Ia discharge rate during fusimotor reflex. 6. It is postulated that static rather than dynamic psi-moto-neurones are involved in the investigated reflex arc.     

Stretch-elicited calcium responses in the intact mouse thoracic aorta

Stretch-elicited intracellular calcium ([Ca(2+)](i)) changes in individual smooth muscle cells in a ring of aorta were measured simultaneously with the force developed by the ring. A phasic increase in [Ca(2+)](i) was observed in 30% of the cells and a sustained one in 10%. Depletion of intracellular calcium store by thapsigargin and caffeine decreased phasic and increased sustained calcium responses. The inhibition of calcium entry either by stretching the aorta in a calcium-free medium or by the inhibition of stretch-activated, non-selective cationic channels by 5 microM GsMtx-4 toxin, decreased the proportion of sustained [Ca(2+)](i) responses but increased transient responses. In this condition, a third of the cells responded to stretch by a bursts of [Ca(2+)](i) spikes. The decrease of calcium influx triggered the generation of burst of calcium spikes after the application of stretch steps to the vascular wall. We conclude that progressive recruitment of smooth muscle cells is the mechanism underlying the force-generating part of the myogenic response. Two types of stretch-elicited calcium responses were observed during the recruitment of the smooth muscle cells. One was a phasic calcium discharge generated by the sarcoplasmic reticulum. The second was a tonic response produced by the activation of the stretch-sensitive cationic channels allowing extracellular Ca(2+) entry.     

The relative sensitivity of Renshaw cells to orthodromic group Ia volleys caused by static stretch and vibrations of extensor muscles.

1. Activity of Renshaw cells monosynaptically excited by ventral root stimulation and disynaptically excited by electric stimulation of the group Ia afferents in the gastrocnemius-soleus (GS) nerve, was recorded in precollicular decerebrate cats. The response of these units to prolonged vibration applied longitudinally to the deefferented GS muscle was then compared with that elicited by static stretch of the homonymous muscle, for comparable frequencies of discharge of the group Ia afferents. 2. Small-amplitude vibration of the GS muscle at 200/sec for one second produced a sudden increase in the discharge rate of Renshaw cells, which gradually decreased within the first 100 msec of vibration to reach steady albeit lower level than that obtained during the first part of vibration. The response of the Renshaw cells during the first 100 msec of vibration (phasic response) and that elicited during the last 500 msec of vibration (tonic response) were evaluated for different frequencies of sinusoidal stretch. The mean increase in the firing frequency per imp./sec in the Ia afferents was also calculated using the total one-second period. 3. The response of Renshaw cells to muscle vibration increased with the frequency of vibration and, over the value of 10/sec, appeared to be linearly related to the frequency of the input, at least up to the frequency of 150/sec. Since vibration was of sufficient amplitude to produce driving of all the primary endings of muscle spindles, the responses were expressed as mean increases in the discharge rate of Renshaw cells per average impulse/sec in the Ia afferents. The discharge of the Renshaw cell increased on the average by 2.90 and 1.08 imp./sec per each imp./sec in the Ia afferents during the phasic and the tonic component of the response respectively, while the response calculated during the whole period of vibration corresponded on the average to 1.45 imp./sec per each imp./sec in the Ia afferents. 4. The Renshaw cells tested above responded also with increasing frequencies of discharge to increasing levels of static extension of the GS muscle. In particular the discharge frequency of Renshaw cells was on the average linearly related to muscle extension, at least for values ranging from 0 to 8 mm. The mean increase in discharge rate as a function of the static extension corresponded on the average to 0.89 imp./sec/mm. Since the discharge rate of the primary endings of muscle spindles recorded from the deefferented GS muscle increased by 2.62 imp./sec/mm, it appears that the mean increase in the discharge rate of Renshaw cells as a function of static extension corresponded to 0.34 imp./sec per each imp./sec in the Ia afferents.     

A role for the cerebellum in the control of limb movement velocity

Accepting, rejecting or modifying the many different theories of the cerebellum's role in the control of movement requires an understanding of the signals encoded in the discharge of cerebellar neurons and how those signals are transformed by the cerebellar circuitry. Particularly challenging is understanding the sensory and motor signals carried by the two types of action potentials generated by cerebellar Purkinje cells, the simple spikes and complex spikes. Advances have been made in understanding this signal processing in the context of voluntary arm movements. Recent evidence suggests that mossy fiber afferents to the cerebellar cortex are a source of kinematic signals, providing information about movement direction and speed. In turn, the simple spike discharge of Purkinje cells integrates this mossy fiber information to generate a movement velocity signal. Complex spikes may signal errors in movement velocity. It is proposed that the cerebellum uses the signals carried by the simple and complex spike discharges to control movement velocity for both step and tracking arm movements.     

The cerebellum and initiation of movement: the stretch reflex

Studies of the stretch reflex in decerebrate cats indicate a phase advance of peak sinusoidal tension in steady-state cycles between 0.1 and 10 Hz. This phase advance is reduced in acute and chronic cerebellectomy, as shown in previous investigations. Also, the augmentation of muscle peak tension in initial sinusoidal stretch cycles at 0.5-5 Hz has been found to be reduced during the time of reflex and motor instability in the several months following cerebellar ablation. This report shows the increased amplitude and phase lead of integrated electromyographic activity in initiating sinusoidal stretch cycles in the decerebrate cat. These reflex aspects are demonstrated in relation to the discharge of neurons in the dorsal spinocerebellar tract and of cerebellar cortical Purkinje cells in initial sinusoidal cycles. The intensity and phase advance of the discharge in dorsal spinocerebellar tract neurons is altered little, but these features are usually increased in Purkinje cells during initial stretches compared to continuous cycling. In terms of overall motor control, these findings are compatible with concepts of movement control, modulated by the cerebellum, in which the discharge of antagonist motor neurons is regulated in concert with that of agonist muscles upon initiation and termination of movement.     

The effects of cold-induced muscle spindle secondary activity on monosynaptic and stretch reflexes in the decerebrate cat.

The effects of muscle spindle secondary ending activity on the stretch reflex were studied in unanesthetized decerebrate cats. Activation of secondary endings was accomplished by reducing the muscle temperature. This has been shown to cause a sustained asynchronous discharge from secondary endings. Cooling of the medial gastrocnemius or lateral gastrocnemius-soleus muscles caused an increase in the phasic and tonic components of their stretch reflexes. Cooling of the relaxed medial gastrocnemius muscle caused similar increases in the components of the stretch reflex of the synergistic lateral gastrocnemius-soleus muscle and an increase in its monosynaptic reflex. It was concluded that the facilitatory autogenetic and synergistic effects of muscle cooling on the stretch and monosynaptic reflexes were brought about by activity in group II afferents from muscle spindle secondary endings and could not be ascribed to any other type of muscle receptor. These results support the concept of an excitatory role for the secondary endings of the muscle spindle in the stretch reflex of the decerebrate cat.     

Response characteristics of single trochanteral campaniform sensilla in the stick insect, Cuniculina impigra

ABSTRACT. The campaniform sensilla on the trochanter of the stick insect, Cuniculina impigra Redtenbacher, were stimulated by slightly bending the leg in the horizontal plane. Single sensory units in the nerve were recorded using glass microelectrodes. These units can be classified into tonic and phasic-tonic receptors. In both cases there were units which increased their discharge frequency during forward movement of the femur, and units which responded to backward movement. No purely phasic receptors were found.     

Comparison between the effect of static contraction and tendon stretch on the discharge of group III and IV muscle afferents.

The exercise pressor reflex is evoked by both mechanical and metabolic stimuli. Tendon stretch does not increase muscle metabolism and therefore is used to investigate the mechanical component of the exercise pressor reflex. An important assumption underlying the use of tendon stretch to study the mechanical component of the exercise pressor reflex is that stretch stimulates the same group III mechanosensitive muscle afferents as does static contraction. We have tested the veracity of this assumption in decerebrated cats by comparing the responses of group III and IV muscle afferents to tendon stretch with those to static contraction. The tension-time indexes as well as the peak tension development for both maneuvers did not significantly differ. We found that static contraction of the triceps surae muscles stimulated 18 of 30 group III afferents and 8 of 11 group IV afferents. Similarly, tendon stretch stimulated 14 of 30 group III afferents and 3 of 11 group IV afferents. However, of the 18 group III afferents that responded to static contraction and the 14 group III afferents that responded to tendon stretch, only 7 responded to both stimuli. On average, the conduction velocities of the 18 group III afferents that responded to static contraction (11.6 +/- 1.6 m/s) were significantly slower (P = 0.03) than those of the 14 group III afferents that responded to tendon stretch (16.7 +/- 1.5 m/s). We have concluded that tendon stretch stimulated a different population of group III mechanosensitive muscle afferents than did static contraction. Although there is some overlap between the two populations of group III mechanosensitive afferents, it is not large, comprising less than half of the group III afferents responding to static contraction.     

The silent period in the stretch response of ia-activated dorsal spino-cerebellar tract neurons to sinusoidal muscle stretch in cats

By means of extracellular recordings of action potentials the stretch responses of single neurons of Clarke's column were analysed. The neurons were monosynaptically activated from Ia afferents of both ipsilateral gastrocnemius muscles. When stretch cycles of more than 0.2 mm amplitude and frequencies above 2 Hz were applied to the gastrocnemius muscles, the discharging was found to cease during the period of stretch release, whereas the average discharge rate was found to increase. In the frequency range between 0.1 and 10 Hz a sinewave of stretch frequency — the response sinewave — fitted to the non-zero bins of cycle histograms described the stretch response at small and large amplitudes equally well. The amount of increase in the average firing rate corresponded quite well to the portion of the response sinewave below the zero discharge rate. This indicates that the occurance of discharge pauses and the relation of the average discharge rate to frequency and amplitude of stretch can be described successfully by a half-wave rectification of the response at zero discharge rate. If one regards the shape of cycle histograms to be a nearly sinusoidal modulation plus a non-linear clipping at zero the application of linear systems analysis is worthwhile in describing the response not only at very small amplitudes but in the whole range of muscle stretch.     

Activity of the climbing fiber innervating various Purkinje cells

Low-amplitude potentials (10-130 microV) related to the action of a distant branch of the climbing fiber, which elicits complex spikes of the reference Purkinje cell were revealed by means of potential averaging synchronously with complex spikes of Purkinje cells in 10 out of 255 paired records of cerebellar Purkinje cells activity and extracellular field potentials at interelectrode distances of 200-1500 microns. These potential waves had a stable form in independent sets of data. In 3 out of 10 cases, the low-amplitude potentials included a slow (about 100 ms in duration) component. In one case, both test and reference electrodes recorded both simple and complex spikes of different Purkinje cells so that complex spikes of both cells were practically synchronous (conditional probability of complex spikes p = 0.97, onset time difference 0.54 ms). Thus for the first time in cerebellar physiology both simple and complex spikes activity of two Purkinje cells controlled by the same climbing fiber was recorded.     

The sensitivity of Renshaw cells to velocity of sinusoidal stretches of the triceps surae muscle.

1. The electrical activity of Renshaw cells monosynaptically excited by ventral root stimulation and disynaptically excited by electric stimulation of the group I afferents in the GS nerve has been recorded and their response to individual sinusoidal stretches of the deefferented GS muscle tested for different amplitudes and durations of the stimulus. 2. The experimental data indicate that the Rensahw cell responses are not only length dependent but also rate dependent. This finding indicates that the same Renshaw cells receive recurrent collaterals of both tonic and phasic motoneurons. 3. The observation that the discharge of Renshaw cells is particularly sensitive to the velocity of stretch suggests that the recurrent collaterals of large phasic motoneurons, which are recruited during high velocity stretches, exert a stronger excitatory action on Renshaw cells than do axon collaterals of the smaller tonic motoneurons, which are selectively stimulated during low velocity stretches.     

Characteristics of the neuronal activity of structures of the hippocampal formation (subiculum)]

Spontaneous and evoked neuronal activity of subiculum was investigated in chronic unanaesthetized rabbits. Rhythmic components of two different types were observed in the spontaneous activity of 58% of units: 4.5-6.0 cps bursts (13%) and 2.0-3.8 cps (45%). Besides their basic frequency, these types of rhythmic activity were differentiated by several additional criteria. 63% of the cells responded to applied sensory stimuli, among which auditory stimuli were the most effective. Multimodal units were slightly more numerous (59%) than unimodal ones. In 50% of the recorded units responses were of tonic type, in 31%--of phasic type, and in 19%--specific on-off responses were observed. In majority of units with repeated stimulations relatively rapid process of habituation developed after a short incremental phase. Interruption of septal afferents resulted in disappearence of 4.5-6.0 cps (theta) bursts, in reduction in the number of tonic responses and in absence of habituation. Similarity of neuronal characteristics in the field CA1 and subiculum is discussed.     

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

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

Functional organization of the electroreceptive midbrain in an elasmobranch (Platyrhinoidis triseriata)

The response properties of 322 single units in the electroreceptive midbrain (lateral mesencephalic nucleus, LMN) of the thornback ray, Platyrhinoidis triseriata, were studied using uniform and local electric fields. Tactile, visual, or auditory stimuli were also presented to test for multimodality. Most LMN electrosensory units (81%) are silent in the absence of stimulation. Those with spontaneous activity fired irregularly at 0.5 to 5 impulses/s, the lower values being more common. Two units had firing rates greater than 10/s. Midbrain electrosensory units are largely phasic, responding with one or a few spikes per stimulus onset or offset or both, but the adaptation characteristics of some neurons are complex. The same neuron can exhibit phasic or phasic-tonic responses, depending upon orientation of the electric field. Tonic units without any initial phasic over-shoot were not recorded. Even the phasic-tonic units adapt to a step stimulus within several seconds. Unit thresholds are generally lower than 0.3 microV/cm, the weakest stimulus delivered, although thresholds as high as 5 microV/cm were recorded, Neuronal responses reach a maximum, with few exceptions, at 100 microV/cm and decrease rapidly at higher intensities. LMN neurons are highly sensitive to stimulus repetition rates: most responded to frequencies of 5 pulses/s or less; none responded to rates greater than 10/s. Three distinct response patterns are recognized. Best frequencies in response to sinusoidal stimuli range from 0.2 Hz (the lowest frequency delivered) to 4 Hz. Responses decrease rapidly at 8 Hz or greater, and no units responded to frequencies greater than 32 Hz. Most LMN neurons have small, well defined excitatory electroreceptive fields (RFs) exhibiting no surround inhibition, at least as detectable by methods employed here. Seventy-eight percent of units recorded had RFs restricted to the ventral surface: of these, 98% were contralateral. The remaining 22% of units had disjunct dorsal and ventral receptive fields. Electrosensory RFs on the ventral surface are somatotopically organized. Anterior, middle, and posterior body surfaces are mapped at the rostral, middle, and caudal levels, respectively, of the contralateral LMN. The lateral, middle, and medial body are mapped at medial, middle, and lateral levels of the nucleus. Moreover, the RFs of all units isolated in a given dorsoventral electrode track are nearly superimposable. About 40% of LMN, measured from the dorsal surface, is devoted to input from ventral electroreceptors located in a small region rostral and lateral to the mouth.(ABSTRACT TRUNCATED AT 400 WORDS)     

Electrical responses of homing pigeon and guinea pig Purkinje cells to pineal indoleamines applied by microelectrophoresis

Summary The effects of microelectrophoretically applied melatonin (aMT), 5-methoxytryptophol (ML), 5-hydroxytryptophol (HL) and noradrenaline (NA) on the electrical activity of cerebellar Purkinje and other cells during both day- and nighttime were studied in urethane-anesthetized intact and pinealectomized homing pigeons and guinea pigs.In the intact pigeon, equal numbers of Purkinje cells were excited and inhibited by aMT and ML, whilst the responses to HL were predominantly inhibitory. The responses varied significantly depending on whether the cells were tested during the day or at night (P<0.001). Pinealectomy abolished the observed day/night differences. In intact pigeons most of the other cerebellar units were inhibited by aMT and ML, whilst HL elicited an excitation in about 50% of the units.In the guinea pig most of the Purkinje cells were inhibited by aMT and ML, whereas HL caused no response in most of the units. No significant 24-h rhythmicity in response to the indoles could be observed. The responses of the other cerebellar units were more complicated. aMT caused an excitation in most of the units, whilst the predominant response caused by ML was inhibition. Only 30% of these units responded to HL; the remainder showed no measurable responses.It is apparent from these studies that pineal indoleamines may play a modulatory role in the cerebellum.Abbreviations ECG electrocardiogram - GABA gamma-aminobutyric acid     

Structure and physiology of the locust femoral chordotonal organ

The connective chordotonal organs (COs) in the femora of the prothoracic and mesothoracic legs of the locust Schistocerca gregaria are divided into two parts, the proximal and the distal scoloparia. The proximal scoloparium contains about 150 small neurons and is anchored to the femoral cuticle. The distal scoloparium contains about 50 larger neurons and is connected at its proximal end to both the cuticle and the flexor tibiae muscle.Records were made from the distal scoloparium, classifying units by spike size. The tibial position/total activity response curve is ∪-shaped but when a small number of units is selected the responses occur only when the tibia is on one side of its centre position. The tonic responses display considerable hysteresis and a degree of adaptation which varies with the tibial angle. Units with phasic and phasic-tonic responses are common and their responsiveness depends on the range of angles the tibia is moved through. The same units respond strongly to flexor tibiae contraction with the tibia either fixed or free, and so may serve as receptors for tension in that muscle.The CO mediates phasic resistance reflexes in all three extensor tibiae motoneurons and tonic reflexes in the extensor ‘slow’ neuron. It is suggested that the very detailed information furnished by the CO is used in a complex way in the control of the femoral muscles.     

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

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

Activity of fusimotor neurons during reflex muscle contraction

The influence of fusimotor activity via the gamma-loop on reflex responses of motoneurons to stretch or vibration stimulation of mm. triceps surae was studied in decerebrate cats. Action potentials of single fusimotor neurons were derived from thin filaments isolated from nerves innervating this muscle group, leaving their main nerve supply intact. Most fusimotor neurons tested were found to be coactivated with motor units during reflex muscle contraction. In the initial period of development of reflex muscle contraction a weak autogenetic inhibitory effect on discharge of fusimotor neurons was found. The results suggest that reduction of the reflex motor signal, leading to a "silent period," is partly the result of a transient decrease in the fusimotor output effect on contracting muscles. A study of changes in fusimotor discharge generation during the ascending phase of reflex muscle contraction may provide data useful for identification of autogenetic reflex influences on these motoneurons and for elucidating the conditions necessary for servoassistance of muscle contractions.Medical Research Institute, Belgrade, Yugoslavia. Translated from Neirofiziologiya, Vol. 16, No. 5, pp. 630–637, September–October, 1984.     

Cyclooxygenase blockade attenuates responses of group III and IV muscle afferents to dynamic exercise in cats

Cyclooxygenase products accumulate in statically contracting muscles to stimulate group III and IV afferents. The role played by these products in stimulating thin fiber muscle afferents during dynamic exercise is unknown. Therefore, in decerebrated cats, we recorded the responses of 17 group III and 12 group IV triceps surae muscle afferents to dynamic exercise, evoked by stimulation of the mesencephalic locomotor region. Each afferent was tested while the muscles were freely perfused and while the circulation to the muscles was occluded. The increases in group III and IV afferent activity during dynamic exercise while the circulation to the muscles was occluded were greater than those during exercise while the muscles were freely perfused (P < 0.01). Indomethacin (5 mg/kg iv), a cyclooxygenase blocker, reduced the responses to dynamic exercise of the group III afferents by 42% when the circulation to the triceps surae muscles was occluded (P < 0.001) and by 29% when the circulation was not occluded (P = 0.004). Likewise, indomethacin reduced the responses to dynamic exercise of group IV afferents by 34% when the circulation was occluded (P < 0.001) and by 18% when the circulation was not occluded (P = 0.026). Before indomethacin, the activity of the group IV, but not group III, afferents was significantly higher during postexercise circulatory occlusion than during rest (P < 0.05). After indomethacin, however, group IV activity during postexercise circulatory occlusion was not significantly different from group IV activity during rest. Our data suggest that cyclooxygenase products play a role both in sensitizing group III and IV afferents during exercise and in stimulating group IV afferents during postexercise circulatory occlusion.     

Modulation of human muscle spindle discharge by arterial pulsations--functional effects and consequences

Arterial pulsations are known to modulate muscle spindle firing; however, the physiological significance of such synchronised modulation has not been investigated. Unitary recordings were made from 75 human muscle spindle afferents innervating the pretibial muscles. The modulation of muscle spindle discharge by arterial pulsations was evaluated by R-wave triggered averaging and power spectral analysis. We describe various effects arterial pulsations may have on muscle spindle afferent discharge. Afferents could be "driven" by arterial pulsations, e.g., showing no other spontaneous activity than spikes generated with cardiac rhythmicity. Among afferents showing ongoing discharge that was not primarily related to cardiac rhythmicity we illustrate several mechanisms by which individual spikes may become phase-locked. However, in the majority of afferents the discharge rate was modulated by the pulse wave without spikes being phase locked. Then we assessed whether these influences changed in two physiological conditions in which a sustained increase in muscle sympathetic nerve activity was observed without activation of fusimotor neurones: a maximal inspiratory breath-hold, which causes a fall in systolic pressure, and acute muscle pain, which causes an increase in systolic pressure. The majority of primary muscle spindle afferents displayed pulse-wave modulation, but neither apnoea nor pain had any significant effect on the strength of this modulation, suggesting that the physiological noise injected by the arterial pulsations is robust and relatively insensitive to fluctuations in blood pressure. Within the afferent population there was a similar number of muscle spindles that were inhibited and that were excited by the arterial pulse wave, indicating that after signal integration at the population level, arterial pulsations of opposite polarity would cancel each other out. We speculate that with close-to-threshold stimuli the arterial pulsations may serve as an endogenous noise source that may synchronise the sporadic discharge within the afferent population and thus facilitate the detection of weak stimuli.