Dominic Stone
The human body is made up of three types of muscles: skeletal, cardiac, and smooth. Skeletal muscles are attached to bones and give the body structure and strength. Cardiac muscles comprise the walls of the heart, allowing blood to be pumped through the vasculature. Smooth muscles are found throughout the body, including in the blood vessels, gastrointestinal tract, bronchioles, uterus, and bladder. Smooth muscles do not contain sarcomeres, which are the smallest functional unit of striated muscle tissue. Sarcomeres are composed of long, fibrous proteins that slide past each other when a muscle contracts or relaxes.
| Characteristics | Values |
|---|---|
| Muscle Tissue Type | Smooth Muscle |
| Sarcomere Definition | The smallest functional unit of striated muscle tissue |
| Sarcomere Composition | Two main protein filaments (actin and myosin) |
| Muscle Contraction | Occurs when actin and myosin filaments slide past each other |
| Calcium Ions | Expose binding sites on actin, allowing myosin to bind |
| Muscle Relaxation | Occurs when muscle fibers return to a low-tension state |
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What You'll Learn
- Smooth muscle fibres use actin and myosin contraction to constrict blood vessels
- Skeletal muscle fibres are organised into groups called fascicles
- Cardiac muscle tissue is a striated muscle fibre under involuntary control
- Myofibrils are composed of repeating sections of sarcomeres
- Sarcomeres are composed of long, fibrous proteins
Smooth muscle fibres use actin and myosin contraction to constrict blood vessels
Mammals have three types of muscles: skeletal, cardiac, and smooth. Skeletal muscles are attached to bones and give the body structure and strength. Cardiac muscles comprise the walls of the heart, allowing blood to be pumped through the vasculature. Smooth muscles, on the other hand, are found throughout the body and serve a variety of functions. They are found in the blood vessels, gastrointestinal (GI) tract, bronchioles, uterus, and bladder.
Smooth muscles are unique in that they can maintain tone for extended periods and often contract involuntarily. They consist of thick and thin filaments that are not arranged into sarcomeres, giving them a non-striated pattern. This is in contrast to skeletal and cardiac muscles, which derive their striated appearance from the arrangement of thick and thin filaments within each sarcomere.
Despite lacking sarcomeres, smooth muscle fibres still use actin and myosin contraction to constrict blood vessels and move the contents of hollow organs in the body. Actin and myosin form continuous chains within the smooth muscle cell, anchored at the dense bodies. The intermediate and thin filaments formed by these chains can stretch to dense bodies located on adjacent smooth muscle cells, forming a mesh-like network.
The interaction between actin and myosin filaments is responsible for muscle contraction, based on the sliding filament model. For a muscle cell to contract, the protein tropomyosin must be moved to uncover the binding sites on the actin. Calcium ions play a crucial role in this process, binding with troponin C molecules and altering the structure of tropomyosin. This forces the binding sites on actin to be exposed, allowing myosin to bind and contraction to occur.
Smooth muscle contraction is regulated differently from skeletal and cardiac muscle. Instead of relying on the binding of calcium to the troponin complex, smooth muscle utilizes calmodulin, an intracellular second messenger that binds calcium. The contraction is sustained as long as calcium remains bound to calmodulin, allowing for prolonged periods of vasoconstriction in blood vessels.
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Skeletal muscle fibres are organised into groups called fascicles
Skeletal muscles are enclosed in connective tissue scaffolding at three levels. Skeletal muscle fibres are organised into groups called fascicles, surrounded by a middle layer of connective tissue called the perimysium. Each muscle fibre is covered by endomysium, and the entire muscle is covered by epimysium. When a group of muscle fibres is "bundled" as a unit within the whole muscle, it is called a fascicle.
Fascicles are covered by a layer of connective tissue called perimysium. Fascicle arrangement is correlated to the force generated by a muscle and affects the muscle's range of motion. Based on the patterns of fascicle arrangement, skeletal muscles can be classified in several ways. For example, parallel muscles have fascicles that are arranged in the same direction as the long axis of the muscle. The majority of skeletal muscles in the body have this type of organisation.
Fascicles can be arranged in different ways, such as parallel, circular, convergent, pennate, fusiform, or triangular. Each arrangement has its own range of motion and ability to do work. For example, a unipennate muscle like the extensor digitorum of the forearm has fascicles on only one side of the tendon, while a bipennate muscle like the rectus femur has fascicles on both sides of the tendon. Multipennate muscles, like the deltoid muscle of the shoulder, have fascicles that insert into multiple tendons, converging on a common tendon.
The organisation of skeletal muscle fibres into fascicles allows the nervous system to trigger specific movements by activating a subset of muscle fibres within a fascicle. Each skeletal muscle fibre is supplied by the axon branch of a somatic motor neuron, which signals the fibre to contract. This is unlike cardiac and smooth muscle, which can contract without nervous system input.
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Cardiac muscle tissue is a striated muscle fibre under involuntary control
There are three types of muscle tissue in the human body: skeletal, cardiac, and smooth muscle. Skeletal muscles are attached to bones and are under voluntary control. Cardiac and smooth muscle are involuntary muscles controlled by the autonomic nervous system.
Cardiac muscle tissue is a type of striated muscle fibre. Striated muscle fibres contain actin and myosin filaments that power contraction and are organised into repeating arrays, called sarcomeres, with a striated microscopic appearance. The sarcomere is the smallest functional unit of striated muscle tissue. It is the repeating unit between two Z-lines, which appear as dark lines under a light microscope.
The interaction between actin and myosin filaments in the A-band of the sarcomere is responsible for muscle contraction. The protein tropomyosin covers the myosin-binding sites of the actin molecules in the muscle cell. For a muscle cell to contract, tropomyosin must be moved to uncover the binding sites on the actin. Calcium ions bind with troponin C molecules, altering the structure of the tropomyosin and forcing it to reveal the cross-bridge binding site on the actin.
Cardiac muscle contraction occurs via excitation-contraction coupling (ECC), which is the process of converting an electrical stimulus into a mechanical response. Calcium-induced calcium release (CICR) involves the conduction of calcium ions into the cardiomyocyte, leading to the further release of ions into the cytoplasm. Calcium ions bind to troponin C molecules, causing tropomyosin to detach from the myosin-binding sites on actin. Actin and myosin then form a cross-bridge, and contraction occurs.
Cardiac muscle tissue is located in the walls of the heart and is responsible for the contractility of the heart and the pumping action.
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Myofibrils are composed of repeating sections of sarcomeres
Smooth muscle tissue lacks sarcomeres. This tissue is found in the blood vessels, gastrointestinal tract, bronchioles, uterus, and bladder.
Now, onto the topic of myofibrils and their composition. Myofibrils are rod-like organelles of a muscle cell. They are composed of long proteins, including actin, myosin, and titin, and other proteins that hold them together. These proteins are organised into thick, thin, and elastic myofilaments, which repeat along the length of the myofibril in sections or units of contraction called sarcomeres.
Sarcomeres are the smallest functional unit of a skeletal muscle fibre. They are the repeating units between two Z-lines, with each sarcomere being around 3 μm in length. The thick myosin filaments and thin actin filaments are organised into repeating arrays, giving the cell its striated appearance. The thick and thin filaments are organised in a parallel fashion along the entire length of the myofibril, with the sarcomeres appearing under the microscope as alternating dark and light bands.
The interaction between actin and myosin filaments in the A-band of the sarcomere is responsible for muscle contraction. When a muscle contracts, the actin is pulled along the myosin towards the centre of the sarcomere, with the two filaments sliding past each other. This shortening of the sarcomeres leads to the contraction of individual skeletal muscle fibres and ultimately the whole muscle.
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 growth in the girth of muscle fibres can occur through the splitting of myofibrils, which can be stimulated by the development of stress on the sarcomere.
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Sarcomeres are composed of long, fibrous proteins
Smooth muscle fibres do not contain sarcomeres. Smooth muscle fibres are found throughout the blood vessels, gastrointestinal tract, bronchioles, uterus, and bladder.
Sarcomeres are the smallest functional unit of skeletal muscle fibres. They are composed of long, fibrous proteins that slide past each other when a muscle contracts or relaxes. The two important proteins that make up sarcomeres are myosin and actin. Myosin forms the thick filaments, while actin forms the thin filaments.
Myosin has a long fibrous tail and a globular head that binds to actin. The thick filaments are anchored at the middle of the sarcomere (the M-line) by a protein called myomesin. The lighter I band regions contain thin actin filaments anchored at the Z-discs by a protein called α-actinin. The thin filaments extend into the A band toward the M-line and overlap with regions of the thick filament. The A band is dark due to the thicker myosin filaments and their overlap with the actin filaments.
The interaction between actin and myosin filaments in the A-band of the sarcomere is responsible for muscle contraction. For a muscle cell to contract, the protein tropomyosin must be moved to uncover the binding sites on the actin. Calcium ions bind with troponin C molecules, altering the structure of the tropomyosin and forcing it to reveal the cross-bridge binding site on the actin.
The length of the actin and myosin filaments (taken together as sarcomere length) affects force and velocity. Longer sarcomeres have more cross-bridges and thus more force, but have a reduced range of shortening.
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Frequently asked questions
Smooth muscle tissue lacks sarcomeres.
A sarcomere is the smallest functional unit of a skeletal muscle fiber. It is composed of two main protein filaments, actin and myosin, which are responsible for muscular contraction.
Smooth muscle tissue is found throughout the body in blood vessels, the gastrointestinal tract, bronchioles, uterus, and bladder. It is under involuntary control by reflexes and the body's autonomic nervous system. Skeletal muscle tissue, on the other hand, is attached to bones and gives the body structure and strength. It is under voluntary control.
