Evidence of an x zone in lobule V of the squirrel monkey (Saimiri sciureus) cerebellum: the distribution of corticonuclear fibers

Sven Ingvar (1889-1947) of Lund University and the Centennial of His Landmark Dissertation on Cerebellar Phylo-Ontogeny

In January 1919, Sven Ingvar (1889-1947) defended his doctoral dissertation (required for the M.D. degree) on cerebellar phylogeny, development, and function at Lund University, Sweden. The work was supervised by Cornelius U. Ariëns Kappers (1877-1946) in Amsterdam and by Karl Petrén (1868-1927) in Lund. A physician of many interests, Ingvar became professor of Practical Medicine in his alma mater. His cerebellar papers, spanning over a decade, are the contributions that gained him international recognition in the neurological sciences. A key discovery was the demonstration, with the Marchi method, of the primary vestibulocerebellar afferent fibers. The merits of his work rest with the use of connections to compare lobes and lobules in different species, and the introduction of the idea of vestibular, spinal, and corticopontine storeys; on the other hand, based on current knowledge, one might take a more critical stance toward the proposition of a posterior lobe as a phylogenetically old structure, and the homolog of the human tonsil. Nonetheless, Ingvar was an early pioneer of the "evo-devo" synthesis (or the field of Evolutionary Developmental Biology, which aims at understanding how developmental processes evolve across species). He studied the comparative anatomy of the cerebellum in over 50 species of reptiles, birds, and mammals and theorized about the spatial relations of phylogenetically older and more recent acquisitions in both the cerebellar and the thalamocortical systems.

Constrained Spherical Deconvolution Tractography Reveals Cerebello-Mammillary Connections in Humans

According to the classical view, the cerebellum has long been confined to motor control physiology; however, it has now become evident that it exerts several non-somatic features other than the coordination of movement and is engaged also in the regulation of cognition and emotion. In a previous diffusion-weighted imaging-constrained spherical deconvolution (CSD) tractography study, we demonstrated the existence of a direct cerebellum-hippocampal pathway, thus reinforcing the hypothesis of the cerebellar role in non-motor domains. However, our understanding of limbic-cerebellar interconnectivity in humans is rather sparse, primarily due to the intrinsic limitation in the acquisition of in vivo tracing. Here, we provided tractographic evidences of connectivity patterns between the cerebellum and mammillary bodies by using whole-brain CSD tractography in 13 healthy subjects. We found both ipsilateral and contralateral connections between the mammillary bodies, cerebellar cortex, and dentate nucleus, in line with previous studies performed in rodents and primates. These pathways could improve our understanding of cerebellar role in several autonomic functions, visuospatial orientation, and memory and may shed new light on neurodegenerative diseases in which clinically relevant impairments in navigational skills or memory may become manifest at early stages.

Zones in the cerebellar cortex: the adventures of one participant in the unfolding story

Prior to the late 1960s, a variety of studies suggested that a general zonal pattern existed within the cerebellar cortex. The hypothesis proposed by Voogd, based on the organization of the subcortical white matter, indicated that this pattern may be very detailed, and he noted that "a further analysis of the corticonuclear projection is still necessary." This brief paper chronicles the approach used by the author to formulate a plan, initiate a large series of experiments (over 250), and follow the sometimes confusing results to finally arrive at an understanding of the details of cerebellar corticonuclear projections. It was discovered that a series of mediolateral cortical zones were present that were topographically related to the underlying cerebellar nuclei, and within each zone, the cortex projected in a rostrocaudal sequence to a specific cerebellar nucleus. The hypothesis proposed by Voogd was fundamentally proven.

The cerebellum in feeding control: possible function and mechanism

Accumulating anatomical, functional, and behavioral studies reveal that the cerebellum is involved in the regulation of various visceral functions including feeding control. Cerebellar lesions may induce alterations in feeding behavior and decreases in body weight. Although the exact mechanisms underlying the cerebellar regulation of food intake is still unclear, a series of studies have demonstrated that there are neural pathways directly and/or indirectly connecting the cerebellum with several important centers for feeding control, such as the hypothalamus. Electrophysiological data suggest that via the direct cerebellohypothalamic projections, the cerebellar outputs may reach, converge, and be integrated with some critical feeding signals including gastric vagal afferents, CCK, leptin, and glycemia on single hypothalamic neurons. Furthermore, recent functional imaging studies provide substantial evidences that hunger, satiation, and thirst are accompanied with a cerebellar activation. Here we describe that the cerebellum may be much more than a movement coordinator and actively participate in feeding control, i.e., it may act as an essential node linking somatic and visceral systems and help to generate an integrated and coordinated somatic-visceral response in feeding behavior.

The cerebellar-hypothalamic circuits: potential pathways underlying cerebellar involvement in somatic-visceral integration

The cerebellum has been considered only as a classical subcortical center for motor control. However, accumulating experimental and clinical evidences have revealed that the cerebellum also plays an important role in cognition, for instance, in learning and memory, as well as in emotional behavior and in nonsomatic activities, such as visceral and immunological responses. Although it is not yet clear through which pathways such cerebellar nonsomatic functions are mediated, the direct bidirectional connections between the cerebellum and the hypothalamus, a high autonomic center, have recently been demonstrated in a series of neuroanatomical investigations on a variety of mammals and indicated to be potential pathways underlying the cerebellar autonomic modulation. The direct hypothalamocerebellar projections originate from the widespread hypothalamic nuclei/areas and terminate in both the cerebellar cortex as multilayered fibers and the cerebellar nuclei. Immunohistochemistry studies have offered fairly convincing evidence that some of these projecting fibers are histaminergic. It has been suggested that through their excitatory effects on cerebellar cortical and nuclear cells mediated by metabotropic histamine H(2) and/or H(1) receptors, the hypothalamocerebellar histaminergic fibers participate in cerebellar modulation of somatic motor as well as non-motor responses. On the other hand, the direct cerebellohypothalamic projections arise from all cerebellar nuclei (fastigial, anterior and posterior interpositus, and dentate nuclei) and reach almost all hypothalamic nuclei/areas. Neurophysiological and neuroimaging studies have demonstrated that these connections may be involved in feeding, cardiovascular, osmotic, respiratory, micturition, immune, emotion, and other nonsomatic regulation. These observations provide support for the hypothesis that the cerebellum is an essential modulator and coordinator for integrating motor, visceral and behavioral responses, and that such somatic-visceral integration through the cerebellar circuitry may be fulfilled by means of the cerebellar-hypothalamic circuits.

Diverging projections of the C2 and D2 olivocorticonuclear cerebellar pathways of the rat

A divergent mediolateral projection to the cerebellar nuclei of the C2 and the D2 olivocorticonuclear cerebellar pathways was found after segregate injections of a tracer (either WGA-HRP or FR or BDA) in the rostral (D2 area) or caudal side (C2 area) of the rat paraflocculus. The C2 olivary area of the cerebellar cortex sends most of its nuclear projection to the nucleus interpositus posterior (classically perceived as the nuclear target of the C2 olivocorticocerebellar pathway) and a smaller contingent of fibres to the parvocellular region of the nucleus lateralis (classically perceived as the nuclear target of the D2 olivocorticocerebellar pathway). The D2 olivary area of the cerebellar cortex sends most of its nuclear projection to the parvocellular region of the nucleus lateralis (classically perceived as the nuclear target of the D2 olivocorticocerebellar pathway) and a smaller contingent of fibres to the magnocellular region of the nucleus lateralis (classically perceived as the nuclear target of the D1 olivocorticocerebellar pathway). The lateral interaction of the D2 and the C2 olivocerebellar pathways could represent the anatomical substrate for the functional integration of different olivocerebellar compartments.

The cerebellar-hypothalamic axis: basic circuits and clinical observations

Experimental studies on a variety of mammals, including primates, have revealed direct and reciprocal connections between the hypothalamus and the cerebellum. Although widespread areas of the hypothalamus project to cerebellum, axons arise primarily from cells in the lateral, posterior, and dorsal hypothalamic areas; the supramammillary, tuberomammillary, and lateral mammillary nuclei; the dorsomedial and ventromedial nuclei; and the periventricular zone. Available evidence suggests that hypothalamocerebellar cortical fibers may terminate in relation to neurons in all layers of the cerebellar cortex. Cerebellohypothalamic axons arise from neurons of all four cerebellar nuclei, pass through the superior cerebellar peduncle, cross in its decussation, and enter the hypothalamus. Some axons recross the midline in caudal areas of the hypothalamus. These fibers terminate primarily in lateral, posterior, and dorsal hypothalamic areas and in the dorsomedial and paraventricular nuclei. Evidence of a cerebellar influence on the visceromotor system is presented in two patients with vascular lesions: one with a small defect in the medial cerebellar nucleus and the other with a larger area of damage involving primarily the globose and emboliform nuclei. Both patients exhibited an abnormal visceromotor response. The second, especially, showed abnormal visceromotor activity concurrent with tremor induced by voluntary movement. These experimental and clinical data suggest that the cerebellum is actively involved in the regulation of visceromotor functions.

[Squirrel monkey--an ideal primate (correction of prmate) model of space physiology]

Investigation of the vestibulo-ocular system of the squirrel monkey was reviewed in consideration of space motion sickness (SMS), or which is recently more often termed as space adaptation syndrome (SAS). Since the first launching of the space satellite, Sputnik [correction of Sputonik] in October 1957, many experiments were carried out in biological and medical fields. A various kind of creatures were used as experimental models from protozoa to human beings. Rats and monkeys are most favorite animals, particularly the non-human primate seems to be the one, because of its phylogenetic relatives akin to the human beings. Chimpanzees, rhesus monkeys, pig tailed-monkeys, red-faced monkeys and squirrel monkeys have been used mostly in American space experiments. Russian used rhesus monkeys. Among these, however, the squirrel monkey has an advantage of the small size of the body, ranging from 600- l000g in adult. This small size as a primate is very advantageous in experiments conducted in a narrow room of the space satellite or shuttle because of its space-saving. The squirrel monkey has another advantage to rear easily as is demonstrated to keep it as a pet. Accordingly, this petit animal provides us a good animal model in biological and medical experiments in space craft. The size of the brain of the squirrel monkey is extraordinary large relative to the body size, which is even superior to that of the human beings. This is partly owed to enlargement of the occipito-temporal cortices, which are forced to well develop for processing a huge amount of audio-visual information indispensable to the arboreal habitant to survive in tropical forest. The vestibular system of the squirrel monkey seems to be the most superior as well, when judged from it relative size of the vestibular nuclear complex. Balancing on swinging twigs or jumping from tree to tree developed the capability of this equilibrium system. Fernandez, Goldberg and his collaborators used the squirrel monkey to elucidate functions of the peripheral vestibular system. A transfer function was proposed to explain the behaviors of regular and irregular unit activity of vestibular nerve fibers. The physiologic characteristics of the second order vestibular neuron was investigated in combination of electrophysiological and micro-morphological way, with using WGA-HRP methods, in relation to somato-motor and eye movements. Interconnections between vestibular neurons and cerebellum, interstitial nucleus of Cajal, oculomotor nuclear complex, superior colliculus and cervical spinal cord were elucidated. In physiological field of the vestibular system, the vestibulo-ocular reflex is well studied and results obtained from the squirrel monkey experiments were reviewed. The squirrel monkey, particularly the Bolivian, is a unique animal in that it is vulnerable to motion sickness induced by visual-motion stimulation with phase mismatch of the two stimuli. Experimental results of labyrinthectomy or bilateral ablation of the maculae staticae led to the conclusion that both semicircular and otolith organs are involved in the genesis of space motion sickness. On the other hand, destruction of the area postrema, acknowledged as the vomiting center to chemical stimulants, produced controversial results. However, it must be pointed out that the a human subject underwent to resection of the area postrema, became insensitive to administration of apomorphine, a well known chemical stimulant of vomiting. Finally the experiments in space revealed the presence of at least two origins of caloric nystagmus, that is, attributable to convection and non-convection current of the endolymphatic fluid.

Nucleus medialis-nucleus interpositus interface: its olivary and cerebellocortical projections in the rat

The nuclear target of the X zone of the cerebellar cortex was identified in rats as clusters of neurons scattered at the interface between the nuclei medialis (NM) and interpositus (NI). In a previous study, we had outlined these target neurons and termed them "interstitial cell groups" (icg). In order to determine whether the icg should be considered as part of either the NM or the medial NI, we analyzed two efferent pathways from the icg: their nucleocortical and nucleoolivary projections. These were compared to their homologues from the NM and the NI. This analysis is based on mapping retrograde cell labeling and anterograde terminal labeling following microinjections of tracers in either the cerebellar cortex, the cerebellar nuclei, or the inferior olive. Nucleocortical projections originating from the icg are of the three types described previously: a "reciprocal" projection to the ipsilateral X zone, a "nonreciprocal" projection to the ipsilateral A zone, and a "symmetrical" projection to the contralateral X zone. These features can be considered as the summed characteristics of the nucleocortical projections from the NM and from the medial NI. Nucleoolivary projections from the icg target the lateral-rostral portion of the dorsal accessory olive as well as the centrocaudal part of the medial accessory olive. These pathways converge with the nucleoolivary projections from the medial NI and from the NM, respectively. The icg receives olivary afferents from both the regions of the dorsal and medial accessory olives to which it projects. On the basis of similarities shown here between the two types of efferents originating from the icg and those from the NM as well as the medial NI, the icg may be regarded as a "mosaic" of neuron clusters alternately belonging to the NM and the medial NI. Therefore, the icg would be reciprocally connected with the inferior olive.

On the caudal extension of the X zone in the cerebellar cortex of the rat

Following a selective injection of biotinylated dextran amine in the nuclear target (the interstitial cell groups, icg) of the X zone of the rat cerebellum, retrogradely labelled Purkinje cells (PCs) were found within a longitudinal strip of cortex, 250 microns in width, 1000 microns lateral to midline. This labelling delineates two compartments in the X zone, one rostral through lobules II-VI, and one caudal through lobules VIII-X. The whole rostrocaudal extent of the icg appears to be the target of PCs from both compartments without any apparent topographical organization.

The X zone and CX subzone of the cerebellum in the rat

The existence of an X zone (lateral to the A zone) and a CX subzone (lateral to the C1 subzone) was documented within the anterior lobe and lobule VI in cats and primates. On the basis of their respective efferent and climbing fibre (CF) afferent connections, delineation of these two cortical subdivisions has been investigated here, in the rat, using small injections of WGA-HRP in the cerebellar cortex. We observe that both X and CX are "fractured" into a rostral and a caudal compartment. The rostral compartment of the X zone extends over lobules IV, V and VI and its caudal compartment over lobules VIII, IX and X. The rostral compartment of the CX subzone seems to be restricted to lobules V and VI, its caudal compartment cannot be topographically distinguished, over lobules IX and X, from the caudal compartment of the X zone. The olivary afferents to the X zone and the CX subzone arise from the horizontal and vertical lamellae of the medial accessory olive: subnucleus a projects into the rostral compartment and lobule VIII of the X zone. Subnuclei b and c project into the rostral compartment of both X and CX. The dorsomedial cell column selectively projects onto the caudal compartment of both X and CX over the vestibulo-cerebellum. The corticonuclear projections of the X zone have been found within the junctional region between the nucleus medialis and the nucleus interpositus (NI), here defined as the interstitial cell groups (icg), the corticonuclear projections of the CX subzone within the medial NI. It is suggested that the icg correspond to clusters of neurones dissociated from the medial aspect of the NI. We therefore consider the X zone and CX subzone of the rat, on the basis of their corticonuclear efferents, as "medial C1" and "lateral C1" subzone, respectively, although both may be regarded as part of the A zone on the basis of their olivary afferents.

Climbing fiber representation of the renal afferent nerve in the vermal cortex of the cat cerebellum

In order to understand how the cerebellum may participate in various autonomic functions, it is necessary to first determine the occurrence and distribution of the various visceral inputs in the cerebellar cortex and their relation to other cerebellar afferents. This study examines the organization of climbing fiber responses (complex spikes) of Purkinje cells elicited by electrical stimulation of the renal afferent nerve and their relationship to climbing fiber responses representing the body surface. Visceral and somatic afferent responses were mapped in the lateral vermal cortex of lobules V to VII in chloralose-anesthetized cats. Extracellular single-unit recordings were made from 628 Purkinje cells, of which 14% had climbing fiber responses induced by renal afferent nerve stimulation. Except for one Purkinje cell, the renal climbing fiber input converged with somatic induced climbing fiber input. Tactile stimulation also elicited 54% of cells, which were unresponsive to the renal afferent nerve stimulation. The occurrence and distribution of the climbing fiber responses elicited by renal afferent nerve stimulation varied between lobules V, VI, and VII for the proportion of responsive units, the onset latencies, and topological organization. More renal responsive units were encountered in lobules V (18%) and VII (17%) than in lobule VI (6%), the average latency of renal climbing fiber responses was significantly longer in lobule VII than in lobules V and VI, and the latencies were also different among various parasagittal planes in lobules V and VII. The proportional representation of various body areas for cells with renal and somatic convergent input was different than for cells with only somatic representation. Proportionally, the forelimb had the greatest representation in lobule V, split receptive fields were frequently represented in lobule VI, and the face was well represented in lobule VII. The results of this study, in conjunction with studies showing climbing fiber representation of the vagal and splanchnic nerves, further substantiate role of the cerebellum in autonomic functions.