Analysis of coupling between optic lobe circadian pacemakers in the cricket Gryllus bimaculatus

The coupling mechanism between weakly coupled two optic lobe circadian pacemakers in the cricket Gryllus bimaculatus was investigated by recording the locomotor activity, under light-dark cycles with various lengths, after the optic nerve was unilaterally severed. The activity rhythm split into two components under the light cycles different from 24 h: one was readily entrained to the light cycle and the other only loosely entrained or freeran. Additional removal of the optic lobe on the intact side resulted in a loss of the entrained component and that on the blinded side caused the reverse effect, indicating that the entrained component was driven by the pacemaker on the intact side and the other by the one on the blinded side. The synchronization between the two components was achieved only in light cycles with a limited length between 23 and 25 h. Without this range, the desynchronization of the components occurred. In the split rhythm, the phase-dependent modulation of the period of freerunning component and the mutual suppression of locomotor activity during the subjective day phase were clearly observed. The suppression was also evident in the lights-on peak that was the masking effect of light. The light cycle with dim light significantly reduced the ratio of animals with the pacemaker coupling as well as the magnitude of the period modulation. These results suggest (1) that the mutual coupling is achieved only when the difference in the periods between the two pacemakers is within an allowable range, (2) that the photic information is also involved in the mechanism of mutual coupling, and (3) that the suppression of activity occurs at the regulatory center for locomotion.Abbreviations CT circadian time - DD constant darkness - LL constant light - LD light to dark cycle - T length of light to dark cycle - freerunning period     

Limiting mechanisms of force production after repetitive dynamic contractions in human triceps surae.

The influence of repetitive dynamic fatiguing contractions on the neuromuscular characteristics of the human triceps surae was investigated in 10 subjects. The load was 50% of the torque produced during a maximal voluntary contraction, and the exercise ended when the ankle range of motion declined to 50% of control. The maximal torque of the triceps surae and the electromyographic (EMG) activities of the soleus and medial gastrocnemius were studied in response to voluntary and electrically induced contractions before and after the fatiguing task and after 5 min of recovery. Reflex activities were also tested by recording the Hoffmann reflex (H reflex) and tendon reflex (T reflex) in the soleus muscle. The results indicated that whereas the maximal voluntary contraction torque, tested in isometric conditions, was reduced to a greater extent (P < 0.05) at 20 degrees of plantar flexion (-33%) compared with the neutral position (-23%) of the ankle joint, the EMG activity of both muscles was not significantly reduced after fatigue. Muscle activation, tested by the interpolated-twitch method or the ratio of the voluntary EMG to the amplitude of the muscle action potential (M-wave), as well as the neuromuscular transmission and sarcolemmal excitation, tested by the M-wave amplitude, did not change significantly after the fatiguing exercise. Although the H and T reflexes declined slightly (10-13%; P < 0.05) after fatigue, these adjustments did not appear to have a direct deleterious effect on muscle activation. In contrast, alterations in the mechanical twitch time course and postactivation potentiation indicated that intracellular Ca(2+)-controlled excitation-contraction coupling processes most likely played a major role in the force decrease after dynamic fatiguing contractions performed for short duration.     

Contraction level-related modulation of corticomuscular coherence differs between the tibialis anterior and soleus muscles in humans

The sensorimotor cortex activity measured by scalp EEG shows coherence with electromyogram (EMG) activity within the 15- to 35-Hz frequency band (β-band) during weak to moderate intensity of isometric voluntary contraction. This coupling is known to change its frequency band to the 35- to 60-Hz band (γ-band) during strong contraction. This study aimed to examine whether such contraction level-related modulation of corticomuscular coupling differs between muscles with different muscle compositions and functions. In 11 healthy young adults, we quantified the coherence between EEG over the sensorimotor cortex and rectified EMG during tonic isometric voluntary contraction at 10-70% of maximal voluntary contraction of the tibialis anterior (TA) and soleus (SOL) muscles, respectively. In the TA, the EEG-EMG coherence shifted from the β-band to the γ-band with increasing contraction level. Indeed, the magnitude of β-band EEG-EMG coherence was significantly decreased, whereas that of γ-band coherence was significantly increased, when the contraction level was above 60% of maximal voluntary contraction. In contrast to the TA, the SOL showed no such frequency changes of EEG-EMG coherence with alterations in the contraction levels. In other words, the maximal peak of EEG-EMG coherence in the SOL existed within the β-band, irrespective of the contraction levels. These findings suggest that the central nervous system regulates the frequency of corticomuscular coupling to exert the desired levels of muscle force and, notably, that the applicable rhythmicity of the coupling for performing strong contractions differs between muscles, depending on the physiological muscle compositions and functions of the contracting muscle.     

Central and peripheral contributions to fatigue in relation to level of activation during repeated maximal voluntary isometric plantar flexions.

This study aimed to investigate central and peripheral contributions to fatigue during repeated maximal voluntary isometric plantar flexions (MVCs). Changes in joint torque, level of activation (LOA), resting twitch amplitude (RT), electromyographic signals (EMG), and presynaptic inhibition of Ia afferents were investigated during 9 bouts of 10 MVCs. MVCs lasted for 2 s and were separated by 1 s. The interval between bouts was 10 s. Electrical stimulation was applied to the tibial nerve; at rest to evoke RTs, M waves, and two (1.5-s interval) H reflexes; with the soleus EMG at 30% of that during MVC to evoke M waves and two H reflexes; and during MVCs to measure LOA. Over the nine bouts, LOA decreased by 12.6% and RT by 16.2%. EMG root mean square during MVCs remained unchanged for the soleus and tibialis anterior muscles, but it decreased for medial gastrocnemius. Peripheral fatigue (decrease in RT) was positively correlated to LOA, whereas central fatigue (decrease in LOA) was not. Depression of both H reflexes suggests that presynaptic inhibition after the first bout was partly induced by homosynaptic postactivation depression of the Ia terminal. The H-reflex-to-M-wave ratio increased with fatigue in both passive and active states, with no change in the ratio of the second H reflex to the first, thereby indicating a decrease of presynaptic inhibition during fatigue. The results indicate that both central and peripheral mechanisms contributed to the fatigue observed during repeated MVCs and that the development of peripheral fatigue was influenced by the level of voluntary activation and initial plantar flexor torque.     

Effects of strength, endurance, and concurrent training on aerobic power and dynamic neuromuscular economy in elderly men

The aim of this study was to investigate the effects of concurrent training on endurance capacity and dynamic neuromuscular economy in elderly men. Twenty-three healthy men (65 ± 4 years) were divided into 3 groups: concurrent (CG, n = 8), strength (SG, n = 8), and aerobic training group (EG, n = 7). Each group trained 3 times a week for 12 weeks, strength training, aerobic training, or both types of training in the same session. The maximum aerobic workload (Wmax) and peak oxygen uptake (VO2peak) of the subjects were evaluated on a cycle ergometer before and after the training period. Moreover, during the maximal test, muscle activation was measured at each intensity by means of electromyographic signals from the vastus lateralis (VL), rectus femoris (RF), biceps femoris long head, and gastrocnemius lateralis to determine the dynamic neuromuscular economy. After training, significant increases in VO2peak and Wmax were only found in the CG and EG (p < 0.05), with no difference between groups. Moreover, there was a significant decrease in myoelectric activity of the RF muscle at 50 (EG), 75 and 100 W (EG and CG) and in the VL for the 3 groups at 100 W (p < 0.05). No change was seen in the electrical signal from the lateral gastrocnemius muscle and biceps femoris. The results suggest specificity in adaptations investigated in elderly subjects, because the most marked changes in the neuromuscular economy occurred in the aerobically trained groups.     

A specific area of the compound eye in the cricket Gryllus bimaculatus sends photic information to the circadian pacemaker in the contralateral optic lobe

The circadian locomotor rhythm of the cricket Gryllus bimaculatus is primarily regulated by a pair of interacting optic lobe circadian pacemaker systems. The interaction involves phase-dependent modulation of the free-running period and phase-dependent suppression of activity. Since photic information has been shown to be involved in the interaction, we examined the regional difference in photoreception for the interaction within cricket compound eyes. The activity rhythm of animals receiving partial reduction of one compound eye combined with severance of the contralateral optic nerve split into entrained and free-running components under a 13-h light to 13-h dark cycle. All the animals operated on showed a phase-dependent suppression of activity, and most animals showed a phase-dependent modulation of the period of the free-running component. However, removal of the dorsocaudal area of the compound eye resulted in a severe reduction of the amplitude of the phase-dependent-period modulation. These results suggest that the dorsocaudal portion of the compound eye is a specific region receiving photic signals that are transmitted to the circadian pacemaker in the contralateral optic lobe and that the phase-dependent suppression of activity is caused by a mechanism separate from that for the period modulation.     

Cholinergic mechanisms related to REM sleep. III. Tonic and phasic inhibition of monosynaptic reflexes induced by an anticholinesterase in the decerebrate cat

The depression of the postural activity induced by intravenous injection of eserine sulphate (0.1 mg/kg), an anticholinesterase, has been studied in precollicular decerebrate cats. The extensor and flexor monosynaptic reflexes elicited by single shock stimulation of the GS, P1-FDHL and DP nerves are tonically depressed during the episodes of postural atonia induced by the anticholinesterase. A further phasic depression of the monosynaptic reflexes occurs during the bursts of rapid eye movements (REM) typical of these episodes. These changes in spinal reflex activity closely resemble the tonic depression of the spinal reflexes described in the unrestrained cats during the desynchronized sleep as well as the phasic depression of the spinal reflexes characteristic of the hypnic bursts of REM. Results obtained after spinal cord section indicate that both the tonic and the phasic depression of the spinal reflexes induced by eserine are due to active inhibitory influences originating from supraspinal structures. A complete bilateral destruction of the vestibular nuclei or limited to the medial and descending vestibular nuclei abolishes not only the cholinergically induced bursts of REM, as reported in a previous paper, but also the related phasic depression of the monosynaptic reflexes. These findings can be related with previous observations showing that a bilateral lesion of the vestibular nuclei abolishes the REM bursts of desynchronized sleep, as well as the related phasic inhibition of the spinal reflexes. The tonic depression of the monosynaptic reflexes induced by the anticholinesterase, on the other hand, remains unmodified by this vestibular lesion. This depression, therefore, can be attributed to supraspinal descending inhibitory volleys originating from extravestibular structures.     

Mechanisms of stepping rhythm formation in decerebrated and chronic spinal cats under epidural stimulation

A study was made of the stepping pattern formation in decerebrated and in chronic spinal cats during epidural stimulation (ES). The hindlimb stepping performance depended on the parameters of ES and afferent input. At non-optimal ES parameters, no stepping was induced, only muscle reflexes followed the stimulation rhythm. Optimized ES (3–5 Hz, 50–100 μA for decerebrated and 20–30 Hz, 150–250 μA for spinal cats) evoked coordinated stepping movements at a natural rate (0.8–1 Hz) accompanied by electromyographic burst activity of the corresponding muscles. In decerebrated cats, the bursts are formed owing to modulation of early responses and the late polysynaptic activity. In chronic spinal cats, this process is mainly due to amplitude modulation of the early responses. Formation of the stepping pattern in decerebrated cats involves spinal interneurons responsible for the polysynaptic activity, which allows its correction based on processing the afferent signals. Activation of this system in chronic spinal cats can be realized by afferent stimulation alone, without ES.     

Reflex modulation in locusts walking on a treadwheel intracellular recordings from motoneurons

Summary In locusts (Locusta migratoria) walking on a treadwheel, afferents of tarsal hair sensilla were stimulated via chronically implanted hook electrodes (Fig. 1). Stimuli applied to the middle leg tarsus elicited avoidance reflexes (Fig. 2). In quiescent animals, the leg was lifted off the ground and the femur adducted. In walking locusts, the response was phase-dependent. During the stance phase, no reaction was observed except occasional, premature triggering of swing movements; stimuli applied near the end of the swing phase were able to elicit an additional, short leg protraction.Central nervous correlates of phase-dependent reflex modulation were observed by recording intracellularly from motoneuron somata in walking animals. As a rule, motoneurons recruited during the swing phase showed excitatory stimulus-related responses around the end of the swing movement, correlated to the triggering of additional leg protractions (Figs. 3, 4, 5). Motoneurons active during the stance phase were often inhibited by tarsal stimulation, some showed only weak responses (Figs. 8, 9, 10). Common inhibitory motoneuron 1 was excited by tarsal stimulation during all phases of the leg movement (Figs. 6, 7). In one type of flexor tibiae motoneuron, a complex response pattern was observed, involving the inversion of stimulus-related synaptic potentials from excitatory, recorded during rest, to inhibitory, observed during long-lasting stance phases (Figs. 11, 12).The results demonstrate how reflex modulation is represented on the level of synaptic input to motoneurons. They further suggest independent gain control in parallel, antagonistic pathways converging onto the same motoneuron as a mechanism for reflex reversal during locomotion.Abbreviations CI ₁ common inhibitory motoneuron (1) - EMG electromyogram - Feti fast extensor muscle of the tibia     

Vestibulospinal influences on lower limb motoneurons

Galvanic vestibular stimulation (GVS) is a research tool used to activate the vestibular system in human subjects. When a low-intensity stimulus (1-4 mA) is delivered percutaneously to the vestibular nerve, a transient electromyographic response is observed a short time later in lower limb muscles. Typically, galvanically evoked responses are present when the test muscle is actively engaged in controlling standing balance. However, there is evidence to suggest that GVS may be able to modulate the activity of lower limb muscles when subjects are not in a free-standing situation. The purpose of this review is to examine 2 studies from our laboratory that examined the effects of GVS on the lower limb motoneuron pool. For instance, a monopolar monaural galvanic stimulus modified the amplitude of the ipsilateral soleus H-reflex. Furthermore, bipolar binaural GVS significantly altered the onset of activation and the initial firing frequency of gastrocnemius motor units. The following paper examines the effects of GVS on muscles that are not being used to maintain balance. We propose that GVS is modulating motor output by influencing the activity of presynaptic inhibitory mechanisms that act on the motoneuron pool.     

A method for the estimation of functional brain connectivity from time-series data

A central issue in cognitive neuroscience is which cortical areas are involved in managing information processing in a cognitive task and to understand their temporal interactions. Since the transfer of information in the form of electrical activity from one cortical region will in turn evoke electrical activity in other regions, the analysis of temporal synchronization provides a tool to understand neuronal information processing between cortical regions. We adopt a method for revealing time-dependent functional connectivity. We apply statistical analyses of phases to recover the information flow and the functional connectivity between cortical regions for high temporal resolution data. We further develop an evaluation method for these techniques based on two kinds of model networks. These networks consist of coupled Rössler attractors or of coupled stochastic Ornstein–Uhlenbeck systems. The implemented time-dependent coupling includes uni- and bi-directional connectivities as well as time delayed feedback. The synchronization dynamics of these networks are analyzed using the mean phase coherence, based on averaging over phase-differences, and the general synchronization index. The latter is based on the Shannon entropy. The combination of these with a parametric time delay forms the basis of a connectivity pattern, which includes the temporal and time lagged dynamics of the synchronization between two sources. We model and discuss potential artifacts. We find that the general phase measures are remarkably stable. They produce highly comparable results for stochastic and periodic systems. Moreover, the methods proves useful for identifying brief periods of phase coupling and delays. Therefore, we propose that the method is useful as a basis for generating potential functional connective models.     

Altered reflex sensitivity after repeated and prolonged passive muscle stretching.

Experiments were carried out to test the effect of prolonged and repeated passive stretching (RPS) of the triceps surae muscle on reflex sensitivity. The results demonstrated a clear deterioration of muscle function immediately after RPS. Maximal voluntary contraction, average electromyographic activity of the gastrocnemius and soleus muscles, and zero crossing rate of the soleus muscle (recorded from 50% maximal voluntary contraction) decreased on average by 23.2, 19.9, 16.5, and 12.2%, respectively. These changes were associated with a clear immediate reduction in the reflex sensitivity; stretch reflex peak-to-peak amplitude decreased by 84. 8%, and the ratio of the electrically induced maximal Hoffmann reflex to the maximal mass compound action potential decreased by 43. 8%. Interestingly, a significant (P < 0.01) reduction in the stretch-resisting force of the measured muscles was observed. Serum creatine kinase activity stayed unaltered. This study presents evidence that the mechanism that decreases the sensitivity of short-latency reflexes can be activated because of RPS. The origin of this system seems to be a reduction in the activity of the large-diameter afferents, resulting from the reduced sensitivity of the muscle spindles to repeated stretch.     

Surface electromyographic assessment of the effect of static stretching of the gastrocnemius on vertical jump performance

The purpose of this study was to investigate the effects of static stretching of the gastrocnemius muscle on maximal vertical jump performance using electromyographic activity (EMG) of the gastrocnemius musculature to record muscle activation during vertical jump performance. Fourteen healthy adults (8 men and 6 women) aged 18-34 years, who were familiar with the vertical jumping task and had no lower extremity injuries or any bone or joint disorders within the past year, served as participants for this study. After a brief warm-up, participants performed the following sequence: (a) three baseline maximal vertical jump trials, (b) 15 minutes of quiet sitting and three 30-second bilateral static stretches of the gastrocnemius muscles, and (c) 3 maximal vertical jump trials. Jump height data were collected using the Kistler force plate, while muscle activity was recorded during the jumping and stretching trials using a Noraxon telemetry EMG unit. Vertical jump height data as well as EMG values were averaged for the 3 trials and analyzed using paired t-tests for pre- and poststretching (alpha = 0.05). Vertical jump height was 5.6% lower when poststretch heights were compared with prestretch heights (t = -4.930, p < 0.005). Gastrocnemius EMG was 17.9% greater when the EMG during poststretch jumps was compared with prestretch jumps (t = 2.805, p < 0.02). The results from this study imply that, despite increased gastrocnemius muscle activity, static stretching of the gastrocnemius muscles had a negative effect on maximal jumping performance. The practical importance concerns coaches and athletes, who may want to consider the potential adverse effects of performing static stretching of the gastrocnemius muscles only before a jumping event, as jump height may be negatively affected. Future research is required to identify the mechanisms that affect vertical jump performance.     

Neuronal activity of the lateral vestibular nucleus of the guinea pig evoked by tilting the animal about the longitudinal axis during locomotion

In experiments on decerebrate guinea pigs, the impulse activity of neurons of the lateral vestibular nucleus evoked by tilting the animal about the longitudinal axis was investigated under conditions of spontaneous and mesencephalon stimulation-evoked locomotor activity. In most investigated neurons, locomotor activity led to changes in their responses to adequate vestibular stimulation. The dominant reaction was intensification of such responses, which was observed in almost all vestibulospinal neurons and in 2/3 of cells not having descending projections. Responses were suppressed only in 1/4 of the neurons not projecting to the spinal cord. The changes in the evoked responses had an amplitude character; the lag of the changes in the discharge frequency relative to the acceleration that caused them was constant. It is suggested that intensification of dynamic reactions of vestibular neurons during locomotion provides maintenance of the animal's equilibrium during movements in space by various gaits and along different trajectories.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 23, No. 5, pp. 541–549, September–October, 1991.     

Vestibulo-ocular reflexes in the adult sea lamprey

Vestibulo-ocular reflexes were elicited in isolated preparations of small adult sea lampreys (Petromyzon marinus). Mechanical stimulation of the labyrinths or electrical stimulation of the vestibular nerves produced stereotyped, conjugated eye movements (Table 1) and appropriate electrical activities in individual extraocular muscles (Fig. 1). No inhibition of discharges in motor nerves was observed during stimulation of opposing reflexes. Likewise, nystagmus was never seen during continued stimulation. The primitive eye reflexes of the lamprey probably correspond to the simple excitatory pathways from single ampullae to individual eye muscles of higher vertebrates.     

Muscle responses and monosynaptic reflexes in falling monkey. Role of the vestibular system.

The free fall has been used in our laboratory as a way to test vestibular function in baboons in order to quantify vestibular compensation in the hemilabyrinthectomized animal. This study presents only those results that concern the contribution of the vestibular system to muscle responses due to sudden fall. EMG activity was recorded from the fully conscious animal using chronic electrodes implanted in various muscles. Spinal monosynaptic reflexes (Hoffmann's and tendon reflexes) were studied in the soleus muscle. Baboons were seated in a special chair suspended from an electromagnet and unexpectedly dropped 90 cm. Experiments were performed in normal, unilateral and bilateral vestibular neurectomized baboons. 1. In normal baboons, results showed a first short-latency response in all tested muscles, followed by a second peak of EMG activity in these muscles. Comparison with data from bilateral vestibular neurectomized baboons demonstrates that normal vestibular function is essential for the appearance of the first peak; the second peak rapidly disappears in our experimental situation where the animal's fall is mechanically braked and interrupted, so the animal does not have to make the postural adjustments necessary for landing, It is suggested that the first peak is concerned with the automatic and reflex control of landing, the second with the voluntary breaking of landing. 2. The modulation of monosynaptic spinal reflexes is closely related to the EMG response in soleus muscle. Facilitation of the H-reflex begins just prior to the onset of the EMG activity and continues as long as the baboon is falling. The T-reflex modulation presents a similar time course except in its early phase where it is depressed. Decrease in T and increase in H-reflexes suggest that the EMG response is most likely due to direct activation of alpha-motoneurons and not by means of the gamma-loop. 3. In unilateral vestibular neurectomized baboons, EMG and reflexological data show the classical asymmetry characterized by a strong decrease of the responses on the side of the lesion, and by a pronounced increase on the contralateral side. It is concluded that this represents the imbalance between the resting discharge of the vestibular neurons, and discloses the influence of labyrinthine afferences at the spinal level. We suggest consequently the use of EMG responses and modulation of spinal reflexes to fall in order to quantify vestibular compensation.     

The effect of active pedaling combined with electrical stimulation on spinal reciprocal inhibition

ObjectivePedaling is widely used for rehabilitation of locomotion because it induces muscle activity very similar to locomotion. Afferent stimulation is important for the modulation of spinal reflexes. Furthermore, supraspinal modulation plays an important role in spinal plasticity induced by electrical stimulation. We, therefore, expected that active pedaling combined with electrical stimulation could induce strong after-effects on spinal reflexes.DesignTwelve healthy adults participated in this study. They were instructed to perform 7 min of pedaling. We applied electrical stimulation to the common peroneal nerve during the extension phase of the pedaling cycle. We assessed reciprocal inhibition using a soleus H-reflex conditioning-test paradigm. The magnitude of reciprocal inhibition was measured before, immediately after, 15 and 30 min after active pedaling alone, electrical stimulation alone and active pedaling combined with electrical stimulation (pedaling + ES).ResultsThe amount of reciprocal inhibition was significantly increased after pedaling + ES. The after-effect of pedaling + ES on reciprocal inhibition was more prominent and longer lasting compared with pedaling or electrical stimulation alone.ConclusionsPedaling + ES could induce stronger after-effects on spinal reciprocal inhibitory neurons compared with either intervention alone. Pedaling + ES might be used as a tool to improve locomotion and functional abnormalities in the patient with central nervous lesion.     

Coupling between respiratory and stepping rhythms during locomotion in decerebrate cats

To determine whether and how the strength of coupling between respiratory and stepping rhythms varies depending on locomotor patterns, correlation analysis was done of diaphragmatic and gastrocnemius muscle activities. In spontaneously breathing cats decerebrated at the precollicular-post-mammillary level, tonic electrical stimulation was delivered to the mesencephalic locomotor region to induce locomotion on a treadmill. Electromyograms were recorded from the left hemidiaphragm and the bilateral gastrocnemius muscles. Various locomotor patterns were elicited by changes in the belt speed of the treadmill and in the intensity of stimulation of the mesencephalic locomotor region. Cross-correlograms between diaphragmatic and gastrocnemius activities showed that coupling was absent or weak when the cats walked slowly. The strength of locomotor-respiratory coupling tended to increase as the mean stepping interval shortened. When the animals were galloping, the respiratory rhythm was entrained 1:1 with the stepping rhythm. This study showed that the strength of coupling between respiratory and stepping rhythms varied depending on the locomotor patterns elicited, especially on whether the animals were running.     

Central Adaptation to Repeated Galvanic Vestibular Stimulation: Implications for Pre-Flight Astronaut Training

Healthy subjects (N = 10) were exposed to 10-min cumulative pseudorandom bilateral bipolar Galvanic vestibular stimulation (GVS) on a weekly basis for 12 weeks (120 min total exposure). During each trial subjects performed computerized dynamic posturography and eye movements were measured using digital video-oculography. Follow up tests were conducted 6 weeks and 6 months after the 12-week adaptation period. Postural performance was significantly impaired during GVS at first exposure, but recovered to baseline over a period of 7–8 weeks (70–80 min GVS exposure). This postural recovery was maintained 6 months after adaptation. In contrast, the roll vestibulo-ocular reflex response to GVS was not attenuated by repeated exposure. This suggests that GVS adaptation did not occur at the vestibular end-organs or involve changes in low-level (brainstem-mediated) vestibulo-ocular or vestibulo-spinal reflexes. Faced with unreliable vestibular input, the cerebellum reweighted sensory input to emphasize veridical extra-vestibular information, such as somatosensation, vision and visceral stretch receptors, to regain postural function. After a period of recovery subjects exhibited dual adaption and the ability to rapidly switch between the perturbed (GVS) and natural vestibular state for up to 6 months.     

Neural pathways involved in mutual interactions between optic lobe circadian pacemakers in the cricketGryllus bimaculatus

The circadian locomotor rhythm of the cricketGryllus bimaculatus is primarily generated by a pair of optic lobe circadian pacemakers. The two pacemakers mutually interact to keep a stable temporal structure in the locomotor activity. The interaction has two principal effects on the activity rhythm, i.e., phase-dependent modulation of the freerunning period and phase-dependent suppression of activity driven by the partner pacemaker. Both effects were mediated by neural pathways, since they were immediately abolished after the optic stalk connecting the optic medulla to the lobula was unilaterally severed. The neural pathways were examined by recording locomotor activity, under a 13 h light to 13 h dark cycle, after the optic nerves were unilaterally severed and the contralateral optic stalk was partially destroyed near the lobula. When the dorsal half of the optic stalk was severed, locomotor rhythm mostly split into two components: one was readily entrained to the given light-dark cycle and the other freeran with a marked fluctuation in freerunning period, where the period of the freerunning component was lengthened or shortened when the onset of the entrained component occurred during its subjective night or day, respectively. The phase-dependent modulation of activity was also observed in both components. However, severance of the ventral half of the optic stalk resulted in appearance only of the freerunning component; neither the phase-dependent modulation of its freerunning period nor the change in activity level was observed. These results suggest that neurons driving the mutual interaction and the overt activity rhythm run in the ventral half of the proximal optic stalk that includes axons of large medulla neurons projecting to the cerebral lobe and the contralateral medulla.Abbreviations LD light dark cycle - freerunning period