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Muscle fibres are found in skeletal, cardiac, and smooth muscles, and they work to cause movement in the body. Skeletal muscle is the most common type of muscle in the human body, comprising 30% to 40% of total body mass. Skeletal muscles are attached to bones by tendons and allow for a wide range of movements and functions. They consist of flexible muscle fibres that contract to enable bones to move. Each muscle can contain thousands of fibres, which are bundled together to form sarcomeres, the fundamental contractile unit of skeletal muscle. The speed at which a muscle contracts is determined by how quickly it acts on ATP, a molecule that releases energy when it breaks down. Muscle fibres can be classified into three types: slow oxidative (SO), fast oxidative (FO), and fast glycolytic (FG).
| Characteristics | Values |
|---|---|
| Types | Skeletal, cardiac, and smooth muscles |
| Composition | Actin (thin filaments), myosin (thick filaments), and support proteins |
| Functions | Cause movement in the body, maintain body temperature, store nutrients, and stabilize joints |
| Contraction | Occurs due to depolarization, a change in electric charge |
| Speed | Depends on how quickly it acts on ATP |
| Types of Skeletal Muscle Fibers | Slow oxidative (SO), fast oxidative (FO), and fast glycolytic (FG) |
| SO Fibers | Slow-twitching fibers with low glycogen content and low fatigue rate |
| FO Fibers | Produce energy using oxygen, resulting in slower fatigue |
| FG Fibers | Produce rapid and forceful contractions, but fatigue quickly |
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Skeletal muscle
The connective tissue sheath covering each fasciculus (bundle of muscle fibres) is known as perimysium, and the innermost sheath surrounding individual muscle fibres is known as endomysium. The outermost sheath of connective tissue covering each muscle is called epimysium. Capillaries and nerve tissue are present within the endomysium to supply the individual muscle fibres. The primary artery supplying blood to a particular skeletal muscle generally courses parallel to the longitudinal axis of the muscle fibre.
The speed at which a skeletal muscle contracts is determined by how quickly it acts on adenosine triphosphate (ATP) molecules, which are used to power the movement of the myosin heads. ATP is a molecule that releases energy when it is broken down. Fast-twitch (FT) fibres break down ATP twice as fast as slow-twitch (ST) fibres. FT fibres are best suited for short-duration, intense movements such as sprinting and weightlifting, while ST fibres are good for long-lasting activities such as running, cycling, and swimming.
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Cardiac muscle
Muscle fibres are found in skeletal, cardiac, and smooth muscles. Cardiac muscle, also called heart muscle or myocardium, is one of three types of vertebrate muscle tissues, the others being skeletal and smooth muscle. It is an involuntary, striated muscle that constitutes the main tissue of the wall of the heart.
The cardiac muscle forms a thick middle layer between the outer layer of the heart wall (the pericardium) and the inner layer (the endocardium). The endocardium is not cardiac muscle and is made up of simple squamous epithelial cells, which form the inner lining of the heart chambers and valves. The pericardium is a fibrous sac surrounding the heart, consisting of the epicardium, pericardial space, parietal pericardium, and fibrous pericardium. The cardiac muscle is responsible for the contractility of the heart and, therefore, the pumping action.
The primary function of cardiac muscle is to pump blood into circulation by generating sufficient force. The mechanism behind each coordinated contraction involves the cardiac muscle and electrical impulses. These contractile functions of the heart require ATP, which can be obtained through various substrates, including fatty acids, carbohydrates, proteins, and ketones.
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Smooth muscle
In the body's organ systems, smooth muscle contributes to the contractile tone of the urinary bladder, ensuring it remains relaxed and only flexes when necessary. Smooth muscle is also responsible for goosebumps, as it causes hairs to stand on end. In the eyes, smooth muscle controls how the pupils dilate or constrict, and it pulls on zonule fibres to adjust the eye's focus.
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Muscle cramps
Muscle fibres are found in skeletal, cardiac, and smooth muscles. They work together with muscles to cause movement in the body. Muscle cramps, also known as muscle spasms, are painful, involuntary contractions or tightening of these muscles. They can occur anywhere in the body, but they are most common in the legs. Cramps can last from a few seconds to several minutes and can be treated with muscle relaxants, stretching, and massage. They usually occur after exercise or at night, and can be more common in pregnant women, people over 65, and people with obesity. While muscle cramps are usually harmless, they can sometimes indicate an underlying neurological condition, especially if accompanied by other symptoms such as muscle weakness or poor coordination. In such cases, it is important to seek medical advice.
To prevent muscle cramps, it is important to stretch before exercising and to stay hydrated. Sports drinks can help replace electrolytes after intense exercise or exercising in the heat. In some cases, medication may be prescribed to prevent cramps, but it may not always be effective and can cause side effects.
While muscle cramps are common and usually harmless, they can cause discomfort and affect daily activities. By understanding the causes and treatments for muscle cramps, individuals can better manage and prevent them.
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Muscle fibre types
Muscle fibres are found in skeletal, cardiac, and smooth muscles. They work together with muscles to cause movement in the body.
Skeletal muscle fibres are the most common type of muscle fibres and are attached to bones by tendons, allowing for skeletal movement. They also provide structural support, help maintain posture, and act as a source of energy during starvation. Skeletal muscle fibres can be classified into three types based on two criteria: the speed of contraction and the method of regenerating ATP.
The first type is slow oxidative (slow twitch or Type I) fibres, which contract slowly and use aerobic respiration (oxygen and glucose) to produce ATP. These fibres are good for endurance activities as they fatigue slowly and can produce long-lasting, low-power contractions. The second type is fast oxidative (fast twitch or Type IIa) fibres, which contract quickly and also use aerobic respiration to generate ATP. The third type is fast glycolytic (fast twitch or Type IIx) fibres, which have rapid contractions and rely on anaerobic glycolysis to produce ATP. Fast glycolytic fibres fatigue quickly and are suitable for activities requiring short bursts of strength.
It is important to note that skeletal muscles in the human body usually contain all three types of fibres, but in varying proportions. The speed of contraction depends on how quickly myosin's ATPase breaks down ATP. Additionally, fibres that use oxygen to produce energy (aerobic respiration) fatigue more slowly than those that don't (anaerobic glycolysis).
Skeletal muscle fibres are susceptible to injuries, such as stretching or tearing, commonly occurring during sports or accidents. They are also influenced by biological factors like age and hormone levels, which can affect muscle hypertrophy.
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Frequently asked questions
Muscle fibers are found in skeletal, cardiac, and smooth muscles.
Skeletal muscles are the most common type of muscle in the body, comprising 30% to 40% of your total body mass. They are attached to bones and allow you to perform a wide range of movements.
Muscle fiber types can be categorized as slow oxidative (SO), fast oxidative (FO), and fast glycolytic (FG). Skeletal muscles typically contain all three types, with varying proportions.
Muscle fibers work with muscles to cause movement in the body. They contract, leading to the movement of bones. Additionally, they provide structural support, help maintain posture, and act as an energy source.
