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Structure of skeletal muscles

Structure of skeletal muscles

This animation demonstrates the fine molecular structure and mechanism of muscles.

Biology

Keywords

muscles, muscle function, motion, contraction, relaxation, muscle work, skeletal muscle, striated muscle, muscle fascicle, muscle fibre, myofibril, epimysium, actin, myosin, muscle protein, moving, tendon, sarcomere, human, biology

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Scenes

Structure of skeletal muscles

  • tendon - It connects muscles to the bone. It is made up of fibrous connective tissue.
  • epimysium - A layer of connective tissue which covers the entire muscle.

Fascicle

  • perimysium - A layer of connective tissue which covers the muscle fascicle.

Muscle fibre

  • cell membrane
  • nucleus - Muscle fibre is formed by the fusion of muscle cells, thus it can be considered a single multinuclear cell.
  • motor nerve fibre - It ends in the motor end plate, where the neurotransmitter is acetylcholine. When the muscle is stimulated, the endoplasmatic reticulum in the muscle fibre releases calcium ions, which causes contractions.
  • endoplasmatic reticulum - It is an important calcium storage. When the muscle is stimulated, the lumen (cavity) of the endoplasmatic reticulum releases calcium ions into the cell plasma, which produces a contraction in the muscle.
  • mitochondrion - An organelle that produces ATP. Muscles require a lot of energy in the form of ATP to function.

Myofibril

  • sarcomere
  • myosin - It is a protein that plays an indispensable role in muscle contractions, together with actin.
  • actin - It is a protein that plays an indispensable role in muscle contractions, together with myosin.
  • Z-line

Actin and myosin

  • Z-line
  • myosin - It is a protein that plays an indispensable role in muscle contractions, together with actin.
  • actin - It is a protein that plays an indispensable role in muscle contractions, together with myosin.
  • H-zone - It only contains myosin. It becomes shorter when the muscle contracts.
  • A-band - It contains myosin. Its length does not change when the muscle contracts.
  • I-band - It only contains actin. It becomes shorter when the muscle contracts.
  • sarcomere

Molecular mechanism

  • myosin head - When bound to actin, its inclination changes and it pulls the actin filament. It needs ATP to function.
  • actin - It is a protein that plays an indispensable role in muscle contractions, together with myosin.
  • ADP
  • phosphate
  • ATP - It is the most important energy source of cells. It splits into ADP and phosphate while releasing energy. The myosin head needs this to release the actin.
  • troponin - It binds calcium ions, which causes the tropomyosin to move and the myosin head binds to the actin.
  • calcium ions - They are released from the endoplasmatic reticulum when the muscle is stimulated. The ions are bound by troponin, which causes the tropomyosin to move and the myosin-binding parts of the actin become uncovered. Myosin then binds to the actin.
  • tropomyosin - It covers the myosin-binding parts of the actin when the muscle is in a relaxed state. When the troponin binds calcium ions, the tropomyosin moves and myosin binds to the actin.
  • Stimulus generates an increase in calcium level.
  • Muscle contraction
  • Muscle relaxation

Animation

  • tendon
  • epimysium
  • perimysium
  • muscle fascicle
  • muscle fibre
  • nucleus
  • actin
  • myosin
  • A-band
  • H-zone
  • I-band
  • myosin head
  • actin
  • ADP
  • phosphate
  • ATP
  • troponin
  • calcium ions
  • tropomyosin

Narration

Human locomotion and movement result from the contraction and relaxation of skeletal muscles.
Muscles are attached to the bones by tendons, tough bands of fibrous connective tissue.
Muscles are covered with epimysium, a layer of connective tissue. Muscles are composed of fascicles, which are covered with perimysium, another layer of connective tissue.

Fascicles are composed of muscle fibres. Muscle fibres are formed by the fusion of muscle cells, and they can therefore be considered single multinuclear cells. They contain actin and myosin, proteins that produce contractions in the muscle.

Actin and myosin are bundled in myofibrils, which have a characteristic, striated structure due to the organisation of the actin and myosin filaments. This is why skeletal muscles are also called striated muscles.

The band that contains myosin is called the A-band. The H-zone, located within the A-band, only contains myosin while the I-band only contains actin.
When the muscle is stimulated, the actin and myosin filaments slide over one another, the I-band and the H-zone become shorter, and the muscle contracts.

The molecular mechanism of muscle contraction is as follows. Stimulation of the muscle results in an increase in the calcium level of muscle fibres.
Calcium ions bind to the troponin, which causes tropomyosin to move and thus the myosin-binding parts of the actin become uncovered. Myosin binds to the actin, ADP and phosphate are released, and the myosin head bends and pushes the actin. Then ATP binds to the myosin head, which releases the actin. The ATP molecule splits into ADP and phosphate, and the myosin binds to the actin again.

This process continues while the calcium level in the muscle fibre is high. When the stimulation is over, the calcium level decreases and the troponin-tropomyosin complex returns to the default state.

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Skeletal muscles form the active part of the locomotor system: they move the bones they are attached to.

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The three types of muscle found in the human body are the smooth, the striated and the cardiac muscle.

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Bones of the lower limbs are connected to the trunk by the pelvis.

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Connective tissues include loose and dense connective tissues, adipose tissue, blood, tendon and bone tissue.

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This animation introduces the most important organ systems of the human body.

Human skeleton

Our body´s internal support structure to which skeletal muscles are attached.

Knee joint

The knee joint is made up by the femur, the tibia and the kneecap.

Layers of the skin; cutaneous senses

The skin is the soft outer covering of our body, its three layers are the epidermis, the dermis and the hypodermis.

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The central nervous system consists of the brain and the spinal cord, the peripheral nervous system consists of nerves and ganglia.

The heart

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Types of surface epithelium

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Types of synovial joints

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Muscles of the upper arm

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The withdrawal reflex

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