Through experimentation with multiple animals, predominantly monkeys, it has been shown that several regions of the brain are particularly active and involved in initiation and preparation of movement.
Two specific membrane potentials, the bereitschaftspotential, or the BP, and contingent negative variation, or the CNV, play a pivotal role in premovement neuronal activity.
Using both the electroencephalography (EEG) and the electromyogram (EMG) recordings, Kornhuber and Deecke were able to identify two components prior to movement onset.
However, over the past couple of decades the early BP is considered to perhaps also be site specific within the supplementary motor area (SMA) and the lateral premotor cortex.
The current consensus is that the early BP starts first in the SMA, including pre-SMA and SMA proper, and then approximately 400ms later in the lateral premotor cortices bilaterally prior to the movement onset, and the late BP starts in the M1 and premotor cortex contralaterally.
The amplitude difference in the late BP was seen over the central region contralateral to the movement, which suggests an important role of M1.
Further experiments also suggest that the bilateral sensorimotor cortices play a role in the preparation of complex movements, along with the SMA.
[3] Some of the first relevant experimentation and subsequent findings about the organization of the primary motor cortex were observed by Wilder Penfield.
This supports the general conclusion that movements and not individual muscles are controlled by the activity of upper motor neurons.
It was found that the force generated by contracting muscles changed as a function of the firing rate of upper motor neurons.
The firing rates of the active neurons often change prior to movements involving very small forces.
More evidence that the lateral pre-motor area is involved in movement selection comes from observations of the effects of cortical damage on motor behavior.
Lesions to this area severely impair the ability of monkeys to perform visually cued conditional tasks.
But, when placed in another setting, the monkey is perfectly capable of performing that movement in a spontaneous, self initiated manner, as a response to the same visual stimulus.
The neurons in area 5 play a role in the initiation and execution of movement and respond at enormously quick speeds.
The neurons on the parietal associative cortex are most strongly involved in programming and execution of voluntary movements.
The early phase of the readiness potential occurs in the supplementary motor region and is involved in the generation of voluntary movement.
Additionally, in observations of goal oriented movements, these neurons fire even when the result is blocked from view.
The research indicated that the mirror motor neurons showed increased activity when the dancers watched the video for the style they have been trained in.
Additionally, the control, non-dancers, showed significantly less brain activation in the mirror motor neurons when watching either type of dance.
[15] Premovement neuronal activity has been widely experimented upon in three major motor fields of the frontal cortex.
The pre-central motor cortex was also identified in this study as having similar neuronal activities as in the PM and SMA.
The long lead neuronal changes were more frequently active during the self paced stimuli than before the triggered movements.
Extracellular recording have shown that these specific neurons increase their rate of discharge before an impending movement.
[7] The neurons present in the global pallidus and substantia nigra are the main output areas of the basal ganglia.
The input of medium spiny neurons to these output areas of the basal ganglia are also GABAergic and therefore inhibitory.
This indirect pathway also involves the subthalamic nucleus (a part of the thalamus), which receives signals from the cerebral cortex.
Thus, this indirect pathway serves to reinforce inhibition by excitatory signals to the GABAergic cells present in the globus pallidus.
[17] This prolonged activity may have to do with impaired function of the basal ganglia, which is thought to send a termination signal to the SMA.
More research studying the effects of the disease on MRCPs (like the BP) and premovement preparatory activity in PD patients is ongoing.