Athena Vale
The heart is a network of cardiac cells that are connected by intercalated discs. These cells are organised into layers of myocardial tissue that wrap around the chambers of the heart. The contraction of these cells results in a decrease in the heart chamber size, which leads to the ejection of blood into the pulmonary and systemic vessels. Cardiac muscles must relax between contractions to allow the ventricles to fill with blood. Tetanus refers to a sustained contraction that occurs when a series of action potentials take place in rapid succession. While tetanus can occur in skeletal muscle, the longer myocardial action potential of the heart helps prevent this sustained contraction.
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What You'll Learn
Cardiac muscle contraction
The primary function of the cardiac muscle is to pump blood into circulation by generating sufficient force. The mechanism behind each coordinated contraction involves the cardiac muscle and electrical impulses. The cardiac muscle must contract with enough force and blood to supply the metabolic demands of the entire body. The heart is made up of three layers—the pericardium, myocardium, and endocardium. The endocardium is not cardiac muscle and is comprised of simple squamous epithelial cells and forms the inner lining of the heart chambers and valves. The pericardium is a fibrous sac surrounding the heart. The myocardium is the cardiac muscle and is responsible for the contractility of the heart and, therefore, the pumping action.
Cardiac muscle cells (cardiomyocytes) are striated, branched, and contain many mitochondria. Each myocyte contains a single, centrally located nucleus surrounded by a cell membrane known as the sarcolemma. The sarcolemma of cardiac muscle cells contains voltage-gated calcium channels, specialized ion channels that skeletal muscle does not possess. Cardiac muscle cells contain branched fibers connected via intercalated discs that contain gap junctions and desmosomes. These interconnections allow the cardiomyocytes to contract together synchronously to enable the heart to work as a pump.
Calcium-induced calcium release (CICR) is a process whereby calcium triggers the release of further calcium from the muscle sarcoplasmic reticulum. CICR creates a "plateau phase" in which the cell's charge stays slightly positive (depolarized) before it becomes more negative as it repolarizes due to the potassium ion influx. The actual mechanical contraction response in cardiac muscle occurs via the sliding filament model of contraction. In the sliding filament model, myosin filaments slide along actin filaments to shorten or lengthen the muscle fiber for contraction and relaxation. The binding of the myosin head to adenosine triphosphate (ATP) pulls the actin filaments to the center of the sarcomere, creating the mechanical force of contraction.
When calcium is no longer bound to troponin-C, the actin-binding site is covered, ending contraction and relaxing the muscle. Intracellular calcium is then removed by the sarcoplasmic reticulum, dropping the intracellular calcium concentration. The troponin complex returns to its inhibiting position on the active site of actin, and the actin filaments return to their initial position, relaxing the muscle.
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Tetanus prevention in the heart
Tetanus is a disease caused by the C. tetani bacteria, which is commonly found in soil, saliva, dust, and manure. The bacteria enter the body through breaks in the skin, such as cuts or puncture wounds, and produce toxins that interfere with normal muscle contractions. While tetanus itself does not spread between people, the bacteria that cause it can enter through direct contact with contaminated wounds or objects.
Cardiac muscle, like all muscle types, can be affected by tetanus. The heart is composed of cardiac cells that are connected to form layers of myocardial tissue wrapped around its chambers. When tetanus affects these muscles, it can result in rapid heart rates and elevated blood pressure, which are symptoms of the disease.
To prevent tetanus in the heart and other parts of the body, vaccination is key. The CDC recommends that adults receive a tetanus booster vaccine every ten years and that anyone with a puncture wound who is uncertain about their vaccination status or has had fewer than three lifetime doses receive a booster as well. This is because recovery from naturally acquired tetanus does not typically result in immunity due to the extreme potency of the tetanospasmin toxin.
In addition to vaccination, it is important to practice good wound care and hygiene to prevent tetanus. This includes promptly cleaning and dressing any wounds, especially if they are contaminated or caused by rusty objects, which are often associated with C. tetani. By taking these preventive measures, the risk of developing tetanus in the heart and other muscles can be significantly reduced.
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Action potentials and tension
The cardiac muscle, or heart muscle, is a highly branched network of cardiac cells that are connected end-to-end by intercalated discs. This muscle is unique in that it contracts and relaxes more than 100,000 times a day at an average heart rate of 70 beats per minute. The rate and strength of these contractions must be variable to meet the body's physiological and pathological challenges.
The cardiac muscle is composed of two types of cells: work cells and pacemaker cells. Work cells have a large, stable resting membrane potential and a prolonged action potential with a plateau phase. In contrast, pacemaker cells have smaller, unstable resting potentials and spontaneously depolarize, generating the intrinsic electrical activity of the heart. These pacemaker cells are found in the sinoatrial (SA) and atrioventricular (AV) nodes, as well as in the bundle of His and some Purkinje cells.
The action potential in cardiac muscle is initiated in the SA node and then spreads across the atria. After a brief delay at the atrioventricular node, the impulse travels through the atrioventricular bundle and bundle branches to the Purkinje fibres, which transmit the impulse to the ventricular muscle cells. The total time from the initiation of the impulse in the SA node to the depolarization of the ventricles is approximately 225 ms.
The action potential in cardiac muscle is characterized by a prolonged plateau phase, lasting around 300 ms, which is significantly longer than the 1 ms plateau phase observed in nerves. This extended plateau phase is due to the influx of calcium ions through slow calcium channels, which is critical for the proper functioning of cardiac muscle. The absolute refractory period for cardiac contractile muscle lasts approximately 200 ms, followed by a relative refractory period of about 50 ms, resulting in a total refractory period of 250 ms. This extended refractory period is essential to prevent premature contractions in the heart, which would be incompatible with life.
In terms of tension, the cardiac muscle exhibits a relationship between load and velocity, similar to other muscle types. The tension in the muscle is equal to the load during shortening and lengthening, except during brief periods of acceleration. The size of the load determines the velocity of shortening, and the muscles that shorten and do external work release more energy as heat and work compared to those that contract under isometric conditions.
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Ventricular muscle
The heart is a network of cardiac cells that are connected by intercalated discs. These cells are arranged into layers of myocardial tissue that wrap around the chambers of the heart. The contraction of these individual heart cells results in a decrease in heart chamber size, which leads to the ejection of blood into the pulmonary and systemic vessels. This process is essential for maintaining blood circulation throughout the body.
Cardiac muscles, including ventricular muscles, have unique properties that differentiate them from skeletal muscles. One notable difference is the regular pattern of contractions in cardiac muscles, which contrasts with the burst-like activity of skeletal muscles. Cardiac muscles, including ventricular muscles, rely on slower reactions to meet their energy requirements due to their lower rate of ATP usage.
The prevention of tetanus in the heart, including the ventricular muscles, is crucial to ensure proper heart function. Tetanus refers to a sustained contraction resulting from a series of rapid action potentials in skeletal muscles. However, cardiac muscles, including ventricular muscles, have a longer myocardial action potential, which helps prevent tetanus. By the time a second action potential occurs, the myocardial cell has almost completely relaxed, allowing for the necessary relaxation between contractions.
In a study on the shrew myocardium, it was observed that tetanus could occur in the ventricular muscle. The right ventricular muscle displayed fused and unfused tetanus when stimulated with a train of stimuli with an internal cycle length of less than 50 to 60 ms. This finding highlights the possibility of tetanus in ventricular muscle under specific conditions.
The understanding of ventricular muscle function and its potential susceptibility to tetanus is essential for maintaining cardiac health and treating various heart conditions. Further research and studies contribute to our knowledge of the complex mechanics of the heart and guide the development of effective treatments and interventions for cardiac-related issues.
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Muscle relaxation
Progressive muscle relaxation (PMR) is a technique that can be used to relieve stress, anxiety, insomnia, and chronic pain. PMR was developed by Edmund Jacobson in the 1920s or 30s and is based on the idea that physical relaxation leads to mental calmness.
PMR involves tightening and relaxing muscle groups in a specific pattern, one group at a time. This technique helps to release tension from muscles and makes the practitioner more aware of the feeling of tension. It is recommended to tense each muscle group and hold for 5 seconds, then exhale and relax for 10 to 20 seconds before moving on to the next muscle group. Most practitioners recommend starting with the lower extremities and ending with the face, abdomen, and chest. However, you can also start at your head and move down your body.
PMR can be practised anywhere from 10 to 20 minutes per day, in a quiet, comfortable area, and with loose, comfortable clothing. It is important to inhale when tensing and exhale when relaxing to avoid holding your breath and causing more tension. This technique can be practised by almost anyone and can be learned from a mental health professional or by following guided audio recordings.
PMR has been found to have therapeutic benefits for various conditions, including headaches, cancer pain, high blood pressure, digestive disturbances, and symptoms of depression and anxiety. It is an excellent tool for learning about the body and the signals it may be sending.
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Frequently asked questions
Tetanus is a sustained contraction that occurs when a series of action potentials occur in rapid succession.
No, cardiac muscles do not show tetanus. This is because cardiac muscles must relax between contractions so the ventricles can fill with blood.
Skeletal muscle operates in bursts of activity, whereas cardiac muscle contracts in a regular pattern.
In cardiac muscles, the long refractory period ensures that the myocardial cell has almost completely relaxed by the time a second action potential can take place, thus preventing tetanus.
