Alec Titan
Sarcomeres are the most basic contractile units in muscles, responsible for producing force through the interaction of actin and myosin. They are composed of two types of filaments: thin filaments containing actin, troponin, and tropomyosin, and thick filaments containing myosin. These filaments slide past each other during muscle contraction, with the thick and thin myofilaments giving skeletal muscles their distinctive striated appearance. The length of sarcomeres is an important indicator of muscle function, with non-uniformities in length contributing to various muscle properties and functions. While the ordered structure of mature sarcomeres is well-established, the mechanism governing their assembly is still a subject of ongoing research and debate.
| Characteristics | Values |
|---|---|
| Definition | A sarcomere is a microscopic segment repeating in a myofibril |
| Composition | Sarcomeres are made up of contractile myofilaments (myosin and actin) and a series of structural proteins (e.g., titin, nebulin, desmin, myomesin, α-actinin, dystrophin, and tropomyosin) |
| Function | Sarcomeres are the most basic contractile units in muscles, responsible for force generation through the interaction of actin and myosin |
| Length | Instantaneous sarcomere length (SL) is an important indicator of the functional properties of muscles, such as force and power |
| Theories of Contraction | The sliding filament theory and the cross-bridge theory are widely accepted, but some studies propose new theories to explain observations that deviate from these paradigms |
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What You'll Learn
Sarcomere length non-uniformities
A sarcomere is the most basic contractile unit in muscles. It is made up of contractile myofilaments (myosin and actin) and a series of structural proteins. The thick (myosin) and thin (actin) myofilaments give skeletal muscles their striated appearance.
The non-uniformity in a myofibril, which is made up of serially arranged sarcomeres, can be characterised by SLs in two distinct groups. One set of sarcomeres relies primarily on active force, while the other set relies exclusively on passive force. These non-uniformities cause the myofibril to produce a force proportional to the filament overlap for the shorter sarcomeres, resulting in a greater force than predicted from an isometric contraction with a uniform SL.
SL non-uniformities have been observed to increase when myofibrils transition from the passive to the active state. This suggests that the mechanisms governing these non-uniformities may be associated with the variable number of contractile proteins and the adjustable stiffness of titin filaments in individual sarcomeres. However, it is unclear if these observations hold true for sarcomeres in whole muscles or if they are specific to isolated fibres and myofibrils.
While the sliding filament and cross-bridge theories have been widely accepted for explaining muscle contraction, there are studies that present results that cannot be readily explained by these theories. These studies propose new theories or models of muscle contraction, highlighting the need for further investigation into the underlying mechanisms of sarcomere length non-uniformities and their functional implications.
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Muscle contraction theories
The sliding filament theory is the most widely accepted explanation for muscle contraction. It proposes that muscle fibres contract when myosin filaments pull actin filaments closer together, causing a shortening of the sarcomeres within the fibre. This movement results in the sliding of actin past myosin, generating muscle tension and force. The myosin filaments use energy from ATP to "walk" along the actin filaments with their cross-bridges, pulling the actin filaments and Z-discs closer together, ultimately shortening the sarcomere.
The cross-bridge theory is another accepted paradigm that focuses on the role of cross-bridges in the sliding of actin filaments. It suggests that the rotation of cross-bridges causes the actin filaments to slide past the myosin filaments. This theory highlights the interaction between the cross-bridges and actin filaments in generating muscle tension.
While these theories provide valuable insights, some studies have presented results that cannot be fully explained by them. For instance, certain forces produced during muscle contractions, particularly at extended sarcomere lengths, exceed those predicted by the sliding filament theory. Additionally, the presence of passive forces modulated by activation and Ca2+ influences the force-length relationship in ways that are not fully accounted for by the existing theories. These findings have led to proposals for new models and theories of muscle contraction, aiming to address the observed discrepancies.
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Sarcomere structure
A sarcomere is a microscopic segment repeating in a myofibril. The fundamental structure of a sarcomere consists of two main contractile myofilaments: thick filaments called myosin and thin filaments called actin. These filaments interact by sliding past each other in response to stimuli, causing muscle contraction.
The thick and thin myofilaments give skeletal muscles their striated appearance, which can be observed with light microscopy. The maximal isometric force that can be generated by sarcomeres depends on the amount of overlap between these filaments, which can be predicted using the theoretical force-length relationship. The instantaneous sarcomere length (SL) and the rate of change in SL are important indicators of the functional properties (e.g., force, power) of a muscle.
Sarcomeres are composed of several proteins organized in a three-dimensional lattice, optimally designed for active and passive force generation. In addition to myosin and actin, several other proteins play crucial roles in regulating, structuring, and functioning sarcomeres. These include tropomyosin, troponin, titin, nebulin, myomesin, α-actinin, and dystrophin.
Tropomyosin, a protein present in thin filaments, regulates the interaction between actin and myosin. In a relaxed muscle fiber, tropomyosin blocks the myosin-binding sites on actin. Troponin, a complex of three proteins attached to tropomyosin, has a binding site for calcium ions. When calcium binds to troponin, it causes a conformational change that moves tropomyosin away from the myosin-binding sites on actin, allowing for the interaction between actin and myosin.
Titin, another important protein, runs from both ends of a sarcomere to its middle, anchoring thick filaments and providing elasticity to the whole unit. Nebulin is a large protein thought to regulate the assembly of actin filaments and, therefore, the length of thin filaments. Myomesin forms the M line, connecting and anchoring the thick filaments, while α-actinin, found in the Z-discs, helps anchor the thin filaments. Dystrophin is a structural protein that connects thin filaments to integral membrane proteins in the sarcolemma, transmitting tension generated by sarcomeres to tendons.
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Sarcomere mechanics
Sarcomeres are the most basic contractile units in muscles. They are made up of contractile myofilaments (actin and myosin) and a series of structural proteins (e.g. titin, nebulin, and desmin). The thick (myosin) and thin (actin) myofilaments give skeletal muscles their striated appearance, which can be observed with light microscopy.
The sliding filament theory and the cross-bridge theory are the two widely accepted theories of muscular contraction. The sliding filament theory proposes that the active force is generated as actin filaments slide past the myosin filaments, resulting in the contraction of an individual sarcomere. The cross-bridge theory, on the other hand, proposes that the sliding of actin filaments is caused by the rotation of cross-bridges.
However, there are studies that show results that cannot be explained by these theories. For example, some studies have shown that forces produced at long sarcomere lengths are higher than those predicted by the sliding filament theory. Additionally, there are passive forces at long sarcomere lengths that can be modulated by activation and Ca2+, which changes the force-length relation.
The instantaneous sarcomere length (SL) and the rate of change in SL are important indicators of the functional properties (e.g. force, power) of a muscle. SL non-uniformities have been thought to be the cause of a variety of muscle properties and functions, such as the residual force enhancement and force depression properties.
Sarcomeres are responsible for the generation of active and passive forces in muscles. The main source of both active and passive tension is the cardiomyocyte, which actively generates force, pumping blood into the vascular system, and then relaxes, allowing the passive filling of the ventricles. The sarcomere dynamics are exemplified by the regulation of titin-based stiffness and the titin life cycle.
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Sarcomere assembly
Sarcomeres are the basic contractile units of striated muscle cells. They are composed of thick and thin protein filaments, with the thick filaments made of myosin protein and the thin filaments made of actin protein. The sarcomere is a repeating unit of interdigitating actin and myosin filaments that serves as the building blocks of the myofibrils in striated muscle cells. Sarcomeres are highly organized structures composed of a series of thick and thin protein filaments that give skeletal and cardiac muscles their characteristic striated appearance.
The assembly of sarcomeres is a complex and ordered process involving an array of structural and associated proteins. While the mechanism of sarcomere assembly is not yet fully understood, particularly in cardiomyocytes, studies have shed light on the process. For instance, it has been observed that in cultured human induced pluripotent stem cell-derived cardiomyocytes (hiCMs), sarcomeres appear several hours after the acquisition of actin-containing muscle stress fibers (MSFs).
The assembly process involves the incorporation of α-actinin into the sarcomere Z-discs, which are located at each end of the sarcomere. The thin actin filaments are attached to these Z-discs, while the thick filaments composed of myosin are found in the center of the sarcomere, forming the A-bands. The coordinated performance of actin and myosin proteins is essential for the function of the sarcomere, enabling muscle contraction and relaxation.
In addition to actin and myosin, other proteins such as troponin and tropomyosin are also present in the sarcomere. These proteins play a regulatory role, preventing the binding of actin and myosin filaments until a signal for muscle contraction and energy via ATP hydrolysis is received. The sarcomere's highly organized structure and the interplay of its various proteins allow it to generate force and signal with other cellular compartments, making it one of the most complex macromolecular assemblies in biology.
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Frequently asked questions
A sarcomere is a microscopic segment repeating in a myofibril. It is made up of two types of filaments: thick filaments (containing myosin) and thin filaments (containing actin, troponin, and tropomyosin).
Sarcomeres are the most basic contractile units in muscles. They produce force through the interaction of actin and myosin.
The force produced by a sarcomere depends on the amount of overlap between the thick and thin myofilaments. The sliding filament theory proposes that sarcomeres shorten during activation by the sliding of actin filaments over myosin filaments.
Sarcomere length (SL) can be measured using phase-contrast light microscopy and non-linear laser microscopy. SL is an important indicator of the functional properties of a muscle, such as force and power.
