Logan Steele
A sarcomere is the basic unit of striated muscles, which are responsible for converting chemical energy released during ATP hydrolysis into mechanical energy. The sarcomere is the fundamental unit within a muscle that is responsible for contraction. It consists of two main protein filaments, actin and myosin, which are the active structures responsible for muscular contraction. The sliding filament theory describes muscular contraction as actin filaments sliding past myosin filaments, resulting in the contraction of an individual sarcomere.
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What You'll Learn
Sarcomere is the basic unit of striated muscles
A sarcomere is the basic unit of striated muscles, which are complex multicomponent biological systems. They are responsible for converting the chemical energy released during ATP hydrolysis into mechanical work.
Sarcomeres are composed of two main protein filaments, actin and myosin, which are the active structures responsible for muscular contraction. The sliding filament theory describes muscular contraction as actin filaments sliding past myosin filaments, resulting in the contraction of an individual sarcomere. The thick myosin filament contains multiple heads, which attach to the thinner actin filaments to create actin-myosin cross-bridges. This process is similar to a cocked spring, with the myosin head binding to the actin filament and producing a power stroke, which slides the actin filament past the myosin, resulting in force generation and the shortening of the sarcomere.
The striated appearance of muscles is due to the alternation of thick-filament-containing (A-Band) and thin-filament-containing (I-band) regions. The A-band is visible as dark transverse lines across myofibers, while the I-band is seen as lightly staining transverse lines. The Z-line, or Z-disc, is formed by actin molecules and appears as dark lines separating sarcomeres. The H-zone is the area between the M-line and Z-disc, containing only myosin. The M-line is a thin line in the centre of the sarcomere, formed by cross-connecting elements of the cytoskeleton.
The length of the sarcomere affects its function, with longer sarcomeres capable of generating more force and velocity. The sarcomere's structure, particularly the overlap of actin and myosin, gives rise to the length-tension curve, which illustrates how sarcomere force output decreases if the muscle is stretched or compressed, affecting the number of cross-bridges that can form.
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Sarcomere consists of two main protein filaments
A sarcomere is the basic contractile unit of a myocyte (muscle fibre). Each sarcomere is composed of two main protein filaments: actin and myosin. These two proteins are the active structures responsible for muscular contraction. Actin is the most abundant protein in most eukaryotic cells and has a pivotal role in muscle contraction as well as in cell movements. It is the essential building block of the microfilament system. Myosin, on the other hand, is one of the three major classes of molecular motor proteins. These two proteins interact in what is known as the sliding filament theory, which proposes that the active force is generated as actin filaments slide past the myosin filaments, resulting in the contraction of an individual sarcomere.
The sliding filament theory was proposed by scientists who, through the use of high-resolution microscopes, visualised the actin and myosin filaments within a sarcomere. They were able to see the length of the sarcomere when relaxed and its shortening as it contracted, and were able to give names to particular zones. The sarcomere can be divided into A bands (or anisotropic bands) and I bands (or isotropic bands). The A bands are also called the dark band and contain the whole thick filament (myosin) as well as the end of actin filaments. The I bands are called the light band and contain only the thin filament (actin). The thin filament lies between the two thick filaments.
The Z disc is the area where two actin filaments connect and transverse the I bands. The sarcomere can also be described as the structure between the two Z discs. The M line contains the protein called myomesin and it marks the centre of the sarcomere. The H zone is the area between the M line and Z disc and contains only the myosin.
The sliding filament theory can be further explained by an analogy. Imagine standing centred between two bookcases, pulling on two ropes—one per arm—which are tied securely to each bookcase. In a repetitive fashion, you pull each rope toward yourself, regrasp it, and then pull again. Eventually, as you progress through the length of the rope, the bookcases move together and approach you. In this example, your arms are similar to the myosin molecules, the ropes are the actin filaments, and the bookcases are the Z discs to which the actin is secured, which make up the lateral ends of a sarcomere.
The interaction of myosin and actin proteins is at the core of our current understanding of sarcomere shortening. Calcium and ATP are cofactors (non-protein components of enzymes) required for the contraction of muscle cells. ATP supplies the energy, and calcium is required by two proteins, troponin and tropomyosin, that regulate muscle contraction by blocking the binding of myosin to filamentous actin. In a resting sarcomere, tropomyosin blocks the binding of myosin to actin. For myosin to bind to actin, tropomyosin must rotate around the actin filaments to expose the myosin-binding sites.
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Sarcomere shortens like a collapsing telescope during contraction
A sarcomere is the basic unit of striated muscles, which are responsible for converting chemical energy into mechanical work. It is the contractile unit of the muscle fibre, composed of two main protein filaments: actin and myosin. These two filaments are responsible for muscular contraction.
The sliding filament theory describes the process of muscular contraction. According to this theory, active force is generated as actin filaments slide past the myosin filaments, resulting in the contraction of an individual sarcomere. The myosin filament contains numerous heads, which attach to the thinner actin filaments to create actin-myosin cross-bridges. This process is similar to a cocked spring, which, when bound to an actin filament, produces a power stroke. The power stroke then slides the actin filament past the myosin, resulting in force generation and the shortening of the sarcomere.
The shortening of the sarcomere during contraction can be likened to a collapsing telescope. The actin filaments at each end of a central myosin filament slide towards the centre of the myosin, in a process known as myosin-actin cycling. The myosin appears to be performing a molecular dance as it reaches forward to bind to the actin, contracts, and then releases the actin before beginning a new cycle.
The contraction of the myosin's S1 region is known as the power stroke, which requires the hydrolysis of ATP (adenosine triphosphate). This process results in the formation of cross-bridges, which extend from the thick myosin filaments to the thin actin filaments. The simultaneous contraction of sarcomeres joined end-to-end throughout a muscle fibre leads to the shortening of the entire muscle.
In summary, the sarcomere is the fundamental unit of muscle contraction, with its actin and myosin filaments generating force through the sliding filament theory. The shortening of the sarcomere during contraction resembles a collapsing telescope, with the actin and myosin filaments working together in a dynamic and coordinated manner.
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The sliding filament theory explains muscular contraction
A sarcomere is the basic contractile unit of muscle fibre, consisting of two main protein filaments: actin and myosin. These proteins are responsible for muscular contraction and performance.
The sliding filament theory explains the mechanism of muscle contraction, based on muscle proteins that slide past each other to generate movement. The sliding filament theory was independently introduced in 1954 by two research teams: one consisting of Andrew Huxley and Rolf Niedergerke from the University of Cambridge, and the other consisting of Hugh Huxley and Jean Hanson from the Massachusetts Institute of Technology. It was originally conceived by Hugh Huxley in 1953.
According to the sliding filament theory, the myosin (thick filaments) of muscle fibres slide past the actin (thin filaments) during muscle contraction, while the two groups of filaments remain at a relatively constant length. This theory is supported by extensive research in muscle physiology, including studies showing the roles of calcium ions and ATP in muscle contractions.
The sliding of the actin and myosin filaments past each other shortens the overall length of the muscle fibre, causing the muscle to contract. This cycle continues as long as calcium ions and ATP are present, leading to sustained muscle contraction. The force of a muscular contraction is determined largely by the number of actin-myosin cross-bridges that are formed.
The sliding filament theory is a widely accepted explanation of the mechanism that underlies muscle contraction. It provides a detailed molecular mechanism explaining how muscle contraction occurs, whereby actin and myosin filaments slide over each other to produce movement in the body.
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Sarcomere is an important target for drug discovery programmes
A sarcomere is the basic contractile unit of muscle fibre, consisting of two main protein filaments: actin and myosin. These proteins are responsible for muscular contraction. The sliding filament theory describes muscular contraction as actin filaments sliding past myosin filaments, resulting in the contraction of an individual sarcomere.
Sarcomeres are an important target for drug discovery programmes as they are the smallest contractile units of heart and skeletal muscles. The actin and myosin filaments in sarcomeres are organised by a network of proteins that combine structural and signalling functions, forming the sarcomeric cytoskeleton. This includes giant proteins such as titin, obscurin, and nebulin, which have been implicated in sarcomere assembly and the regulation of muscle contractile and metabolic adaptation.
For a long time, sarcomeres were not considered a promising target for drug discovery. However, recent advances in muscle biology and the identification of novel inotropic drugs that act on actin or myosin filaments suggest that small molecule drugs can penetrate the sarcomere and produce medically useful responses in vivo. The development of myosin activators also indicates that sarcomeric enzyme activation is a promising avenue for drug discovery.
Furthermore, many diseases of the heart and skeletal muscle, such as heart failure and muscle atrophy, lack specific and effective treatments. As sarcomeres are the smallest contractile units of these muscles, they can be targeted for the development of pharmacological treatments for these diseases. The identification of key genes important for muscle homeostasis through scientific discovery and human genetic research has also yielded potential targets for drug discovery programmes aimed at hereditary and acquired muscle diseases.
The target-based approach to drug discovery can be effective in developing novel treatments for a validated target. However, the process of target validation is complex and uncertain. Combination drugs that impact multiple targets simultaneously may be more effective in controlling complex disease systems and are less prone to drug resistance.
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Frequently asked questions
A sarcomere is the basic unit of striated muscles, which are responsible for converting chemical energy released during ATP hydrolysis into mechanical work.
A sarcomere is made of two main protein filaments: actin and myosin.
The sliding filament theory is a model that describes muscular contraction. In this theory, active force is generated as actin filaments slide past the myosin filaments, resulting in contraction of an individual sarcomere.
The function of a sarcomere is to generate muscular force through the contraction of actin and myosin filaments. This contraction shortens the muscle, resulting in movement.
