To begin with…
Studies have shown that the motor loop through the basal ganglia helps initiate conscious movement. One model has shown that furthered inhibition of the thalamus via the basal ganglia underlies what is known as hypokinesia or the reduction of movement. Contrarily, decreased output by the basal ganglia leads to hyperkinesia or the excess of movement.
Now the basal ganglia consists of several structures ,which includes the caudate nucleus, the putamen, the globus pallidus and the subthalamic nigra. Some neuroscientists also include the substantia nigra as part of the basal ganglia even though technically it is part of the midbrain. This is because the substantia nigra plays a quintessential role in the control of movement. Together with the putatmen, the caudate nucleus makes up the striatum, which is the target of cortical input to the basal ganglia. The globus pallidus controls output to the thalamus, which helps create a loop of information from the cortex back to the cortex.
A simplified version of the motor loop:
Cortex -> Striatum -> Globus pallidus -> Thalamus -> Cortex
The impulse that drives the motor loop originates from the cortex (the frontal, parietal and prefrontal) and forms an excitatory connection with the putamen. The putamen cells then form an inhibitory connection with neurons in the globus pallidus, which then forms an inhibitory connection with the thalamus. More specifically, a part of the thalamus known as the ventral lateral nucleus or VLo. The VLo then forms the thalamocortical connection with the supplementary motor area or SMA, which is a medial region of cortical area 6 that directly sends axons to motor units.
This may seem counter-intuitive. Why would an inhibitory signal cause the activation of a motor unit? Well, basically at rest, neurons in the globus pallidus are active. Because the neurons globus pallidus are active, this inhibits the activity of the neurons in the thalamus, specifically in the VLo. So, when the impulse from the cortex excites the putamen, the neurons globus pallidus are inhibited. The inhibition of these neurons allows the VLo to become active or excited. The activation of the neurons in the VLo sends neural activity via the thalamocortical connection to the SMA.
Bear, Mark F., Barry W. Connors, and Michael A. Paradiso. Neuroscience: Exploring the Brain. Philadelphia, PA: Lippincott Williams & Wilkins, 2007. Print.