Basal ganglia as a sensory gating devise for motor control.

Basal ganglia lie between the cerebral cortex and the thalamus, and have dense fiber connections between them. These connections form 4-5 distinct loops or circuits to allow parallel processing of information. Among them, the most intensively studied is the motor loop, which comprises 2 distinct direct and indirect pathways. The direct pathway disinhibits the powerful inhibition of the internal segment of the globus pallidus/substantia nigra pars reticulata upon thalamic ventrolateral nuclei with a net result of facilitatory influence upon the motor cortex. By contrast, the indirect pathway exerts an inhibitory effect. Overall this dual system provides a center (excitatory)-surround (inhibitory) mechanism to focus its effect on selected cortical neurons. Although putative transmitters, inhibitory or excitatory nature of these projections and their receptors are mostly known, the functional role of the loop in motor control is not precisely understood. Several lines of evidence have recently been presented to support the view that this center-surround mechanism is used to focus the output to a specific group of muscles required for performing a specific task. This operation is made possible through opening the sensory channel for the expected sensory feed-back afferents during movement. Thus one of the important functions of basal ganglia seems to be the gating of sensory input for motor control.