Dominic Stone
Muscle fascicles and myofibrils are both structures that make up skeletal muscle. A muscle fascicle is a group of muscle cells (fibres) that are grouped together in parallel within a connective tissue sheath called the perimysium. Myofibrils, on the other hand, are long contractile fibres that are made up of thick and thin myofilaments. These myofilaments are composed of proteins such as actin, myosin, and titin, and they give the muscle its striped appearance. The arrangement of these structures within skeletal muscles contributes to their functional capabilities, force generation, and range of motion.
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What You'll Learn
Myofibrils are the contractile elements in muscle cells
Myofibrils are indeed the contractile elements in muscle cells. They are long contractile fibres, with groups of myofibrils running parallel to each other on the long axis of the myocytes (long single multinucleated cells that combine to form the muscle). The myocytes themselves also run parallel to each other on the long axis of the cell.
Myofibrils are made up of thick and thin myofilaments, which give the muscle its striped appearance. The thick filaments are composed of strands of the protein myosin, and the thin filaments are made up of the protein actin, along with two other muscle regulatory proteins, tropomyosin and troponin. Under the influence of adenosine triphosphate (ATP), actin and myosin form a contractile compound, actomyosin, which is required for muscle contraction.
The sarcomere is the smallest functional unit of a skeletal muscle fibre and is a highly organised arrangement of contractile, regulatory, and structural proteins. It is the fundamental unit of contraction and the shortening of these individual sarcomeres leads to the contraction of the individual muscle fibres, and ultimately, the whole muscle. Myofibrils are composed of sarcomeres linked in series. The sarcomeres have light and dark regions that give the cell its striated appearance.
The number of myofibrils in a muscle fibre varies with its contractile type and cross-sectional area. Each myofibril is 1 to 3 μm in diameter and has a polygonal shape. The myofibrils make up more than 50% of the total protein in the cell.
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Muscle fascicles are bound together by fascia to form skeletal muscle
Skeletal muscles are composed of connective tissue, blood vessels, and nerves. They are made up of hundreds or even thousands of muscle fibres bundled together and wrapped in a connective tissue covering. Each muscle fibre is a single cylindrical muscle cell, and they are organised into groups called fascicles. Each bundle of muscle fibres, or fascicles, is surrounded by a layer of connective tissue called the perimysium.
The perimysium is the middle layer of connective tissue in skeletal muscle, with the endomysium being the innermost layer and the epimysium being the outermost. The endomysium surrounds each individual muscle fibre within a fascicle, and the epimysium surrounds the entire muscle. The connective tissue covering provides support and protection for the muscle cells, allowing them to withstand the forces of contraction.
Muscle fascicles are bound together by the perimysium to form skeletal muscle. The perimysium is a layer of connective tissue that surrounds each bundle of muscle fibres, or fascicles, within the skeletal muscle. It is the middle layer of connective tissue, with the endomysium being the innermost layer and the epimysium being the outermost layer. The perimysium provides support and protection for the muscle fibres, and it also contains larger vessels, nerve trunks, and sensory neuromuscular spindles.
The arrangement of fascicles in skeletal muscle determines the type of movement a muscle can make. Fascicles can be arranged in different ways, including parallel, circular, convergent, pennate, fusiform, and triangular. The arrangement of fascicles also affects the muscle's range of motion and ability to generate force. For example, circular muscles act as sphincters, closing orifices, while parallel muscles have fascicles that are arranged in the same direction as the long axis of the muscle.
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Muscle fascicles are composed of muscle cells called myocytes
A muscle fascicle is a bundle of skeletal muscle fibres surrounded by a layer of connective tissue called the perimysium. Each muscle fascicle typically contains only one type of muscle cell, either type I or type II fibres. However, in some cases, they may contain a mixture of both types.
Muscle fibres are composed of myofibrils, which are, in turn, made up of sarcomeres. Sarcomeres are the smallest functional unit of a skeletal muscle fibre and are highly organised arrangements of contractile, regulatory, and structural proteins. The striated appearance of skeletal muscle fibres is due to the arrangement of thick and thin myofilaments within each sarcomere. These myofilaments are composed of actin and myosin proteins, which slide over one another during muscle contraction, causing the Z-bands to move closer together and resulting in the shortening of sarcomeres and the contraction of individual muscle fibres.
The sarcolemma is the plasma membrane of a muscle fibre, and the cytoplasm is referred to as sarcoplasm. The sarcolemma is encased by the endomysium, a thin connective tissue layer of collagen and reticular fibres that surrounds each muscle fibre. The endomysium, along with the perimysium and epimysium, provides support and protection to the muscle fibres, allowing them to withstand the forces of contraction.
Each muscle fibre, or muscle cell, is a single cylindrical muscle cell. An individual skeletal muscle may be made up of hundreds or even thousands of these muscle fibres bundled together and wrapped in a connective tissue covering. This arrangement of muscle fibres into fascicles allows the nervous system to trigger specific movements by activating a subset of muscle fibres within a fascicle.
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Myocytes contain small strands called myofibrils
Myocytes, or muscle cells, contain small strands called myofibrils. These myofibrils are composed of proteins and are responsible for regulating muscle contraction and relaxation. They are the machinery or motor that drives contraction and relaxation in muscles. Myofibrils are approximately 1 μm in diameter and can range from 50 per myocyte in fetal muscles to 2000 per myocyte in the muscles of untrained adults.
Myofibrils are made up of thick and thin myofilaments, which give muscles their striped appearance. The thick filaments are composed of strands of the protein myosin, while the thin filaments are made up of the protein actin, along with the regulatory proteins tropomyosin and troponin. These thick and thin filaments are arranged in a hexagonal pattern and are divided into thick and thin myofilaments, which are parallel to the long axis of the muscle fibres.
The smallest fundamental contractile unit of the myofibril is called the sarcomere. Sarcomeres are highly organised arrangements of contractile, regulatory, and structural proteins. They are the smallest functional unit of a skeletal muscle fibre and are responsible for the contraction of individual skeletal muscle fibres. Sarcomeres are connected in series within the myofibril, and their shortening leads to the contraction of the muscle fibre.
The sarcomere is defined as the region of a myofibril between two cytoskeletal structures called Z-discs, or Z-lines. The Z-discs are composed of α-actinin, a structural support protein. The thick filaments are anchored at the middle of the sarcomere, or the M-line, by a protein called myomesin. The thin filaments, composed primarily of actin, are attached to the Z-discs and extend into the centre of the sarcomere.
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Myofibrils are surrounded by sarcoplasm (muscle cell cytoplasm)
Myofibrils are the long fibres that make up a muscle fibre or cell. They are composed of myofilaments of actin, myosin, and other associated proteins, which are organised into sarcomeres, the functional contractile region of the myocyte. The sarcomere is the smallest functional unit of a skeletal muscle fibre and is a highly organised arrangement of contractile, regulatory, and structural proteins. The actin and myosin filaments slide over each other to cause the shortening of sarcomeres and the cells to produce force, leading to muscle contraction.
Myofibrils are surrounded by sarcoplasm, the muscle cell cytoplasm. The sarcoplasm makes up 30-40% of the fibre volume and contains elements of the transverse (T) tubular system and sarcoplasmic reticulum (SR), mitochondria, lysosomes, glycogen granules, and often, fat droplets. The sarcoplasm is rich in myoglobin and glycogen, which store the glucose and oxygen required for energy generation.
The sarcoplasmic reticulum acts as a sink of Ca+ ions, which are released upon signalling from the transverse tubules. The T tubules play an important role in supplying the myocyte with Ca+ ions, which are key for muscle contraction. The sarcoplasm also contains the terminal cisternae, which are expanded regions of the sarcoplasmic reticulum that store calcium. The gap between the terminal cisternae and T tubules is bridged by a tetragonal arrangement of foot processes called junctional feet, which maintain the architecture of the triad during contraction and mediate the coupling of sarcolemmal excitation with the release of calcium for muscle contraction.
The sarcolemma is the plasma membrane of the muscle fibre or cell and surrounds the sarcoplasm. It is similar to other cell membranes but forms deep, tubular invaginations in skeletal muscle. The sarcolemma maintains the intracellular milieu, actively transports substrates into the muscle cell, serves as a docking location for proteins, and transmits neural excitatory impulses that lead to muscle contraction.
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Frequently asked questions
A muscle fascicle is a group of muscle cells (fibres) that are grouped together in parallel within a connective tissue sheath called the perimysium. A whole muscle belly is then composed of all the muscle fascicles grouped together by an outer layer of connective tissue called the epimysium.
A myofibril is a long contractile fibre of a muscle cell. Each myofibril has a diameter of between 1 and 2 micrometres (μm). They are composed of long proteins, including actin, myosin, and titin.
A muscle fascicle is a group of muscle cells (fibres) that are bundled together, whereas a myofibril is a long contractile fibre of a muscle cell.
The fascicular organisation of muscle fascicles is common in muscles of the limbs, allowing the nervous system to trigger a specific movement of a muscle by activating a subset of muscle fibres within a fascicle of the muscle.
Myofibrils are responsible for muscle contractions. They are composed of thick and thin myofilaments that slide over one another, causing the muscle to contract.
