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The heart is a network of cardiac cells that are connected by intercalated discs. The cells are arranged in layers of myocardial tissue that wrap around the chambers of the heart. The heart, therefore, consists mostly of muscle. However, unlike skeletal muscle, the heart exhibits rhythmic contractions. Tetanus is 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 to prevent this sustained contraction.
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What You'll Learn
The heart's contractility and rhythmicity
The heart is a network of highly branched cardiac cells that are connected end-to-end by intercalated discs. These cells are organised into layers of myocardial tissue that wrap around the chambers of the heart. The heart consists mostly of muscle, with myocardial cells (collectively called the myocardium) arranged in ways that set the heart apart from other muscle types.
The heart's contractility is influenced by the arrangement of myocardial cells and their ability to generate force and shortening in bands of muscle. This results in a decrease in heart chamber size and the consequent ejection of blood into the pulmonary and systemic vessels. The calcium ions play a crucial role in this process, as they are released by the terminal cisternae during muscle stimulation and are then removed from the sarcoplasm by the sarcoplasmic reticulum, a process that requires energy from the breakdown of ATP.
The rhythmicity of the heart is equally important, as it ensures the coordination of the heart's pumping action. The heart exhibits rhythmic contractions, which means it can adjust the amount of blood pumped per minute (cardiac output) to meet the varying metabolic needs of peripheral tissues, including the muscle, kidney, brain, skin, liver, gastrointestinal tract, and the heart itself.
Preventing tetanus in the heart is crucial because cardiac muscles must relax between contractions to allow the ventricles to fill with blood. The longer myocardial action potential of the myocardium helps prevent sustained contractions, known as tetanus, from occurring in the heart.
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Calcium ions and their role in tetanus
The role of calcium ions in tetanus is a complex topic that has been the subject of numerous scientific studies. While the focus of these studies varies, many involve experiments on frog skeletal muscle fibres.
One such study examined the effect of tetanus duration on the free calcium during the relaxation of frog skeletal muscle fibres. It was observed that with increasing tetanus duration, the rate of decline of calcium ions decreased during the first period of muscle relaxation. This decrease in the rate of decline is believed to be due to an intracellular calcium buffer becoming loaded with calcium during the tetanus. During the second period, the concentration of calcium ions was elevated above resting levels, and the mean level depended on the tetanus duration. A long-lasting elevation in calcium ions was also observed during the third period, reflecting the release of calcium from the intracellular calcium buffer.
Another study used electron-probe analysis to investigate calcium release and ionic changes in the sarcoplasmic reticulum of tetanized muscle. It was found that during a 1.2-second tetanus, approximately 59% of the calcium content in the terminal cisternae (TC) was released, leading to a significant increase in the total cytoplasmic calcium concentration. This release of calcium was accompanied by a notable uptake of magnesium and potassium into the TC. However, the amount of calcium released exceeded the total measured cation accumulation.
Additionally, the role of calcium ions in tetanic seizures or tetany, which is characterised by involuntary muscle contractions, has also been explored. Tetany is typically caused by a deficiency of calcium ions, resulting in increased neuronal membrane permeability to sodium ions. This, in turn, leads to progressive depolarization and an increased likelihood of action potentials, ultimately causing contractions of peripheral skeletal muscles.
In summary, the studies outlined above highlight the critical role of calcium ions in the development and progression of tetanus. By understanding the complex interactions between calcium ions, muscle fibres, and neuronal activity, scientists can gain valuable insights into the mechanisms underlying tetanus and explore potential therapeutic interventions.
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The difference between skeletal and heart muscle
Cardiac muscle, also called heart muscle or myocardium, is one of three types of vertebrate muscle tissues, the others being skeletal muscle and smooth muscle. Cardiac muscles are found in the heart and are self-stimulating, with an intermediate speed of contraction and energy requirements. They are responsible for performing involuntary muscular movements.
On the other hand, skeletal muscles are attached to bones all over the body and do not self-stimulate. They have a high speed of contraction and energy requirements, and they are responsible for performing voluntary muscular movements.
The two types of muscle can be differentiated based on their structure, functions, and other features. For example, the sarcolemma, transverse tubules, and sarcoplasmic reticulum serve as major sources of contraction-dependent calcium in both cardiac and skeletal muscle, but the mechanisms by which calcium is made available to and utilized by the myofibrils differ between the two types. In skeletal muscle, excitation-contraction coupling is considered to be mediated by a voltage-dependent charge movement within the region of the sarcolemma (T-tubule): junctional sarcoplasmic reticulum. In contrast, excitation in cardiac muscle is linked to contraction by a process of calcium-induced calcium release, with an influx of calcium into the cell that triggers the release of more calcium from the sarcoplasmic reticulum to activate the myofibrils.
Additionally, cardiac muscle cells are roughly rectangular when viewed through a microscope, and they are joined at their ends by intercalated discs to form long fibers. Intercalated discs are complex adhering structures that connect the single cardiomyocytes to an electrochemical syncytium, allowing action potentials to spread between cardiac cells by permitting the passage of ions between them. In contrast, skeletal muscle becomes a multicellular syncytium during embryonic development.
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The role of the myocardium
The myocardium is a vital component of the human heart, playing a critical role in its structure and function. It is one of the three major categories of muscles in the human body, alongside smooth and skeletal muscles. The myocardium is the middle muscular layer of the heart, composed of specialised muscle cells called cardiomyocytes. These cells possess unique characteristics that enable the myocardium to contract and relax in a synchronised manner.
The myocardium's primary function is to facilitate the contraction and relaxation of the heart walls. This contraction and relaxation process is essential for receiving and pumping blood into the systemic circulation. The myocardium contracts in response to the electrical rhythms generated by the heart, as detected by an electrocardiogram (ECG or EKG). This electrical activity is initiated by pacemaker cells, specifically the sinoatrial (SA) and atrioventricular (AV) nodes, which spontaneously send electrical impulses throughout the heart.
The specialised cardiomyocytes of the myocardium have distinct cellular features, including intercalated discs with gap junctions. These gap junctions facilitate fast cell-to-cell communication, enabling the myocardium to act as a functional unit with coordinated contractions. The myocardium's contractile function is further supported by the presence of sarcomeres, which allow for its contractility. However, unlike skeletal muscle, the myocardium's contractions are involuntary and occur independently of conscious control.
Additionally, the myocardial cells serve as a scaffold for the heart chambers, providing structural support and maintaining the shape of the heart. The myocardium lies between the inner endocardium, which forms the inner lining of the heart chambers and valves, and the outer epicardium, which makes up the visceral pericardium surrounding and protecting the heart. Together, these layers ensure the heart's proper functioning and protect it from external influences.
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Tetanus in the mammalian heart
Studies have been conducted to investigate the occurrence of tetanus in the mammalian heart, specifically examining the shrew myocardium. These studies have provided valuable insights into the understanding of tetanus in ventricular muscle.
One key finding is that tetanus can occur in the shrew ventricular muscle when subjected to specific conditions. During experiments, a regular twitch with a cycle length of 2000 ms was followed by a train of stimuli with an internal cycle length of less than 50 to 60 ms. This resulted in the development of both unfused and fused tetanus. The action potentials were generated at the same frequency as the stimuli within the train. As the duration of the train increased, the tension during tetanus surpassed the tension of the preceding regular twitch.
Similar characteristics of tetanus were also observed during an arrhythmic train, which was occasionally triggered spontaneously by a regular stimulus. Simultaneous recordings of the action potential and the twitch revealed that the shrew ventricular muscle is susceptible to tetanization. This occurs because the action potential duration is significantly shorter than the duration of the mechanical event, allowing the action potential to terminate before any mechanical activity commences.
Additionally, studies have compared the shrew ventricular muscle with that of a guinea pig. It was found that the shrew ventricular muscle exhibits shorter tension development and relaxation times, as well as shorter twitch durations. Furthermore, the ventricular myosin of the shrew was discovered to possess high Ca2+-activated ATPase activity and is composed of α-type heavy chains, specifically the V1 variety of myosin. These findings contribute to our understanding of the mechanical properties and myosin type in heart muscle.
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Frequently asked questions
Tetanus is a sustained contraction that occurs when a series of action potentials occur in rapid succession.
It is important to prevent tetanus in the heart because myocardial muscles must relax between contractions so the ventricles can fill with blood.
The longer myocardial action potential of the myocardium helps prevent tetanus.
Skeletal muscle and heart muscle differ in that heart muscle exhibits rhythmic contractions.
Tetanus can occur in the shrew myocardium, in which the ventricular action potential is similar to that of skeletal muscle.
