These topics are basically looking at how action potentials in smooth muscle cells differ from action potentials in skeletal muscle cells (so I am assuming you know what action potentials are). Most smooth muscle cells do not have sodium channels. They have two kinds of calcium (Ca++) channels involved in action potentials. One type called "ligand gated Ca++ channels" open in response to neurotransmitters or certain hormones. The other type of Ca++ channel in smooth muscle membranes are "voltage gated Ca++ channels." When a smooth muscle cell undergoes action potential in response to hormone or neurotransmitter it happens in these basic steps... 1. The first type of channel (ligand gated) is stimulated by neurotransmitter or hormone and Ca++ enters the cell, causing the inside of the cell to become more positive. 2. Once the inside of the cell becomes positive enough the voltage gated Ca++ channels open and this causes the spike (so that is spike potential) the membrane quickly becomes very positive on the inside. 3. Slow voltage gated potassium (K+) channels then bring the membrane back towards resting potential (more negative on the inside) by allowing K+ to leave the cell. Slow wave rhythm: Some smooth muscle cells have channels that cause the membrane to continuously become more positive on the inside until the voltage gated Ca++ channels are activated and an action potential spike is observed. So, these cells activate themselves in a rhythmic manor. After each action potential they return to being negative on the inside and the cycle repeats. These cells are "auto rhythmic" For plateau phase... in some smooth muscle cells the calcium channels stay open longer, so the action potential displays a rounded or flattened top (stays positive longer)... that's the plateau. I hope this helps :)