Cardiac Muscle
Structurally, cardiac muscle, also known as myocardium, has a striated appearance due to the arrangement of fibers that allow for muscle contractions. The myocardium is a very aerobic muscle and depends heavily on an uninterrupted blood supply to deliver oxygen to the heart muscle. As described by Dr. Kathryn Lewis in "Sensible ECG Analysis," the cells of the heart muscle have unique properties that allow the heart to function as a distinctive system. These myocardial cells, called cardiomyocytes, have the characteristics of automaticity, conductivity, contractility and excitability. The first two characteristics are unique to cardiac muscle, whereas the latter two are common to other muscle types.
Automaticity
Automaticity is a characteristic unique to cardiac muscle and refers to the heart's ability to generate its own signal to contract. Rather than receiving input from the central nervous system, at rest the heart relies on pacemaker cells located in the right upper chamber to spontaneously generate electrical signals, which lead to the rhythmic contractions known as heartbeats. The rate of the heartbeat is based on how long it takes the pacemaker cells to fire, reset and fire again. Interestingly, these pacemaker cells function in such as a way as to prevent the heart from holding a contraction for a long period of time. If the heart muscle were to sustain a contraction for a long period, it would not be able to adequately deliver blood and nutrients to the rest of the body. The inability of the myocardium to hold a contraction, or continuously fire without resetting, is a protective mechanism built into pacemaker cells.
Conductivity
In order to transmit the electrical signal that is generated in the upper right quadrant of the heart to the rest of the myocardium, the muscle fibers must be able to conduct electricity. Cardiac muscle has the ability to pass the electrical signal from one fiber to the next until it has spread throughout the entirety of the heart. Once each fiber has been given the signal, the heart will contract as a whole. Contracting in this fashion allows for a significant amount of force to be generated by the two lower chambers of the heart, which will allow blood to be delivered to the lungs and throughout the whole body. Without conductivity, each muscle fiber would need to have its own pacemaker and would likely disrupt the synchronicity, rhythm and efficiency of myocardial contractions.
Contractility
Contractility is the ability of the heart to generate tension, or produce force, in order to eject blood from the heart. It is, in essence, the physical expression of the electrical signals initiated by the pacemaker cells and passed throughout the heart muscle. The text "Cardiovascular Physiology" describes the mechanisms by which the amount of force generated by the heart can be regulated by a variety of factors such as the amount of blood that fills up the chambers of the heart and signals, such as norepinephrine, released from nerve endings. Both factors will increase the strength of cardiac contractions and allow for greater force production.
Excitability
Although cardiac muscle can generate its own electrical signal, the pacemaker cells fire at a very steady rate. Because of this, any increases in heart rate have to be governed by an external stimulus. The heart's ability to respond to an additional stimulus and change its rate of contraction is known as excitability. Just as norepinephrine increases the contractility of the heart, it also increases the rate of contraction of the heart muscle. The characteristic of excitability is vital in allowing the body to more rapidly deliver adequate amounts of oxygen and nutrients in times of physical stress, such as during exercise.
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