1. What the Platysma Is, and Why Neck Bands Are Muscle Rather Than Skin
The platysma is a wide, thin sheet of muscle running from the upper chest and shoulder area, up across the front of the neck, and attaching along the lower jaw and lower face. It sits just beneath the skin rather than deep among other structures, which is why its movement shows so readily on the surface.
Its fibers do not form one continuous curtain in most people. There is usually a gap toward the midline, and the inner borders on either side of that gap are the edges that become visible. With age and years of repeated contraction those edges thicken and separate from the tissue around them, standing out as vertical cords.
That origin is the most useful thing to understand before you consider treatment. The cords are muscle. They are not folds of loose skin and they are not fat. If you clench your jaw hard and the cords jump out sharply, that is the pattern neurotoxin interrupts.
The platysma also pulls downward as a matter of course. Because it attaches along the jaw, every contraction tugs at the jawline and the corners of the mouth. One anatomical fact therefore explains both treatments here: relax the standing edges and the cords soften, relax the pull along the jaw and the jawline border gets some help.