A good question. Myogenic pertaining to muscles means being able to contract on its own. Ok.
Both muscle types are capable of voluntary "contraction", but you have to understand that the way in which it happens is not the same. As you know, for cardiac muscle, it relies on the SA node. However, for smooth muscle, there is no SA node, but rather depends on several factors. For instance, peristalsis, which is the wavelike contracting motion of the esophagus, is the result of the "voluntary contraction" of the smooth muscle due to STRETCHING when you swallow food. While there is no SA node in smooth muscle, what caused it to contract was stretch receptors.
What is important to understand is that while these muscles are capable of contracting by themselves, the nervous system does play a role. For example, the nervous system controls blood pressure due to the contraction/relaxation of smooth muscles located in the blood vessels. So there you have it, the nervous system controlling both the cardiac muscles and the smooth muscles.
While we look for clear-cut answers in our struggle to learn for the DAT, it is important to understand the body in a bigger realm, rather than just clear cut answers, which in many cases, is not always true. A rather lenghty response, hope it clears it up though.
