## 3 Results
### Regional Subdivisions of the Substantia Nigra  
The SNc and SNr exhibit marked differences in neurochemistry, connectivity, and function. The SNc contains dopaminergic neurons whose axons form ascending projections (i.e., nigrostriatal and mesolimbic) that modulate the activity of striatal circuits [28]. In contrast, the SNr predominantly consists of GABAergic neurons that function as output cells of the basal ganglia. These neurons are analogous to GPi, in that they send inhibitory signals to the motor and associative nuclei of the thalamus and to the brainstem [64]. Tractography has enabled the differentiation of distinct functional subcomponents of the SN based on their connectivity. [PERSON] et al. (2010) [70] performed DTI segmentation of the SN and divided it into two parts, corresponding to the SNc and SNr. The inner segment (SNc) demonstrated predominant connections with the dorsal striatum (putamen, caudate), globus pallidus, anterior nuclei of the thalamus, and via thalamic pathways with the prefrontal cortex. In contrast, the outer segment (SNr) has been shown to be predominantly connected with the posterior and ventral thalamus, the dorsal striatum, and pathways directed to the premotor and primary motor cortex. A complementary approach to the segmentation of dopaminergic structures is connectivity-driven parcellation, a method which [PERSON] et al. (2020) [71] applied to the SNc and the adjacent VTA. Utilizing probabilistic tractography and clustering analysis, the researchers categorized the dopaminergic midbrain into three functionally distinct domains: the limbic, prefrontal, and sensorimotor domains. These domains exhibit topographical overlap with the dorsoventral gradient. This approach is consistent with the established functional subdivisions of dopamine neurons in primates and underscores the significant heterogeneity of the SNc/VTA in humans. Another study by [PERSON] et al. (2013) [72] divided the SN according to connections to the striatum. This revealed that the dorsomedial SN preferentially innervates the ventral striatum (NAc), whereas the ventrolateral SN connects to the dorsal striatum (putamen and caudate). In addition to the classical division between dorsal and ventral SN projections to the striatum, a recent study using probabilistic tractography demonstrated increased structural connectivity between the VTA and the SN and NAc in individuals who carry the fat mass and obesity-associated (FTO) risk gene. This connectivity was found to be significantly higher than in the control group and was also associated with higher levels of motor impulsivity. As demonstrated by the authors, the structural connection between the SN/VTA and the NAc partially mediated the relationship between genetic risk and impulsive behavior [73]. A study by [PERSON] et al. (2022) [65] utilizing GQI tractography described four subcomponents of the AL, including the GPSN branch, which is the connection between the globus pallidus and the SN. The present study demonstrated that this pathway runs laterally to the other outputs of the AL and is directed specifically towards the SN. This is evidenced by the presence of a special anatomical bundle connecting these two structures. The presence of the GPSN lends support to the hypothesis that the SN is not merely an exit point (SNr) or an entry node (SNc), but may also be involved in the integration of signals from other nuclei of the basal ganglia, including the globus pallidus. As illustrated in Table 1, the key structural connections of the SN are outlined, including their anatomical course, functional significance, and related clinical implications.