Localization of focal potentials evoked in the red nucleus and the ventrolateral nucleus of the thalamus by electrical stimulation of the cerebellar nuclei

Brain activation during maintenance of standing postures in humans

The regulatory mechanism of bipedal standing in humans remains to be elucidated. We investigated neural substrates for maintaining standing postures in humans using PET with our mobile gantry PET system. Normal volunteers were instructed to adopt several postures: supine with eyes open toward a target; standing with feet together and eyes open or eyes closed; and standing on one foot or with two feet in a tandem relationship with eyes open toward the target. Compared with the supine posture, standing with feet together activated the cerebellar anterior lobe and the right visual cortex (Brodmann area 18/19), and standing on one foot increased cerebral blood flow in the cerebellar anterior vermis and the posterior lobe lateral cortex ipsilateral to the weight-bearing side. Standing in tandem was accompanied by activation within the visual association cortex, the anterior and posterior vermis as well as within the midbrain. Standing with eyes closed activated the prefrontal cortex (Brodmann area 8/9). Our findings confirmed that the cerebellar vermis efferent system plays an important role in maintenance of standing posture and suggested that the visual association cortex may subserve regulating postural equilibrium while standing.

Where is the self? A neuroanatomical theory of consciousness

The enigmatic nature of the experience of self-awareness is examined in the light of recent discoveries and, on this basis, combined with inferences derived introspectively from the experience of the phenomenon itself; a specific physical locus of this experience within the human brains is deduced-proposed. The fundamental premise in this work is that whereever conscious self-awareness is generated, the neuronal structure(s) involved must continually have access to an extremely precise representation of information derived from the sense of vision plus a great variety of other kinds of information so as to permit it to make decisions regarding actions (movements and their implementation) that promote the survival and perpetuation of the biological system in which the self is generated. First, a definitve set of criteria that define most of the inputs to and operations carried out by the self-awareness entity were assembled. This ensemble of functions was then compared with the connections and possible roles of specific neuroanatomical structures described in published literature, particularly the recent literature and particularly that concerned with the sense of vision. It was discovered that only one brain structure receives the prerequisite information from the sense of vision plus information derived from cortical memory stores plus a variety of other relevant sources needed to generate a coherent sense of selfness. This structure is the superior colliculus of the tectum. The superior colliculi not only receive a highly precise retinotopic representation of inputs to the eyes, but also receive inputs from a great variety of other structures, including many areas of the cerebral cortex, vestibular inputs, auditory inputs, "affective" inputs, and inputs that putatively define the positions of the eyes and of the head. This information, it is deduced, not only allows this structure to generate a continuing synthesis of representations of the self-vs.-environment, but also allows a part of it to assess the significance (probable meaning) of these integrated inputs with respect to the selection of an implementation of actions that serve the interests of the physical structure in which the self-experience is generated. The function of memory in this system not only involves the continually updated representation of where the self is with respect to items and objects in its environment, but also provides means through which the relevance of recorded experiences representing the past may be caused to affect the decision-making process.(ABSTRACT TRUNCATED AT 400 WORDS)

Converging cerebellofugal inputs to the thalamus. I. Mapping of monosynaptic field potentials in the ventrolateral nucleus of the thalamus

The extent of thalamic projections from punctate sites in the cerebellar nuclei was examined in 22 acutely prepared cats by mapping monosynaptic field potentials evoked in the ventrolateral (VL) nucleus by stimulation of the interpositus and dentate nuclei (IN and DN). The monosynaptic field potentials were evoked in the VL by low current stimulating pulses applied at high frequency to these cerebellar nuclei. Quantification of the projections was possible since the conditions of stimulation and recording were strictly controlled. The incoming volley recorded in the brachium conjunctivum caudally to the VL was also analysed. It was composed of two amplitude peaks with different latencies, corresponding to two groups of fibres conducting at 60-90 m/s and 20-25 m/s respectively. The negative field potentials in VL also showed two amplitude peaks and different latencies. The differences in latency between the first and second peaks in the presynaptic and postsynaptic field potentials are compatible with the possibility that both groups of afferent fibres may be monosynaptically connected to VL relay cells. The cerebello-thalamic projections were mapped and their density gradient was established. Two or three small thalamic strips of dense projections surrounded by a large zone of weaker projections were observed to emerge from each punctate cerebellar site. In the discussion of the functional significance of these findings, it is suggested that this organization might constitute a modulatable morphological support for a mechanism focalizing the cerebello-cortical inputs.

An electron microscopic study of the corticorubral fibers after neonatal deep cerebellar nuclear lesions in albino rats

After a lesion in the sensorimotor and adjacent cortex in normal adult rats, degenerating terminals showing the dense reaction form asymmetrical contacts with spines, dendrites of various sizes, soma and other axonal terminals. Filamentous degeneration is also present. After neonatal deep cerebellar nuclear lesions involving the dentate nucleus and the adjacent interposed nucleus, the cerebrocorticorubral fibers form similar synaptic contacts with somatic, dendritic and axonal profiles. The incidence of axo-dendritic contacts on spine is reduced, while that of axo-dendritic contacts on small, medium-sized and large dendrites and axo-somatic contacts is increased.

Synaptic organization of the cerebello-thalamo-cerebral pathway in the cat. II. Input-output organization of single thalamocortical neurons in the ventrolateral thalamus

Input-output neural organization of single thalamocortical (T-C) neurons in the ventrolateral nucleus (VL) of the thalamus was investigated using an intracellular recording technique in the anesthetized cat. Stimulation of the dentate (DN) and the interpositus (IN) nuclei produced monosynaptic unitary EPSPs of large amplitude in T-C neurons projecting to the motor cortex or area 6 over the entire mediolateral region of VL. The thalamic projections from DN and IN are very wide and there is a considerable overlap between the dentate and the interpositus projection areas in VL. And in this overlapping area, a considerable number of T-C neurons (50%) receive inputs from both DN and IN. More than 40% of T-C neurons were antidromically activated from widely separated electrodes in the motor cortex, indicating that the cortical arbolization of single T-C neurons is very wide and the number of these neurons with widely divergent projections is considerably large.

Synaptic organization of the cerebello-thalamo-cerebral pathway in the cat. I. Projection of individual cerebellar nuclei to single pyramidal tract neurons in areas 4 and 6

The neural connections of the dentate (DN) and the interpositus (IN) nuclei to the motor cortex and area 6 were investigated by recording intracellular postsynaptic potentials from fast and slow pyramidal tract neurons (PTNs) in the anesthetized cat. Localized stimulation of DN and IN produced di- or polysynaptic EPSPs in fast and slow PTNs in the "forelimb area" of the motor cortex and area 6. The effects of stimulation of the two cerebellar projections were essentially the same, although some regional difference of their relative strength was noted. In these cortical areas, the majority of fast and slow PTNs received convergent inputs from both DN and IN. By examining the interaction of DN- and IN-evoked EPSPs, spatial facilitation and occlusion at the level of the thalamus were demonstrated. Therefore, it was concluded that at least a portion of the convergence of the dentate and the interpositus inputs occurred at the level of the ventrolateral nucleus of the thalamus.

An electron microscopic study of the cerebellorubral connections after neonatal lesions in the sensorimotor and adjacent cortex in the albino rat

After a single right deep cerebellar nuclear (DCN) lesion in the dentate and adjacent interposed nuclei in adult rats, most degenerating cerebellorubral terminals in the rostral parvicellular portion of the contralateral red nucleus make contact with medium-sized and large dendrites. A few of them make contact with small dendrites, somata and other axonal terminals. The degenerating axons and terminals show the dense type of reaction. After left neonatal sensorimotor and adjacent cortical lesions followed by a right DCN lesion 2-10 months later, the pattern of most synaptic contacts resembles that in normal adult rats receiving only DCN lesions. There are, however, noteworthy differences. The incidence of axodendritic contact on dendritic spines is greatly increased. There is also an increase in the incidence of two terminals in close proximity and in contact with other neuronal profiles, and of one terminal in contact with more than one dendritic profile. The incidence of axodendritic contact on medium-sized and large dendrites is slightly reduced. The possibility of a taking over of cerebrocorticorubral projections by cerebellorubral fibers and of axonal and dendritic sprouting is suggested. In animals receiving double lesions, the filamentous type of degeneration could be seen. Degenerating dendrites occur in animals receiving single or double lesions.

A quantitative electron microscope study of cerebellar axon terminals on the magnocellular red nucleus neurons in the cat

In the red nucleus (RN) of the cat, the bouton covering ratio (BCR: the ratio of whole somatic surface length and that covered with axon terminals) and the density of axon terminals in contact with somatic profiles (DAST: the number of axosomatic terminals per micron of somatic surface membrane) were calculated in each neuronal somatic profile over 60 micron in diameter. The mean BCR was 61.4 +/- 1.43 (S.E. M.)%. The mean DAST of axosomatic terminals filled with spherical synaptic vesicles (S-terminals) was 27.7 +/- 0.95, and that of terminals with pleomorphic and/or flattened vesicles (F-terminals) was 10.3 +/- 1.12. Subsequently, sequential changes of the BCR and DAST of intact terminals were examined in the RN deafferented from the cerebellorubral fibers. The mean BCR and DAST were decreased most markedly during the survival period of 4-7 days; thus decrease was chiefly due to degeneration of S-terminals (BCR: 16.7 +/- 1.17 %, DAST of S-terminals: 7.1 +/- 1.12, DAST of F-terminals: 5.8 +/- 1.22). In the RN 11-63 days after the operation, both the BCR and DAST tended to re-increase slightly and the majority of the re-increased terminals appeared to be F-terminals. Possible meanings of this re-increase of axosomatic terminals are discussed.

Brachium conjunctivum-red nucleus synaptic system in the baboon

Electrophysiological properties of the brachium conjunctivum-red nucleus (BC-RN) synaptic system were studied in barbiturate-anesthetized baboon (Papio). Topographic recordings from the mesencephalon demonstrated that most of the BC-evoked activity was restricted to an area histologically identified as red nucleus; however, some brachium-evoked activity was recorded from the surrounding mesencephalic reticular formation. Short latency BC-evoked activity was also recorded from the pons in the region of nucleus reticularis tegmenti pontis. The majority of the BC fibers were found to conduct at a rate of 44 m/sec; a second group of BC fibers with a slower conduction velocity of 23 m/sec was also observed. Brachium-evoked responses recorded from magnocellular and pravicellular RN neurons were short latency responses consistent with monosynaptic activation of these RN neurons by the BC fibers. The BC-RN synaptic system was found to be a very secure synaptic system and could transmit activity at high rates of stimulation with little or no failure. The responsiveness of the BC fibers was found to be similar to that of optic nerve fibers and pyramidal tract fibers, both of which have been characterized as being similar to peripheral A fibers. The responsiveness of the BC-RN synaptic system began to decrease 5 msec after a single or repetitive transmission and was reduced to about 50% of normal responsiveness at 34 msec. This period of reduced postsynaptic responsiveness was associated with a reduction in presynaptic input to RN and suggest that a disfacilitation at the level of the deep cerebellar nuclei may be in part responsible for the subnormal responsiveness of the BC-RN synaptic system.

Arousal Effects on Evoked Activity in a "Nonsensory" System

Responses from motor cortex evoked by cerebellar stimulation have been recorded with chronically implanted electrodes from cats in different states of arousal. The response, which in the waking cat consists of a short latency biphasic component followed by one or more slower waves, was attenuated, or abolished completely, in association with electroencephalographic and behavioral signs of decreased arousal. In contrast, responses in primary somatosensory cortex evoked by stimulating the bulbar trigeminal nucleus were enhanced during periods of decreased alertness.