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Muscle structure refers to the composition of the three types of muscles in the body: skeletal, cardiac, and smooth muscle. Skeletal muscles, which are under the direct control of the cerebral cortex of the brain, are voluntary muscles that allow for a wide range of movements and functions. They are composed of muscle fibres, also known as myocytes, which are stimulated by motor neurons to contract. Cardiac and smooth muscles are involuntary muscles that are controlled by the autonomic nervous system. The structure of muscle tissue is one of the four major tissue types, providing movement and heat generation to the organs of the body.
| Characteristics | Values |
|---|---|
| Muscle types | Skeletal, cardiac and smooth muscle |
| Most common muscle type | Skeletal muscle |
| Skeletal muscle composition | Skeletal muscle is made up of hundreds or thousands of muscle fibres bundled together and wrapped in connective tissue |
| Skeletal muscle shape | Varies depending on fascicular architecture and fibre length |
| Skeletal muscle size | Varies from a few hundred to hundreds of thousands of muscle fibres |
| Skeletal muscle mass | Makes up 30-40% of total body mass |
| Skeletal muscle control | Voluntary |
| Cardiac muscle control | Involuntary |
| Smooth muscle control | Involuntary |
| Muscle fibre composition | Myofibrils, actin, myosin, troponin, tropomyosin, titin, nebulin |
| Muscle fibre shape | Long and cylindrical |
| Muscle fibre size | Up to 100 μm in diameter and 30 cm in length |
| Muscle fibre function | Contraction |
| Muscle fibre structure | Sarcolemma (plasma membrane), sarcoplasm (cytoplasm) |
| Myofibril structure | Striated pattern of light and dark regions, composed of sarcomeres |
| Sarcomere structure | Contractile, regulatory and structural proteins |
| Sarcomere function | Contraction of sarcomeres leads to contraction of muscle fibres |
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What You'll Learn
Skeletal muscle structure
Skeletal muscles are the most common type of muscle in the body, comprising 30% to 40% of total body mass. They are attached to bones and allow us to perform a wide range of movements and functions. These muscles are voluntary, meaning we control how and when they work.
Each skeletal muscle is made up of hundreds or thousands of muscle fibres bundled together and wrapped in a connective tissue covering. Each muscle fibre is a single cylindrical muscle cell. The connective tissue covering provides support and protection for the delicate muscle cells and allows them to withstand the forces of contraction. It also provides pathways for the passage of blood vessels and nerves.
The connective tissue covering is composed of different layers: the outermost layer is called the epimysium, which divides the muscle into compartments. Each compartment contains a bundle of muscle fibres called a fasciculus, which is surrounded by a layer of connective tissue called the perimysium. Within each fasciculus, each individual muscle fibre is surrounded by a layer of connective tissue called the endomysium, the innermost layer.
The sarcomere is the smallest functional unit of a skeletal muscle fibre. It is a highly organised arrangement of contractile, regulatory, and structural proteins. The shortening of these individual sarcomeres leads to the contraction of individual skeletal muscle fibres and ultimately the whole muscle. Before a skeletal muscle fibre can contract, it must receive an impulse from a nerve cell. An artery and at least one vein accompany each nerve that penetrates the epimysium of a skeletal muscle.
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Cardiac muscle structure
There are three types of muscles in the body: skeletal, cardiac, and smooth muscle. Cardiac muscle, also called myocardium or heart muscle, is an involuntary, striated muscle that constitutes the main tissue of the wall of the heart. The heart wall is a three-layered structure with a thick layer of myocardium sandwiched between the inner endocardium and the outer epicardium (also known as the visceral pericardium). The endocardium lines the cardiac chambers, covers the cardiac valves, and joins with the endothelium that lines the blood vessels connected to the heart. The outer epicardium forms part of the pericardial sac that surrounds, protects, and lubricates the heart.
The individual cardiac muscle cell (cardiomyocyte) is a tubular structure composed of chains of myofibrils, which are rod-like units within the cell. The myofibrils consist of repeating sections of sarcomeres, which are the fundamental contractile units of the muscle cells. Sarcomeres are composed of long proteins that organize into thick and thin filaments, called myofilaments. Thin myofilaments contain the protein actin, and thick myofilaments contain the protein myosin. The myofilaments slide past each other as the muscle contracts and relaxes. Neighbouring cardiomyocytes are joined together at their ends by intercalated discs to create a syncytium of cardiac cells. Intercalated discs enable the rapid transmission of electrical impulses through the network, allowing the syncytium to act in a coordinated contraction of the myocardium.
Cardiac muscle cells are surrounded by an extracellular matrix produced by supporting fibroblast cells. Cardiac fibroblasts are vital supporting cells within cardiac muscle. They are largely responsible for creating and maintaining the extracellular matrix that surrounds the cardiomyocytes. Fibroblasts play a crucial role in responding to injury, such as a myocardial infarction.
Cardiac muscle is under involuntary control, with its own pacemaker cells like the sinoatrial (SA) node that spontaneously depolarizes. These depolarizations occur at a consistent pace, but the pacemaker cells can also receive input from the autonomic nervous system to adjust the heart rate depending on the body's requirements.
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Smooth muscle structure
Smooth muscle is one of the three main types of muscle in the body, along with skeletal and cardiac muscle. Unlike skeletal muscle, smooth muscle is involuntary, meaning it works without conscious thought. Smooth muscle is found throughout the body and is involved in many vital functions, including blood circulation, digestion, eyesight, and more.
Smooth muscle is a type of muscle tissue that works automatically and is involved in the body's automated self-management processes. It gets its name from its smooth appearance, which is different from most other types of muscle. Smooth muscle exists in many places inside the body, including the walls of blood vessels and hollow internal organs, such as the stomach, intestines, and uterus. It is also found in the walls of passageways, including arteries and veins of the cardiovascular system, and in the tracts of the urinary, respiratory, and reproductive systems.
In the cardiovascular system, smooth muscle helps regulate blood flow by controlling the diameter of the vessels. Vascular smooth muscle cells, also known as mural cells, wrap around larger vessels and influence blood flow, endothelial network growth, and vessel stability. Smooth muscle activation can lead to the formation of pulmonary hypertension and the development of asthma when it results in airway obstruction.
Smooth muscle is also found in the skin, where it causes hairs to stand on end, as with goosebumps. In the eyes, smooth muscle pulls on fibers to control focus and the dilation and constriction of the pupils. In the uterus, smooth muscle contractions occur during labor. Smooth muscle is susceptible to various conditions, including injuries, paralysis, and anti-smooth muscle autoantibodies, where the immune system attacks and damages the smooth muscle.
Smooth muscle fibers group in branching bundles, allowing for stronger contractions than in striated muscles. The actin filaments are stretched between dense bodies in the cytoplasm and attachment plaques at the cell membrane, with myosin filaments between them. Intermediate filaments such as desmin and vimentin provide additional support to the cell structure. Smooth muscle cells contract slower than skeletal muscle cells but are stronger, more sustained, and require less energy.
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Muscle fibres
The plasma membrane of a muscle fibre is called the sarcolemma, and the cytoplasm is referred to as sarcoplasm. Within a muscle fibre, proteins are organised into organelles called myofibrils, which run the length of the cell and contain sarcomeres connected in series. The sarcomere is the smallest functional unit of a skeletal muscle fibre and is responsible for the contraction of individual skeletal muscle fibres. The arrangement of thick and thin myofilaments within each sarcomere gives skeletal muscle fibres their striated appearance.
There are three types of muscle fibres: slow oxidative (SO), fast oxidative (FO), and fast glycolytic (FG). Most skeletal muscles contain all three types, but the proportions vary. Muscle fibres can adapt to changing demands by altering their size or fibre type composition, which forms the basis for physical therapy interventions aimed at improving a patient's force development or endurance.
The speed of contraction in muscle fibres depends on the rate at which myosin's ATPase hydrolyzes ATP to produce cross-bridge action. Fast fibres have a quicker contraction rate, making them more suitable for sprinting, while slow fibres are better for endurance sports. The number of slow and fast-twitch fibres in an individual is determined by genetics and can be influenced by training.
It is important to note that muscle fibres cannot be increased in number through exercise. Instead, muscles grow larger through a combination of muscle cell growth, the addition of new protein filaments, and the contribution of undifferentiated satellite cells.
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Muscle fascicles
Skeletal muscle is the most common type of muscle in the body, comprising 30% to 40% of total body mass. Each skeletal muscle is made up of hundreds or even thousands of muscle fibres bundled together and wrapped in a connective tissue covering. Each bundle of muscle fibres is called a fasciculus, or muscle fascicle.
A muscle fascicle is a group of muscle cells (fibres) that are grouped together in parallel within a connective tissue sheath called the perimysium. Each muscle fibre is covered by endomysium, and the entire muscle is covered by epimysium. The fascicles are then grouped together by an outer layer of connective tissue called the epimysium to form a whole muscle belly. The fascicles of skeletal muscles are macroscopically visible and based on their orientation, skeletal muscles can be arranged into four basic structural patterns: circular, parallel, convergent, and pennate.
The arrangement of fascicles in the skeletal muscle affects the muscle's range of motion and ability to work. 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. Some parallel muscles are flat sheets that expand at the ends to make broad attachments, such as the sartorius. When a muscle has a widespread expansion over a sizable area and the fascicles come to a single, common attachment point, the muscle is called convergent. An example of a convergent muscle is the pectoralis major, which converges on the intertubercular groove and greater tubercle of the humerus via a tendon.
Pennate muscles blend into a tendon that runs through the central region of the muscle for its whole length, similar to the quill of a feather, with the muscle fascicles arranged like feathers. A unipennate muscle has fascicles located on one side of the tendon, for example, the extensor digitorum of the forearm. A bipennate muscle, such as the rectus femurs, has fascicles on both sides of the tendon, similar to the arrangement of a single feather. Multipennate muscles have fascicles that insert on multiple tendons, tapering towards a common tendon, like multiple feathers converging on a central point. An example of a multipennate muscle is the deltoid muscle of the shoulder.
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Frequently asked questions
Muscles are specialised cells known as muscle cells, myocytes, or muscle fibres. They are responsible for providing movement and heat generation to the organs of the body. There are three types of muscles in the body: skeletal, cardiac, and smooth muscle.
Skeletal muscles are voluntary muscles, meaning an individual can control how and when they move and work. They are the most common type of muscle in the body, comprising 30% to 40% of total body mass. Skeletal muscles are attached to bones and allow for a wide range of movements and functions.
Skeletal muscles are made up of muscle fibres, or myofibers, which are long and cylindrical in shape. Each skeletal muscle fibre is a single cylindrical muscle cell, and each muscle can contain thousands of fibres. These fibres are bundled together and wrapped in a connective tissue covering called the epimysium. The fascicles, or bundles, of skeletal muscles can be arranged into four basic structural patterns: circular, parallel, convergent, and pennate.
