Cutaneous afferent responses in interpositus neurones of the cat

A Novel Paired Somatosensory-Cerebellar Stimulation Induces Plasticity on Cerebellar-Brain Connectivity

The cerebellum receives and integrates a large amount of sensory information that is important for motor coordination and learning. The aim of the present work was to investigate whether peripheral nerve and cerebellum paired associative stimulation (cPAS) could induce plasticity in both the cerebellum and the cortex. In a cross-over design, we delivered right median nerve electrical stimulation 25 or 10 ms before applying transcranial magnetic stimulation over the cerebellum. We assessed changes in motor evoked potentials (MEP), somatosensory evoked potentials (SEP), short-afferent inhibition (SAI), and cerebellum-brain inhibition (CBI) immediately, and 30 min after cPAS. Our results showed a significant reduction in CBI 30 minutes after cPAS, with no discernible changes in MEP, SEP, and SAI. Notably, cPAS10 did not produce any modulatory effects on these parameters. In summary, cPAS25 demonstrated the capacity to induce plasticity effects in the cerebellar cortex, leading to a reduction in CBI. This novel intervention may be used to modulate plasticity mechanisms and motor learning in healthy individuals and patients with neurological conditions.

Representation of movement velocity in the rat's interpositus nucleus during passive forelimb movements

The interpositus nucleus (IN) receives a large amount of sensory information from the limbs and, in turn, elaborates signals for movement control. In this paper, we tried to gather evidence on the possibility that neurons in the IN may elaborate sensory representations of the forelimb kinematics and, particularly, of the movement velocity vector. For this purpose, the forepaw of anesthetized rats was attached to a computer-controlled robot arm displaced passively along two types of trajectories (circular and figure eight), with the limb joints unconstrained. The firing activity of single cells was recorded and related to limb position and the two components of the movement velocity vector, namely, movement speed and direction. By using multiple regression analysis, we found that 12 out of 85 (14%) neurons were modulated by position, 18 out of 85 (21%) neurons were modulated by direction, 24 out of 85 (28%) neurons were modulated by movement speed, and 31 out of 85 (37%) neurons were sensitive to the full movement velocity vector. Most of the neurons modulated only by the speed component of the velocity vector (19 out of 24) were located in the posterior portion of the IN, whereas neurons in the anterior portion were mostly related to both components of the velocity vector. These results suggest that sensory information related to whole-limb movement velocity may be encoded by the IN, indicating also that the posterior interpositus may preferentially represent movement speed.

Processing of limb kinematics in the interpositus nucleus

Neural representations of limb movement kinematic parameters are common among central nervous system structures involved in motor control, such as the interpositus nucleus of the cerebellum. Much experimental evidence indicates that neurons in the interpositus may encode limb kinematic parameters both during active, voluntary actions and during limb motion imposed passively, which entrains only sensory afferents. With respect to the sensory processing of information related to movement kinematics, we show that interpositus neuronal activity can parse out the directional from the scalar component (i.e., the movement speed) of the velocity vector. Moreover, a differential role for the anterior and posterior portion of interpositus in encoding these parameters emerged from these data, since the activity of the posterior interpositus was specifically associated to changes of movement speed. Limb movement representations in the interpositus nucleus may be instrumental for the control of goal-directed movements such as shaping hand during grasping or precise foot placement during gait. Finally, we discuss the idea that sensory information about the movement kinematics contribute to both feedback and anticipatory processes for limb movement control.

Reactive control of precision grip does not depend on fast transcortical reflex pathways in X-linked Kallmann subjects

It has been shown that subjects maintain grasp stability by automatically regulating grip force in response to loads applied tangentially to a manipulandum held using a precision grip. Signals from cutaneous mechanoreceptors convey the information necessary for both the initiation and scaling of responses. The central neural pathways that support these grip reactions are unknown. However, the latency of the increase in force is similar to that of 'long-latency' transcortical reflexes recorded from muscles following muscle stretch or electrical stimulation of digital nerves. This study assessed the importance of fast transcortical pathways for reactive grip responses by examining these responses in subjects with X-linked Kallmann's syndrome (XKS). Subjects were selected whose corticospinal projection, as assessed by magnetic brain stimulation, is essentially ipsilateral, and in whom the long-latency reflex components following digital nerve stimulation are only found contralateral to the stimulated side. Despite this anomaly of the fast corticospinal pathway, these XKS subjects responded in the same way as control subjects; grip response latencies were similar and responses were appropriately scaled. However, the non-operating hand of these XKS subjects often mirrored the grip force changes of the operating hand. Reflex force mirroring was most marked during the first 50 ms and the force output was always less than 20 % of that of the operating hand. We conclude, firstly, that somatosensory driven precision grip responses that support grasp stability do not depend on fast conducting corticospinal pathways in these subjects and, secondly, that such responses do not use those 'long-latency' reflex pathways probed by cutaneomuscular reflexes elicited by electrical stimulation of digital nerves.

GABA-ergic transmission in deep cerebellar nuclei

gamma-Aminobutyric acid (GABA) is the inhibitory transmitter released at Purkinje cell axon terminals in deep cerebellar nuclei (DCN). Neurons in DCN also receive excitatory glutamatergic inputs from the inferior olive. The output of DCN neurons, which depends on the balance between excitation and inhibition on these cells, is involved in cerebellar control of motor coordination. Plasticity of synaptic transmission observed in other areas of the mammalian central nervous system (CNS) has received wide attention. If GABA-ergic and/or glutamatergic synapses in DCN also undergo plasticity, it would have major implications for cerebellar function. In this review, literature evidence for GABA-ergic synaptic transmission in DCN as well as its plasticity are discussed. Studies indicate that fast inhibitory postsynaptic potentials (IPSPs) and currents (IPSCs) in neurons of DCN are mediated by GABAA receptors. While GABAB receptors are present in DCN, they do not appear to be activated by Purkinje cell axons. The IPSPs undergo paired-pulse, as well as frequency-dependent, depressions. In addition, tetanic stimulation of inputs can induce a long-term depression (LTD) of the IPSPs and IPSCs. Excitatory synapses do not appear to undergo long-term potentiation or LTD. The LTD of the IPSP is not input-specific, as it can be induced heterosynaptically and is associated with a reduced response of DCN neurons to a GABAA receptor agonist. Postsynaptic Ca2+ and protein phosphatases appear to contribute to the LTD. The N-methyl-D-aspartate receptor-gated, as well as the voltage-gated Ca2+ channels are proposed to be sources of the Ca2+. It is suggested that LTD of GABA-ergic transmission, by regulating DCN output, can modulate cerebellar function.

Possible CS and US pathways for rabbit classical eyelid conditioning: electrophysiological evidence for projections from the pontine nuclei and inferior olive to cerebellar cortex and nuclei

Projections from the lateral region of the pontine nuclei and the dorsal accessory inferior olive to both cerebellar cortex and cerebellar dentate/interpositus nuclei were electrophysiologically examined using single-pulse stimulation and single-unit and population recording. Stimulation of the pontine nuclear region activated population potentials and single units recorded in both cerebellar cortex and deep nuclei. Pontine-evoked activity in cerebellar cortex (Larsell's lobule HVI and adjacent areas) was rather well-defined and strong while pontine-evoked activity in the deep cerebellar nuclei seemed relatively more diffuse and weaker. Short onset latencies for both single units and population potentials were found suggesting direct projections. Similar to previous studies, inferior olive stimulation evoked short-latency responses in cerebellar cortex and nuclei thus suggesting direct projections. More pontine- and olivary-evoked activity was seen in cortex than in the nuclei with slightly more olivary-evoked potentials per recording electrode penetration observed than pontine-evoked activity. Our findings suggest that cortical and nuclear regions of the cerebellum receive converging projections from the pontine nuclei and inferior olive, projections that may carry information about stimuli used during classical conditioning. These findings are discussed in terms of cerebellar circuits that may be involved in classical eyelid conditioning.

An excitatory pathway through dorsal columns to rubrospinal cells in the cat

A somaesthetic excitatory pathway to rubrospinal cells, via other than cortico- or cerebello-rubral relays, has been demonstrated in cats, using intracellular recording techniques. After lesions of the rostral part of the cerebral cortex and the cerebellum, electrical stimulation of the contralateral forepaw and low intensity stimulation of the dorsal columns produced excitatory post-synaptic potentials (e.p.s.p.s) in rubrospinal cells, which often reached the threshold for firing bursts of spikes. These somaesthetic e.p.s.p.s are composed of a burst of small elementary e.p.s.p.s which progressively create a depolarization with a long-lasting time course. Evidence has been obtained that the pathway runs through the dorsal columns to reach the red nucleus after a relay. Stimulation applied to the dorsal columns gives rise to the response with very low threshold current, close to 20 microA. Also, the firing pattern of lemniscal fibres to single-shock dorsal column stimulation has been correlated with the time course of excitation of the rubrospinal cells. As calculated from latencies of e.p.s.p.s the conduction velocity in the investigated pathway is over 55 m/s in the peripheral nerve fibres, indicating that thick, myelinated fibres are involved. It is suggested that the fast somaesthetic loop might, via the rubrospinal tract, participate in exploratory motor behaviour and in the adaptation of gait to the environment.

An HRP and autoradiographic study of the projection from the cerebellar cortex to the nucleus interpositus anterior and nucleus interpositus posterior of the cat

The recently developed anatomical techniques of retrograde transport of the enzyme horseradish peroxidase (HRP), anterograde transport of tritiated amino acid, and intracellular injections of HRP were used to study the organization of the corticonuclear projection to the nucleus interpositus anterior (NIA) and the nucleus interpositus posterior (NIP) of the cat. Injections of HRP into the NIA and the NIP revealed that the major areas of the cortex which provided afferents to these two nuclei were the intermediate cortex of the anterior lobe (IAL) and the paramedian lobule (PML). There were, however, significant differences in the distribution of Purkinje (Pk) cells which projected to each nucleus. The NIA received afferents from all areas of the IAL while the NIP projection area was restricted to a band located at the medi-almost aspect of the lobe. All areas of the PML, in particular the intermediate folia, projected to the NIP, while the Pk cells which sent axons to the NIA were restricted to the rostral and caudal folia of this lobule. The projection from each area was somatotopically organized. The axons of intracellularly stained Pk cells were followed to their termination in the NIA and NIP confirming the results obtained with the two extracellular techniques. An attempt was made to examine the organization of the corticonuclear projection at the single cell level in the PML. Pk cells located in the same sagittal plane appeared to terminate in the same area of the same nucleus while Pk cells located not more than 500 micrometers medial or lateral to each other terminated in different nuclei. Basically, the organization of the corticonuclear projection from the IAL is longitudinally organized while the PML has a much more complex arrangement in which the Pk cells projecting to the NIA and NIP are interspersed.

Dependence of the activity of interpositus and red nucleus neurons on sensory input data generated by movement

Cats performed flexion movements of the forearm, and the discharge of interpositus and red nucleus neurons was examined for relationships to the motion and to the EMG activity of agonist and antagonist muscles. It is shown that, over a wide range in the time course of the motion, the activity of the neurons is covariant with both the EMG and the movement parameters, in particular, the velocity. Also, the discharge of the interpositus neurons is modulated in phase with the velocity of passive movement. It is concluded that during fast, intentional movements, sensory input data generated by motion is a major determinant of the output of the interpositus nucleus. Furthermore, the results are consistent with the concept that this output provides a continuous modulation of spinal segmental mechanisms by way of the red nucleus and rubrospinal tract.

Electrophysiological and horseradish peroxidase studies of precerebellar afferents to the nucleus interpositus anterior. II. Mossy fiber system

Inputs to the nucleus interpositus anterior (NIA) of the cat from precerebellar nuclei which are thought to give rise to mossy fibers were studied using electrophysiological and anatomical techniques. Stimulation of one of these precerebellar nuclei, the lateral reticular nucleus (LRN) evoked monosynaptic EPSPs in NIA neurons. These EPSPs were followed by polysynaptic IPSPs and late depolarization mediated by the response of the cerebellar cortex. Similar responses were occasionally seen following stimulation of the brachium pontis (BP). When horseradish peroxidase was injected into the NIA, labeled cells were found in the magnocellular and parvicellular LRN, the external cuneate nucleus (ECN), the pontine nuclei and the perihypoglossal nuclei. There was no evidence for a direct projection of the nucleus reticularis pontis to the NIA. It was suggested that most of the tonic excitation of NIA neurons is provided by the LRN and ECN.

Afferents to the cerebellar lateral nucleus. Evidence from retrograde transport of horseradish peroxidase after pressure injections through micropipettes

HRP was injected by pressure from glass capillary micropipettes unilaterally into the lateral nucleus of rat so as to encompass the entire nucleus, but without spread into the interpositus nuclei. The cells of origin of the afferents to the lateral nucleus were studied after retrograde transport of the HRP. The reticulotegmental nucleus of the pons was labelled bilaterally and is the major source of crossed and uncrossed reticular imputs. The pontine nuclei also provide extensive crossed and uncrossed afferents. The inferior olive gives a large crossed olivo-lateral nucleus projection and a minor uncrossed input. The trigeminal nuclear complex--the nucleus of the spinal tract and the mesencephalic, principal sensory, and motor nuclei--all provide uncrossed afferents. The rostral portion of the lateral reticular nucleus gives a small crossed and uncrossed projection while the perihypoglossal nuclei and the dorsal parabrachial body give crossed afferents to the lateral nucleus. The norepinephrine afferent system from the locus coeruleus is represented by one or two heavily labelled cells and the serotonin raphe systems come from at least five raphe subgroups, the dorsal, superior centralis, pontis, obscurus and magnus nuclei. No evidence was found for commissural fibers between ipsilateral or contralateral cerebellar nuclei, or afferent axons from the spinocerebellar nuclei and the paramedian retricular nucleus. The significance of these sources of afferent imputs to the lateral cerebellar nucleus is discussed. The question is raised of the direct relationship between size of terminal axonal arborization and the quantity of HRP granules present in a cell retrograde transport. The limitations of the HRP method for detecting subtle local differences in the distribution of afferents within the heterogeneous groups of neurons in the lateral nucleus are discussed.

Effects of afferent volleys from the limbs on the discharge patterns of interpositus neurones in cats anaesthetized with alpha-chloralose

1. In cats anaesthetized with alpha-chloralose, micro-electrodes have been used to record the discharge patterns of single neurones in the region of the nucleus interpositus. 2. Almost all cells tested could be antidromically invaded following electrical stimulation of the contralateral red nucleus, showing that they were cerebellar efferent neurones. 3. A little over half of the interpositus neurones were spontaneously active, usually at rates of less than 20 impulses/sec. 4. About 40% of the cells had no spontaneous activity, although they gave brisk responses to electrical stimulation of cutaneous nerves. Such silent units were encountered most frequently in the earlier stages of an experiment, but a number were found more than 15 hr after the beginning of an experiment. 5. Stimulation of cutaneous and mixed nerves of the fore and hind limbs provoked impulse discharges of the cells and also produced phases of deceleration of the resting discharge of spontaneously firing cells. 6. The typical response of an interpositus neurone consisted of a short latency (6-35 msec) discharge, usually separated from a long latency (50-500 msec) discharge by a period of inhibition or return to the resting discharge rate. The two phases of excitation appeared to be independently generated, since in a number of cells one phase appeared without the other. In addition, the later phase of excitation was abolished in all cells tested by a small dose of pentobarbitone which produced very little effect on the earlier phase. The long latency response was quantitatively much greater, sometimes consisting of 50 or more impulses in a response which lasted several hundred msec, but was very variable from one trial to another. 7. The long latency discharge and sometimes the preceding inhibition could readily be mimicked by single shock stimulation of the region of the contralateral inferior olive. Short latency discharges were, however, rarely evoked by olivary stimulation. 8. It is suggested that the short latency responses of the interpositus neurones were a result of synaptic excitation via cerebellar afferents, while the ensuing inhibition was a result of post-synaptic inhibition resulting from the Purkinje cell excitation due to the afferent volleys. It is suggested that the long latency excitation is due to the afferent volleys. It is suggested that the long latency excitation is due at least in part to disinhibition resulting from long pauses in Purkinje cell firing following their activation by climbing fibre afferents. 9. The possibility that these long latency responses have a physiological significance in relation to locomotion is discussed.

Responses of fastigial nucleus neurones to stimulation of the caudate nucleus in the cat

1. Extracellular records were made of single unit activity evoked in the fastigial nucleus (FN) by electrical stimulation of the caudate nucleus (CN) in cats anaesthetized with sodium thiopentone. 2. Single shock stimulation evoked bilaterally complex responses having up to three components. These were, in temporal order, with percent of units exhibiting them, (a) a short burst of evoked spikes with either a latency of smaller than 5-5 msec and no associated field potential (6 percent) or a latency of 7-5-20 msec and associated with a prominent negative-going field potential (36 percent); (b) suppression of spontaneous discharges for a period of 20-150 msec (90 percent); (c) a resumption of spike discharges with a transient increase in frequency lasting for 25-500 msec (66 percent). 3. Changes in component (c) of the response patterns of some units were noted during repetitive stimulation The nature of the change depended on the laterality of the FN with respect to the stimulated CN. 4. Mechanisms which might account for the responses are discussed, but it is emphasized that some of the results cannot yet be satisfactorily explained.

Cerebellar responses to teleceptive stimuli in alert monkeys

Discharges of single Purkinje cells in the intermediate and lateral zones of the cerebellar cortex and of neurons in the interpositus and dentate nuclei were recorded in alert monkeys during the presentation of intense auditory and visual stimuli. Concomitant monitoring of the electromyogram (EMG) demonstrated that these stimuli evoked characteristic startle responses in most instances. Firing patterns of cerebellar nuclear cells to auditory stimuli could be categorized into four types, the most common of which consisted of a short-latency acceleration of discharge, followed by a decrease in activity, and in most cells by a later period of facilitation. Simple spike discharge patterns of Purkinje cells consisted largely of prolonged increases or decreases in firing rate, although more complex patterns were seen. In almost 50% of the Purkinje cells tested, complex spikes were evoked by the auditory stimuli. Comparison of simple spike responses of Purkinje cells and of the discharges of cerebellar nuclear cells to auditory and visual inputs revealed that, except for a longer latency, the discharge pattern evoked by flash stimuli was identical to that evoked by sound in all instances. By contrast, in about one-third of the Purkinje cells with related complex spike discharge, complex spikes were evoked by stimuli of only a single modality. Comparison of the times of changes in nuclear and Purkinje cell activity suggests that the initial change in nuclear cell discharge was due to an increase in mossy fiber activity, while the subsequent decrease resulted from Purkinje cell inhibition evoked by mossy and climbing fiber inputs. The absence of increases in nuclear cell discharge at the time of most decreases in Purkinje activity indicates that removal of Purkinje inhibition does not have a major effect on the discharge rates of individual nuclear cells. The data also suggest that excitation of nuclear cells via climbing fiber collaterals played only a minor role in influencing their discharge. Since most EMG changes occurred after or at about the same time as the initial changes in cerebellar discharge, it is unlikely that the initial changes in cerebellar activity were a result of feedback from contracting muscles. It is proposed that the similar discharge patterns of cerebellar neurons to auditory and visual input results from a convergence of these inputs on a structure which projects to the cerebellum as mossy fibers.

The cerebellum as a computer: patterns in space and time

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