Consciousness and Name-Form

In the twelve nidanas in the dependent origination it is said that through ignorance arises constructive activities (Sankskara). I do understand that. But the fact that I am still having trouble understanding is how Sankara causes the consciousness to arise and how consciousness cause the name and form to arise. I do understand that cause of name-form the consciousness arises as these two are very interdependent. But my question is as the above. 1. How does Sankskara cause consciousness to arise? 2. How does consciousness cause the name-form to arise?