The cerebellar network: From structure to function and dynamics (original) (raw)
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Since the discoveries of Camillo Golgi and Ramón y Cajal, the precise cellular organization of the cerebellum has inspired major computational theories, which have then influenced the scientific thought not only on the cerebellar function but also on the brain as a whole. However, six major issues revealing a discrepancy between morphologically inspired hypothesis and function have emerged. (1) The cerebellar granular layer does not simply operate a simple combinatorial decorrelation of the inputs but performs more complex non-linear spatio-temporal transformations and is endowed with synaptic plasticity. (2) Transmission along the ascending axon and parallel fibers does not lead to beam formation but rather to vertical columns of activation. (3) The olivo-cerebellar loop could perform complex timing operations rather than error detection and teaching. (4) Purkinje cell firing dynamics are much more complex than for a linear integrator and include pacemaking, burst–pause discharges, and bistable states in response to mossy and climbing fiber synaptic inputs. (5) Long-term synaptic plasticity is far more complex than traditional parallel fiber LTD and involves also other cerebellar synapses. (6) Oscillation and resonance could set up coherent cycles of activity designing a functional geometry that goes far beyond pre-wired anatomical circuits. These observations clearly show that structure is not sufficient to explain function and that a precise knowledge on dynamics is critical to understand how the cerebellar circuit operates.
Organizational Principles of Cerebellar Neuronal Circuitry
News in physiological sciences : an international journal of physiology produced jointly by the International Union of Physiological Sciences and the American Physiological Society, 1998
We review our recent studies of cerebellar neuronal organization, emphasizing that consideration of organizational features of cerebellar circuitry represents a necessary step toward the understanding of how the cerebellum does what it does, in terms of both its internal information processing and its interaction with other motor structures.
Cerebellar structure and function: Making sense of parallel fibers
Human Movement Science, 2002
Many parts of the brain have to cooperate in a finely tuned way in order to generate coordinated motor output. Parameters of these cooperations are adjusted during early childhood development and years of motor learning later in life. The cerebellum plays a special role in the concert of these brain structures. With the unusual geometrical arrangement of its neuronal elements, especially of parallel fibers and Purkinje cells the cerebellum is a selective and sensitive detector of a specific class of spatio-temporal activity patterns in the mossy fiber system: sequences of excitatory input which ÔmoveÕ along the direction of parallel fibers at about 0.5 m/s, i.e. the speed of spike conductance in parallel fibers. Precise spatio-temporal neuronal activity patterns have been shown to occur in two major sources of afference to the cerebellum, the neocortex and the sensory feedback system. Based on our own experimental work and the above-mentioned findings we suggest that the cerebellum detects specific spatio-temporal activity patterns which trigger learned cerebellar output related to motor control and which contributes to the control of precise timing of muscle contraction.
Cerebellar Nuclei: Key Roles for Strategically Located Structures
The Cerebellum, 2010
With the exception of vestibular information, cerebellar nuclei represent the unique source of output of the cerebellar circuitry. The fastigial (FN), globose/emboliform (interpositus, IN), and dentate (DN) nuclei receive inhibitory GABAergic signals from Purkinje neurons and send back fibers to the cerebellar cortex. The numerous GABA-A inhibitory synapses between cerebellar cortex and cerebellar nuclei allow responses to high-frequency Purkinje cell firing . Cerebellar nuclei receive excitatory collaterals of mossy fibers and climbing fibers, especially via AMPA and NMDA receptors . A subset of small neurons in cerebellar nuclei project to the inferior olivary complex, providing a feedback to the inferior olive. Cerebellar nuclei thus integrate the converging excitatory and inhibitory signals to provide the final output of the cerebellar circuits. Each cerebellar nucleus has a separate somatotopic representation of the body . The projections to different cerebral cortical areas originate from distinct areas of cerebellar nuclei. In particular, the DN is spatially divided into a motor and nonmotor zone, with a closed loop from the nucleus to the cerebral cortex and back to the nucleus. Cerebellar nuclei control differentially the medial and lateral motor systems and their functions [4]. The vestibular and FN are concerned with the control of eye movements, control of head orientation, stance, and gait. FN can be functionally divided into rostral and caudal components [4, 5]. The rostral portion is involved in the control of somatic musculature, head orientation, and eye-gaze shifts [4]. The caudal FN plays key roles in saccade generation and smooth pursuit [6]. The IN is particularly active during the modulation of various reflexes and sensory feedback [7]. The eyeblink responses are typically associated with a modulation of activity in behaving animals [8]. The intermediate cortex and the IN fire in relation to the antagonist muscle group [9, 10], in agreement with a role in damping the limb oscillations and compensation of errors [11]. The IN participates in the excitability of the stretch reflexes [12]
2001
The functional partitioning of the cerebellar cortex depends on the projection patterns of its afferent and efferent neurons. However, the entire morphology of individual projection neurons has been demonstrated in only a few classes of neurons in the vertebrate CNS. To investigate the contribution of the projection pattern of individual olivocerebellar axons to the cerebellar functional compartmentalization, we labeled individual olivocerebellar axons, which terminate in the cerebellar cortex as climbing fibers, with biotinylated dextran amine injected into the inferior olive in the rat, and completely reconstructed the entire trajectories of 34 olivocerebellar axons from serial sections of the cerebellum and medulla. Single axons had seven climbing fibers on average, which terminated at similar distances from the midline in a single or in multiple lobules. Cortical projection areas of adjacent olivary neurons were clustered as narrow but separate longitudinal segments and often innervated by collaterals of single neurons. Comparison of the cerebellar distribution of olivocerebellar axons arising from different sites within a single olivary subnucleus indicated that slightly distant neurons projected to complementary sets of such segments in a single longitudinal band. Several of these longitudinal bands formed a so-called parasagittal zone innervated by a subnucleus of the inferior olive. Single olivocerebellar axons projected rostrocaudally to segments within a single band but did not project mediolaterally to multiple bands. These results suggest fine substructural organization in the cerebellar compartmentalization that may represent functional units.