The doctor's visit has to be reported as well as the condition being treated. Medications have to be reported if you are CURRENTLY taking them. Refer to the FAA's guide for Medical Examiners p.28.
While taking disqualifying medications for the treatment of an acute illness, the airman must not exercise the privileges of his medical unless cleared to do so by the FAA.
My interpretation is that you do not need to report the medication unless you are currently taking it or routinely need it for a chronic condition. While you are on the muscle relaxant, you should refrain from flying. To be certain, your AME should be able to answer that question for you before you start the med.
I hope that helps.