Comprised of elongated cells with multiple nuclei, cardiac muscle tissue appears striated under the microscope. Yet, unlike other striated muscles in the body, cardiac muscle controls an involuntary action, similar to smooth muscle tissue. The rhythmically contracting cardiac muscle tissue is essentially under the control of the heart’s pacemaker, the sinoatrial node. However, a number of chemical substances may affect the action of the tissue, many of which are utilized for medical purposes.