Generation of Rhythmic Contractions

The electrical coupling among cardiac muscle fibers can explain how contraction occurs synchronously in all the fibers of a chamber. We will now consider the control mechanisms responsible for the repetitive contractions that characterize the beating of the heart. If a heart is removed from the body and placed in an appropriate artificial environment, it will continue to contract repetitively even though it is isolated from the nervous system and the rest of the body. By contrast, a skeletal muscle isolated under similar conditions will not contract unless its nerve is activated. The rhythmic activity of the heart muscle is an inherent property of the individual muscle fibers making up the heart, and this constitutes another important difference between cardiac muscle fibers and skeletal muscle fibers. This difference can be demonstrated dramatically in experiments in which muscle tissue is dissociated into individual cells, which are placed in a dish isolated from each other and from the influence of any other cells, like nerve cells. Under these conditions, muscle cells from skeletal muscles are quiescent; they do not contract in the absence of their neural input. Cells from cardiac muscle, however, continue to contract rhythmically even in isolation. Thus, rhythmic contractions of heart muscle are due to built-in properties of the cardiac muscle cells. Before we can examine the membrane mechanism underlying this autorhythmicity, it will be necessary to look first at the action potential of cardiac muscle cells. In keeping with the different behavior of cardiac cells, this action potential has some different characteristics from the action potentials of neurons or skeletal muscle cells.

0 0

Post a comment