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Muscle fibres are found in the three types of muscle tissue in the human body: skeletal, cardiac, and smooth muscle. Skeletal muscle is the most common type of muscle in the body, comprising around 30-40% of total body mass. These muscles are attached to bones by tendons and enable a wide range of movements and functions. They are also under our voluntary control. Cardiac muscles, on the other hand, are found only in the heart and are involuntary, contracting in a coordinated manner to facilitate the heartbeat. Smooth muscles are also involuntary and are found in internal organs and eyes, performing functions like moving food through the digestive tract and adjusting pupil size. Each type of muscle fibre has unique characteristics and functions, contributing to the overall movement and stability of the human body.
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What You'll Learn
Skeletal muscle fibres
Human skeletal muscle is made up of cells known as myocytes, or muscle fibres. Each muscle fibre consists of a single muscle cell, and when grouped together, they facilitate the movement of limbs and tissues.
Type 2 fibres are further divided into subtypes: type 2A and type 2B. Type 2A fibres are considered fast-twitch (FT) and type 2B fibres are also referred to as superfast or super-fast-twitch fibres. Type 2 fibres do not use oxygen to generate energy. Instead, they store energy that can be used for short bursts of movement. Type 2B fibres contain very few mitochondria and appear white. They are used for quick and forceful movements such as weightlifting, sprinting and jumping.
The contractile unit of skeletal muscle fibres is the sarcomere, which runs from Z line to Z line. The striated appearance of skeletal muscle fibres is due to the arrangement of two contractile proteins: actin (thin filament) and myosin (thick filament). The outermost sheath of connective tissue covering each muscle is called the epimysium. Each muscle is made up of groups of muscle fibres called fascicles, which are surrounded by a layer of connective tissue called the perimysium.
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Cardiac muscle fibres
Muscle fibres consist of a single muscle cell and help to control the physical forces within the body. Cardiac muscle fibres, also known as myocardium, are one of the three major categories of muscles in the human body, the other two being smooth muscle and skeletal muscle. Cardiac muscle fibres are only found in the heart and are responsible for its pumping and relaxing functions. They are involuntary, meaning that they are automatic and a person cannot control them.
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Smooth muscle fibres
Smooth muscle plays a crucial role in various organ systems in the body. In the digestive system, it helps move food through the digestive tract and aids in nutrient collection. In the urinary system, it facilitates the elimination of waste and toxins, maintaining the contractile tone in the urinary bladder. Smooth muscle is also essential for regulating blood pressure and tissue oxygenation in the cardiovascular system. It is found in the walls of arteries and veins, where it helps regulate blood flow and oxygen supply to tissues.
Additionally, smooth muscle is involved in the respiratory system, where it helps to relax the airway smooth muscle in asthmatic patients. In the reproductive system, it is found in the tracts of the reproductive organs. Smooth muscle fibres are even present in the eyes, where they play a role in adjusting the size of the pupil and the shape of the lens.
At the cellular level, smooth muscle consists of thick and thin filaments of actin and myosin, which are not arranged into sarcomeres. These filaments form continuous chains within the smooth muscle cells, anchored at dense bodies. The actin filaments are stretched between these dense bodies and attachment plaques at the cell membrane, while the myosin filaments lie between them. The calcium-containing sarcoplasmic reticulum aids in sustaining contraction. Smooth muscle cells are innervated by the visceral nervous system, allowing them to contract and relax spontaneously.
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Muscle fibre types
Skeletal muscle fibres, commonly referred to as muscles, are part of the voluntary muscular system. They are attached to bones by tendons, allowing for skeletal movement. Skeletal muscle fibres can be classified based on two criteria: the speed of contraction and how they regenerate ATP.
There are three main types of skeletal muscle fibres: slow oxidative (Type I), fast oxidative (Type IIa), and fast glycolytic (Type IIx). Slow oxidative fibres contract slowly and use aerobic respiration to produce ATP. They are used for endurance activities such as running, cycling, or swimming. On the other hand, fast oxidative fibres contract quickly and also use aerobic respiration to generate ATP. They produce higher tension contractions than slow oxidative fibres.
Fast glycolytic fibres, also known as fast-twitch fibres, have the fastest contraction speed among the three types. They rely on anaerobic glycolysis to generate ATP quickly and produce powerful, high-tension contractions. However, they fatigue quickly and are only suitable for short-duration activities. These fibres are responsible for rapid and forceful movements.
It is important to note that most skeletal muscles contain all three types of fibres but in varying proportions. The presence of different fibre types allows for the wide variety of capabilities that human muscles exhibit. Additionally, muscle fibres can adapt to changing demands by modifying their size or fibre type composition. This adaptability forms the basis for physical therapy interventions aimed at improving patients' force development and endurance.
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Muscle fibre cells
Muscle fibres consist of a single muscle cell, which help to control the physical forces within the body. When grouped together, they facilitate the organised movement of limbs and tissues. Each muscle fibre contains smaller units made up of repeating thick and thin filaments, which cause the muscle tissue to be striated (or have a striped appearance).
Skeletal muscle fibres are classified into two types: type 1 and type 2. Type 2 is further broken down into subtypes 2A and 2B. Type 1 fibres are slow-twitch fibres, which use oxygen to generate energy for movement. Type 2A and 2B fibres are fast-twitch fibres, which do not use oxygen to generate energy. Instead, they store energy that can be used for short bursts of movement. Type 2 fibres have less mitochondria and appear white.
Slow-twitch fibres are good for long-lasting activities such as holding a posture, stabilising bones and joints, and endurance activities like running, cycling, or swimming. They are slow to fatigue. On the other hand, fast-twitch fibres are used to produce rapid, forceful contractions for quick, powerful movements. However, they fatigue quickly and can only be used for short periods.
Smooth muscle cells control involuntary movements such as peristalsis contractions in the oesophagus and stomach. They are found in the walls of hollow organs, including the stomach, intestines, bladder, and uterus, as well as in the walls of blood vessels and in the tracts of the respiratory, urinary, and reproductive systems. Smooth muscle cells have a single nucleus and are non-striated, giving them a more uniform appearance.
Cardiac muscle fibres are striated and are only found in the heart. They have their own rhythm and are branched and interconnected. Special cells called pacemaker cells generate the impulses that cause cardiac muscle to contract, which can speed up or slow down as necessary.
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Frequently asked questions
Skeletal muscle fibers are the most common type of muscle fiber in the body, comprising 30% to 40% of total body mass. They are attached to bones by tendons and allow for a wide range of movements and functions.
Cardiac muscle fibers are only found in the heart. They contract in a coordinated way to allow the heart to beat.
Smooth muscle fibers are found in internal organs and the eyes. They are involuntary, meaning you can't control them. They perform functions such as moving food through the digestive tract and changing the size of your pupils.
