The heart is essentially a big muscle. The cardiac muscle of the heart is responsible for the contractions that pump blood through the body.
Cardiac muscle is a specialized muscle only found in the heart. It is striated with narrow dark and light bands, similar to skeletal muscle. However, the bands are narrower and much shorter than skeletal muscle.
One unique characteristic of cardiac muscle is that it does not fatigue. This allows the muscle to keep contracting, or beating, without getting tired like typical skeletal or smooth muscle. Unfortunately, cardiac muscle cannot tolerate lack of oxygen. It can be easily damaged due to lack of oxygen.
Although the heart is an involuntary muscle, contraction is not triggered by the autonomic nervous system like other involuntary muscles. The action potential triggering the heartbeat is generated within the heart itself. The sino-atrial node, or pacemaker, in the right atrium initiates the heartbeat. However, autonomic nervous system motor nerves can control the rate or strength of the heart beat. This allows the body to increase or decrease the heart rate in situations requiring more oxygen and nutrients to the body, such as during stress or exercise.