Cardiac muscle fibers Cardiac muscle is its own specialized type of muscle tissue. It is under involuntary control of the central nervous system (CNS), much like the smooth muscles that move food through the intestines, constrict the arteries, or dilate the pupils of the eyes. Compared to skeletal muscle fiber, cardiac muscle fibers are shorter and branched, allowing them to be closely arranged next to each other. This form helps the muscle fibers contract at the same time as the fibers adjacent to them do. Similar to skeletal muscle, cardiac muscle contracts via the pulling of thin actin filaments by thicker strands of myosin. Cardiac muscle is also striated, similar to skeletal muscle, because of the actin–myosin interactions. The actin–myosin bonding is inhibited by two proteins, including troponin, which blocks actin and myosin until it is drawn away by the introduction of calcium into the sarcomere. This is why calcium channel blockers reduce the forceful contraction of the ventricle (Thompson & Ozemek, 2023). Like numerous structures within the body, the heart is composed of three distinguishable layers. These layers serve to separate the delicate inner tissue from the heart and protect from irritation of the immediate environment. The heart’s three distinct layers include (Thompson & Ozemek, 2023): Electrical control of the heart muscle The heart is a little bit bigger than a human fist. The heart, like any muscle, needs to receive an electrical signal to contract. Skeletal muscles do not function until they receive a signal from the central nervous system. Electrical signals that activate muscles travel very quickly (approximately 1/800th of a second). At this speed, the electrical signal would cause every muscle fiber in the heart to contract simultaneously. The heart is so specialized that it contains its own ability to generate a nerve signal. It would be very bad to have the atria and ventricles contracting at the same time, as the timing of the chambers relaxing and contracting is crucial. It is important for the atria to squeeze from the top down and for the ventricles to contract from the bottom up (Thompson & Ozemek, 2023). The sinoatrial (SA) node is considered the pacemaker of the heart. It generates the electrical signal that begins the atrial contraction. Electrical impulse spreads from the sinus node throughout the left and right atria, causing them to contract and push their volume of blood into the ventricles. The atrioventricular (AV) node is the delay/relay station. It holds the electrical signal for a fraction of a second. The bundle of His is made up of conductive cells that begin taking the signal down the intraventricular septum. The bundle branches contain conductive cells that split into left and right branches. They carry the signal to the bottom of the ventricles, which causes the ventricles to contract. This contraction forces them to push their blood volume out into Blood pressure When a chamber of the heart relaxes and fills with blood, the process is called diastole . Conversely, when the chamber contracts and pumps blood, the process is called systole . Diastole and systole comprise the cardiac cycle. At rest, both phases of the cardiac cycle occur 60 to 80 times each minute. The diastole phase takes up over two-thirds of this time. Upon exertion, the ratio levels and time spent in systole approach the time spent in diastole. This brings us to the concept of blood pressure. Blood pressure is an outward force of the blood pushing against the arterial walls at any given moment. The body must have enough blood
● Pericardium ( peri = “outer”) : The pericardium is the thin outer layer of the connective tissue that provides protection to the heart. The pericardium contains coronary arteries and serous fluid to cool, protect, and lubricate the organ. ● Myocardium ( myo = “muscle”) : The myocardium is the thick, muscular middle layer. The myocardium is the working muscle of the heart. The myocardial cells are shorter than skeletal muscle, allowing the myocytes to contract simultaneously. ● Endocardium ( endo = “inner”) : The endocardium is very thin, smooth inner layers that line the heart chambers and surface of the valves. The endocardium facilitates blood flow.
Self-Assessment Quiz Question #5 The outermost layer of the heart is the:
a. Pericardium. b. Myocardium. c. Endocardium. d. Exocardium.
the general circulation. The Purkinje fibers are special cells that carry the signal across the ventricles from bottom to top (similar to squeezing toothpaste up the tube; Thompson & Ozemek, 2023). See Image 6 to better visualize and trace out the path of electrical charge through the heart. Image 6: Pat of Electrical Change in the Heart
pressure to keep the blood moving through the arteries. With inadequate blood pressure (hypotension), the blood isn’t flowing adequately, and the cells will not get oxygen. Conversely, with too much pressure (hypertension), the blood is pushing out too hard on the artery walls. Fluid will always push harder against its container as it’s being pumped in. This makes the heart work harder and may cause it to grow enlarged. High blood pressure can also damage the tunica interna, the delicate inner lining of the arteries. This can lead to numerous health issues (American Heart Association, 2023).
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