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The human body contains three types of muscle tissue: skeletal, smooth, and cardiac. Skeletal muscles, which are attached to the bones, are the most common type of muscle in the body, making up 30-40% of our total body mass. They are also known as striated muscles due to their striped appearance under a microscope. This striated appearance is caused by the regular arrangement of contractile proteins (actin and myosin) along the length of the myofibrils. The sarcomeres, which are the functional units of skeletal muscle, are all the same length and are composed of repeating bands of dark A bands and light I bands. The giant protein titin is responsible for determining the resting sarcomere length and ensuring that the A band remains in the middle of the sarcomere during contraction. Another protein, obscurin, may also play a role in maintaining the cross-striated appearance of skeletal muscle.
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What You'll Learn
Repeating bands of proteins
Skeletal muscles are the most common type of muscle in the human body, accounting for between 30% and 40% of total body mass. They are attached to the bones and control locomotion and any movement that can be consciously controlled. When viewed under a microscope, skeletal muscle tissue has a striped or striated appearance.
The striated appearance of skeletal muscle is caused by the repeating bands of the proteins actin and myosin that are present along the length of myofibrils. Myofibrils are composed of smaller structures called myofilaments, which include thick filaments and thin filaments. Thick filaments occur only in the A band of a myofibril, while thin filaments attach to a protein in the Z disc called alpha-actinin and occur across the entire length of the I band and partway into the A band. The region where thick and thin filaments overlap has a dense appearance due to the minimal space between them. Notably, the extraction of myosin from myofibrils abolishes the A bands, and further removal of actin leaves only the Z discs, resulting in a loss of striation.
The giant protein titin, discovered in the 1970s, is essential for maintaining the striated appearance of skeletal muscle. Titin, an elastic protein about 1 µm in length, runs from the Z disc to the middle of the A band, ensuring that each sarcomere is the same length. It also determines the resting elasticity of the sarcomere, maintains the position of the A band in the centre of the sarcomere during contraction, and likely influences thick-filament length. Another protein, obscurin, may also play a role in maintaining the cross-striated appearance of skeletal muscle.
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Actin and myosin filaments
The striated appearance of skeletal muscle is caused by repeating bands of the proteins actin and myosin, which are present along the length of myofibrils. Myofibrils are cylindrical bundles of actin and myosin filaments.
The sliding filament theory, proposed in 1954, states that muscle tension is generated by the sliding of actin past myosin. Actin is tethered to structures called Z discs or Z bands, located at the lateral ends of each sarcomere. When the actin filament length shortens, so does the sarcomere and, consequently, the muscle.
The globular end of each myosin protein nearest actin, called the S1 region, has multiple hinged segments that facilitate contraction. The S1 region bends, allowing myosin to "walk" along actin. The tail region of myosin (S2) exhibits flexibility and rotates in conjunction with the S1 contraction. Myosin reaches forward, binds to actin, contracts, releases actin, and then reaches forward again to bind to actin in a new cycle—this process is known as myosin-actin cycling.
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The role of titin
Skeletal muscles are long and cylindrical in appearance, and when viewed under a microscope, they exhibit a striped or striated appearance. This is due to the regular arrangement of contractile proteins known as actin and myosin. These proteins form repeating bands, with dark A bands and light I bands running along the length of myofibrils, resulting in the striated appearance of skeletal muscles.
Myofibrils are composed of smaller structures called myofilaments, including thick filaments and thin filaments. Thick filaments are found only in the A band of a myofibril, while thin filaments attach to a protein called alpha-actinin in the Z disc and extend across the I band and partially into the A band. The region where thick and thin filaments overlap creates a dense appearance due to the close packing of the filaments.
The giant protein titin plays a crucial role in maintaining the striated appearance of skeletal muscles. Titin is the third most abundant protein in vertebrate striated muscles, constituting approximately 10-15% of the total muscle protein pool. It is involved in several essential functions that contribute to the striated appearance:
- Determining Resting Sarcomere Length: Titin is responsible for determining the resting length of sarcomeres, the functional units of skeletal muscle. By ensuring that all sarcomeres are the same length, titin contributes to the uniform striated pattern.
- Centering the A Band: Titin helps maintain the position of the A band in the middle of the sarcomere during contraction. This positioning of the A band is crucial for the striated appearance.
- Thick Filament Length Determination: Titin likely plays a role in determining the length of thick filaments. By influencing the length of the thick filaments within the A band, titin indirectly affects the overall striated pattern.
- Stabilization of Thick Filaments: Titin binds to the thick filaments, providing stability and preventing their depolymerization. This stabilizing role helps maintain the structural integrity of the striated pattern.
- Molecular Ruler: Titin may act as a molecular ruler, influencing the length of the thick filaments and, consequently, the overall organization of the sarcomere.
- Stiffness Adjustment: Titin adjusts its stiffness in response to activation-dependent calcium levels and active force demands. This ability to modulate stiffness contributes to the mechanical performance of striated muscles.
- Passive Force Contribution: While titin's passive force contribution may vary depending on muscle type, it can potentially influence the mechanical properties of striated muscles during contraction and force production.
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Sarcomere length
Sarcomeres are the smallest functional unit of striated muscle tissue. They are composed of long, fibrous proteins that slide past each other when a muscle contracts or relaxes. The length of a sarcomere is determined by the length of the actin and myosin filaments it contains. The sarcomere length affects the force and velocity of the muscle—longer sarcomeres have more cross-bridges and thus more force, but have a reduced range of shortening.
The absolute sarcomere length non-uniformities were greatest for the longest average sarcomere lengths. In a study by Pincheira et al. (2021), median sarcomere lengths in the biceps femoris ranged from about 2.5 to 4.0 µm across their subjects. Another study on rabbit psoas myofibrils divided the sarcomeres into three groups based on their initial average sarcomere lengths: short (2.7 µm), intermediate (3.2 µm), and long (3.6 µm).
The protein titin plays a crucial role in determining the resting sarcomere length, ensuring that the A band remains in the middle of the sarcomere during contraction, and possibly determining thick-filament length. Titin is the largest single highly elasticated protein found in nature, providing binding sites for numerous proteins. It acts as a molecular ruler and a blueprint for the assembly of the sarcomere.
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Skeletal muscle vs cardiac muscle
Skeletal muscle and cardiac muscle are both types of striated muscle tissue. When viewed under a microscope, skeletal muscle tissue has a striped or striated appearance. This is caused by the regular arrangement of contractile proteins (actin and myosin) that are present along the length of myofibrils. Dark A bands and light I bands repeat along the myofibrils, and the alignment of myofibrils in the cell causes the entire cell to appear striated or banded.
Skeletal muscles are attached to bones all over the body and are responsible for performing voluntary muscular movements. They are under voluntary control and have a high speed of contraction and energy requirement.
Cardiac muscle cells are located in the walls of the heart and are responsible for performing involuntary muscular movements. They are self-stimulating and have an intermediate speed of contraction and energy requirement. Cardiac contractions pump blood throughout the body and maintain blood pressure.
Both skeletal and cardiac muscles have a striated appearance due to the presence of repeating bands of the proteins actin and myosin. However, there are several differences between the two types of muscle. Firstly, skeletal muscles are under voluntary control, while cardiac muscles are involuntary. Secondly, skeletal muscles have a high speed of contraction and energy requirement, while cardiac muscles have an intermediate speed and energy requirement. Thirdly, skeletal muscles are attached to bones and responsible for movements like breathing and posture maintenance, while cardiac muscles are located in the heart and responsible for pumping blood throughout the body.
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Frequently asked questions
Striated muscles are the most common type of muscle in the human body. They make up between 30% and 40% of total body mass. They are attached to the bones and are responsible for functions like chewing, swallowing, breathing, and moving your bones.
The striated appearance of skeletal muscles is due to repeating bands of the proteins actin and myosin that are present along the length of myofibrils. Dark A bands and light I bands repeat along myofibrils, and the alignment of myofibrils in the cell causes the entire cell to appear striated or banded.
Shoulder muscles, hamstring muscles, and abdominal muscles are all examples of skeletal muscles. Cardiac muscles are also striated.
