Distribution of cerebellar terminations and their relation to other afferent terminations in the ventral lateral thalamic region of the monkey

Subcortical input to the smooth and saccadic eye movement subregions of the frontal eye field in Cebus monkey

We have recently identified two functional subregions in the frontal eye field (FEF) of the Cebus monkey, a smooth eye movement subregion (FEFsem) and a saccadic subregion (FEFsac). The thalamic input to these two subregions was studied and quantified to gain more information about the influence of the cerebellum and basal ganglia on the oculomotor control mechanisms of the cerebral cortex. A recent study using transneuronal transport of virus demonstrated that there are neurons in the basal ganglia and cerebellum that project to the FEFsac with only a single intervening synapse (Lunch et al., 1994). In the present study, we concentrated on the thalamic input to the FEFsem to define possible basal ganglia-thalamus-cortex and cerebellum-thalamus-cortex channels of information flow to the FEFsem. We localized the functional subregions using low threshold microstimulation, and retrogradely transported fluorescent tracers were then placed into the FEFsem and FEFsac. The neurons that project to the FEFsem are distributed in (1) the rostral portion of the ventral lateral nucleus, pars caudalis, (2) the caudal portion of the ventral lateral nucleus, pars caudalis, (3) the mediodorsal nucleus, (4) the ventral anterior nucleus, pars parvocellularis, and (5) the ventral anterior nucleus, pars magnocellularis. In contrast, the large majority of neurons that project to the FEFsac are located in the paralaminar region of the mediodorsal nucleus. The FEFsac and FEFsem thus each receive neural input from both basal ganglia-receiving and cerebellar-receiving cell groups in the thalamus, but each receives input from a unique combination of thalamic nuclei.

Multiarchitectonic and stereotactic atlas of the human thalamus

To improve anatomical definition and stereotactic precision of thalamic targets in neurosurgical treatments of chronic functional disorders, a new atlas of the human thalamus has been developed. This atlas is based on multiarchitectonic parcellation in sections parallel or perpendicular to the standard intercommissural reference plane. The calcium-binding proteins parvalbumin (PV), calbindin D-28K (CB), and calretinin (CR) were used as neurochemical markers to further characterize thalamic nuclei and delimit subterritories of functional significance for stereotactic explorations. Their overall distribution reveals a subcompartmentalization of thalamic nuclei into several groups. Predominant PV immunostaining characterizes primary somatosensory, visual and auditory nuclei, the ventral lateral posterior nucleus, reticular nucleus (R), and to a lesser degree also, lateral part of the centre median nucleus, and anterior, lateral, and inferior divisions of the pulvinar complex. In contrast, CB immunoreactivity is prevalent in medial thalamic nuclei (intralaminar and midline), the posterior complex, ventral posterior inferior nucleus, the ventral lateral anterior nucleus, ventral anterior, and ventral medial nuclei. The complementary distributions of PV and CB appear to correlate with distinct lemniscal and spinothalamic somatosensory pathways and to cerebellar and pallidal motor territories, respectively. Calretinin, while overlapping with CB in medial thalamic territories, is also expressed in R and limbic associated anterior group nuclei that contain little or no CB. Preliminary analysis indicates that interindividual nuclear variations cannot easily be taken into account by standardization procedures. Nevertheless, some corrections in antero-posterior coordinates in relation to different intercommissural distances are proposed.

Dissociation in the neural control of single-joint and multi-joint movements in the thalamic ataxia syndrome

We report a patient presenting with a right thalamic ataxia syndrome following a hemorrhage located in the left lateral and posterior thalamus. We investigated the fast goal-directed movements of the wrists (single-joint movements) and the fast pointing movements in the upper limbs (multi-joint movements). On the right side, single-joint movements were markedly hypermetric and characterized by an asymmetry in kinematics, an abnormality of ballistic movements which is considered to be a fundamental cerebellar disorder. By contrast, rapid multi-joint movements were only very slightly impaired. These results suggest that ballistic movements of the wrist are under the strong influence of the cerebello-thalamo-cortical pathway, while rapid pointing multi-joint movements in upper limb are mostly influenced by another pathway emerging from the lateral cerebellum, possibly the dentato-rubral or the dentato-reticular projections in the brainstem. The roles of these neuroanatomical pathways in the control of fast single-joint and multi-joint movements are discussed.

Cerebellar output channels

The cerebellum has long been regarded as involved in the control of movement, in part through its connections with the cerebral cortex. These connections were thought to combine inputs from widespread regions of the cerebral cortex and "funnel" them into the motor system at the level of the primary motor cortex. Retrograde transneuronal transport of herpes simplex virus type I has recently been used to identify areas of the cerebral cortex that are "directly" influenced by the output of the cerebellum. Results suggest that cerebellar output projects via the thalamus to multiple cortical areas, including premotor and prefrontal cortex, as well as the primary motor cortex. In addition, the projections to different cortical areas appear to originate from distinct regions of the deep cerebellar nuclei. These observations have led to the proposal that cerebellar output is composed of a number of separate "output channels." Evidence from functional imaging studies in humans and single neuron recording studies in monkeys suggests that individual output channels are concerned with different aspects of motor or cognitive behavior.

Context-response linkage

Brindley (1969) proposed that we initially generate movements "consciously," under higher cerebral control. As the movement is practiced, the cerebellum learns to link within itself the context in which the movement is made to the lower level movement generators. Marr, (1969) Albus (1971), and Ito (1972) proposed that the linkage is established by a special input from the inferior olive, which plays upon an input-output element within the cerebellum during the period of the learning. When the linkage is complete, the occurrence of the context (represented by a certain input to the cerebellum) will trigger (through the cerebellum) the appropriate motor response. The "learned" movement is distinguished from the "unlearned" conscious movement by being automatic, rapid, and stereotyped. Another important variable can be added to the idea of the context-response linkage: novel combinations of downstream elements. With regard to the motor system, this could explain how varied combinations of muscles may become active in precise time-amplitude specifications so as to produce coordinated movements appropriate to specific contexts. This chapter further extends this idea to the premotor parts of the brain and their role in cognition. These areas receive influences from the cerebellum and are active both in planning movements that are to be executed and in thinking about movements that are not be executed. Evidence shows that the cerebellar output extends even to what has been characterized as the ultimate frontal planning area, the "prefrontal" cortex, area 46. The cerebellum thus may be involved in context-response linkage and in response combination even at these higher levels. The implication would be that, through practice, an experiential context would automatically evoke a certain mental action plan. The plan would be in the realm of thought and could-but need not-lead to execution. The specific cerebellar contribution would be one of the context linkage and the shaping of the response through trial and error learning. The prefrontal and premotor areas could still plan without the help of the cerebellum, but not so automatically, rapidly, stereotypically, so precisely linked to context, or so free or error. Nor would their activities improve optimally with mental practice.

Hand/face border as a limiting boundary in the body representation in monkey somatosensory cortex

Horizontal intracortical connections may form one substrate for representational plasticity in somatosensory cortex. Electrophysiological mapping demonstrated the finer details of the representations of the hand, lower jaw, neck, and face in area 3b of normal macaque monkeys. Injections of two fluorescent tracers then defined the extent to which horizontal connections crossed from the face into the hand representations and vice versa in area 3b. Connections are widely distributed within cortical representations of skin areas innervated by cervical nerves or by the trigeminal nerve but do not cross a border defined by the anterior limit of the representation of skin innervated by the second cervical nerve. This border separates the representation of the muzzle, innervated only by the mandibular nerve, and the representation of the lower jaw and neck region, innervated by the second and third cervical nerves but overlapped by the mandibular nerve. Thus, the muzzle representation lacks connections with the hand and with the lower jaw and neck representations, but the representations of the hand and of the lower jaw and neck are strongly interconnected. Overlap of the hand and of the lower jaw and neck representations and of their horizontal intracortical connections may form one basis for expansions of the lower jaw representation into that of the hand when peripheral input from the hand is lost. Lack of connections with the rest of the face representation may limit this spread.

Adrenergic receptors in Alzheimer's disease brain: selective increases in the cerebella of aggressive patients

In this study, the distribution and concentration of beta1, beta2, and alpha2 adrenergic receptors were examined in the frontal cortex, hypothalamus, and cerebellum of Alzheimer's disease (AD) and age-matched control human brains by receptor autoradiography. The purpose of this study was to detect changes in adrenergic receptor concentrations in key areas of the brain known to affect behavior. For these studies, [125I]iodopindolol ([125I]IPIN) was used to visualize total beta adrenergic sites (with ICI-89,406 and ICI-118, 551 as subtype-selective antagonists to visualize beta2 and beta1 receptors, respectively). [3H]UK-14,304 was used to localize the alpha2 sites. Essentially no significant difference in adrenergic receptor concentration was found between total AD cases taken together and control patients. It was found, however, that there were important distinctions within the AD group when cases were subdivided according to the presence or absence of aggression, agitation, and disruptive behavior. Aggressive AD patients had markedly increased (by approximately 70%) concentrations of alpha2 receptors in the cerebellar cortex compared with nonaggressive patients with similar levels of cognitive deficit. The levels of cerebellar alpha2 receptors in aggressive AD patients were slightly above the healthy elderly controls, suggesting that these receptors are preserved and perhaps increased in this subgroup of AD. beta1 And beta2 adrenergic receptors of the cerebellar cortex showed smaller but significant ( approximately 25%) increases in concentration in aggressive AD subjects versus both nonaggressive AD patients and controls. No significant differences were found in adrenergic receptor concentrations within the frontal cortex or hypothalamus. These results point out the importance of distinguishing behavioral subgroups of AD when looking for specific neurochemical changes. These autoradiographic results may reflect the importance of the cerebellum in behavioral control.

Projection from the cerebellar lateral nucleus to precerebellar nuclei in the mossy fiber pathway is glutamatergic: a study combining anterograde tracing with immunogold labeling in the rat

The pontine nuclei (PN) and the nucleus reticularis tegmenti pontis (NRTP) are sources of an excitatory projection to the cerebellar cortex via mossy fibers and a direct excitatory projection to the cerebellar nuclei. These precerebellar nuclei, in turn, receive a feedback projection from the cerebellar nuclei, which mostly originate in the lateral nucleus (LN). It has been suggested that the feedback projection from the LN partially uses gamma-aminobutyric acid (GABA) as a transmitter. We tested this hypothesis by using a combination of anterograde tracing (biotinylated dextran amine injection into the LN) and postembedding GABA and glutamate immunogold histochemistry. The pattern of labeling in the PN and the NRTP was compared with that of cerebellonuclear terminals in two other target structures, the parvocellular part of the nucleus ruber (RNp) and the ventromedial and ventrolateral thalamus (VM/VL). The projection to the inferior olive (IO), which is known to be predominantly GABAergic, served as a control. A quantitative analysis of the synaptic terminals labeled by the tracer within the PN, the NRTP, and the VL/VM revealed no GABA immunoreactivity. Only one clearly labeled terminal was found in the RNp. In contrast, 72% of the terminals in the IO were clearly GABA immunoreactive, confirming the reliability of our staining protocol. Correspondingly, glutamate immunohistochemistry labeled the majority of the cerebellonuclear terminals in the PN (88%), the NRTP (90%), the RNp (93%), and the VM/VL (63%) but labeled only 5% in the IO. These data do not support a role for GABAergic inhibition either in the feedback systems from the LN to the PN and the NRTP or within the projections to the RNp and the VM/VL.

Toward an agreement on terminology of nuclear and subnuclear divisions of the motor thalamus

The nomenclature most commonly applied to the motor-related nuclei of the human thalamus differs substantially from that applied to the thalamus of other primates, from which most knowledge of input-output connections is derived. Knowledge of these connections in the human is a prerequisite for stereotactic neurosurgical approaches designed to alleviate movement disorders by the placement of lesions in specific nuclei. Transfer to humans of connectional information derived from experimental studies in nonhuman primates requires agreement about the equivalence of nuclei in the different species, and dialogue between experimentalists and neurosurgeons would be facilitated by the use of a common nomenclature. In this review, the authors compare the different nomenclatures and review the cyto- and chemoarchitecture of the nuclei in the anterolateral aspect of the ventral nuclear mass in humans and monkeys, suggest which nuclei are equivalent, and propose a common terminology. On this basis, it is possible to identify the nuclei of the human motor thalamus that transfer information from the substantia nigra, globus pallidus, cerebellum, and proprioceptive components of the medial lemniscus to prefrontal, premotor, motor, and somatosensory areas of the cerebral cortex. It also becomes possible to suggest the principal functional systems involved in stereotactically guided thalamotomies and the functional basis of the symptoms observed following ischemic lesions in different parts of the human thalamus.

Primary motor cortex influences on the descending and ascending systems

The motor cortex plays a crucial role in the co-ordination of movement and posture. This is possible because the pyramidal tract fibres have access both directly and through collateral branches to structures governing eye, head, neck trunk and limb musculature. Pyramidal tract axons also directly reach the dorsal laminae of the spinal cord and the dorsal column nuclei, thus aiding in the selection of the sensory ascendant transmission. No other neurones in the brain besides pyramidal tract cells have such a wide access to different structures within the central nervous system. The majority of the pyramidal tract fibres that originate in the motor cortex and that send collateral branches to multiple supraspinal structures do not reach the spinal cord. Also, the great majority of the corticospinal neurones that emit multiple intracraneal collateral branches terminate at the cervical spinal cord level. The pyramidal tract fibres directed to the dorsal column nuclei that send collateral branches to supraspinal structures also show a clear tendency to terminate at supraspinal and cervical cord levels. These facts suggest that a substantial co-ordination between descending and ascending pathways might be produced by the same motor cortex axons at both supraspinal and cervical spinal cord sites. This may imply that the motor cortex co-ordination will be mostly directed to motor responses involving eye-neck-forelimb muscle synergies. The review makes special emphasis in the available evidence pointing to the role of the motor cortex in co-ordinating the activities of both descending and ascending pathways related to somatomotor integration and control. The motor cortex may function to co-operatively select a unique motor command by selectively filter sensory information and by co-ordinating the activities of the descending systems related to the control of distal and proximal muscles.

Cerebellar somatotopic representation and cerebro-cerebellar interconnections in ataxic patients

Different methods of functional neuroimaging were used for studying somatotopic encoding of function in the cerebellum and for investigating cerebro-cerebellar interconnections in patients with cerebellar degeneration. fMRI showed, that the center of activation for hand function was located in the intermediate hemispheric portion of Larsell lobules H IV-V. Foot movements activated areas medial and anterior to the corresponding hand areas within Larsell lobules II-III. Changed function in motor cortices could be demonstrated in patients with cerebellar degeneration as compared to normal controls by recording movement-related cortical potentials (BP). In patients the motor potential was almost lacking and transcranial magnetic stimulation demonstrated enhancement of inhibitory mechanisms (prolonged postexcitatory inhibition) in the motor cortex. PET-findings suggested, that both effects are correlated to increased activity of inhibitory interneurons. Cerebellar patients showed increased activation in relation to movements in the SMA and basal ganglia and reduced activation in the cerebellum and lateral premotor areas. It could be speculated, that compensatory mechanisms are the reason for a stronger activation of the medial premotor system, including SMA, in patients with cerebellar degeneration. On the basis of our results it appears, that the cerebellum facilitates the lateral premotor system areas much more than it does the medial areas.

Toward an agreement on terminology of nuclear and subnuclear divisions of the motor thalamus

The nomenclature most commonly applied to the motor-related nuclei of the human thalamus differs substantially from that applied to the thalamus of other primates, from which most knowledge of input-output connections is derived. Knowledge of these connections in the human is a prerequisite for stereotactic neurosurgical approaches designed to alleviate movement disorders by the placement of lesions in specific nuclei. Transfer to humans of connectional information derived from experimental studies in nonhuman primates requires agreement about the equivalence of nuclei in the different species, and dialogue between experimentalists and neurosurgeons would be facilitated by the use of a common nomenclature. In this review, the authors compare the different nomenclatures and review the cyto- and chemoarchitecture of the nuclei in the anterolateral aspect of the ventral nuclear mass in humans and monkeys, suggest which nuclei are equivalent, and propose a common terminology. On this basis, it is possible to identify the nuclei of the human motor thalamus that transfer information from the substantia nigra, globus pallidus, cerebellum, and proprioceptive components of the medial lemniscus to prefrontal, premotor, motor, and somatosensory areas of the cerebral cortex. It also becomes possible to suggest the principal functional systems involved in stereotactically guided thalamotomies and the functional basis of the symptoms observed following ischemic lesions in different parts of the human thalamus.

The cerebellum and cognitive function: implications for rehabilitation

OBJECTIVE

To characterize the cognitive effects of isolated cerebellar lesions.

DESIGN

Review of two inpatient cases.

SETTING

The rehabilitation unit of a tertiary general hospital.

PATIENTS OR OTHER PARTICIPANTS

Two patients with acute ischemic strokes who had solitary cerebellar infarcts.

INTERVENTIONS

Assessment with standard neuropsychological tests. Scores were compared with patients' premorbid levels and standardized test norms. A classical conditioning eyeblink paradigm was performed.

MAIN OUTCOME MEASURES

Neuropsychological measures of intellectual and executive functions, learning and memory, visual-spatial abilities, language functioning, fine motor speed, and dexterity.

RESULTS

Test findings suggested lesion-associated deficits in higher aspects of cognition (visuospatial reasoning, verbal and visual memory, and intellectual and executive functions). These functions are not usually associated with the fundamentally motoric role of the cerebellum.

CONCLUSIONS

(1) Lesions in the cerebellum can be associated with impairments in higher cognitive functioning. (2) Such effects may be severe enough for a diagnosis of dementia under current diagnostic criteria. (3) These rehabilitation patients may benefit from comprehensive cognitive examination to determine if cognitive effects will detract from their participation. (4) Further research is needed to localize which cerebellar areas affect which cognitive abilities.

The relationship between monkey ventrolateral thalamic nucleus activity and kinematic parameters of wrist movement

Extracellular recordings were made from single neurones in the cerebellar thalamus (75 neurones) and the VPLc (44 neurones) of four conscious moving monkeys. The experiment was designed to establish the discharge of ventrolateral thalamic neurones encodes information about kinematic parameters. The animals were trained to resist unexpected perturbations of the wrist and to perform skilled, voluntary wrist movements, producing stereotyped reflex and active movements with a wide range of durations and amplitudes. Statistical analysis of the discharge pattern associated with individual trials of movement was performed. In various maintained wrist positions there was a significant correlation (P < 0.05) between frequency of tonic discharge and joint position in 40% of the cerebellar thalamic neurones and in 54% of VPLc neurones. The phasic neuronal discharge associated with stereotyped movement often appeared "velocity-or acceleration-like'. However, statistical analyses revealed that the phasic discharge of only a small percentage of cerebellar thalamic and VPLc neurones was correlated with duration of movement, peak velocity or acceleration. The percentage of cerebellar thalamic neurones with discharge correlated to kinematic parameters was much greater when an unexpected change in the gain between joint angle and screen display led to errors in tracking the target. It is concluded that the cerebello-thalamo-cortical (CTC) pathway, carries information regarding maintained joint position but not velocity or acceleration of movement during a stereotyped task. However, the CTC pathway has a greater capacity to signal information about movement velocity and acceleration when there is a mismatch between the intended and actual movement.

Thalamic input to mesial and superior area 6 in the macaque monkey

The aim of the present study was to define the origin of the thalamocortical projections to each of the mesial and superior area 6 areas. To this purpose, restricted injections of neuronal tracers were made into areas F3, F6, F2, and F7 after physiological identification of the injection sites. The results showed that each of these areas receives afferents from a set of thalamic nuclei and that this set differs, qualitatively and quantitatively, according to the injected area. The main inputs to F3 [supplementary motor area properly defined (SMA-proper)] originate in the nuclei ventral lateral, pars oralis (VLo), ventral posterior lateral, pars oralis (VPLo), and ventral lateral, pars caudalis (VLc) as well as in caudal parts of the VPLo and VLc (VPLo/VLc complex). F6 (pre-SMA) is mainly the target of nucleus ventral anterior, pars parvocellularis (VApc), and area X of Olszewski. The input to F2 originates mainly in the VPLo/VLc complex, in VLc, and in VLo. The dorsal part of F7 (supplementary eye field) mainly receives from area X, VApc, and nucleus ventral anterior, pars magnocellularis (VAmc), whereas the ventral F7 is connected with VApc, area X, VLc, and the VPLo/VLc complex. All of the injected areas receive a strong projection from the medial dorsal nucleus (MD). It is concluded that each cortical area is a target of both cerebellar and basal ganglia circuits. F3 and F2 are targets of the so-called "motor" basal ganglia circuit and a cerebellar circuit originating in dorsorostral sectors of dentate and interpositus nuclei. In contrast, F6 and ventral F7 receive a basal ganglia input mainly from the so-called "complex" circuit and a cerebellar input originating in the ventrocaudal sectors of dentate and interpositus nuclei. Finally, with respect to the rest of F7, dorsal F7 also receives a basal ganglia input from the "oculomotor circuit."

Enhancement of inhibitory mechanisms in the motor cortex of patients with cerebellar degeneration: a study with transcranial magnetic brain stimulation

The excitatory state of the primary motor cortex can be studied by measuring either the postexcitatory inhibition after transcranial magnetic single stimulation (pI-S) or the refractory period with magnetic double stimulation (rP-D). The cerebellum may influence the excitability of the motor cortex by cerebellar inputs and outputs from side loops of transcortical projections. Therefore, we studied pI-S and rP-D in 24 patients with autosomal dominant cerebellar ataxia or idiopathic cerebellar ataxia, who were allocated to one group (Group A) with mild to moderate ataxia (n = 11) and to another group (Group B) with severe ataxia (n = 13). The results were compared with those obtained in 21 normal age-matched control subjects. The central motor conduction time (CMCT) was delayed in approximately half of the patients, demonstrating that the degenerative process, beyond the cerebellum, also affects the pyramidal tract. Mean CMCT was significantly delayed only in patients of Group B. pI-S was prolonged in 10 of our 24 patients; incidence of pathology in pI-S did not differ between the two patient groups. In 5 patients with normal CMCT, pathological pI-S results were found. Mean pI-S was prolonged in the whole patient group and in both subgroups as well. rP-D was prolonged in two patients of Group B only, but mean rP-D was significantly prolonged in the whole patient group. Prolonged postexcitatory inhibition and refractory period may be a consequence of a transient facilitation of cortical inhibitory interneurons, which results in a decreased excitability of primary motor cortex in patients with cerebellar degeneration.

Origin of thalamocortical projections to the presupplementary motor area (pre-SMA) in the macaque monkey

The presupplementary motor area (pre-SMA) is a recently defined cortical motor area that is located immediately rostral to the supplementary motor area (SMA) and is considered to play more complex roles in motor control than the SMA. In the present study, we examined the distribution of cells of origin of thalamocortical projections to the pre-SMA in the macaque monkey. Under the guidance of intracortical microstimulation mapping, the retrograde tracer biotinylated dextran amine was injected into the pre-SMA. Retrogradely labeled neurons were distributed primarily in the parvicellular division of the ventroanterior nucleus (VApc), oral division of the ventrolateral nucleus (VLo), area X, and mediodorsal nucleus (MD). Some labeled neurons were also observed in the medial and caudal divisions of the ventrolateral nucleus. The results indicate that the pre-SMA may receive not only basal ganglia inputs via the VApc, VLo, and MD, but also a cerebellar input via the X.

Projections from the cerebellar interposed and dorsal column nuclei to the thalamus in the rat: a double anterograde labelling study

It is generally agreed that cerebellar and lemniscal pathways project to largely separate areas of the thalamus and influence different functional areas of the cerebral cortex. Cerebellar afferents arise from neurones in the deep cerebellar nuclei and terminate in the ventral lateral group of thalamic nuclei or the "motor thalamus," whereas lemniscal afferents arise from the dorsal column nuclei and terminate in the adjacent ventral posterior group of thalamic nuclei or "sensory thalamus." However, it remains unclear whether or not these pathways converge onto thalamic neurones in the border zone between motor and sensory thalamus. The aim of this study was to compare directly the locations of cerebellar interposed and dorsal column nuclei terminals in the rat thalamus by using a double anterograde labelling technique. Microinjections of dextran-tetramethylrhodamine and dextran-fluorescein were made into the interposed and dorsal column nuclei, and labelled terminals in the thalamus were examined in the same sections. The labelled cerebellar and lemniscal terminals were located in separate areas throughout most of the ventral lateral and ventral posterior lateral nuclei, and there was only a limited region around the rostral border between these nuclei where the two groups of terminals came in close proximity to each other. In this common projection zone, however, cerebellar and lemniscal terminals seldom intermingled, and they mostly occupied separate, discreet areas. The results show that cerebellar and lemniscal fibres do indeed project to the border zone between the sensory and cerebellar thalamic nuclei, but they show practically no overlap in this region and are likely to influence separate thalamic neurones.

Comparison of cerebellothalamic and pallidothalamic projections in the monkey (Macaca fuscata): a double anterograde labeling study

To address the question of segregated projections from the internal segment of the globus pallidus (GPi) and the cerebellar nuclei (Cb) to the thalamus in the monkey, we employed a double anterograde labeling strategy combining the anterograde transport of horseradish peroxidase conjugated to wheat germ agglutinin (WGA-HRP) with biotinylated dextran amine (BDA) transport. The tissue was processed sequentially for WGA-HRP, and then BDA immunohistochemistry using two different chromogens. Since the two labels were easily distinguishable on the same histological section, the interrelationship between the cerebellar and pallidal projection systems could be directly evaluated. We found that both the cerebellothalamic and pallidothalamic label consisted of dense plexuses of labeled fibers and swellings in a patch-like configuration. The patches or foci of labeling were distributed either as dense single label or as interdigitating patches of double label. We found dense single label in the central portion of the ventral anterior nucleus pars principalis (VApc) and the ventral lateral nucleus pars oralis (VLo) following the GPi injections or in the central portion of the ventral posterior lateral nucleus pars oralis (VPLo) and nucleus X (X) following the cerebellar nuclei injections. Complementary interdigitating patches of WGA-HRP and BDA labeling were found primarily in transitional border regions between thalamic nuclei. On occasion, we found overlap of both labels. We observed a gradient pattern in the density of the pallidothalamic and cerebellothalamic projections. The pallidothalamic territory included VApc, VLo, and the ventral lateral nucleus pars caudalis (VLc), with the density of these projections decreasing along an anterior to posterior gradient in the thalamus. Occasional patches of pallidal label were found in VPLo and nucleus X. Conversely, the density of cerebellothalamic projections increased along the same gradient, with the cerebellothalamic territory extending anteriorly beyond the cell-sparse zones of VPLo, X, and VLc to include VLo and VApc also. These data suggest that although the cerebellar and pallidal projections primarily occupy separate thalamic territories, individual thalamic nuclei receive differentially weighted inputs from these sources.

Central mechanisms of tremor

Physiologic and pathologic tremors are mechanistically classified into two broad groups: (1) those produced by oscillation in sensorimotor loops, so-called mechanical-reflex tremors, and (2) those produced by the oscillatory properties of central neuronal networks. This review provides a contemporary perspective of tremor pathophysiology while acknowledging that no form of tremor is understood completely. Indeed, the origin of oscillation in most forms of tremor is undefined, and in many instances the underlying pathology is unknown.