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Skeletal muscle

Group 1: Skeletal Muscle Structure and Anatomy – Skeletal muscles are organs of the vertebrate muscular system. – Muscle cells of skeletal muscles are longer […]

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Group 1: Skeletal Muscle Structure and Anatomy

– Skeletal muscles are organs of the vertebrate muscular system.
– Muscle cells of skeletal muscles are longer than other muscle tissue types.
– Skeletal muscle tissue is striated due to sarcomere arrangement.
– Skeletal muscles are voluntary and controlled by the somatic nervous system.
– Human body has over 600 skeletal muscles.
– Men have around 61% more skeletal muscle than women.
– Muscles are in bilaterally-placed pairs.
– Tendons attach muscles to bones for movement.
– Muscle compartments are separated by deep fascia.
– Muscle fibers are multinucleated with myonuclei.
– A muscle fiber can contain hundreds to thousands of nuclei.
– Myosatellite cells can provide additional myonuclei for growth or repair.
– Muscles attach to tendons at the musculotendinous junction.

Group 2: Muscle Fiber Types and Characteristics

– Muscle fibers are broadly categorized as Type I (slow) and Type II (fast).
– Type II fibers include IIA (oxidative) and IIX (glycolytic) divisions.
– Fiber types have distinct metabolic, contractile, and motor properties.
– Most human skeletal muscles contain all three fiber types.
– Fiber types differ in contraction speed, ATP production, and fatigue resistance.
– Muscle fibers can be classified based on color, twitch capabilities, and ATPase.
– Type I fibers appear red due to high myoglobin levels.
– Type II fibers are white and rely more on glycolytic enzymes.
– Fast twitch fibers contract and develop tension faster than slow twitch fibers.
– Slow twitch fibers generate ATP through aerobic energy transfer.

Group 3: Muscle Fiber Growth, Naming, and Architecture

– Exercise stimulates myofibril increase in muscle cells.
– Well-exercised muscles develop more mitochondria, myoglobin, and capillaries.
– Muscle cells cannot divide to produce new cells.
– Adult muscle cells are fewer than in newborns.
– Muscle fibers grow when exercised and shrink when not in use.
– Muscle names indicate size, shape, action, location, orientation, and heads.
– Fixator muscles stabilize prime movers during movement.
– Biceps have two heads, triceps have three, and quadriceps have four.
– Muscle names can indicate location, fascicle orientation, and number of heads.
– Muscle architecture variations include fusiform, strap, convergent, circular, and parallel muscles.

Group 4: Muscle Fiber Typing Methods and Evolution

– Fiber typing methods include ATPase staining and immunohistochemical staining.
– Myosin ATPase activity determines fiber type, while myosin heavy chain staining determines MHC fiber type.
– Most multicellular animals have both slow-twitch and fast-twitch muscle fibers.
– Organisms can shift fiber type proportions through training and environmental responses.
– Different animals have varying muscle fiber type proportions based on muscle group purposes.
– Genetic sequences are preserved across species but may serve different functions.
– The Prdm1 gene regulates slow twitch fiber formation in zebrafish and mice.
– Sox6 indirectly influences the down-regulation of slow muscle genes in mice.

Group 5: Muscle Contraction, Energy Consumption, and Efficiency

– Muscle primary function is contraction.
– Muscle fibers stimulated by motor neurons for contraction.
– Regulatory proteins troponin and tropomyosin enable muscle contraction.
– ATP is stored in resting muscle and used up quickly during contraction.
– Creatine phosphate generates more ATP for about 15 seconds.
– Glucose breakdown in aerobic respiration produces most ATP for muscles.
– Human muscle efficiency in rowing and cycling ranges from 18% to 26%.
– Vertebrate muscles produce 25–33N of force per square centimeter.
– Commands from the brain control voluntary and involuntary muscle movements.
– Proprioception is the sense of body awareness and position in space.

Skeletal muscle (Wikipedia)

Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other types of muscle tissue, and are often known as muscle fibers. The muscle tissue of a skeletal muscle is striated – having a striped appearance due to the arrangement of the sarcomeres.

Skeletal muscle
A top-down view of skeletal muscle
SynonymsSkeletal striated muscle / Striated voluntary muscle
SystemMuscular system
Latinmuscularis skeletalis
Anatomical terminology

Skeletal muscles are voluntary muscles under the control of the somatic nervous system. The other types of muscle are cardiac muscle, which is also striated, and smooth muscle, which is non-striated; both of these types of muscle tissue are classified as involuntary, or, under the control of the autonomic nervous system.

A skeletal muscle contains multiple fascicles – bundles of muscle fibers. Each individual fiber, and each muscle is surrounded by a type of connective tissue layer of fascia. Muscle fibers are formed from the fusion of developmental myoblasts in a process known as myogenesis resulting in long multinucleated cells. In these cells the nuclei, termed myonuclei, are located along the inside of the cell membrane. Muscle fibers also have multiple mitochondria to meet energy needs.

Muscle fibers are in turn composed of myofibrils. The myofibrils are composed of actin and myosin filaments called myofilaments, repeated in units called sarcomeres, which are the basic functional, contractile units of the muscle fiber necessary for muscle contraction. Muscles are predominantly powered by the oxidation of fats and carbohydrates, but anaerobic chemical reactions are also used, particularly by fast twitch fibers. These chemical reactions produce adenosine triphosphate (ATP) molecules that are used to power the movement of the myosin heads.

Skeletal muscle comprises about 35% of the body of humans by weight. The functions of skeletal muscle include producing movement, maintaining body posture, controlling body temperature, and stabilizing joints. Skeletal muscle is also an endocrine organ. Under different physiological conditions, subsets of 654 different proteins as well as lipids, amino acids, metabolites and small RNAs are found in the secretome of skeletal muscles.

Skeletal muscles are substantially composed of multinucleated contractile muscle fibers (myocytes). However, considerable numbers of resident and infiltrating mononuclear cells are also present in skeletal muscles. In terms of volume, myocytes make up the great majority of skeletal muscle. Skeletal muscle myocytes are usually very large, being about 2–3 cm long and 100 μm in diameter. By comparison, the mononuclear cells in muscles are much smaller. Some of the mononuclear cells in muscles are endothelial cells (which are about 50–70 μm long, 10–30 μm wide and 0.1–10 μm thick), macrophages (21 μm in diameter) and neutrophils (12-15 μm in diameter). However, in terms of nuclei present in skeletal muscle, myocyte nuclei may be only half of the nuclei present, while nuclei from resident and infiltrating mononuclear cells make up the other half.

Considerable research on skeletal muscle is focused on the muscle fiber cells, the myocytes, as discussed in detail in the first sections, below. However, recently, interest has also focused on the different types of mononuclear cells of skeletal muscle, as well as on the endocrine functions of muscle, described subsequently, below.

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