Final answer:
M1 cells project to the globus pallidus internal segment, the substantia nigra pars reticulata, and the thalamus.
Step-by-step explanation:
The three subcortical motor structures that M1 cells project to are the globus pallidus internal segment (GPi), the substantia nigra pars reticulata (SNr), and the thalamus. The M1 cells send axons from the primary motor cortex (M1) to these structures to regulate movement.