Rostro-caudal distribution of reticulospinal projections from different brainstem nuclei in the lamprey

The reticulospinal (RS) system in the lamprey is responsible for the control of locomotion, postural corrections and steering. To perform these functions, the RS system has to affect different muscular compartments along the body axis selectively. In this study, the possibility that RS neurones in different nuclei may project to different parts of the spinal cord, was investigated. The rostro-caudal extent of single RS axons was defined by stimulating them antidromically while recording from their cell body. All recorded mesencephalic RS neurones projected to the caudal tip of the spinal cord. Of the rhombencephalic RS neurones, 26% of the recorded neurones did not reach the caudalmost fourth of the spinal cord and this proportion varied between the anterior (18%), middle (17%) and posterior (36%) rhombencephalic reticular nuclei. For these RS axons, the level of termination covered the whole rostro-caudal extent of the spinal cord. No correlation was found between the length of an axon and its conduction velocity or between the length of an axon and the rostro-caudal position of its cell body in the nuclei.     

Continuous theta-burst stimulation to primary motor cortex reduces the overproduction of forces following removal of visual feedback

Forward models, generated from the efference copies of motor commands, are thought to monitor the accuracy of ongoing movement. By comparing predicted with actual afferent information, forward models also aid in the differentiation of self-produced movements from externally generated ones. Many have proposed that a consequence of this comparison is attenuation of the predicted component of incoming sensory signals. Previous work from our laboratory has shown that following the removal of an external visual reference, discrete sequential forces exceed target values. Forces produced at the fingertip were perceived as weaker, which lead to a systematic, compensatory over-production of the magnitudes required. The relatively new repetitive TMS protocol of continuous theta-burst stimulation (cTBS) has been shown to reliably depress cortical excitability for a period following stimulation. If sensory attenuation mechanisms were responsible for the overproduction of forces found in our previous results, we hypothesized that reducing cortical excitability of M1 through application of cTBS would induce discrepancy between the efference copy generated and motor output produced. As a result, we expected the overproduction of forces following visual feedback removal would be reduced after receiving cTBS. Participants produced series of pinch grip forces in time to a metronome and to visually specified force magnitudes. Visual feedback of force output was extinguished 10 s into experimental trials and participants performed continued responses for the remaining 10 s. Results confirmed our hypothesis. Mean peak force and constant error were greater and more positive in the absence of visual feedback regardless of stimulation condition; however, the magnitude of increase was significantly reduced following cTBS compared with baseline and sham conditions. Variability was not differentially affected by stimulation condition, increasing only with removal of visual feedback contingent upon the larger forces produced in these trials. Our findings provide further evidence to support the idea that TBS may differentially affect motor output and efference copy generation.     

Centrifugal innervation of the lamprey retina. Light- and electron microscopic and electrophysiological investigations

Centrifugal fibers and their synaptic connections were studied in retinas of the lamprey Lampetra fluviatilis. The morphological analysis of retinofugal and retinopetal elements was performed after their horseradish peroxidase (HRP) filling through either the cut optic nerve in isolated retina preparations or after intracerebral HRP injections. In flat-mounted retinas, labeled ganglion cell bodies with their dendritic arborizations as well as centrifugal axons were found. The topography of labeled ganglion cell bodies and fibers in semi-thin plastic sections is described. The electron microscopic analysis revealed that the centrifugal terminals synapse either upon unlabeled somata or profiles containing synaptic vesicles (PCSVs). In more rare cases these boutons seem to establish synaptic contacts on ganglion cell dendrites. The target cell bodies were located within the inner part of the inner nuclear layer, whereas postsynaptic dendrites and PCSVs were mainly observed in the outer portion of the internal synaptic layer. Stimulation of the optic nerve in isolated retinas produced antidromic responses in 23 neurons and in 9 of these cells, an antidromic spike was followed by a postsynaptic potential (PSP). Ten cells yielded no antidromic response, but showed PSPs sometimes associated with spikes. The morphological and physiological evidence obtained indicate that these PSP-generating cells were activated synaptically by centrifugal fibers and that in the lamprey retina, these fibers make contacts either with dendrites or somata of amacrine cells and probably with ganglion cell dendrites.     

Bending the lamprey spinal cord causes a slowly-decaying increase in the frequency of fictive swimming

It is well known that rhythmic lateral bending of the isolated lamprey spinal cord/notochord can entrain the central pattern generator (CPG) for locomotion. During entrainment, the CPG's frequency is equal to the bending frequency. We report here that bending can also have a slowly-decaying excitatory effect on the CPG's frequency. Experiments were performed in which the caudal end of a 30-50 segment piece of spinal cord/notochord was repeatedly rhythmically bent for 0.5-12 cycles. A slowly-decaying excitation was said to be present if after the termination of bending the CPG's frequency was significantly greater than baseline and decayed back to baseline with a time constant of one or more cycles. In 14 of 16 animals, a slowly-decaying excitation could be evoked by bending. In five of the nine animals tested, this slowly-decaying excitation could be evoked with bending frequencies both faster and slower than the baseline frequency. Depending on the animal, the slowly-decaying excitation could be elicited by as little as one-half cycle of bending and by bending amplitudes as small as 6-21 degrees peak deflection. We interpret these data as evidence of a second effect of bending distinct from the phase-dependent effect that produces entrainment.     

Presence of retina-specific proteins in the lamprey pineal complex

The pineal complex of river lamprey reacted with the antisera raised against retina specific proteins including bovine opsin, chick visinin and frog light-sensitive cyclic GMP phosphodiesterase (PDE). Immunoreactive materials stained with anti-opsin were evenly located at the outer segment of photoreceptor cells in the pineal organ and also found in the parapineal organ. Although anti-visinin stained the pineal and parapineal photoreceptor cells, the immunopositive photoreceptor cells were observed only at the lateral portion and not at the medial portion of the pineal organ. No immunoreactive materials were found in the pineal complex by the anti-PDE, whereas the anti-PDE reacted with photoreceptor cells of the retinal tissue. The data suggest that the pineal and parapineal retinas of lamprey contain opsin- and visinin-like proteins with different distribution in their photoreceptor cell layer as found in the lamprey retinal tissue.     

Evidence for functional regeneration in the adult lamprey spinal cord following transection

We report here that following partial spinal transections in adult lampreys, the fibers of the spinal cords can regenerate and restore some intersegmental coordination to the central pattern generator for locomotion, as tested in the isolated cord preparation. However, the regeneration by this test is not successful in all animals.     

The spinal 5-HT system contributes to the generation of fictive locomotion in lamprey

Activation of NMDA receptors evokes sustained fictive locomotion in the isolated spinal cord of the sea lamprey Petromyzon marinus (P. marinus), but in the river lamprey Lampetra fluviatilis (L. fluviatilis) the ventral root activity is often irregular. A previous study showed that the number of 5-HT immunoreactive fibres, neurones and varicosities are much lower in the spinal cord of L. fluviatilis than in P. marinus. To further analyse the underlying mechanisms, the present study investigated the role of the 5-HT system in stabilising fictive locomotion. In P. marinus a blockade of 5-HT1A receptors by spiperone reversibly increased the frequency and the coefficient of variation. This implies that there is an endogenous release of 5-HT during fictive locomotion that is important for the generation of locomotor activity. In L. fluviatilis bath applied NMDA or -glutamate evoked in most cases irregular activity. An addition of 5-HT (0.5–2 μM) rapidly stabilised the burst generation and led to a sustained fictive locomotion. In a split-bath configuration, NMDA administered to the rostral part of the spinal cord in P. marinus evoked fictive locomotion in both the rostral part and the first few segments of the caudal part. When spiperone was added to the caudal part, the burst activity changed into tonic activity within 10 min. Taken together, these results indicate that activity in the intrinsic 5-HT system in the lamprey spinal locomotor network contributes significantly to the rhythm generation. The quantitative differences with regard to the 5-HT plexus between P. marinus and L. fluviatilis may account for the observed discrepancy between the two species.     

Threshold position control signifies a common spatial frame of reference for motor action and kinesthesia

Abundance of muscle spindles is most likely related to gradual recruitment and functional specialization of motor units, as well as to their fundamental role in reflex intermuscular interaction and cooperation with other sensory systems. Spindle afferents per se usually convey ambiguous kinesthetic information to the brain. Experimental data indicate that the nervous system cannot use efferent copies, i.e., pre-programmed imitations of motor commands to muscles to overcome this ambiguity and form adequate position sense. Instead, position sense becomes adequate when proprioceptive signals are interpreted in reference to the threshold limb position set by the brain. By resetting the threshold position, the nervous system not only appropriately adjusts kinesthetic sense but also causes motor action. This brief analysis illustrates not only that action and perception are coupled [J.J. Gibson, The Senses Considered as Perceptual Systems. George Allen and Unwin Ltd., London, 1968; W.H. Warren, The dynamics of perception and action. Psychol. Rev. 113 (2006) 358-89] but also that they are accomplished in the same spatial frame of reference selected and manipulated by the brain.     

An improvement in perception of self-generated tactile stimuli following theta-burst stimulation of primary motor cortex

Recent studies have shown that self-generated tactile sensations are perceived as weaker than the same sensations externally generated. This has been linked to a central comparator mechanism that uses efference copy to attenuate the predictable component of sensory inputs arising from one's own actions in order to enhance the salience of external stimuli. To provide a quantitative measure of this attenuation, a force-matching task was developed in which subjects experience a force applied to their finger and are then required to match the perceived force by actively pushing on the finger using their other hand. The attenuation of predictable sensory input results in subjects producing a larger active force than was experienced passively. Here, we have examined the effects of a novel rTMS protocol, theta-burst stimulation (TBS), over primary motor cortex on this attenuation. TBS can alter the excitability of motor cortex to incoming activity. We show that application of a 20s continuous train of TBS, that depresses motor cortex, significantly improves performance in a force-matching task. This suggests that the TBS intervention disturbed the predictive process that uses efference copy signals to attenuate predictable sensory input. A possible explanation for the effect is that TBS has a differential effect on the populations of neurones that generate motor output in M1 than on those neural structures that are involved in generating an efference copy of the motor command.     

Combined retrograde labeling and calcium imaging in spinal cord and brainstem neurons of the lamprey

Neurons in the brainstem and spinal cord of the lamprey were retrogradely labeled with Calcium Green-dextran, an indicator dye that increases its fluorescence when intracellular calcium levels increase. Optical signals could be recorded from these labeled neurons during spinal cord stimulation, nerve stimulation, or spontaneous activity, up to 4 days after dye application and for distances of 5–14 mm away from the application site. Optical signals were enhanced by 4-AP, a potassium channel blocker, and blocked by cadmium, a calcium channel blocker. Taken together, the results suggest that the optical signals recorded from labeled neurons were due to calcium influx during electrical activity. Thus, retrograde labeling with calcium indicator dyes may provide a general purpose method for simultaneously monitoring the activity-related changes of intracellular calcium in anatomically identified groups of neurons in the lamprey nervous system.     

Adaptation of feedforward movement control is abnormal in patients with cervical dystonia and tremor

Objective

It is under debate whether the cerebellum plays a role in dystonia pathophysiology and in the expression of clinical phenotypes. We investigated a typical cerebellar function (anticipatory movement control) in patients with cervical dystonia (CD) with and without tremor.

Role of glutamate receptor subtypes in the lamprey respiratory network

The respiratory role of glutamate receptors was investigated in the isolated lamprey brain preparation by analyzing the changes in respiratory activity induced by bath application of specific antagonists of ionotropic and metabotropic glutamate receptors. The results show that these antagonists differentially affect the pattern of breathing and provide the first evidence that both ionotropic and metabotropic glutamate receptors are involved in neurotransmission within the lamprey respiratory network.     

A new population of neurons with crossed axons in the lamprey spinal cord

Neurons with contralateral, rostrally and caudally projecting axons were studied in whole mounts of lamprey spinal cord using retrograde labelling techniques with fluorescent dextran-amines, cobalt-lysine or horseradish peroxidase. A previously unknown large population (180-300 cells per hemisegment) of small (less than 25 microns) cells with contralateral projecting axons is described. Their axons extend over less than 5 segments rostrally or caudally. The number of these cells per segment was relatively constant in the rostral half of the spinal cord, but increased significantly in the caudal half. In comparison, medium-sized cells with contralateral axons corresponding to previously identified premotor interneurons were far less numerous (14-21 per hemisegment) and their axons extended more than 5 segments. Contralaterally projecting edge cells (intraspinal stretch receptor neurons) with principal rostral or caudal axons plus short collaterals in the other direction were distributed throughout the length of the spinal cord, whereas large and giant cells with a varied morphology were found in the caudal half.     

The axon reaction of lamprey spinal interneurons

Axotomy and partial denervation of giant interneurons (GIs) and lateral cells (LCs) were produced by complete spinal transection in the larval lamprey spinal cord. Both cell types demonstrated a reduction in cytoplasmic basophilia, increase in cell size, nuclear eccentricity, and formation of a chromophilic nuclear cap. This was quantified in the case of cell diameter. During the first 8 weeks of recovery, the GIs with the largest diameters were found progressively further from the scar and this peak change moved at approximately 0.5 mm/day. The increase in size of GIs remained up to 20 weeks post-transection, long after the time required for their axons to regenerate across the scar and form functioning synapses. GIs injected intracellularly with horseradish peroxidase (HRP) also showed this increase in diameter as well as a simplification of their dendritic trees. Intracellular recordings from GIs revealed changes in the frequency and amplitude of spontaneous synaptic input. In the first two weeks after transection, spontaneous excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs) were less frequent than in control cells. After 6 weeks of recovery they became more frequent than in control cells. EPSPs predominated in axotomized GIs, while in control cells they constituted only 36% of the total of spontaneous potentials. A reversible increase in the amplitude of these EPSPs occurred at 3-4 weeks of recovery time. The resting membrane potential was significantly reduced by the 6th week after transection and returned to normal after the 22nd week.     

Brain segmentation and trigeminal projections in the lamprey; with reference to vertebrate brain evolution

Vertebrate brains exhibit remarkable diversity in each animal group, reflecting evolutionary changes at the molecular-level developmental program of the nervous system that vertebrates have experienced. We focused on the developmental morphological plan of the brain to understand the evolutionary scenario that led to the above diversity. By comparing the organization of the brain of non-vertebrate chordates, cyclostomes and gnathostomes, a step-wise modification of brain patterning programs becomes apparent. Furthermore, by labeling the lamprey oral region, the somatotopic projections of the trigeminal nerve that enter into the hindbrain become visible. Finally, by combining the knowledge on rhombomere segments and neuronal projections, the evolutionary relationship between somatotopy and brain segmentation are discussed.     

Organization of left–right coordination in the mammalian locomotor network

Neuronal circuits involved in left–right coordination are a fundamental feature of rhythmic locomotor movements. These circuits necessarily include commissural interneurons (CINs) that have axons crossing the midline of the spinal cord. The properties of CINs have been described in some detail in the spinal cords of a number of aquatic vertebrates including the Xenopus tadpole and the lamprey. However, their function in left–right coordination of limb movements in mammals is poorly understood. In this review we describe the present understanding of commissural pathways in the functioning of spinal cord central pattern generators (CPGs). The means by which reciprocal inhibition and integration of sensory information are maintained in swimming vertebrates is described, with similarities between the three basic populations of commissural interneurons highlighted. The subsequent section concentrates on recent evidence from mammalian limbed preparations and specifically the isolated spinal cord of the neonatal rat. Studies into the role of CPG elements during drug-induced locomotor-like activity have afforded a better understanding of the location of commissural pathways, such that it is now possible, using whole cell patch clamp, to record from anatomically defined CINs located in the rhythm-generating region of the lumbar segments. Initial results would suggest that the firing pattern of these neurons shows a greater diversity than that previously described in swimming central pattern generators. Spinal CINs play an important role in the generation of locomotor output. Increased knowledge as to their function in producing locomotion is likely to provide valuable insights into the spinal networks required for postural control and walking.     

Functional regeneration of descending brainstem command pathways for locomotion demonstrated in the in vitro lamprey CNS

The lamprey brainstem contains a ‘command system’ which descends into the spinal cord to activate motor networks and initiate locomotion. In the present study, partial lesions were made in the rostral spinal cord in order to spare various tracts and determine which tracts carry the descending command signal to the spinal cord. Sparing the medial areas of the rostral spinal cord usually blocked both sensory-evoked and spontaneous locomotion, while sparing the lateral regions of the rostral spinal cord did not abolish voluntary locomotor activity. Either the ventrolateral or dorsolateral spinal tracts could support the initiation of locomotion. Brainstem structures rostral to the mesencephalon were not necessary for the initiation of locomotor behavior. The data indicate that the lateral spinal tracts contain a significant part of the descending command pathway for locomotion. In contrast, the medial spinal tracts were neither necessary nor usually sufficient to support locomotor behavior, suggesting that the larger reticulospinal Muller cells, which project in these tracts, do not contribute significantly to the initiation of locomotion.     

GABAB-ergic modulation of burst rate and intersegmental coordination in lamprey: experiments and simulations

Neuromodulators which influence the operation of neural networks act as a rule via several cellular and synaptic mechanisms. Activation of GABA_B-receptors in the lamprey spinal cord reduce both calcium currents and the peak amplitude of the post-spike afterhyperpolarization (AHP). Activation of GABA_B-receptors reduce the segmental alternation rate and modifies the intersegmental coordination when the spinal locomotor circuits are activated by NMDA. Using physiological experiments we find that a reduced AHP is not sufficient to account for the observed reduction of the burst rate. Computer simulations revealed that either a reduced AHP or calcium current could alter the phase coordination between the segments similar to earlier experiments.     

The body unbound: vestibular-motor hallucinations and out-of-body experiences

Among the varied hallucinations associated with sleep paralysis (SP), out-of-body experiences (OBEs) and vestibular-motor (V-M) sensations represent a distinct factor. Recent studies of direct stimulation of vestibular cortex report a virtually identical set of bodily-self hallucinations. Both programs of research agree on numerous details of OBEs and V-M experiences and suggest similar hypotheses concerning their association. In the present study, self-report data from two on-line surveys of SP-related experiences were employed to assess hypotheses concerning the causal structure of relations among V-M experiences and OBEs during SP episodes. The results complement neurophysiological evidence and are consistent with the hypothesis that OBEs represent a breakdown in the normal binding of bodily-self sensations and suggest that out-of-body feelings (OBFs) are consequences of anomalous V-M experiences and precursors to a particular form of autoscopic experience, out-of-body autoscopy (OBA). An additional finding was that vestibular and motor experiences make relatively independent contributions to OBE variance. Although OBEs are superficially consistent with universal dualistic and supernatural intuitions about the nature of the soul and its relation to the body, recent research increasingly offers plausible alternative naturalistic explanations of the relevant phenomenology.     

Intracortical inhibition and facilitation with unilateral dominant, unilateral nondominant and bilateral movement tasks in left- and right-handed adults

PURPOSE: To investigate intracortical inhibition and facilitation in response to unilateral dominant, nondominant and bilateral biceps activation and short-term upper extremity training in right- and left-handed adults. METHODS: Paired-pulse transcranial magnetic stimulation was used to measure intracortical excitability in motor dominant and nondominant cortices of 26 nondisabled adults. Neural facilitation and inhibition were measured in each hemisphere during unilateral dominant, nondominant and bilateral arm activation and after training in each condition. RESULTS: No differences were seen between right- and left-handed subjects. Intracortical facilitation and decreased inhibition were seen in each hemisphere with unilateral activation/training of contralateral muscles and bilateral muscle activation/training. Persistent intracortical inhibition was seen in each hemisphere with ipsilateral muscle activation/training. Inhibition was greater in the nondominant hemisphere during dominant hemisphere activation (dominant arm contraction). CONCLUSION: Strongly dominant individuals show no difference in intracortical responses given handedness. Intracortical activity with unilateral and bilateral arm activation and short-term training differs based on hemispheric dominance, with the motor dominant hemisphere exerting a larger inhibitory influence over the nondominant hemisphere. Bilateral activation and training have a disinhibitory effect in both dominant and nondominant hemispheres.