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The human body is composed of around 600 muscles, which are made up of thousands of small fibres woven together. These fibres are soft tissue, and they stretch and press together to move our organs and body. There are three types of muscle tissue in vertebrates: skeletal, smooth, and cardiac. Skeletal muscles are attached to bones by tendons and are responsible for voluntary movements. Smooth muscles are found within the walls of organs and structures such as the uterus and blood vessels, and they contract involuntarily. Cardiac muscle, also known as myocardium, makes up the middle layers of the heart and contract involuntarily.
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What You'll Learn
- Skeletal muscle is made of bundles of muscle fibres called myofibers
- Cardiac muscle is striated, like skeletal muscle, and is found only in the heart
- Smooth muscle is non-striated and involuntary, and is found in the walls of organs
- Muscle fibres are made of contractile proteins actin and myosin, which interact to cause movement
- Muscle fibres are also made of support proteins, including titin, desmin, myomesin, and nebulin
Skeletal muscle is made of bundles of muscle fibres called myofibers
Skeletal muscle is a highly organised tissue composed of bundles of muscle fibres called myofibers. Each myofiber is a muscle cell with its basic cellular unit, the sarcomere. 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 sarcomere is defined as the region of a myofibril (a bundle of muscle fibres) contained between two cytoskeletal structures called Z-discs (or Z-lines or Z-bands). The striated appearance of skeletal muscle fibres is due to the arrangement of thick and thin myofilaments within each sarcomere. The dark striated A band is composed of thick filaments containing myosin, while the lighter I band regions contain thin actin filaments anchored at the Z-discs by a protein called α-actinin.
The interaction of these proteins results in muscle contraction. When a skeletal muscle fibre is activated by a motor neuron, cross-bridges form between the thick and thin filaments, and the thin filaments are pulled, sliding past the thick filaments within the sarcomere. This shortens the sarcomere, leading to the contraction of individual skeletal muscle fibres and ultimately the whole muscle.
Each skeletal muscle contains multiple fascicles, which are bundles of muscle fibres. These fibres are formed from the fusion of developmental myoblasts in a process known as myogenesis, resulting in long, multinucleated cells. The nuclei, or myonuclei, are located along the inside of the cell membrane, and each nucleus supports the volume of cytoplasm in that particular section of the myofiber.
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Cardiac muscle is striated, like skeletal muscle, and is found only in the heart
Muscle is a soft tissue that is composed of thousands of small fibres woven together. There are three types of muscle tissue in the body: skeletal, smooth, and cardiac.
Cardiac muscle, also called the myocardium, is one of the three major categories of muscles found within the human body. It is found only in the walls of the heart and makes up the thick middle layer of the heart. It is composed of cardiac muscle cells, which perform highly coordinated actions that keep the heart pumping and blood circulating throughout the body.
Cardiac muscle is striated, like skeletal muscle, and contains sarcomeres in highly regular arrangements of bundles. The stripes occur due to alternating filaments that comprise myosin and actin proteins. The dark stripes indicate thick filaments that comprise myosin proteins, while the thin, lighter filaments contain actin. When a cardiac muscle cell contracts, the myosin filament pulls the actin filaments toward each other, causing the cell to shrink.
Unlike skeletal muscle, cardiac muscle is under involuntary control. The individual cardiac muscle cell (cardiomyocyte) is a tubular structure composed of chains of myofibrils, which are rod-like units within the cell. The outside of the cardiomyocyte is surrounded by a plasma membrane called the sarcolemma. Cardiomyocytes are the individual cells that make up the cardiac muscle. The primary function of cardiomyocytes is to contract, which generates the pressure needed to pump blood through the circulatory system.
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Smooth muscle is non-striated and involuntary, and is found in the walls of organs
Smooth muscle is one of the three major types of muscle tissue, the others being skeletal and cardiac muscle. It is non-striated, meaning it has no sarcomeres and therefore no stripes. Smooth muscle is also involuntary, and its movement is not consciously controlled. It is found in the walls of organs and structures such as the oesophagus, stomach, intestines, bronchi, uterus, urethra, bladder, blood vessels, and the arrector pili in the skin that control the erection of body hair. It is also present in the urinary tract, the digestive tract, and the tracts of the respiratory, urinary, and reproductive systems.
Smooth muscle consists of thick and thin filaments that are not arranged into sarcomeres, giving it a non-striated pattern. It contains a large amount of actin and myosin, the main proteins involved in muscle contraction. Actin filaments attach to dense bodies spread throughout the cell. The calcium-containing sarcoplasmic reticulum aids in sustaining contraction. Smooth muscle is fusiform, round in the centre, and tapers at each end. It can tense and relax and has greater elastic properties than striated muscle. This is important in organ systems like the urinary bladder, where contractile tone must be preserved.
Smooth muscle is controlled by the autonomic nervous system, which uses hormones, neurotransmitters, and other receptors to control it spontaneously. It is also regulated by the central nervous system and innervation from the peripheral plexus or endocrine (hormonal) activation. Smooth muscle contributes to many different tissues throughout the body, including the regulation of blood vessels.
Smooth muscle is found in the walls of hollow visceral organs, except the heart, and appears spindle-shaped. It is broadly divided into two types: single-unit and multi-unit smooth muscle. Most smooth muscle is of the single-unit type, and it contracts in a coordinated fashion, causing the whole muscle to contract or relax.
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Muscle fibres are made of contractile proteins actin and myosin, which interact to cause movement
Cardiac muscle tissue is found only in the walls of the heart as myocardium. It is an involuntary muscle, meaning it contracts and relaxes without our conscious awareness. Smooth muscle is also involuntary and is found within the walls of organs and structures such as the oesophagus, stomach, intestines, bronchi, uterus, urethra, bladder, blood vessels, and the arrector pili in the skin.
Each muscle fibre is made up of blocks of proteins called myofibrils, which contain a specialised protein (myoglobin) and molecules to provide the oxygen and energy required for muscle contraction. Myofibrils contain filaments that fold together when given the signal to contract. This shortens the length of the muscle fibre, which, in turn, shortens the entire muscle if enough fibres are stimulated at the same time. The two most significant myofilaments are actin and myosin filaments, which are arranged distinctively to form various bands on the skeletal muscle.
Actin filaments are double-helical structures, known as filamentous-actin (F-actin), composed of monomeric G-actin units. The G-actin exhibits polarity and creates a positive and negative end within the sarcomere, with the positive end situated toward the terminal end of the sarcomere. Myosin proteins are composed of two regions: light meromyosin and heavy meromyosin. Light meromyosin binds other light meromyosin regions to anchor myosin at the M line. Heavy meromyosin is further subdivided into two regions: the S-1 portion, or the myosin head, binds actin and contains an ATPase portion, while the S-2 portion is the location of the power stroke.
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Muscle fibres are also made of support proteins, including titin, desmin, myomesin, and nebulin
Muscle fibres are composed of thousands of small fibres woven together. These fibres are made of contractile proteins called actin and myosin, which interact to cause movement. Muscle fibres are also made of support proteins, including titin, desmin, myomesin, and nebulin.
Support proteins within the sarcomere include titin, desmin, myomesin, C protein, nebulin, and plectin. Plectin tethers the Z discs of adjacent myofibrils to each other. Desmin helps maintain myofibril alignment, connects to the cytoskeleton and other structural elements within the cell, and distributes contractile force. Myomesin and C protein are myosin-binding proteins that function to tether and stabilise myosin at the M line.
Titin is found at the Z disc and anchors myosin longitudinally within the sarcomere. A single molecule of titin stretches from the Z line to the M line with its entire N-terminus in the Z line. Titin molecules of adjacent sarcomeres overlap in the Z line and M line. The portion of titin in the I band is believed to be elastic and to act as a molecular ruler, having a role in passive tension during stretching of the myofibrils. Three to six titin molecules are associated with each myosin filament and there may be lateral associations with actin. It has a binding site for calpain-3.
Nebulin is a very long protein that is associated with the thin filaments. It is specific to skeletal muscle and is oriented in reverse with its C-terminus anchored in the Z line and extending into the I band. It makes side-to-side contact with titin and may have a role in regulating the length of the actin filaments. It may also serve as a template for thin-filament assembly and may interact with tropomyosin.
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Frequently asked questions
The human body has about 600 muscles.
Muscles are made of thousands of small fibres woven together. These fibres are called myofibrils, which contain many contractile units known as sarcomeres.
Sarcomeres are the fundamental contractile units of a skeletal muscle. They consist of a central M line, with thick myofilaments of myosin attached to either side.
Myofibrils are composed of actin thin filaments and myosin thick filaments and support proteins. They are bundled tightly together and wrapped in a thin transparent membrane called a perimysium.
The three main types of muscle are skeletal, smooth and cardiac.
