Response of Renshaw cells to sinusoidal stretch of hindlimb extensor muscles.

1. Renshaw cells responding disynaptically to electrically induced group I volleys in the intact gastrocnemius-soleus (GS) nerve, were submitted to small-amplitude, high-frequency vibration applied longitudinally to the deefferented GS muscle in precollicular decerebrate cats. 2. Vibration of the GS muscle at 200/sec, 180 mu peak-to-peak amplitude for 80-100 msec produced a sudden increase in the discharge rate of Renshaw cells, which gradually decreased within 25-50 msec to reach a steady level higher than that recorded in the absence of vibration. 3. Excitation of Renshaw cells appeared at a threshold amplitude of vibration (at 200-250/sec) of 5-20 mu and increased to a maximum value for amplitudes of about 70-80 mu, i.e., when all the primary endings of the spindles from the GS muscle had been driven by the stimulus. Recruitment of the secondary endings of the muscle spindles, due to large amplitude muscle vibration, did not modify the response of the Renshaw cells to the mechanically induced group Ia volleys. 4. These findings were obtained with the GS muscle pulled at 8 mm of initial extension. A threshold response of Renshaw cells to vibration appeared at 4 mm of static stretch, while maximal responses occurred at 8 mm. No further increase and actually a slight decrease in the response appeared for initial extensions of the muscle of 10-12 mm. 5. For a given vibration amplitude, the response of the Renshaw cells increased with increasing frequencies of vibration to reach the maximum at frequencies of 150-250/sec. Bursts of Renshaw cell discharges synchronous to each stroke of vibrator occurred only for low frequencies of stimulation (less than 25/sec). 6. It is concluded that vibration of the GS muscle represents a very effective method in exciting the Renshaw cells and that this response depends upon selective stimulation of homonymous motoneurons monosynaptically excited by the orthodromic volleys originating from the primary endings of the corresponding muscle spindles.     

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.     

Characterization of muscle spindle afferents in the adult mouse using an in vitro muscle-nerve preparation

We utilized an in vitro adult mouse extensor digitorum longus (EDL) nerve-attached preparation to characterize the responses of muscle spindle afferents to ramp-and-hold stretch and sinusoidal vibratory stimuli. Responses were measured at both room (24°C) and muscle body temperature (34°C). Muscle spindle afferent static firing frequencies increased linearly in response to increasing stretch lengths to accurately encode the magnitude of muscle stretch (tested at 2.5%, 5% and 7.5% of resting length [Lo]). Peak firing frequency increased with ramp speeds (20% Lo/sec, 40% Lo/sec, and 60% Lo/sec). As a population, muscle spindle afferents could entrain 1:1 to sinusoidal vibrations throughout the frequency (10-100 Hz) and amplitude ranges tested (5-100 μm). Most units preferentially entrained to vibration frequencies close to their baseline steady-state firing frequencies. Cooling the muscle to 24°C decreased baseline firing frequency and units correspondingly entrained to slower frequency vibrations. The ramp component of stretch generated dynamic firing responses. These responses and related measures of dynamic sensitivity were not able to categorize units as primary (group Ia) or secondary (group II) even when tested with more extreme length changes (10% Lo). We conclude that the population of spindle afferents combines to encode stretch in a smoothly graded manner over the physiological range of lengths and speeds tested. Overall, spindle afferent response properties were comparable to those seen in other species, supporting subsequent use of the mouse genetic model system for studies on spindle function and dysfunction in an isolated muscle-nerve preparation.     

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.     

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.     

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).     

Responses of nucleus z neurons to vibration of hindlimb extensor muscles in the decerebrate cat.

1. Small sinusoidal changes of length, applied longitudinally to the ddifferented GS muscle, were uused as a specific stimulus to activate the muscle spindle receptors in precollicular decerebrate cats. In order to determine the relative contribution of the primary and secondary endings of muscle spindles to the response of the nucleus z neurons, the effects of muscle vibration on this unit activity were studied under conditions in which the segmental monosynaptic reflexes produced by stimulation of the primary endings of muscle spindles were continuously monitored. 2. Vibration of the GS muscle (at 180-200/sec and amplitudes up to 250-320 mu) affected the frequency and pattern of discharge in 50 out of 168 units recorded from the lower medulla...     

The role of different size vestibulospinal neurons in the static control of posture

1. In addition to giant cells, originally described by Deiters, the lateral vestibular nucleus contains also medium- and small-size cells. The role that these neurons exert in the static control of posture has been investigated in precollicular decerebrate cats in which the resting discharge of spontaneously active vestibulospinal neurons projecting to lumbosacral segments of the spinal cord (IVS neurons) has been related to the cell size inferred on the basis of the conduction velocity of their axons. 2. In control experiments, the IVS neurons with slower axonal conduction velocity and, by inference, having thinner axons and smaller cell bodies differed from those having faster conduction velocity by displaying a higher resting discharge rate and a relatively regular interspike interval distribution, i.e. a lower coefficient of variation (CV). 3. The resting discharge of the IVS neurons, which corresponded on the average to 24.5 +/- 15.7, S.D. imp./sec, in control experiments, increased significantly to 44.1 +/- 23.8, S.D. imp./sec after ablation of the cerebellar vermis and the fastigial nuclei, leading to a great increase in postural activity, while the proportion of regularly discharging units (with the lowest CV) increased. Moreover, the negative correlation between resting discharge of all the recorded IVS neurons and the conduction velocity of the corresponding axons, which was quite slight in the experiments with the cerebellum intact, greatly increased after partial cerebellectomy. This finding was due to a prominent increase in resting discharge of the small-size IVS neurons, while the discharge of the large-size IVS neurons was, on the average, comparable to that obtained in the controls. It appears, therefore, that the cerebellum exerts a prominent tonic inhibitory influence on the small-size IVS neurons, which are thus responsible for the great increase in decerebrate rigidity after cerebellectomy. 4. The resting discharge rate of the IVS neurons was not, on the average, greatly modified after ipsilateral acute (aVN) and chronic vestibular neurectomy (cVN) with respect to the controls. However, the proportion of regularly discharging units (with the lowest CV) decreased after aVN, but increased after cVN. The relation found in control experiments, i.e. the faster the conduction velocity of VS axon the lower was the unit discharge at rest, was lost after aVN, due to a decrease in resting discharge rate of the slow neurons. The mean discharge rate of these units, however, recovered after cVN, so that the negative correlation between resting discharge rate and axonal conduction velocity was reestablished.(ABSTRACT TRUNCATED AT 400 WORDS)     

Static input-output relations in the spinal recurrent inhibitory pathway

The static discharge rate of Renshaw cells (studied in deafferented, intercollicularly decerebrate cats) has a nonlinear dependence on the frequency of trains of stimulus impulses to -motor axons in the ventral root. This dependence is well described by a rectangular hyperbola that approaches saturation with increasing stimulus frequency. The tendency to saturate is independent of the number of motor axons exciting a Renshaw cell. On average, the stimulus frequency at which the discharge rate reaches half its saturation value lies between 10 and 15 Hz. The effect of Renshaw cell activity — measured as the antidromic inhibition of individual -motoneurons — reflects the form of the static frequency characteristics. An electric circuit analog of the Renshaw cell membrane is presented which serves to explain the qualitative features of the static input-output relations; the nonlinearity is the result of synapses with linear properties acting together at the cell membrane.Dedicated to Professor R. Granit, Stockholm, on the occasion of his 80th birthday     

Molecular determinants of mechanosensation in the muscle spindle

The muscle spindle (MS) provides essential sensory information for motor control and proprioception. The Group Ia and II MS afferents are low threshold slowly-adapting mechanoreceptors and report both static muscle length and dynamic muscle movement information. The exact molecular mechanism by which MS afferents transduce muscle movement into action potentials is incompletely understood. This short review will discuss recent evidence suggesting that PIEZO2 is an essential mechanically sensitive ion channel in MS afferents and that vesicle-released glutamate contributes to maintaining afferent excitability during the static phase of stretch. Other mechanically gated ion channels, voltage-gated sodium channels, other ion channels, regulatory proteins, and interactions with the intrafusal fibers are also important for MS afferent mechanosensation. Future studies are needed to fully understand mechanosensation in the MS and whether different complements of molecular mediators contribute to the different response properties of Group Ia and II afferents.     

Effects of hypoxia on the discharge of group III and IV muscle afferents in cats.

The reflex pressor response evoked by static muscular contraction is widely believed to be caused by the stimulation of group III and IV afferents. Although the specific nature of the contraction-induced stimulus to these thin-fiber afferents is unknown, they are thought to be stimulated in part by a condition arising from a mismatch between blood supply and demand in the exercising muscle. Hypoxia, a condition found in skeletal muscle during such a mismatch, may stimulate these afferents. We have therefore tested the hypothesis that perfusion of the triceps surae muscles with hypoxic blood stimulates group III and IV afferents in barbiturate-anesthetized cats. We found that 3-3.5 min of hypoxia with the triceps surae muscles at rest significantly (P < 0.05) increased the average discharge rate of contraction-sensitive group IV afferents but had no effect on the average discharge rate of contraction-sensitive group III afferents. Hypoxia had only trivial effects on the discharge of contraction-insensitive group III and IV afferents. Hypoxia stimulated 4 of 11 contraction-sensitive group IV afferents and 2 of 13 contraction-sensitive group III afferents. The responses of the afferents stimulated by hypoxia were small in magnitude. Hypoxia with the muscles at rest appeared to have no effect on either hydrogen or lactate ion concentrations in the femoral venous blood. In addition, hypoxia increased the responses to contraction in only 3 of 22 group III and 4 of 21 group IV afferents tested. We conclude that muscle tissue hypoxia is a minor stimulus to afferents that sense a mismatch between blood supply and demand during static contraction.     

Effect of the amplitude and frequency of sinusoidal stimulation on the harmonic distortion of the response of sensory terminations of muscle spindles

Harmonic distortion (HD) from 1,055 responses of muscle spindles sensory endings to sinusoidal stretches (frequency range 0.0008 to 0.8333 Hz, amplitude range 0.019 to 3.09 mm) has been studied in the cat soleus muscle. Sixty-six per cent were primary afferents (Ia) and 34% secondary (II). HD mean value (0.28) did not show any significant differences between both types of endings. Analysis of variance for HD versus stimulation amplitude showed a greater HD when stretch amplitudes were beyond 1.599 mm or less than 0.031 mm on primary afferents (p less than 0.001) and less than 0.070 mm on secondary (p less than 0.001). The effect of stimulus frequency was also significant (p less than 0.01 Ia and p less than 0.001 II), however only at 0.8333 Hz and in secondary endings HD was significantly higher. The silent period in the response, at release of stretch, caused by half wave rectification could explain about 50% of measured HD.     

Responses of muscle spindles of tenotomized and hypertrophied muscles to stretching and vibration

Responses of muscle spindles of tenotomized and hypertrophied muscles to stretching and vibration were investigated. During constant stretching of the muscles with a load of 100 g the spontaneous activity of the primary endings in the control muscle was 17±1.5 spikes/sec, in the hypertrophied muscle it was unchanged, and after tenotomy it was increased to 26±1.5 spikes/sec. The discharge frequency of the secondary endings was unchanged under these circumstances. Responses of primary and secondary endings of spindles of the tenotomized muscle during the dynamic and static phases of stretching were higher in frequency than responses of spindles of normal muscles. The discharge frequency of the primary endings in the hypertrophied muscle also was increased during both phases of stretching. Responses of secondary endings of the spindles of the hypertrophied muscle were indistinguishable under these circumstances from responses of normal muscles. Primary endings of spindles of tenotomized and hypertrophied muscles, just as normally, reproduced frequencies of vibration stimulation up to 2000 Hz, but some increase in the discharge frequency was observed in the secondary endings at this time.I. M. Sechenov Institute of Evolutionary Physiology and Biochemistry, Academy of Sciences of the USSR, Leningrad. Translated from Neirofiziologiya, Vol. 8, No. 3, pp. 311–317, September, 1976.     

Static sensitivity of primary muscle spindle endings

Static discharges were studied in 75 primary endings of passive muscle spindles during stepwise stretching of the cat triceps surae muscle. Afferents conducting excitation with velocities of between 72 and 115 m/sec, with high dynamic sensitivity, and with static thresholds below 8 mm were chosen. The muscle was stretched by 10 mm relative to the completely relaxed state with a step of 0.8 mm. Spike discharges were recorded 40 sec after each stretching for 30 sec and the mean frequency was calculated. Comparison of static and differential static responses for different units, of the "muscle length-mean discharge frequency" dependence, and of the static thresholds showed that a linear (under 4.5 spikes/sec/mm) or steady increase in the mean discharge frequency to 40 spikes/sec took place in only 20% of primary endings with a probability of more than 0.7 for each step of muscle stretching. In most primary endings a narrow range of sensitivity to a change in the static length of the muscle was found. It is suggested that the "poor" static sensitivity was due either to high static thresholds or to the absence of increases in mean discharge frequency despite continued stretching.I. P. Pavlov Institute of Physiology, Academy of Sciences of the USSR, Leningrad. Translated from Neirofiziologiya, Vol. 13, No. 5, pp. 540–548, September–October, 1981.     

Detection and analysis of recurrent inhibition of firing motoneurons in man

During regular firing of "small" motor units, activated during weak voluntary contraction of the human soleus muscle, thick efferent fibers of n. tibialis were stimulated (a small M response was evoked, in which the small units did not participate). Peristimulus histograms of potentials of single motor units were constructed and the effect of stimulation on interspike interval duration was analyzed. The firing rate of the motor units was 4.5–7.6 spikes/sec. Stimulation of the nerve led to a sharp decrease in probability of their discharge or even complete temporary cessation of firing, i.e., it had a well marked inhibitory effect (lasting 10–20 msec). The latent period of inhibition (35–40 msec) was only a little longer than the latent period of the monosynaptic reflex of the soleus muscle. The effect of an inhibitory volley on duration of the interspike interval of the motor units depended on the time when the volley arrived during the interval. Lengthening of the interval was observed only if the inhibitory volley arrived in the second half or at the end of the interval. It is concluded that inhibition of firing of small motor units is due to Renshaw cells, activated on stimulation of axons of large motoneurons. The efficiency of a short (compared with the duration of the interspike interval) inhibitory volley reaching a motoneuron firing at low frequency characteristic of its adequate activation, is discussed.Institute for Problems of Information Transmission, Academy of Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 16, No. 1, pp. 88–96, January–February, 1984.     

Visual responses in crayfish 3. Further studies on transmission through the brain

Responses to illumination of the eyes of the crayfish were studied by gross recording from one of the circumesophageal connectives. Two-thirds of the spontaneous activity at this level of the CNS consists of ascending activity, which is eliminated by cutting a connective posterior to the recording electrode. An average of about eight fibers in a connective responded to 1 sec illumination of the homolateral eye. The fibers were of four types: pure-on units, on-sustained units, on-and-off units and pure-off units. The average latency was 74 msec for the on-response and 26 msec for the off-response. The latency of responses to 10 μsec flashes of increasing intensity shortened from 72 to 52 msec. This was demonstrated to be mainly a peripheral effect since ERG latency showed a parallel reduction while the ERG-connective response interval remained more nearly constant at 40–50 msec. ERG amplitude, frequency, and usually the duration of the connective spike discharge increased at greater stimulus intensities, yet the average number of responding fibers was greatest at intermediate intensities. The results indicate minimal processing of response patterns by the brain.     

Proprioception during voluntary movement

In the last decade, a number of laboratories have accumulated data on the firing of single afferent fibres from muscle and skin during movement in awake cats, monkeys and human subjects. While there is general agreement on the firing behaviour of skin afferents and tendon organ (Ib) afferents during movement, there remains a significant divergence of opinion regarding the way in which the response of muscle spindle afferents (Ia and II) to length changes is modified by fusimotor action (e.g., alpha-gamma linkage versus "fusimotor set"). The controversies surrounding the fusimotor system have tended to overshadow the emergence of several important characteristics of proprioceptive behaviour, corroborated in separate laboratories. (i) Mean Ia firing rates during active movements are nearly always higher than at rest. Thus, activation of the fusimotor system is reserved for the control of, or preparation for, movement. In animals, there is now strong evidence that there is usually a tonic component of fusimotor action during rhythmical movements. (ii) During fast, unloaded movements (peak muscle speeds, 0.2 resting lengths/s or more), the firing of both Ia and II afferents usually increases during lengthening and decreases during shortening. Ib afferents fire during even the most rapid active shortening of their parent muscles. (iii) During powerful shortening contractions performed against significant loads, Ia firing is often appreciable, suggesting that there is at least some underlying alpha-gamma coactivation. (iv) During fast imposed muscle stretches, Ia afferents respond with segmented bursts of firing (threshold speed for segmentation, 0.5-1.0 resting length/s). Ib afferents show far less segmentation of discharge under similar circumstances.(ABSTRACT TRUNCATED AT 250 WORDS)     

Computer simulation of the impulse pattern of muscle spindle afferents under static and dynamic conditions

Summary A simple model has been employed to describe and interprete measurements from deefferented muscle spindle afferents with static and dynamic stimulation; the model simulates the generator potential of the spindle and the time dependent change of sensitivity at the impulse generating membrane. The properties of the model in transforming the steady and time dependent analogue signals into impulse patterns are demonstrated, and the influence of the various parameters on the response characteristics have been investigated. Results from simulations are compared with experimental data, and it is shown that the impulse patterns of secondary muscle spindle afferents can be simulated quantitatively. The frequency distributions of impulse intervals and different sequential dependencies within the impulse patterns are analysed.

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Zusammenfassung Zur Beschreibung und Deutung von Messungen an deefferentierten Muskelspindelafferenzen unter statischer und dynamischer Reizung wird ein einfaches Modell verwendet, das haupts?chlich das Generatorpotential des Mechanoreceptors und die zeitabh?ngige ?nderung der Empfindlichkeit an der impulserzeugenden Membran simuliert. Die Eigenschaften des Modells bei der Transformation konstanter und zeitabh?ngiger Analogsignale in Impulsfolgen werden dargestellt und die Einflüsse der verschiedenen Modellparameter untersucht. Im Vergleich der Simulationsergebnisse mit experimentellen Daten wird gezeigt, da? die Impulsmuster sekund?rer Muskelspindelafferenzen quantitativ simuliert werden k?nnen. Die dabei verwendeten Parameter werden angegeben. Analysisert werden die H?ufigkeitsverteilungen der Impulsintervalle und verschiedene sequentielle Abh?ngigkeiten innerhalb der Impulsfolgen.

     

Functional characteristics of the intraspinal spread of viscerosomatic activity.

The functional characteristics of the intraspinal spread of the dual-component viscerosomatic reflex response were studied in chloralose-anaesthetized cats. In the region of the direct inputs of splanchnic afferents into the spinal cord, the initial part of the early (propriospinal) component is evoked by afflux from the extraspinal pathway in the sympathetic chain and has the shortest latent period. At segment Th8 to Th12 level this was 5.2--8.1 msec; cranially and caudally from this level it was longer. Activity spreading from the adjacent segments via intraspinal longitudinal systems also participates in the origin and course of the early component of the efferent discharge. The rate of ascending intraspinal irradiation in the thoracic region was 3.2--12 m/sec (6.6 +/- 2.4 m/sec). The rate of the descending spread of propriospinal activity in the thoracic region via intraspinal pathways was 3.6--18.3 m/sec (12.5 +/- 4.5 m/sec); in the lumbar region it was significantly lower -- 2.9--19.3 m/sec (7 +/- 4.5 m/sec). The latent period of the later component varied from 20 to 30 msec and displayed a cranio-caudal increase in length. The rate of the descending spread of the later component in the thoracic and lumbar cord did not differ significantly (20 +/- 10 m/sec and 22 +/- 13 m/sec respectively). The continuous splanchnic discharge in the lumbosacral region is correlated to the different rate of the intraspinal spread of the early and the late response and to their overlapping.     

Muscle afferent modulation by stimulation of motor and supplementary motor cortex in cats

Local stimulation in the zone of motor representation of the cat hind limb in the postcruciate cortex (area 4) modulates afferent activity of flexor spindles of the foot. An initial pause, connected with contraction of extrafusal fibers, is observed in this activity. After the muscle has returned to its original length, a sharp rise of discharge frequency develops followed by a return to its initial level. Similar phases, but less marked, are observed in secondary afferents. Stimulation of contralateral and ipsilateral regions of the medial precruciate cortex (area 6) causes selective, intensive, and prolonged facilitation of discharge of type Ia units followed by an after-effect, without involving extrafusal muscle fibers. Since influences of the premotor supplementary cortex on lumbar gamma motoneurons are relatively independent of influences coupled with activation of the alpha system on muscle afferents from the motor cortex, a specific role of area 6 in the regulation of segmental excitability of the gamma system can be postulated.