The heart muscle is the muscle that is responsible for the pumping of the heart.
(heart = cardia, cardiac = from the heart, cordial = from the heart, pre-cordial = before the heart; i.e. the chest)
A. Comparing a skeletal action potential with a Cardiac action potential :
The action potential of the cardiac muscle is similar on many points with the action potential of the skeletal muscle but also differs on some important points.
The upstroke of the cardiac action potential (= depolarization) is caused by opening of sodium -channels and influx of sodium-ions into the cardiac cell. This is similar to what occurs in the skeletal muscle.
The down stroke of the cardiac action potential (= repolarization) is caused by opening of potassium -channels and efflux of potassium-ions out of the cardiac cell. This is also similar to what occurs in the skeletal muscle.
Between the depolarization and the repolarization, the potential stays at about 0 mV for a relatively long time, approximately 100-300 milliseconds (0.1 - 0.3 seconds). This potential that lasts between the de- and the re-polarization is called the “plateau” (pronounced “pla-too”; originally a French word which means a flat area on top of a mountain).
During this plateau-phase, the calcium channels are opened and there is a calcium-influx. This calcium is used in the subsequent contraction of the muscle to bind the myosin cross-bridges to the actin molecules.
B. Cardiac Action Potential and Cardiac Contraction:
As with skeletal muscle, the action potential in a heart cell will initiate contraction of that cell.
One important consequence of a plateau in the heart action potential is that the action potential duration is much longer than in skeletal muscles.
The cardiac action potential can range between 100 and 300 msec (=milliseconds) instead of 4-8 msec in a typical skeletal muscle cell.
As shown in figure A, the contraction in the heart muscle occurs during the action potential (while in skeletal muscles the contraction occurs after the action potential).
Another consequence of a long action potential duration is that the refractory period of the cardiac action potential is much longer than in skeletal muscles. Instead of 4-8 milliseconds, the refractory period in the heart may last as long as 300 milliseconds.
Therefore, in the heart, the contraction is finished before a second action potential can be generated, as shown in figure B.
In figure C, the second action potential is initiated immediately after the refractory period of the first action potential and even then, the contraction of the first is finished before the beginning of the second.
In other words, the contractions in the heart can not summate (as they can in skeletal muscles; temporalsummation).
C. Some additional notes relating to cardiac muscle:
There are therefore in cardiac muscles at least three different ion-channels: Na+, K+ and Ca2+ channels involved in creating an action potential
And, as in other excitable tissues, the K+ channels are mainly responsible for the resting potential.
Because there is no temporal summation in cardiac muscles, there can be no tetanic contractions in cardiac muscle.
There are also no motor units in the heart. In fact the action potentials in the heart do not come from the nerve cells at all but are self-generated in a specialized area of the heart called the sinus node. This is the pacemaker of the heart.
By the way, cardiac muscles, under the microscope, also show a striated pattern (=anisotropy), similar to skeletal muscle cells.
Please note that this is only a very brief introduction to the cardiac muscle. A much more elaborate presentation will be given at a later time.