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Muscle fibres are single muscle cells that work together to generate movement in the body and its internal organs. There are three types of muscle tissue in the body: skeletal, smooth, and cardiac. Skeletal muscle is the most common type, comprising 30% to 40% of total body mass. Each skeletal muscle consists of thousands of muscle fibres bundled together and wrapped in connective tissue. These fibres can be classified into two types: Type 1 and Type 2, with the latter further divided into subtypes 2A and 2B. The number of slow and fast-twitch fibres in the body varies between individuals and is determined by genetics. Muscle fibres can contain droplets of fat in the sarcoplasm, and the sarcoplasm also contains most of the muscle fibre's soluble proteins.
| Characteristics | Values |
|---|---|
| Types | Skeletal, cardiac, and smooth muscles |
| Skeletal Muscle Composition | Made up of thousands of muscle fibers wrapped together by connective tissue sheaths |
| Skeletal Muscle Connective Tissue Sheaths | Epimysium, perimysium, and endomysium |
| Skeletal Muscle Fiber Composition | Myofibrils containing multiple myofilaments |
| Myofilaments | Actin and myosin filaments |
| Skeletal Muscle Functions | Controls the voluntary movements of the body |
| Skeletal Muscle Examples | Shoulder muscles, hamstring muscles, and abdominal muscles |
| Cardiac Muscle Characteristics | Branched and interconnected |
| Cardiac Muscle Functions | Generate impulses that cause the cardiac muscle to contract, facilitating the beating of the heart |
| Smooth Muscle Functions | Move food through the digestive tract and change the sizes of the pupil |
| Muscle Fiber Types | Slow oxidative (SO), fast oxidative (FO), and fast glycolytic (FG) |
| Muscle Fiber Characteristics | Can adapt to changing demands by changing size or fiber type composition |
| Type 1 Muscle Fibers | Utilize oxygen to generate energy for movement; have a higher density of mitochondria, making them dark |
| Type 2A Muscle Fibers | Can use oxygen to generate energy but contain less mitochondria, making them light |
| Type 2B Muscle Fibers | Do not use oxygen; store energy for short bursts of movement and contain very little mitochondria, making them white |
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What You'll Learn
- Skeletal muscle fibres contain myofibrils, mitochondria and myoglobin
- Cardiac muscle fibres contain pacemaker cells
- Smooth muscle fibres are found in the internal organs and eyes
- Muscle fibres contain soluble proteins, lipids and acetylcholine
- Skeletal muscle fibres are made of actin and myosin filaments
Skeletal muscle fibres contain myofibrils, mitochondria and myoglobin
Skeletal muscles are the most common type of muscle in the human body, comprising 30% to 40% of total body mass. They are attached to the bones and allow us to perform a wide range of movements and functions. Skeletal muscles are voluntary, meaning we control how and when they work.
Skeletal muscle fibres are composed of myofibrils, which are rod-like organelles of a muscle cell. Each muscle fibre contains several hundred to several thousand myofibrils. These myofibrils are made up of actin (thin filaments), myosin (thick filaments), and support proteins. The arrangement of actin and myosin gives skeletal muscle its microscopic striated appearance and creates functional units called sarcomeres. The sarcomere is the region of a myofibril contained between two cytoskeletal structures called Z-discs (or Z-lines/Z-bands). The thick filaments are anchored at the middle of the sarcomere (the M-line) by a protein called myomesin.
When a muscle contracts, the actin is pulled along the myosin towards the centre of the sarcomere, causing the two filaments to overlap. This leads to the shortening of the sarcomere and the muscle cell, resulting in force generation. The actin and myosin filaments themselves do not change length but slide past each other.
Skeletal muscle fibres also contain mitochondria, which are energy-generating organelles. Type 1 fibres have a higher density of mitochondria, while Type 2A fibres contain less, and Type 2B fibres have the least amount. The presence of mitochondria gives Type 1 fibres a darker appearance compared to the lighter Type 2 fibres.
Additionally, skeletal muscle fibres contain myoglobin, a protein that stores oxygen. Type IIb fibres, also known as white fibres, have low myoglobin content, contributing to their lighter colour.
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Cardiac muscle fibres contain pacemaker cells
Muscle fibres are single muscle cells that work together to generate movement in the body. There are three types of muscle tissue in the body: skeletal, smooth, and cardiac. Cardiac muscle fibres have their own rhythm, which is generated by special cells called pacemaker cells.
Pacemaker cells are highly specialised myocardial cells that directly control heart rate. They generate the impulses that cause cardiac muscle to contract, typically at a constant pace, but with the ability to speed up or slow down as necessary. This property is known as autorhythmicity. The pacemaker cells respond to signals from the autonomic nervous system to increase or decrease the heart rate. They also respond to various hormones that modulate heart rate to control blood pressure.
Pacemaker cells are comprised of sinoatrial (SA) and atrioventricular (AV) nodes, which are known to fire spontaneously, sending electrical activity throughout the heart. They do not require stimulation to initiate their action. This autorhythmicity occurs because of funny current channels, which allow sodium ions to leak continuously into the cell, slowly raising the membrane potential until a certain threshold is reached, causing depolarization of the cell. This subsequently opens calcium channels, causing calcium ions to enter the cell, further raising the membrane potential.
The action potential is then conducted down the cardiac conduction system as an electrical impulse and also between one cardiomyocyte to another through gap junctions. This conduction helps the heart to contract in a synchronized fashion. The wave of contraction that allows the heart to work as a unit, called a functional syncytium, begins with the pacemaker cells.
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Smooth muscle fibres are found in the internal organs and eyes
Smooth muscle fibres are found in the walls of internal organs and the eyes. They are one of the three types of muscle tissues in the human body, the other two being skeletal and cardiac. Smooth muscle is involuntary, meaning it works without conscious thought. It is also non-striated, giving it a more uniform appearance than skeletal muscle.
Smooth muscle is found in the walls of hollow internal organs, such as the stomach, liver, pancreas, bladder, uterus, and intestines. It is also present in the walls of passageways, including arteries and veins of the cardiovascular system. Additionally, it is found in the tracts of the urinary, respiratory, and reproductive systems.
In the eyes, smooth muscle fibres act to change the size of the pupil and the shape of the lens. They are also found in the skin, where they allow hair to stand up in response to cold temperatures or fear.
Smooth muscle consists of thick and thin filaments of actin and myosin, the main proteins involved in muscle contraction. These filaments are not arranged into sarcomeres, resulting in a non-striated pattern. Smooth muscle cells contract more slowly than skeletal muscle cells but are stronger, more sustained, and require less energy. They also have greater elastic properties, which are important in organ systems like the urinary bladder, where contractile tone must be preserved.
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Muscle fibres contain soluble proteins, lipids and acetylcholine
Muscle fibres are single muscle cells that are grouped together to generate movement in the body and internal organs. They are found in three types of muscle tissue: skeletal, smooth, and cardiac. The muscle fibres in these tissues have different characteristics and qualities. For instance, skeletal muscle fibres are attached to the skeleton by tendons and control the body's voluntary movements, such as walking and bending over. Smooth muscle fibres, on the other hand, are involuntary and are found in internal organs and eyes, where they perform functions like moving food through the digestive tract. Finally, cardiac muscle fibres have their own rhythm, contracting in response to impulses generated by special cells called pacemaker cells.
Muscle fibres contain soluble proteins, lipids, and acetylcholine. Soluble proteins, such as troponin I and myosin, have different functional properties in muscle fibres, influencing characteristics like elasticity and moisture retention. The composition and functionality of muscle fibres seem to play a crucial role in lipid metabolism. For example, certain exercise modalities may improve metabolic dyslipidemia by favourably affecting skeletal muscle mass, fibre composition, and functionality. Additionally, muscle fibre composition has been linked to metabolic pathologies, with abnormal compositions observed in individuals with obesity, diabetes, and metabolic dysfunctions.
Acetylcholine is a neurochemical that acts as a neurotransmitter, allowing neurons to communicate with each other and with specialised cells like myocytes. It is most commonly associated with the neuromuscular junction, where motor neurons located in the ventral spinal cord synapse with muscles to activate them. Acetylcholine intervenes in various physiological functions, including regulating cardiac contractions, intestinal peristalsis, and glandular secretion.
The presence of soluble proteins, lipids, and acetylcholine in muscle fibres highlights the complex nature of these cellular components and their integral role in the body's overall functioning.
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Skeletal muscle fibres are made of actin and myosin filaments
Skeletal muscles are one of the three significant muscle tissues in the human body, alongside cardiac and smooth muscle. They are the most common type of muscle in the body, comprising around 30% to 40% of total body mass. Skeletal muscles are attached to bones by tendons, allowing for a wide range of voluntary movements.
Skeletal muscle fibres are composed of myofibrils, which are cylindrical bundles of two types of filaments: thick filaments of myosin and thin filaments of actin. Each myofibril is organised as a chain of contractile units called sarcomeres, which are responsible for the striated appearance of skeletal and cardiac muscle. The sarcomeres are the fundamental contractile unit of a skeletal muscle, and they shorten to produce force. The actin and myosin filaments slide over each other within the sarcomeres, causing the skeletal muscle fibres to contract.
The contraction of skeletal muscle fibres is triggered by nerve impulses, which stimulate the release of calcium ions from the sarcoplasmic reticulum. This increased concentration of calcium ions signals muscle contraction via the action of two accessory proteins bound to the actin filaments: tropomyosin and troponin. Tropomyosin is a fibrous protein that binds lengthwise along the groove of actin filaments, while troponin uses calcium ion binding to regulate when the myosin heads form cross-bridges to the actin filaments.
The interaction between actin and myosin is essential for muscle contraction and plays a central role in cell biology. Myosin is a molecular motor that converts chemical energy in the form of ATP to mechanical energy, generating force and movement. Actin, in association with myosin, is responsible for various types of cell movements, including muscle contraction and cell division.
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Frequently asked questions
Muscle fibers are single muscle cells. They are grouped together to generate movement in your body and internal organs.
There are three types of muscle fibers: slow oxidative (SO), fast oxidative (FO), and fast glycolytic (FG). Most skeletal muscles contain all three types, but in varying proportions.
SO fibers contract slowly and use aerobic respiration (oxygen and glucose) to produce ATP. They are resistant to fatigue and can produce low power contractions over long periods.
FO and FG muscle fibers contract quickly and are responsible for producing rapid, powerful movements. FG fibers, in particular, do not use aerobic metabolism and do not have substantial numbers of mitochondria.
Skeletal muscles are the most common type of muscle in the body and make up 30-40% of total body mass. Examples include shoulder muscles, hamstring muscles, and abdominal muscles.
