Figure 2.

Diagram describing the frontostriatal motor loop controlling motor function under physiological and parkinsonian states. The prefrontal cortex (PFC) participates in cognitive control and planning of movements. The premotor cortex organizes sequences of body actions, and the primary motor cortex is responsible for executing them. Excitatory signals, which are initiated by cortical glutamatergic neurons, project from the PFC to the premotor cortex, and then to the motor cortex through several subcortical structures. Then, the resultant signals received by the pyramidal cells of the motor cortex go to the motor neurons of the spinal cord. Together, this is called the frontostriatal motor loop. Midbrain dopaminergic neurons play an essential role in modulating the signals that go along the frontostriatal motor loop. Changes in the direct inhibitory (initiated by D2 receptors) and indirect (D1 receptors) pathways under parkinsonian states due to the loss of dopaminergic neurons in the SNc are indicated. GABA, γ-aminobutyric acid; SNc, substantia nigra pars compacta; GPe, globus pallidus external; GPi, globus pallidus internal; STN, subthalamic nucleus; PPN, peripeduncular nucleus.