GABAergic output of the basal ganglia

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Abstract

Using GABAergic outputs from the SNr or GPi, the basal ganglia exert inhibitory control over several motor areas in the brainstem which in turn control the central pattern generators for the basic motor repertoire including eye–head orientation, locomotion, mouth movements, and vocalization. These movements are by default kept suppressed by tonic rapid firing of SNr/GPi neurons, but can be released by a selective removal of the tonic inhibition. Derangement of the SNr/GPi outputs leads to either an inability to initiate movements (akinesia) or an inability to suppress movements (involuntary movements). Although the spatio-temporal patterns of individual movements are largely innate and fixed, it is essential for survival to select appropriate movements and arrange them in an appropriate order depending on the context, and this is what the basal ganglia presumably do. To achieve such a goal, however, the basal ganglia need to be trained to optimize their outputs with the aid of cortical inputs carrying sensorimotor and cognitive information and dopaminergic inputs carrying reward-related information. The basal ganglia output to the thalamus, which is particularly developed in primates, provides the basal ganglia with an advanced ability to organize behavior by including the motor skill mechanisms in which new movement patterns can be created by practice. To summarize, an essential function of the basal ganglia is to select, sort, and integrate innate movements and learned movements, together with cognitive and emotional mental operations, to achieve purposeful behaviors. Intricate hand–finger movements do not occur in isolation; they are always associated with appropriate motor sets, such as eye–head orientation and posture.

Section snippets

Outputs of the basal ganglia

The output of the basal ganglia is issued largely (but not exclusively) from the substantia nigra pars reticulata (SNr) and the internal segment of the globus pallidus (GPi) (Carpenter, 1981). The output neurons in the SNr and GPi share two important features: (1) they are GABAergic and inhibitory (Uno and Yoshida, 1975; Di Chiara et al., 1979; Yoshida and Omata, 1979), and (2) they fire tonically and rapidly (DeLong and Georgopoulos, 1981). The tonic firing is present even in vitro slice

An example of basal ganglia-controlled functions — Saccadic eye movement

A big advantage of using saccadic eye movement as a behavioral measure is that its neuronal mechanisms have been studied extensively so that the basal ganglia mechanisms can be studied more or less independently from motor execution mechanisms. Let me first describe what aspects of saccadic eye movement are controlled by motor execution mechanisms in the brainstem, which is situated downstream to the basal ganglia.

Saccadic eye movement (or saccade) is an extremely fast and simultaneous rotation

Functions of basal ganglia GABAergic outputs

So far I have been describing a neural mechanism of behavioral selection. However, for the mechanism to be functional, it must be operated appropriately. One way to answer this question is to examine the behavioral context in which the mechanism is deployed. An experimental objective would then be to examine whether the key neurons constituting the mechanism (in this case SNr neurons) change their activity selectively in that context. This is a difficult task because the experimenter has to

Basal ganglia GABAergic control of basic movements

So far I have been discussing the function of the basal ganglia in relation to the control of saccadic eye movement, which is mediated by the SNr–SC connection. It is then tempting to hypothesize that other kinds of body movements or behaviors may also be controlled directly by the basal ganglia. Noteworthy in this respect is the fact that the basal ganglia send their outputs to many areas in the midbrain and the brainstem, which contain neural mechanisms of specific motor behaviors. Although

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