Complex sentence: It is a major element of the basal ganglia system where it is a major element of the basal ganglia core; made up of the striatum and its direct targets: pallidum and nigra.
Split into simpler sentences: It is a major element of the basal ganglia system. There, it is a major element of the basal ganglia core made up of the striatum and its direct targets: pallidum and nigra.