Activity of fusimotor neurons during reflex muscle contraction

The influence of fusimotor activity, via the gamma-loop, on the reflex response of motor neurons to stretch or vibration of triceps surae muscles was investigated in decerebrated cats. Action potentials of single fusimotor neurons were recorded from fine branches cut from nerves innervating triceps surae muscles leaving their main nerve supply intact. Most of the examined fusimotor neurons were found to be coactivated with motor neurons during reflex contraction of the muscles. Week autogenetic inhibitory influence on the discharge of fusimotor neurons was established at the beginning of the reflex tension rise in the muscles. A decrease of the reflex motor signal leading to the so-called silent period may, on the ground of this finding, results partially from the transiently lowered fusimotor outflow to the contracting muscles. Investigation of the changes in fusimotor spike occurrence during the rising phase of reflex muscle contraction seemed to be valuable in the estimation of both the autogenetic reflex influence on these motoneurons and conditions required for servoassistance.     

The responses of muscle spindles to small, slow movements in passive muscle and during fusimotor activity

We have previously shown that movement detection thresholds at the human elbow joint were less than a degree of joint rotation in the passive limb but were higher if they were measured while subjects co-contracted elbow muscles [A.K. Wise, J.E. Gregory, U. Proske, J. Physiol., 508 (1998) 325-330]. Here we report observations on the responses of muscle spindles of the soleus muscle of the anaesthetised cat to determine their ability to signal small length changes in the passive muscle and during a contraction, under conditions resembling those of the human experiments. After appropriate conditioning of the muscle to control for history effects, primary endings of muscle spindles showed thresholds to ramp stretch at 20 micrometers s-1 of between less than 5 micrometers and 15 micrometers, which translates to 0.05 degrees -0.15 degrees of human elbow joint rotation. Thresholds were much higher following conditioning to introduce slack in the muscle. Since during a voluntary contraction there is likely to be alpha:gamma co-activation, responses of spindles were also recorded during slow stretches (100 micrometers at 20 micrometers s-1) during static fusimotor stimulation, dynamic fusimotor stimulation, combined fusimotor stimulation and fusimotor plus skeletomotor stimulation. Invariably, responses to passive stretch were larger than during motor stimulation. It is concluded that spindles are sensitive enough to signal fractions of a degree of elbow joint rotation and that the rise in threshold observed during a voluntary contraction may be accounted for by the actions of fusimotor and skeletomotor axons on spindle stretch responses.     

Changes in reflex responses of the genioglossus muscle during sleep in rabbits

Changes in reflex responses in the genioglossus (GG) muscle evoked by electrical stimulation of the inferior alveolar nerve and GG muscle tone (background activity, BGA) were investigated during sleep-wakefulness stages in rabbits. The GG muscle showed two types of electromyographic activity patterns: a respiration-related phasic activity and non-respiration-related activity. GG reflex responses and BGA exhibited a stage-dependent decrease as they were constantly suppressed from quiet wakefulness to non-rapid eye movement sleep to rapid eye movement sleep (REMS). Degree of suppression of reflexes was much larger than that of BGA regardless of GG activity patterns. When amplitude of reflex responses was compared between with and without rapid eye movements during REMS, no difference between the conditions was noted. These results suggest that excitability of the GG muscle is affected by sleep stages by not only a modulation of excitability in motoneurons but also in interneurons involved in the reflex arc.     

Summing responses of cat soleus muscle spindles to combined static and dynamic fusimotor stimulation

This is a study of the summation of responses of primary endings of muscle spindles to combined static and dynamic fusimotor stimulation in the soleus muscle of the anaesthetised cat. Summation, expressed as a summation coefficient, K, was measured under a variety of conditions including (1) at several, fixed muscle lengths using steady rates of stimulation, (2) using ramp-shaped increases in stimulation rates, (3) during passive stretches after muscle conditioning, and (4) during combined stretch plus stimulation. The predominant effect observed was occlusion, that is, the combined response was less than the sum of the two individual responses. The calculated mean K value for responses at fixed length was 0.156 (±0.005 S.E.M.). It was hypothesised that summation arose from electrotonic spread of generator current between the afferent terminals, either directly, or as a result of mechanical interactions between the contracting intrafusal fibres. Summation for responses from pairs of static fusimotor fibres gave a larger mean K value, 0.340 (±0.020 S.E.M.). These findings were interpreted in terms of a model of the muscle spindle where responses to dynamic fusimotor stimulation arise at one impulse generating site, and static fusimotor responses arise at another.     

Summation of responses of cat muscle spindles to combined static and dynamic fusimotor stimulation

This is a study of the process of interaction between the responses of muscle spindles to stimulation of two fusimotor fibres. Combined stimulation of a static and a dynamic fusimotor fibre supplying the same muscle spindle in the soleus muscle of the anaesthetised cat gave a response which was larger than from stimulating each fibre separately, but less than their sum. A similar summation process was observed with pairs of static fusimotor fibres. The mean summation coefficient for the responses to stimulation of 14 pairs of static fusimotor fibres was 0.29 (range 0.14–0.52; S.D. 0.09), while for 42 static:dynamic pairs it was 0.30 (range 0.07–0.89; S.D. 0.20). Mechanisms considered for the summation process were probabilistic mixing of impulse traffic from two or more impulse generators within the terminals of the primary ending of the spindle, the spread of generator current from one encoding site to another and mechanical interactions between contracting intrafusal fibres. In an experiment where single static and dynamic fusimotor fibres were stimulated together, and then stimulation of the static fibre stopped, the size of the continuing dynamic response was larger than when the dynamic fibre had been stimulated alone. This finding suggested some kind of mechanical interaction between the contracting intrafusal fibres and implies that static and dynamic fusimotor effects within a spindle cannot be considered to be entirely independent of one another.     

Ventricular reflex interactions during transmural myocardial ischemia

The role of left ventricular receptors with sympathetic afferent fibers in the reflex response to myocardial ischemia is controversial, particularly in the canine model. Previous experiments have shown that reflex excitatory responses mediated by left ventricular sympathetic afferents can be detected in sinoaortic denervated and vagotomized dogs during transmural myocardial ischemia. The purpose of these experiments was to determine if reflex excitatory responses occur in dogs with intact left ventricular vagal afferents. Experiments were performed in 27 chloralose-anesthetized dogs with sinoaortic denervation. Responses of efferent renal sympathetic nerve activity, arterial, and left atrial pressures to transmural and non-transmural inferoposterior myocardial ischemia were measured before and after interruption of left ventricular sympathetic afferents by stellectomy. The adequacy of sympathetic deafferentation was assessed by measurement of renal nerve responses to epicardial bradykinin. Prior to stellectomy, excitatory responses were observed in 10 animals and inhibitory responses in 9 animals. The remaining animals had no responses and were excluded from analysis. In the excitatory group, reflex increases in renal nerve activity during both transmural and non-transmural inferoposterior ischemia were abolished by stellectomy and not replaced by inhibitory responses. In the inhibitory group, non-transmural inferoposterior ischemia elicited greater reflex decreases in renal nerve activity when left ventricular sympathetic afferents were intact. After stellectomy, transmural ischemia elicited greater reflex inhibition of renal nerve activity. Renal nerve responses to epicardial bradykinin were abolished by stellectomy. These results indicate that reflex excitatory responses mediated by left ventricular receptors with sympathetic afferent fibers can be elicited in dogs with intact vagal afferents. These excitatory responses are most apparent during transmural myocardial ischemia. In dogs with inhibitory responses to coronary occlusion, activation of sympathetic afferents during transmural ischemia appears to attenuate reflex inhibitory responses mediated by left ventricular vagal afferents.     

Intracellular responses of raphe magnus neurons during the jaw-opening reflex evoked by tooth pulp stimulation

Neurons in the nucleus raphe magnus (RM) may play an important role in the modulation of nociception. To determine how RM neurons are activated during a nociceptive reflex, the intracellular responses of raphe neurons were studied during the jaw-opening reflex (JOR) elicited by tooth pulp shock in lightly anesthetized cats. Tooth pulp stimulation produces reflex EMG activation of the digastric muscle at a latency of 7-11 ms, resulting in jaw opening. Tooth pulp shock that elicits the JOR also produces an EPSP in a subset of raphe neurons. This EPSP consists of an early small depolarization that occurs at a latency of 10-15 ms followed by a larger depolarization at a latency of 20-60 ms. In all cases the latency to EPSP is longer than the latency to digastric EMG onset. Electrical stimulation of the 4 paws elicits oligosynaptic EPSPs in the same cells at a latency of 16-20 ms. Electrical train stimulation of the midbrain periaqueductal gray region (PAG) suppresses the JOR. Single shock stimulation at the same PAG sites that suppress the JOR evokes monosynaptic EPSPs in the large majority of raphe neurons recorded. In all cases, the threshold for EPSP is below the threshold for suppression of the JOR. The EPSP amplitude is a direct function of PAG stimulus intensity and there is temporal summation of EPSPs evoked by paired PAG shocks. At condition-test intervals of 40-90 ms, train stimulation of PAG suppresses the tooth pulp-evoked EPSP in raphe neurons. The threshold for EPSP suppression occurs at a PAG stimulation intensity below that required for suppression of the JOR. The present findings provide evidence that RM neurons may play an important role in the modulation of the tooth pulp-evoked JOR, but only after the initial withdrawal reflex has occurred.     

Assessment of fusimotor contribution to reflex reinforcement in humans

The contribution of the fusimotor system to reflex reinforcement such as the Jendrassik manoeuvre was investigated by recording single unit activity with tungsten electrodes from muscle spindle afferent nerves in unanaesthetized normal human subjects. Muscle spindle afferent activity was recorded before, during, and after the reinforcement test. When the leg muscles remained relaxed during the Jendrassik manoeuvre, spindle activity recorded in the tibial nerve was accelerated. Also in the median nerve, activity from muscle spindle afferent fibres was increased during a remote contraction of the ipsilateral quadriceps muscle. Comparing the time course of the phasic reflex reinforcement and the muscle spindle facilitation during the remote contraction, a marked after-effect was recorded in both responses. Present results show an increased spontaneous muscle spindle activity in relaxed muscles during a remote muscle contraction, and provide evidence for the contribution of the fusimotor system to the enhancement of phasic reflexes by reinforcement manoeuvres.     

Variability of interspike intervals of human motor neurons during voluntary muscle contraction and tonic vibration reflex]

Firing of motor units of human soleus, triceps brachii and rectus femoris muscles was studied. Standard deviations of interspike intervals against mean intervals were plotted during voluntary muscle contraction and tonic vibration reflex. There was no significant difference between the results obtained under these conditions.     

Antidromic responses and reflex activity of single salivatory neurons in the cat

Single salivatory neurons of the brain stem of the urethane-chloralose anesthetized cat were identified by their antidromic responses to stimulation of the chorda tympani. The antidromically identified neurons were recorded in the lateral reticular formation of the brain stem between the spinal trigeminal nucleus and the vestibular complex. As suggested by previous anatomic work, the salivatory neurons appear to be diffusely distributed within the region. The identified neurons responded synaptically to stimulation of the lingual nerve, but this reflex activity was not mediated by an input from the taste fibers.     

Summing responses of cat soleus muscle spindles to combined static and dynamic fusimotor stimulation(1)

This is a study of the summation of responses of primary endings of muscle spindles to combined static and dynamic fusimotor stimulation in the soleus muscle of the anaesthetised cat. Summation, expressed as a summation coefficient, K, was measured under a variety of conditions including (1) at several, fixed muscle lengths using steady rates of stimulation, (2) using ramp-shaped increases in stimulation rates, (3) during passive stretches after muscle conditioning, and (4) during combined stretch plus stimulation. The predominant effect observed was occlusion, that is, the combined response was less than the sum of the two individual responses. The calculated mean K value for responses at fixed length was 0.156 (+/-0.005 S.E.M.). It was hypothesised that summation arose from electrotonic spread of generator current between the afferent terminals, either directly, or as a result of mechanical interactions between the contracting intrafusal fibres. Summation for responses from pairs of static fusimotor fibres gave a larger mean K value, 0.340 (+/-0.020 S.E.M.). These findings were interpreted in terms of a model of the muscle spindle where responses to dynamic fusimotor stimulation arise at one impulse generating site, and static fusimotor responses arise at another.     

Intermittent stimulation of fusimotor fibres during slow stretch of muscle spindles in the cat

Responses of muscle spindles of the peroneus tertius muscle of the cat were recorded during intermittent fusimotor stimulation, applied during slow stretch, after muscle conditioning by stimulation of the nerve at the test length or at a length 2.5 mm longer. Some static and all dynamic axons evoked afferent bursts whose amplitude was relatively independent of the level of stretch response. Other static axons produced bursts that grew in size with the stretch.     

Discharge patterns of cat raphe neurons during sleep and waking

Discharge patterns of 21 single neurons of median and magnus raphe have been chronically recorded with tungsten microelectrodes stereotaxically placed in freely behaving unrestrained, unanesthetized cats. These neurons in the median and magnus raphe fired slowly during slow-wave sleep (SWS, 5.66 ± 4.24/sec), faster during awake (AWA, 11.11 ± 7.56/sec) and fastest during paradoxical sleep (PS, 15.87 ± 8.46/sec). Student'st-test (single-tailed) indicated that differences between SWS and AWA, SWS and PS were highly significant, and was significant between AWA and PS. The ratio of the firing rate of PS to SWS varied from 1.4 to 5 for the entire group. During PS the raphe units had a tendency to fire in burst. Interspike interval and joint interval distribution histograms (JIDH) also showed pronounced differences between the patterns of raphe activity during PS and SWS. Twelve units recorded from raphe magnus began to discharge more rapidly about 10–20 sec before the onset of the hippocampal theta rhythm characteristic of PS. This enhanced unit activity decreased concomitantly with the cessation of theta activity. These data indicate that neurons in raphe magnus might participate in the generation and maintenance of theta activity in dorsal hippocampus (DH) during PS. The lower single unit activity during SWS in comparison to AWA seems to disagree with Jouvet's ‘active serotonergic control of the SWS’; the increased raphe magnus activity prior to and during theta rhythm in DH gives some support to his ‘possible participation of the serotonergic caudal raphe system in PS genesis’.     

Effects of muscle relaxant E-0646 on human stretch reflex and responses

The experiment examined the effects of a muscle relaxant E-0646 on short latency stretch reflex (SLR) and long latency stretch responses (LLRs) of lower leg muscles in 5 healthy male subjects, aged 24-41 years. An AC servomotor produced an ankle rotation of comfortably seated subjects in the direction to stretch the triceps surae muscle for the SLR and the tibialis anterior muscle for the LLRs. Following control measurements, after the oral administration of 50, 100, 150, and 300 mg of E-0646, the SLR and LLRs were recorded every 2 hours for 4 or 6 hours with simultaneous blood sampling to measure E-0646 plasma levels. Electromyograms (EMGs) of the soleus and tibialis anterior muscles were recorded with surface electrodes on each muscle belly. A precision potentiometer was used to measure the ankle angle. The soleus SLR, evoked by a torque pulse (7 msec duration) of one of 5 different amplitudes, decreased 2 hours after the 300 mg administration. A SLR decrease was also found in a simple relationship between the integrated SLR EMG and the angular velocity at 20 msec after the torque onset, in which the slope and intercept of the relationship changed. The plasma E-0646 level roughly correlated with the SLR decrease. No significant change of the SLR was observed when dosage was less than 150 mg. The LLRs of the tibialis anterior muscle were tested under the instruction "resist when perturbed" during a weak isometric contraction against a constant plantar flexion torque.(ABSTRACT TRUNCATED AT 250 WORDS)     

Human muscle afferent responses to tendon taps. 2. Effects of variations in fusimotor bias.

The effect of varying the fusimotor bias on the muscle spindle responses to light tendon taps has been studied in normal human volunteers using surface electrodes at the wrist for recording whole nerve activity. Reinforcement manoeuvres were found to increase the sensitivity of the afferent responses to the mechanical stimulus. Such sensitisation was found to be exhibited more commonly as a decrease in the latency of the peak of the afferent waveform than as an increase in amplitude. Increase in amplitude of the response was seen in cases where the subject was well relaxed and the test muscle quiescent. A change in furimotor drive was also achieved by asking the subjects to close their eyes voluntarily during the test, thus depriving themselves of the visual feedback. The results under these conditions were found to be variable, though showing considerable changes from control recordings. The effect of reinforcement manoeuvres may perhaps result in increasing the dynamic fusimotor drive. Such an effect may be simulated on occluding the blood supply to the test muscle since ischaemia produces an immediate rise in the rate of afferent discharge. The method of recording is suggested as a convenient technique for clinical use.