Direct Pathway

The Direct Pathway (also known as the direct striatonigral pathway) is one of the pathways in the basal ganglia circuitry that is involved in the control and modulation of movements^[1]. The Direct Pathway is primarily associated with the facilitation or enhancement of voluntary movements^[2].

Outlined below is the simplified sequence of events in the Direct Pathway:

1. Initiation: The process commences in the cerebral cortex, specifically the motor and sensory areas, where the decision to execute a voluntary movement originates^[4].

2. Transmission to Striatum: The generated information is transmitted to the striatum, the basal ganglia's main input nucleus. Composed of medium spiny neurons (MSNs), the striatum is GABAergic, meaning they release the inhibitory neurotransmitter gamma-aminobutyric acid^[5].

3. Inhibition of GPi/SNr: In the direct pathway, these MSNs project directly to the internal segment of the globus pallidus (GPi) and the substantia nigra pars reticulata (SNr). This connection is inhibitory, owing to the GABAergic nature of the MSNs^[6].

4. Reduction of Thalamic Inhibition: The GPi and SNr, primary output structures of the basal ganglia, maintain a continuous inhibitory influence on the thalamus, specifically the ventral anterior and ventral lateral nuclei. This inhibitory signal prevents the thalamus from activating the motor cortex and initiating movement^[7].

5. Excitation of Motor Cortex: Upon activation, the striatal neurons in the direct pathway inhibit the GPi/SNr, reducing their inhibitory influence on the thalamus. Consequently, the thalamus can send excitatory signals to the motor cortex, initiating the desired movement^[8].

In short, the direct pathway's function is to 'disinhibit' the thalamus, thereby enabling it to excite the motor cortex and produce movement^[9].

The direct pathway works alongside the indirect pathway (sometimes called the 'NoGo' pathway) to maintain a balance in motor control. The indirect pathway suppresses unwanted movements, essentially counterbalancing the direct pathway^[11]. Together, both pathways contribute to the facilitation of smooth, coordinated voluntary movements^[12].

While this description is a simplified overview, the actual process involves a more complex interplay of brain regions, neuronal types, and neurotransmitters. However, it provides an understanding of the direct pathway's role and function.