Crosstalk between the canonical and non-canonical transforming growth factor-beta1 (TGF-β1) and myostatin pathways. Once the TGF-β1 or myostatin ligands bind to the appropriate type I and type II receptors, cross-phosphorylation of the type I receptor occurs, leading to the phosphorylation of downstream effectors. In the canonical pathway, the type I receptor phosphorylates Smad2/3, which then binds to Smad4 and translocates into the nucleus to act as transcription factors. In the non-canonical pathway, the type I receptor phosphorylates proteins that are involved in the activation of the mitogen-activated protein kinases (MAPKs). Activated MAPKs can then regulate transcription factors and/or the Smad proteins through direct interactions or via downstream proteins.