Inhibition of gamma motoneurone discharge by contraction of the homonymous muscle in the decerebrated cat

Distribution of glycinergic terminals on lumbar motoneurons of the adult cat: an ultrastructural study

The distribution of glycine-like immunoreactivity on cat lumbar motoneurons was examined in electron microscopy, using pre-embedding immunocytochemistry. In the dorsolateral portion of the ventral horn, numerous labeled axon terminals were presynaptic to somatic and dendritic profiles of alpha-motoneurons. Most of the glycinergic boutons contained pleomorphic vesicles and showed symmetrical contacts. On the somatic and proximal dendritic compartments, glycinergic terminals accounted for, respectively, 24.6 and 26.6% of the total number of terminals. There were very few glycinergic terminals on gamma-motoneurons. Immunoreactive axons, dendrites and cell bodies were also observed near the motoneurons. These results support the view that glycine plays a major role in the inhibition of alpha-motoneurons and suggest that inhibitory mechanisms occur on the soma as well as on dendrites.

Autogenetic inhibition from contraction receptors in the decerebrate cat

1. Autogenetic inhibition from contraction receptors was measured by eliciting contractions of the soleus muscle in the decerebrate cat. Inhibitory feedback was detected when the tension increment f, produced by stimulating motor fibres in the presence of a background reflex contraction, was smaller than the tension d elicited by the same stimulus in the absence of reflex action. Tendon vibration was applied throughout to clamp primary spindle afferents at a constant firing rate, thereby preventing spindle unloading from disfacilitating the reflex contraction. 2. The reduction in tension d--f varied roughly linearly with the size of the tension stimulus f. Feedback gain was proportional to d--f/f, i.e. the ratio of inhibited tension to stimulus tension. It was computed by averaging over several measurements obtained with stimuli of different sizes, and ranged between 0 and 0.88 in ten animals. The average gain, 0.39, implies that voluntary muscle force is reduced by approximately 27% through the direct inhibition of alpha-motoneurones from homonymous contraction receptors. 3. Inhibitory feedback gain did not appear to co-vary with the background reflex contraction. When measured without vibration, however, a positive covariance did emerge, suggesting that this is due to unloading of muscle spindles, either by extrafusal muscle shortening or by inhibition of fusimotor neurones. 4. Inhibited tension varied linearly with the estimated increment in Ib afferent firing. On the assumption that group Ib afferents carried the entire inhibitory signal, inhibitory feedback gain measured with vibration was used to predict the size of the gain if vibration had not been applied. Feedback gain calculated in this way was reduced by still did not vary with reflex tension. 5. In one animal with signs of brain stem trauma, feedback gain was increased to around six. It is argued that inhibitory feedback in the intact animal can rise to comparable values, as a result both of convergence of signals from different muscles and of supraspinal facilitation.

Statistical limits for detecting change in the cumulative sum derivative of the peristimulus time histogram

The peristimulus time histogram (psth) provides a means of correlating the discharges of neurones with other events. The cumulative sum (cusum) derived from the psth facilitates the detection of small changes in the psth that may be obscured by random fluctuations in counts. The cusum integrates differences from the mean control level of counts in the psth. Any signal in the data that is related to the stimulus appears as a slope in the cusum. Psth's constructed from the rhythmic discharges of single neurones are shown to contain periodical fluctuations in counts that arise from refractoriness. This periodicity results in a cusum which deviates less from the horizontal line than predicted from a Poisson distribution of points. The more regular the spike train, i.e., the lower the coefficient of variation of the distribution of interspike intervals, the flatter is the cusum. The theory of stochastic point processes is used to derive an algorithm for calculating the best approximation of variance of the cusum. Significance limits set at 3 standard deviations of the cusum are shown to provide a good fit to cusums for unit discharges over a wide range of coefficients of variation (0.09-0.60).

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

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

Effects of triceps stretch by ankle flexion on intact afferents and efferents of gastrocnemius in the decerebrate cat

We describe the effects on group I and II afferents as well as alpha- and gamma-efferents of gastrocnemius muscles elicited by stretch and release of the triceps surae muscle performed by a dorsiflexion movement, a maintained dorsiflexion and release of the ankle joint. The experiments were made in decerebrated cats in which the neural loops between the muscles and the spinal cord were intact. Multi-unit discharges of each kind of fibre were obtained, electronically on-line, from two monopolar electrodes 4 mm apart on a small branch of the gastrocnemius lateralis or medialis nerve. Sensory and motor impulses were separated according to the opposite directions of their propagation, then both groups were further subdivided according to conduction velocity. The stretch reflex induced by ankle flexion was associated in eighteen out of nineteen experiments with reflex changes in the activity of the gamma-axon population of the gastrocnemius nerve branch. Facilitatory, depressor and biphasic (facilitatory-depressor) effects were observed, the particular type seeming to depend on the level of gamma tone and gamma-motoneurone size. Flexion release led to cessation of the alpha stretch reflex and to a rebound firing of the gamma-axon population. The lengthening reaction and the clasp-knife phenomenon occurred in certain preparations and were associated with reflex inhibition of the gamma-motoneurones. The over-all responses of group I fibres to triceps stretch showed dynamic and static components, whereas those of the group II fibres were, except in two examples, almost devoid of dynamic sensitivity. During release of stretch and as a result of the simultaneous rebound in gamma activity, the afferent discharges showed a very short pause, no pause, or even a rebound. The origins of the discharges of groups I and II muscle afferents and the factors (muscle length, active and passive muscle tension, reflex changes in fusimotor drive) influencing the activity of the receptors involved are considered as far as possible. Some aspects of the mechanisms of the fusimotor reflex are discussed.

Autogenetic inhibition of extensor gamma-motoneurones revealed by electrical stimulation of group I fibres in the cat

Forty functionally single gamma-efferents (20-42 m/s) to the triceps surae were isolated in ventral root filaments of the decerebrated and paralysed cat in order to study the effects of group I muscle afferents on their own fusimotor neurones. All the efferents studied were spontaneously active. During splitting the continuity of the efferent fibre was preserved so that the destination of the target muscle of the efferent could be determined by antidromic stimulation of the muscle nerve using the collision block technique. Thereafter the filament was cut so that the reflex response, uncontaminated by antidromic impulse invasion, could be recorded from the central end. Sixteen of forty gamma-efferents to the triceps were inhibited by repetitive stimulation (range -3 to -40 impulses/s) of the homonymous nerve within the group I range. Raising the stimulus strength above the group II threshold produced no further increase in inhibition. Twelve of these sixteen cells were also tested by stretching the triceps; ten showed marked inhibition, and two were not influenced. In ten of the sixteen inhibited cells, the autogenetic inhibition at maximum group I stimulus strength was larger than the maximum antidromic inhibition elicited by stimulation of the remainder of the cut ventral roots L7 and S1. Since both effects were additive, it is concluded that Renshaw inhibition is at least not solely responsible for the autogenetic inhibition. Consistent with this assumption is the observation that some cells receiving electrically evoked autogenetic inhibition were not susceptible to inhibition induced by small-amplitude vibration of the triceps. Since small-amplitude vibration is known to excite most of the Ia afferents of the vibrated triceps, Ib afferents must be involved in the autogenetic fusimotor inhibition. A contribution of Ia afferents to the autogenetic inhibition (via alpha-motoneurones and Renshaw cells or via Ib inhibitory interneurones) seems likely since inhibition, induced by small-amplitude vibration, was detectable in many of the cells receiving autogenetic group I inhibition. All of these cells were susceptible to antidromic inhibition. Ten of the forty cells tested responded with tonic facilitation to the homonymous nerve stimulation. Some arguments favour the view that static gamma-motoneurones are involved in the low-threshold autogenetic inhibition. The results strongly support earlier work suggest a regulatory function of low-threshold muscle receptors on their own gamma-motoneurones.

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

The reflex actions, elicited by graded electrical stimulation of hind-limb muscle, skin and joint nerves were studied in an extended series of experiments in extra-and intracellular recordings from 120 lumbar gamma-motoneurones of cats anaesthetized with chloralose. The present report deals with the action of group I muscle afferent fibres, which was examined in ninety-five gamma-cells. Of the gamma-cells 83% were classified as either static or dynamic by stimulation in the mesencephalic area for dynamic control. The general responsiveness (i.e. number of input nerves with effect/number of input nerves tested) of the cells was very high (89.9%). The responsiveness to stimulation of group I muscle afferent fibres was extremely low, both in flexor and extensor gamma-motoneurones and irrespective of whether they were static or dynamic. There was no difference, as regards the low incidence of group I muscle reflex action, between stimulation of autogenetic and heteronymous nerves. Among the rare reflex effects elicited from group I muscle fibres inhibition was four times as frequent as excitation. Inhibitory effects could be provoked from the autogenetic as well as from the heteronymous nerves. In contrast, excitatory effects were almost always autogenetic. The scarcity of group I muscle action on gamma-motoneurones is in striking contrast to the well known and powerful reflex actions of Ia and Ib fibres on alpha-motoneurones. These findings are discussed in relation to the concept of alpha-gamma linkage, and it is concluded that skeletomotor and fusimotor activity cannot be rigidly linked under all conditions.

Closely coupled excitation of gamma-motoneurones by group III Muscle afferents with low mechanical threshold in the cat

1. The reflex responses of gamma-motoneurones to discharges of muscle receptors innervated by Group III axons have been examined in hind-limb muscles of decerebrated and spinal cats.2. Electrical stimulation of the gastrocnemius medialis nerve at a strength sufficient to excite Group III axons caused excitation of gastrocnemius lateralis or soleus gamma-motoneurones. The excitation was more prominent in the spinal animal.3. Excitation of either silent or tonically firing gamma-motoneurones was a secure, driven type of response consisting of one or two spikes occurring at fairly fixed latency in response to a single stimulus. Eighteen out of thirty-nine gamma-motoneurones studied showed such excitation.4. Subtracting peripheral conduction times of the gamma-motoneurone impulse and the earliest component of the Group III volley from the latency of reflex excitation gave a range of central delays of 1.8-4.8 msec (mean 3.0 msec).5. Inhibitory Group III effects were also seen but were less pronounced than the excitation.6. The origin of the receptors connected to the Group III axons whose discharge causes driven excitation of gamma-motoneurones was investigated.7. Isometric twitch contractions of a muscle caused pronounced facilitation or excitation of homonymous and heteronymous gamma-motoneurones. Excitation occurred close to the peak or during relaxation of the twitch with a range in latency of 30-110 msec. The muscles studied were triceps surae, flexor digitorum and hallucis longus.8. Pressure or light taps applied to the gastrocnemius medialis muscle caused an increase in discharge frequency in twenty out of twenty-eight gamma-motoneurones of the same muscle (four were inhibited). The response to a steady stimulus adapted but could outlast it by many seconds. Pressure was more effective when applied to the proximal or distal parts of the muscle.9. Fifteen out of thirty-eight gastrocnemius medialis receptors having Group III axons (conduction velocities 5-27 m/sec) were found to discharge one, or occasionally two, spikes to twitch contractions of the parent muscle at latencies of 30-80 msec.10. Eight out of ten receptors with Group III axons that responded to contraction had low thresholds to pressure and taps, whereas the majority of the twenty-three Group III units which were insensitive to contraction had high thresholds to such stimuli. Receptive fields of all Group III units were confined to either the proximal third or distal third of gastrocnemius medialis or, in a few instances, the Achilles tendon.11. We conclude that discharges in Group III axons from receptors which respond to non-noxious, low threshold mechanical stimuli cause a tightly coupled excitation of gamma-motoneurones. The likely contribution of this reflex to the control of movement is discussed.

Uptake, localization and release of serotonin in the chick retina

1. High pressure liquid chromatography and electrochemical detection have shown that serotonin exists in the chick retina (127 ng/g wet weight). No other indoleamine was identified. 2. Immunofluorescent histological studies showed that the endogenous serotonin was localized apparently in those cell bodies and processes which took up exogenous [3H]serotonin as revealed by autoradiography. These serotonergic neurones can be destroyed by injecting kainic acid into the eye. 3. Isolated chick retina accumulated exogenous [3H]serotonin. Kinetic analysis revealed the presence of two saturable uptake systems: a 'high affinity' mechanism with an apparent Km of 5.9 X 10(-8) M and a Vmax of 0.143 X 10(-13) mol/mg wet weight . min and a low affinity mechanism with an apparent Km2 of 1.8 X 10(-3) M and Vmax of 0.12 X 10(-9) mol/mg wet weight . min. 4. The uptake of serotonin was temperature-sensitive and sodium-dependent and Lilly 110140 and chlorimipramine were potent inhibitors of the amine uptake. 5. Autoradiographic studies indicated that neuronal processes associated with the innermost and outermost areas of the inner plexiform layer and perikarya situated in the inner nuclear layer are the sites which accumulated exogenous [3H]serotonin. 6. [3H]Serotonin accumulated in the retina was released by increasing the external K+ concentration. This release was Ca2+-dependent. Additionally, autoradiographic studies show that [3H]serotonin taken up by the serotonin neurones was also released by Ca2+-dependent K+ depolarization of the retina.

Fusimotor reflexes in triceps surae elicited by natural stimulation of muscle afferents from the cat ipsilateral hind limb

1. Experiments were performed in forty-one cats anaesthetized with chloralose.2. The aim of the study was to investigate whether activity in stretch-sensitive muscle receptors may cause reflex effects in fusimotor neurones.3. Activity in fusimotor neurones was studied indirectly by recording from primary and secondary muscle spindle afferents of the triceps surae muscle. The mean rate of firing of the afferents as well as either dynamic index (during ramp extension) or modulation (during sinusoidal extension) was determined. This was done under control conditions, with the posterior biceps-semitendinosus muscles relaxed, and under test conditions, with the same muscles extended.4. All together, seventy-one primary afferents were studied quantitatively. Pure or predominantly dynamic effects were observed in twenty-two, pure or predominantly static effects in nine and no statistically significant effects in forty of the units. Amongst seven secondary afferents studied, two showed weak fusimotor activation, the other five were not influenced.5. Electrical stimulation of the posterior biceps-semitendinosus or medial gastrocnemius nerves at group II strength was observed to cause dynamic fusimotor reflexes on a number of occasions.6. The reflex effects observed were, on many occasions, recorded in spinalized preparations.7. The reflex effects were not accompanied by any detectable e.m.g. activity in triceps, as judged from surface e.m.g. recordings. The reflex effects observed are therefore tentatively ascribed to activation of gamma-motoneurones, yet a contribution from beta-motoneurones cannot wholly be excluded.8. On the basis of available evidence concerning reflex connexions to gamma-motoneurones from various muscle afferents, it is suggested that the effects observed were caused by activation of muscle spindle secondary endings.

The effect of dantrolene sodium on intrafusal muscle fibres in the rat soleus muscle

1. The action of the skeletal muscle relaxant drug, dantrolene sodium, given intravenously, on the intrafusal fibres of the soleus muscle of the urethane-anaesthetized rat has been investigated. The experiments were made on functionally single spindle afferents and gamma fusimotor fibers isolated in dorsal and ventral roots respectively. 2. Dantrolene sodium was without effect on the discharge of primary and secondary afferents from the passive muscle spindle, nor were the dynamic indices of these endings affected. 3. Intrafusal muscle contraction was measured indirectly by means of the spindle afferent discharge. 4. The intrafusal muscle twitch contraction, as measured by means of the amplitude of the frequencygram, was depressed more slowly to a lesser extent than was the twitch of the extrafusal contraction. 5. Intrafusal contraction resulting from tetanic stimulation of the gamma fibre was depressed by dantrolene sodium to an extent dependent upon the stimulation frequency. At frequencies of 10, 25 and perhaps 50 Hz the depression was complete, that is, no afferent response was evoked; at 200 Hz stimulation, the depression was minimal (or non-existent). 6. For a muscle spindle primary ending under dynamic gamma activation dantrolene sodium caused a reduction of dynamic index whereas for the ending under static activation it caused an increase. 7. The significance of the findings in terms of the clinical use of the drug is considered.

Pattern of 'non-reciprocal' inhibition of motoneurones by impulses in group Ia muscle spindle afferents in the cat

1. Inhibitory post-synaptic potentials (i.p.s.p.s) evoked by adequate stimulation of group Ia muscle spindle afferents of triceps surae and plantaris and by near-threshold electrical stimulation of quadriceps and hamstring nerves were recorded in a number of motoneurone species. The aim of the study was to compare the pattern of non-reciprocal Ia inhibitory actions on hind-limb motoneurones with the pattern of inhibition evoked from group Ib tendon organ afferents.2. In all the motoneurone species analysed in which i.p.s.p.s were evoked by electrical stimulation maximal for both group Ia and Ib afferents of triceps surae and plantaris, they were also evoked when these muscles were stretched and the amplitude of the stretch (10-35 mum) was below threshold for Ib afferents; 70-100% of motoneurones with Ib i.p.s.p.s showed stretch-evoked i.p.s.p.s. The stretch-evoked i.p.s.p.s appeared with latencies compatible with disynaptic and trisynaptic linkage. Since these latencies were too short to allow their mediation by group II afferents the i.p.s.p.s are attributed to a selective action of Ia afferents. The i.p.s.p.s did not appear after the nerves to triceps surae and plantaris had been cut.3. Electrical stimulation of quadriceps and hamstring nerves which was near threshold for Ia afferents and well below threshold for either the Ib component of the incoming volley or group II afferents, similarly evoked non-reciprocal i.p.s.p.s. They were found in those motoneurones in which inhibition was evoked by stimulation maximal for group I afferents. Such Ia i.p.s.p.s were evoked both in homonymous motoneurones and in motoneurones of four other hind-limb muscles. Their latencies corresponded to di- and trisynaptic coupling.4. In some motoneurones of the pretibial flexors (anterior tibial, extensor digitorum longus and peroneus longus), disynaptic i.p.s.p.s evoked from triceps surae and/or plantaris which were depressed by a conditioning ventral root stimulation (i.e. Ia reciprocal i.p.s.p.s) were followed by trisynaptic i.p.s.p.s which were not depressed in this way (Ia ;non-reciprocal' i.p.s.p.s). It thus appears that the same motoneurones may be inhibited by impulses in group Ia afferents via different spinal pathways.5. The study leads to the conclusion that the non-reciprocal inhibition from group Ia muscle spindle afferents operates in parallel with the inhibition from group Ib tendon organ afferents in all motoneurone species tested.

Autogenetic effects of muscle contraction on extensor gamma motoneurones in the cat

The effect of isometric twitch contractions of hind limb muscles on the discharge of gamma motoneurones has been studied in decerebrated cats with thoracic spinal cord section. Contractions of flexor digitorum longus (FDL) or gastrocnemiussoleus (GS) strongly inhibited the background discharge of their homonymous gamma motoneurones. In contrast, contraction of FDL generally caused less inhibition, or did not affect the discharge, of the synergist GS gamma motoneurones. Both the autogenetic and synergist inhibition were considerably weaker in decerebrated cats with intact spinal cords. The inhibition lasted 20 to 50 ms and occurred principally during the rising phase of contraction. It could be followed or terminated by a weaker period of facilitation during relaxation of the muscle. Shortening the muscle so that no active tension developed during contraction either abolished or substantially curtailed the inhibition. We discuss the probability that the inhibition is a spinal segmental reflex brought about by impulses generated in Ib afferents of tendon organs. The inhibition is under tonic inhibitory control in the decerebrated cat when the spinal cord is intact. Unlike the Ib inhibition of alpha motoneurones that of gamma motoneurones has a particularly strong autogenetic component.

The effects of intrapulmonary carbon dioxide and airway pressure on phrenic activity in the dog [proceedings]

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