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Abstract
Motor thalamus (Mthal) is implicated in the control of movement because it is strategically located between motor areas of the cerebral cortex and motor-related subcortical structures, such as the cerebellum and basal ganglia (BG). The role of BG and cerebellum in motor control has been extensively studied but how Mthal processes inputs from these two networks is unclear. Specifically, there is considerable debate about the role of BG inputs on Mthal activity. This review summarizes anatomical and physiological knowledge of the Mthal and its afferents and reviews current theories of Mthal function by discussing the impact of cortical, BG and cerebellar inputs on Mthal activity. One view is that Mthal activity in BG and cerebellar-receiving territories is primarily “driven” by glutamatergic inputs from the cortex or cerebellum, respectively, whereas BG inputs are modulatory and do not strongly determine Mthal activity. This theory is steeped in the assumption that the Mthal processes information in the same way as sensory thalamus, through interactions of modulatory inputs with a single driver input. Another view, from BG models, is that BG exert primary control on the BG-receiving Mthal so it effectively relays information from BG to cortex. We propose a new “super-integrator” theory where each Mthal territory processes multiple driver or driver-like inputs (cortex and BG, cortex and cerebellum), which are the result of considerable integrative processing. Thus, BG and cerebellar Mthal territories assimilate motivational and proprioceptive motor information previously integrated in cortico-BG and cortico-cerebellar networks, respectively, to develop sophisticated motor signals that are transmitted in parallel pathways to cortical areas for optimal generation of motor programmes. Finally, we briefly review the pathophysiological changes that occur in the BG in parkinsonism and generate testable hypotheses about how these may affect processing of inputs in the Mthal.
Highlights
Motor thalamus (Mthal) encompasses thalamic nuclei that are strategically located between motor areas of the cerebral cortex and two subcortical networks, the basal ganglia (BG) and the cerebellum, generally considered to be related to the complex cognitive and proprioceptive control of movement, respectively (Middleton and Strick, 2000)
Mthal acts as a “super-integrator” of motor information converging from cortex and BG, and from cortex and cerebellum, rather than a relay of driver signals as is thought to occur in the sensory thalamus
We focus on the possible roles of BG inputs in the Mthal because BG activity is profoundly altered in Parkinson’s disease (PD) and the cerebellum is a major input to Mthal, there is a paucity of data reporting changes in PD
Summary
Motor thalamus (Mthal) encompasses thalamic nuclei that are strategically located between motor areas of the cerebral cortex and two subcortical networks, the basal ganglia (BG) and the cerebellum, generally considered to be related to the complex cognitive and proprioceptive control of movement, respectively (Middleton and Strick, 2000). Consistent with a driver role, cerebellar afferents in Mthal form large boutons that mainly synapse on primary dendrites (Rinvik and Grofova, 1974; Kultas-Ilinsky and Ilinsky, 1991; Aumann et al, 1994; Sawyer et al, 1994b; Kuramoto et al, 2011; Rovo et al, 2012) These anatomical features are corroborated by intracellular studies showing that stimulation of cerebellar afferents produces strong, fast, excitatory events in Mthal neurons, that are even faster than cortical ones (Uno et al, 1970; Shinoda et al, 1985; Sawyer et al, 1994a). To significantly advance our understanding of how the Mthal processes information underlying the control of movement, it is important that future studies are conducted in mammals executing movement tasks
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