The Cerebrocerebellar System
Introduction
If there is a cerebellar contribution to nonmotor function, particularly to cognitive abilities and affective states, then there must be corresponding anatomic substrates that support this. The cerebrocerebellar circuit consists of a feedforward, or afferent limb, and a feedback, or efferent limb. The feedforward limb is composed of the corticopontine and pontocerebellar mossy fiber projections; the feedback loop is the cerebellothalamic and thalamocortical pathways (Fig. 1). Our conceptual approach (Schmahmann, 1991) holds that the cerebellum modifies behaviorally relevant information that it has received from the cerebral cortex via the corticopontine pathway and then redistributes this now “cerebellar-processed” information back to the cerebral hemispheres. For this reason, both limbs (feedforward, and feedback) of this cerebrocerebellar circuit are essential to the discussion of the cerebellar contribution to nonmotor processing.
A second feedforward system links the cerebral cortex with the red nucleus, from where the central tegmental tract leads to the inferior olivary nucleus and then through the climbing fiber system to the cerebellar cortex. This second afferent arc has more restricted relevance to the discussion of the relationship between the cerebellum and cognition. Input from serotonin, norepinephrine and dopamine containing brain stem structures constitutes another substantial source of cerebellar afferents.
This chapter describes the neural circuitry postulated to subserve the cerebellar contribution to nonmotor processing, particularly cognitive and affective modulation. The information presented here is derived from experiments in the nonhuman primate, and whereas there is ample precedent to extrapolate this to an understanding of similar systems in humans, the inherent limitations of this approach are readily acknowledged (cf. H. G. Leiner and A. L. Leiner, this volume).
Section snippets
Corticopontine Projections
The cerebral cortex projection to the basilar pons (the corticopontine pathway) is the obligatory first stage in the feedforward limb of the cerebrocerebellar loop. The corticopontine pathway originates in neurons in layer Vb of the cerebral cortex (Glickstein et al., 1985), the axons of which enter the internal capsule, descend into the cerebral peduncle, and terminate around neurons that occupy the ventral half of the pons. Based in part on its cellular architecture, the basilar pons of the
The Feedback Limb of the Cerebrocerebellar System
The feedback loop of the cerebrocerebellar system is composed of the cerebellar corticonuclear projection, efferents from deep cerebellar nuclei en passant through the red nucleus to the thalamus, and the thalamocortical relay (Fig. 1). The intricacies of the cerebellar cortex itself are beyond the scope of this discussion except to state that elegant models of cerebellar function (Marr, 1969; Albus, 1971; Ito, 1982) have been based on the structural consistency of the cortex and its physiology
Climbing Fibers and Cognition: Is There an Anatomic Substrate?
A central feature of the Marr (1969)-Albus (1971) theory of motor learning is the interaction between mossy fiber and climbing fiber systems. It has been suggested that learning is an important mechanism whereby the cerebellum also modulates nonmotor behavior. Mossy fibers to the cerebellum arise largely from neurons in the basilar pons. The inferior olive is the sole source of the climbing fiber input to the cerebellum. The cerebral afferents of the pontine (mossy fiber) and olivary (climbing
Conclusions
The new understanding of the anatomy of the cerebrocerebellar system presented in this chapter is consistent with the hypothesis that the cerebellum is incorporated into the neural circuitry subserving cognitive and affective operations. The anatomic, circuitry that links the associative and paralimbic cerebral cortices with the cerebellum appears to be directed in both a feedforward and a feedback manner. Those cerebral areas that commit efferents to the cerebellum via the corticopontine
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