The great gate: control of sensory information flow to the cerebellum

Developmentally Unique Cerebellar Processing Prioritizes Self- over Other-Generated Movements

Animals must distinguish the sensory consequences of self-generated movements (reafference) from those of other-generated movements (exafference). Only self-generated movements entail the production of motor copies (i.e., corollary discharges), which are compared with reafference in the cerebellum to compute predictive or internal models of movement. Internal models emerge gradually over the first three postnatal weeks in rats through a process that is not yet fully understood. Previously, we demonstrated in postnatal day (P) 8 and P12 rats that precerebellar nuclei convey corollary discharge and reafference to the cerebellum during active (REM) sleep when pups produce limb twitches. Here, recording from a deep cerebellar nucleus (interpositus, IP) in P12 rats of both sexes, we compared reafferent and exafferent responses with twitches and limb stimulations, respectively. As expected, most IP units showed robust responses to twitches. However, in contrast with other sensory structures throughout the brain, relatively few IP units showed exafferent responses. Upon finding that exafferent responses occurred in pups under urethane anesthesia, we hypothesized that urethane inhibits cerebellar cortical cells, thereby disinhibiting exafferent responses in IP. In support of this hypothesis, ablating cortical tissue dorsal to IP mimicked the effects of urethane on exafference. Finally, the results suggest that twitch-related corollary discharge and reafference are conveyed simultaneously and in parallel to cerebellar cortex and IP. Based on these results, we propose that twitches provide opportunities for the nascent cerebellum to integrate somatotopically organized corollary discharge and reafference, thereby enabling the development of closed-loop circuits and, subsequently, internal models.

Developmentally unique cerebellar processing prioritizes self-over other-generated movements

Animals must distinguish the sensory consequences of self-generated movements (reafference) from those of other-generated movements (exafference). Only self-generated movements entail the production of motor copies (i.e., corollary discharges), which are compared with reafference in the cerebellum to compute predictive or internal models of movement. Internal models emerge gradually over the first three postnatal weeks in rats through a process that is not yet fully understood. Previously, we demonstrated in postnatal day (P) P8 and P12 rats that precerebellar nuclei convey corollary discharge and reafference to the cerebellum during active (REM) sleep when pups produce limb twitches. Here, recording from a deep cerebellar nucleus (interpositus, IP) in P12 rats of both sexes, we compared reafferent and exafferent responses to twitches and limb stimulations, respectively. As expected, most IP units showed robust responses to twitches. However, in contrast with other sensory structures throughout the brain, relatively few IP units showed exafferent responses. Upon finding that exafferent responses occurred in pups under urethane anesthesia, we hypothesized that urethane inhibits cerebellar cortical cells, thereby disinhibiting exafferent responses in IP. In support of this hypothesis, ablating cortical tissue dorsal to IP mimicked the effects of urethane on exafference. Finally, the results suggest that twitch-related corollary discharge and reafference are conveyed simultaneously and in parallel to cerebellar cortex and IP. Based on these results, we propose that twitches provide opportunities for the nascent cerebellum to integrate somatotopically organized corollary discharge and reafference, thereby enabling the development of closed-loop circuits and, subsequently, internal models.

Designing AAV Vectors for Monitoring the Subtle Calcium Fluctuations of Inferior Olive Network in vivo

Adeno-associated viral (AAV) vectors, used as vehicles for gene transfer into the brain, are a versatile and powerful tool of modern neuroscience that allow identifying specific neuronal populations, monitoring and modulating their activity. For consistent and reproducible results, the AAV vectors must be engineered so that they reliably and accurately target cell populations. Furthermore, transgene expression must be adjusted to sufficient and safe levels compatible with the physiology of studied cells. We undertook the effort to identify and validate an AAV vector that could be utilized for researching the inferior olivary (IO) nucleus, a structure gating critical timing-related signals to the cerebellum. By means of systematic construct generation and quantitative expression profiling, we succeeded in creating a viral tool for specific and strong transfection of the IO neurons without adverse effects on their physiology. The potential of these tools is demonstrated by expressing the calcium sensor GCaMP6s in adult mouse IO neurons. We could monitor subtle calcium fluctuations underlying two signatures of intrinsic IO activity: the subthreshold oscillations (STOs) and the variable-duration action potential waveforms both in-vitro and in-vivo. Further, we show that the expression levels of GCaMP6s allowing such recordings are compatible with the delicate calcium-based dynamics of IO neurons, inviting future work into the network dynamics of the olivo-cerebellar system in behaving animals.

The logic behind neural control of breathing pattern

The respiratory rhythm generator is spectacular in its ability to support a wide range of activities and adapt to changing environmental conditions, yet its operating mechanisms remain elusive. We show how selective control of inspiration and expiration times can be achieved in a new representation of the neural system (called a Boolean network). The new framework enables us to predict the behavior of neural networks based on properties of neurons, not their values. Hence, it reveals the logic behind the neural mechanisms that control the breathing pattern. Our network mimics many features seen in the respiratory network such as the transition from a 3-phase to 2-phase to 1-phase rhythm, providing novel insights and new testable predictions.

Corollary discharge in precerebellar nuclei of sleeping infant rats

In week-old rats, somatosensory input arises predominantly from external stimuli or from sensory feedback (reafference) associated with myoclonic twitches during active sleep. A previous study suggested that the brainstem motor structures that produce twitches also send motor copies (or corollary discharge, CD) to the cerebellum. We tested this possibility by recording from two precerebellar nuclei—the inferior olive (IO) and lateral reticular nucleus (LRN). In most IO and LRN neurons, twitch-related activity peaked sharply around twitch onset, consistent with CD. Next, we identified twitch-production areas in the midbrain that project independently to the IO and LRN. Finally, we blocked calcium-activated slow potassium (SK) channels in the IO to explain how broadly tuned brainstem motor signals can be transformed into precise CD signals. We conclude that the precerebellar nuclei convey a diversity of sleep-related neural activity to the developing cerebellum to enable processing of convergent input from CD and reafferent signals.

Respiratory rhythm generation in vivo

The cellular and circuit mechanisms generating the rhythm of breathing in mammals have been under intense investigation for decades. Here, we try to integrate the key discoveries into an updated description of the basic neural processes generating respiratory rhythm under in vivo conditions.

Cutaneous and periodontal inputs to the cerebellum of the naked mole-rat (Heterocephalus glaber)

The naked mole-rat (Heterocephalus glaber) is a small fossorial rodent with specialized dentition that is reflected by the large cortical area dedicated to representation of the prominent incisors. Due to naked mole-rats’ behavioral reliance on the incisors for digging and for manipulating objects, as well as their ability to move the lower incisors independently, we hypothesized that expanded somatosensory representations of the incisors would be present within the cerebellum in order to accommodate a greater degree of proprioceptive, cutaneous, and periodontal input. Multiunit electrophysiological recordings targeting the ansiform lobule were used to investigate tactile inputs from receptive fields on the entire body with a focus on the incisors. Similar to other rodents, a fractured somatotopy appeared to be present with discrete representations of the same receptive fields repeated within each folium of the cerebellum. These findings confirm the presence of somatosensory inputs to a large area of the naked mole-rat cerebellum with particularly extensive representations of the lower incisors and mystacial vibrissae. We speculate that these extensive inputs facilitate processing of tactile cues as part of a sensorimotor integration network that optimizes how sensory stimuli are acquired through active exploration and in turn adjusts motor outputs (such as independent movement of the lower incisors). These results highlight the diverse sensory specializations and corresponding brain organizational schemes that have evolved in different mammals to facilitate exploration of and interaction with their environment.

Removal of default state-associated inhibition during repetition priming improves response articulation

Behavior is a product of both the stimuli encountered and the current internal state. At the level of the nervous system, the internal state alters the biophysical properties of, and connections between, neurons establishing a "network state." To establish a network state, the nervous system must be altered from an initial default/resting state, but what remains unclear is the extent to which this process represents induction from a passive default state or the removal of suppression by an active default state. We use repetition priming (a history-dependent improvement of behavioral responses to repeatedly encountered stimuli) to determine the cellular mechanisms underlying the transition from the default to the primed network state. We demonstrate that both removal of active suppression and induction of neuron excitability changes each contribute separately to the production of a primed state. The feeding system of Aplysia californica displays repetition priming via an increase in the activity of the radula closure neuron B8, which results in increased bite strength with each motor program. We found that during priming, B8 received progressively less inhibitory input from the multifunctional neurons B4/5. Additionally, priming enhanced the excitability of B8, but the rate at which B8 activity increased as a result of these changes was regulated by the progressive removal of inhibitory input. Thus, the establishment of the network state involves the induction of processes from a rested state, yet the consequences of these processes are conditional upon critical gating mechanisms actively enforced by the default state.

The missing link: evolution of the primate cerebellum

The cerebellum has too often been seen as the "little brain," subservient to the "big brain," the cerebrum. That is changing, as neuroimaging uncovers the cerebellum as the "missing link" in the neurological underpinnings of many cognitive domains. Connections between the neocortex and the cerebellum are now more precisely defined, with functionally localized areas of cerebellar cortex understood for cognitive tasks in humans. Comparative volumetric studies of the primate cerebellum have isolated some elements of circuitry, and our field is moving toward a better integration with the neurosciences in a systematic comparative framework. The next decade may show great advances, as relatively noninvasive techniques of neuroimaging have the potential to build a comparative model of the evolution of primate neurocircuitry.

Effects of vestibular rehabilitation on multiple sclerosis-related fatigue and upright postural control: a randomized controlled trial

BACKGROUND

Fatigue and impaired upright postural control (balance) are the 2 most common findings in people with multiple sclerosis (MS), with treatment approaches varying greatly in effectiveness.

OBJECTIVES

The aim of this study was to investigate the benefits of implementing a vestibular rehabilitation program for the purpose of decreasing fatigue and improving balance in patients with MS.

DESIGN

The study was a 14-week, single-blinded, stratified blocked randomized controlled trial.

SETTING

Measurements were conducted in an outpatient clinical setting, and interventions were performed in a human performance laboratory.

PATIENTS

Thirty-eight patients with MS were randomly assigned to an experimental group, an exercise control group, or a wait-listed control group.

INTERVENTION

The experimental group underwent vestibular rehabilitation, the exercise control group underwent bicycle endurance and stretching exercises, and the wait-listed control group received usual medical care.

MEASUREMENTS

Primary measures were a measure of fatigue (Modified Fatigue Impact Scale), a measure of balance (posturography), and a measure of walking (Six-Minute Walk Test). Secondary measures were a measure of disability due to dizziness or disequilibrium (Dizziness Handicap Inventory) and a measure of depression (Beck Depression Inventory-II).

RESULTS

Following intervention, the experimental group had greater improvements in fatigue, balance, and disability due to dizziness or disequilibrium compared with the exercise control group and the wait-listed control group. These results changed minimally at the 4-week follow-up. Limitations The study was limited by the small sample size. Further investigations are needed to determine the underlying mechanisms associated with the changes in the outcome measures due to the vestibular rehabilitation program.

CONCLUSION

A 6-week vestibular rehabilitation program demonstrated both statistically significant and clinically relevant change in fatigue, impaired balance, and disability due to dizziness or disequilibrium in patients with MS.

The unipolar brush cell: a remarkable neuron finally receiving deserved attention

Unipolar brush cells (UBC) are small, glutamatergic neurons residing in the granular layer of the cerebellar cortex and the granule cell domain of the cochlear nuclear complex. Recent studies indicate that this neuronal class consists of three or more subsets characterized by distinct chemical phenotypes, as well as by intrinsic properties that may shape their synaptic responses and firing patterns. Yet, all UBCs have a unique morphology, as both the dendritic brush and the large endings of the axonal branches participate in the formation of glomeruli. Although UBCs and granule cells may share the same excitatory and inhibitory inputs, the two cell types are distinctively differentiated. Typically, whereas the granule cell has 4-5 dendrites that are innervated by different mossy fibers, and an axon that divides only once to form parallel fibers after ascending to the molecular layer, the UBC has but one short dendrite whose brush engages in synaptic contact with a single mossy fiber terminal, and an axon that branches locally in the granular layer; branches of UBC axons form a non-canonical, cortex-intrinsic category of mossy fibers synapsing with granule cells and other UBCs. This is thought to generate a feed-forward amplification of single mossy fiber afferent signals that would reach the overlying Purkinje cells via ascending granule cell axons and their parallel fibers. In sharp contrast to other classes of cerebellar neurons, UBCs are not distributed homogeneously across cerebellar lobules, and subsets of UBCs also show different, albeit overlapping, distributions. UBCs are conspicuously rare in the expansive lateral cerebellar areas targeted by the cortico-ponto-cerebellar pathway, while they are a constant component of the vermis and the flocculonodular lobe. The presence of UBCs in cerebellar regions involved in the sensorimotor processes that regulate body, head and eye position, as well as in regions of the cochlear nucleus that process sensorimotor information suggests a key role in these critical functions; it also invites further efforts to clarify the cellular biology of the UBCs and their specific functions in the neuronal microcircuits in which they are embedded. High density of UBCs in specific regions of the cerebellar cortex is a feature largely conserved across mammals and suggests an involvement of these neurons in fundamental aspects of the input/output organization as well as in clinical manifestation of focal cerebellar disease.

Encoding of oscillations by axonal bursts in inferior olive neurons

Inferior olive neurons regulate plasticity and timing in the cerebellar cortex via the climbing fiber pathway, but direct characterization of the output of this nucleus has remained elusive. We show that single somatic action potentials in olivary neurons are translated into a burst of axonal spikes. The number of spikes in the burst depends on the phase of subthreshold oscillations and, therefore, encodes the state of the olivary network. These bursts can be successfully transmitted to the cerebellar cortex in vivo, having a significant impact on Purkinje cells. They enhance dendritic spikes, modulate the complex spike pattern, and promote short-term and long-term plasticity at parallel fiber synapses in a manner dependent on the number of spikes in the burst. Our results challenge the view that the climbing fiber conveys an all-or-none signal to the cerebellar cortex and help to link learning and timing theories of olivocerebellar function.

Angiotensin II AT(1) receptor blockade selectively enhances brain AT(2) receptor expression, and abolishes the cold-restraint stress-induced increase in tyrosine hydroxylase mRNA in the locus coeruleus of spontaneously hypertensive rats

Spontaneously hypertensive rats, a stress-sensitive strain, were pretreated orally for 14 days with the AT(1) receptor antagonist candesartan before submission to 2 h of cold-restraint stress. In non-treated rats, stress decreased AT(1) receptor binding in the median eminence and basolateral amygdala, increased AT(2) receptor binding in the medial subnucleus of the inferior olive, decreased AT(2) binding in the ventrolateral thalamic nucleus and increased tyrosine hydroxylase mRNA level in the locus coeruleus. In non-stressed rats, AT(1) receptor blockade reduced AT(1) receptor binding in all areas studied and enhanced AT(2) receptor binding in the medial subnucleus of the inferior olive. Candesartan pretreatment produced a similar decrease in brain AT(1) binding after stress, and prevented the stress-induced AT(2) receptor binding decrease in the ventrolateral thalamic nucleus. In the locus coeruleus and adrenal medulla, AT(1) blockade abolished the stress-induced increase in tyrosine hydroxylase mRNA level. Our results demonstrate that oral administration of candesartan effectively blocked brain AT(1) receptors, selectively increased central AT(2) receptor expression and prevented the stress-induced central stimulation of tyrosine hydroxylase transcription. The present results support a role of brain AT(1) and AT(2) receptors in the regulation of the stress response, and the hypothesis that AT(1) receptor antagonists may be considered as potential therapeutic compounds in stress related disorders in addition to their anti-hypertensive properties.

Disrupted cerebellar development in preterm infants is associated with impaired neurodevelopmental outcome

The unfavorable impact of prematurity on the developing cerebellum was recently recognized, but the outcome after impaired cerebellar development as a prematurity-related complication is hitherto not adequately documented. Therefore we compared 31 preterm patients with disrupted cerebellar development to a control group of 31 gender and gestational age matched premature infants with normal cerebellar development. Supratentorial brain injuries during the neonatal period were comparable between the groups. At a minimum age of 24 months motor and mental development was assessed by standardized tests. Disrupted cerebellar development was associated with significantly poorer scores both in the subtests for neuromotor (p < 0.001) and mental development (p < 0.001), respectively. Mixed CP was diagnosed in 48% of affected patients, whereas none of the patients of the control group had mixed CP. Microcephaly and epilepsy were significantly related to disrupted cerebellar development. Preterm patients with disrupted cerebellar development exhibit poorer outcome results in all investigated variables. The role of the cerebellum in neurodevelopment after prematurity seems to be underestimated so far.

Distribution and phenotypes of unipolar brush cells in relation to the granule cell system of the rat cochlear nucleus

In most mammals the cochlear nuclear complex (CN) contains a distributed system of granule cells (GCS), whose parallel fiber axons innervate the dorsal cochlear nucleus (DCN). Like their counterpart in cerebellum, CN granules are innervated by mossy fibers of various origins. The GCS is complemented by unipolar brush (UBCs) and Golgi cells, and by stellate and cartwheel cells of the DCN. This cerebellum-like microcircuit modulates the activity of the DCN's main projection neurons, the pyramidal, giant and tuberculoventral neurons, and is thought to improve auditory performance by integrating acoustic and proprioceptive information. In this paper, we focus on the rat UBCs, a chemically heterogeneous neuronal population, using antibodies to calretinin, metabotropic glutamate receptor 1alpha (mGluR1alpha), epidermal growth factor substrate 8 (Eps8) and the transcription factor T-box gene Tbr2 (Tbr2). Eps8 and Tbr2 labeled most of the CN's UBCs, if not the entire population, while calretinin and mGluR1alpha distinguished two largely separate subsets with overlapping distributions. By double labeling with antibodies to Tbr2 and the alpha6 GABA receptor A (GABAA) subunit, we found that UBCs populate all regions of the GCS and occur at remarkably high densities in the DCN and subpeduncular corner, but rarely in the lamina. Although GCS subregions likely share the same microcircuitry, their dissimilar UBC densities suggest they may be functionally distinct. UBCs and granules are also present in regions previously not included in the GCS, namely the rostrodorsal magnocellular portions of ventral cochlear nucleus, vestibular nerve root, trapezoid body, spinal tract and sensory and principal nuclei of the trigeminal nerve, and cerebellar peduncles. The UBC's dendritic brush receives AMPA- and NMDA-mediated input from an individual mossy fiber, favoring singularity of input, and its axon most likely forms several mossy fiber-like endings that target numerous granule cells and other UBCs, as in the cerebellum. The UBCs therefore, may amplify afferent signals temporally and spatially, synchronizing pools of target neurons.

In vivo mouse inferior olive neurons exhibit heterogeneous subthreshold oscillations and spiking patterns

In vitro whole-cell recordings of the inferior olive have demonstrated that its neurons are electrotonically coupled and have a tendency to oscillate. However, it remains to be shown to what extent subthreshold oscillations do indeed occur in the inferior olive in vivo and whether its spatiotemporal firing pattern may be dynamically generated by including or excluding different types of oscillatory neurons. Here, we did whole-cell recordings of olivary neurons in vivo to investigate the relation between their subthreshold activities and their spiking behavior in an intact brain. The vast majority of neurons (85%) showed subthreshold oscillatory activities. The frequencies of these subthreshold oscillations were used to distinguish four main olivary subtypes by statistical means. Type I showed both sinusoidal subthreshold oscillations (SSTOs) and low-threshold Ca(2+) oscillations (LTOs) (16%); type II showed only sinusoidal subthreshold oscillations (13%); type III showed only low-threshold Ca(2+) oscillations (56%); and type IV did not reveal any subthreshold oscillations (15%). These subthreshold oscillation frequencies were strongly correlated with the frequencies of preferred spiking. The frequency characteristics of the subthreshold oscillations and spiking behavior of virtually all olivary neurons were stable throughout the recordings. However, the occurrence of spontaneous or evoked action potentials modified the subthreshold oscillation by resetting the phase of its peak toward 90 degrees . Together, these findings indicate that the inferior olive in intact mammals offers a rich repertoire of different neurons with relatively stable frequency settings, which can be used to generate and reset temporal firing patterns in a dynamically coupled ensemble.

Cerebellar control of the inferior olive

A subpopulation of neurones in the cerebellar nuclei projects to the inferior olive, the source of the climbing fibre input to the cerebellum. This nucleo-olivary projection follows the zonal and, probably also, the microzonal arrangement of the cerebellum so that closed loops are formed between the neurones in the olive, the cerebellar cortex and the nuclei. The nucleo-olivary pathway is GABAergic, but several investigators argue that its main effect is to regulate electrotonic coupling between cells in the inferior olive rather than inhibit the olive. However, there is now strong evidence that the nucleo-olivary fibres do inhibit the olive. Three functions have been suggested for this inhibition: (i) feedback control of background activity in Purkinje cells, (ii) feedback control of learning, and (iii) gating of olivary input in general. Evidence is consistent with (i) and (ii). Activity in the nucleo-olivary pathway suppresses both synaptic transmission and background activity in the olive. When learned blink responses develop, the blink related part of the olive is inhibited while blinks are produced. When the nucleo-olivary pathway is interrupted, there is a corresponding increase in complex spike discharge in Purkinje cells followed by a strong suppression of simple spike firing. Stimulation of the pathway has the opposite results. It is concluded that the nucleo-olivary fibres are inhibitory and that they form a number of independent feedback loops, each one specific for a microcomplex, that regulate cerebellar learning as well as spontaneous activity in the olivo-cerebellar circuit.

Motor Coding in Floccular Climbing Fibers

The climbing fibers (CFs) that project from the dorsal cap of the inferior olive (IO) to the flocculus of the cerebellar cortex have been reported to be purely sensory, encoding “retinal slip.” However, a clear oculomotor projection from the nucleus prepositus hypoglossi (NPH) to the IO has been shown. We therefore studied the sensorimotor information that is present in the CF signal. We presented rabbits with visual motion noise stimuli to break up the tight relation between instantaneous retinal slip and eye movement. Strikingly, the information about the motor behavior in the CF signal more than doubled that of the sensory component and was time-locked more tightly. The contribution of oculomotor signals was independently confirmed by analysis of spontaneous eye movements in the absence of visual input. The motor component of the CF code is essential to distinguish unexpected slip from self-generated slip, which is a prerequisite for proper oculomotor learning.

Rhythmic Ca2+Oscillations Drive Sinoatrial Nodal Cell Pacemaker Function to Make the Heart Tick

Excitation-induced Ca(2+) cycling into and out of the cytosol via the sarcoplasmic reticulum (SR) Ca(2+) pump, ryanodine receptor (RyR) and Na(+)-Ca(2+) exchanger (NCX) proteins, and modulation of this Ca(2+)cycling by beta-adrenergic receptor (beta-AR) stimulation, governs the strength of ventricular myocyte contraction and the cardiac contractile reserve. Recent evidence indicates that heart rate modulation and chronotropic reserve via beta-ARs also involve intracellular Ca(2+) cycling by these very same molecules. Specifically, sinoatrial nodal pacemaker cells (SANC), even in the absence of surface membrane depolarization, generate localized rhythmic, submembrane Ca(2+) oscillations via SR Ca(2+) pumping-RyR Ca(2+) release. During spontaneous SANC beating, these rhythmic, spontaneous Ca(2+) oscillations are interrupted by the occurrence of an action potential (AP), which activates L-type Ca(2+) channels to trigger SR Ca(2+) release, unloading the SR Ca(2+) content and inactivating RyRs. During the later part of the subsequent diastolic depolarization (DD), when Ca(2+) pumped back into the SR sufficiently replenishes the SR Ca(2+) content, and Ca(2+)-dependent RyR inactivation wanes, the spontaneous release of Ca(2+) via RyRs again begins to occur. The local increase in submembrane [Ca(2+)] generates an inward current via NCX, enhancing the DD slope, modulating the occurrence of the next AP, and thus the beating rate. Beta-AR stimulation increases the submembrane Ca(2+) oscillation amplitude and reduces the period (the time from the prior AP triggered SR Ca(2+) release to the onset of the local Ca(2+) release during the subsequent DD). This increased amplitude and phase shift causes the NCX current to occur at earlier times following a prior beat, promoting the earlier arrival of the next beat and thus an increase in the spontaneous firing rate. Ca(2+) cycling via the SR Ca(2+) pump, RyR and NCX, and its modulation by beta-AR stimulation is, therefore, a general mechanism of cardiac chronotropy and inotropy.

Hemicerebellectomy blocks the enhancement of cortical motor output associated with repetitive somatosensory stimulation in the rat

Repetitive peripheral stimulation is associated with an enhancement of the intensity of corticomotor responses. We analysed the effects of hemicerebellectomy on the modulation of cortical motor output associated with repetitive electrical stimulation of the sciatic nerve in the rat. Hemicerebellectomy blocked the enhancement of the corticomotor response. The cerebellum is a key player in this form of short-term plasticity.

Beyond Bowditch: the convergence of cardiac chronotropy and inotropy

The ability of the heart to acutely beat faster and stronger is central to the vertebrate survival instinct. Released neurotransmitters, norepinephrine and epinephrine, bind to beta-adrenergic receptors (beta-AR) on pacemaker cells comprising the sinoatrial node, and to beta-AR on ventricular myocytes to modulate cellular mechanisms that govern the frequency and amplitude, respectively, of the duty cycles of these cells. While a role for sarcoplasmic reticulum Ca(2+) cycling via SERCA2 and ryanodine receptors (RyR) has long been appreciated with respect to cardiac inotropy, recent evidence also implicates Ca(2+) cycling with respect to chronotropy. In spontaneously beating primary sinoatrial nodal pacemaker cells, RyR Ca(2+) releases occurring during diastolic depolarization activate the Na(+)-Ca(2+) exchanger (NCX) to produce an inward current that enhances their diastolic depolarization rate, and thus increases their beating rate. beta-AR stimulation synchronizes RyR activation and Ca(2+) release to effect an increased beating rate in pacemaker cells and contraction amplitude in myocytes: in pacemaker cells, the beta-AR stimulation synchronization of RyR activation occurs during the diastolic depolarization, and augments the NCX inward current; in ventricular myocytes, beta-AR stimulation synchronizes the openings of unitary L-type Ca(2+) channel activation following the action potential, and also synchronizes RyR Ca(2+) releases following depolarization, and in the absence of depolarization, both leading to the generation of a global cytosolic Ca(i) transient of increased amplitude and accelerated kinetics. Thus, beta-AR stimulation induced synchronization of RyR activation (recruitment of additional RyRs to fire) and of the ensuing Ca(2+) release cause the heart to beat both stronger and faster, and is thus, a common mechanism that links both the maximum achievable cardiac inotropy and chronotropy.

Abnormal gating of somatosensory inputs in essential tremor

OBJECTIVE

To study whether sensorimotor cortical areas are involved in Essential Tremor (ET) generation.

BACKGROUND

It has been suggested that sensorimotor cortical areas can play a role in ET generation. Therefore, we studied median nerve somatosensory evoked potentials (SEPs) in 10 patients with definite ET.

METHODS

To distinguish SEP changes due to hand movements from those specifically related to central mechanisms of tremor, SEPs were recorded at rest, during postural tremor and during active and passive movement of the hand. Moreover, we recorded SEPs from 5 volunteers who mimicked hand tremor. The traces were further submitted to dipolar source analysis.

RESULTS

Mimicked tremor in controls as well as active and passive hand movements in ET patients caused a marked attenuation of all scalp SEP components. These SEP changes can be explained by the interference between movement and somatosensory input ('gating' phenomenon). By contrast, SEPs during postural tremor in ET patients showed a reduction of N20, P22, N24 and P24 cortical SEP components, whereas the fronto-central N30 wave remained unaffected.

CONCLUSIONS

Our findings suggest that in ET patients the physiological interference between movement and somatosensory input to the cortex is not effective on the N30 response. This finding thus indicates that a dysfunction of the cortical generator of the N30 response may play a role in the pathogenesis of ET.

The role of the cerebellum in preparing responses to predictable sensory events

Despite numerous studies on the effects of lesions of the mammalian cerebellum on coordination, adaptation and learning, the precise nature of this structure's contribution to motor control remains controversial. This paper reviews the results of a series of behavioural studies with monkeys trained to make rapid, accurate sequences of responses to visual targets. The effects of discrete cerebellar lesions on the performance of these animals is discussed in the light of recent theories about how the cerebellum might be concerned with learning to anticipate certain kinds of sensory events. Additional studies are considered that advocate sensory prediction as a fundamental cerebellar function that could contribute to many of the behavioural processes with which the cerebellum has been implicated. In particular, it is demonstrated how such information could be employed in the augmentation of motor learning by the formation of expectations about the sensory feedback arising from movements and interactions with the environment. Whilst it is argued that the cerebellum may not be unique in being able to perform such functions, comparative anatomical studies suggest that it may operate with an unequalled degree of temporal precision. Such precision forms the signature of skilled motor acts.