sarcomere

Examples of sarcomere in the following topics:

• Microscopic Anatomy

  • Intercalated disks transmit electrical action potentials between sarcomeres.
  • A sarcomere is the basic unit of muscle tissue in both cardiac and skeletal muscle.
  • Sarcomeres appear under the microscope as striations, with alternating dark and light bands.
  • Sarcomeres are connected to a plasma membrane, called a sarcolemma, by T-tubules, which speed up the rate of depolarization within the sarcomere.
  • Actin molecules are bound to the Z-disc, which forms the borders of the sarcomere.

• Sliding Filament Model of Contraction

  • In the sliding filament model, the thick and thin filaments pass each other, shortening the sarcomere.
  • To understand the sliding filament model requires an understanding of sarcomere structure.
  • A sarcomere is defined as the segment between two neighbouring, parallel Z-lines.
  • Titin molecules are thought to play a key role as a molecular ruler maintaining parallel alignment within the sarcomere.
  • The amount of force and movement generated generated by an individual sarcomere is small.

• Skeletal Muscle Fibers

  • Skeletal muscles are composed of striated subunits called sarcomeres, which are composed of the myofilaments actin and myosin.
  • These proteins are organized into regions termed sarcomeres, the functional contractile region of the myocyte.
  • Within the sarcomere actin and myosin, myofilaments are interlaced with each other and slide over each other via the sliding filament model of contraction.
  • The regular organization of these sarcomeres gives skeletal and cardiac muscle their distinctive striated appearance.
  • The sarcomere is the functional contractile region of the myocyte, and defines the region of interaction between a set of thick and thin filaments.

• Force of Muscle Contraction

  • Muscles exist in this state to optimize the force produced during contraction, which is modulated by the interlaced myofilaments of the sarcomere.
  • When a sarcomere contracts, myosin heads attach to actin to form cross-bridges.
  • This results in sarcomere shortening, creating the tension of the muscle contraction.
  • If a sarcomere is stretched too far, there will be insufficient overlap of the myofilaments and the less force will be produced.
  • In mammals, there is a strong overlap between the optimum and actual resting length of sarcomeres.

• Control of Muscle Tension

  • Neural control initiates the formation of actin–myosin cross-bridges, leading to the sarcomere shortening involved in muscle contraction .
  • Maximal tension occurs when thick and thin filaments overlap to the greatest degree within a sarcomere.
  • If a sarcomere at rest is stretched past an ideal resting length, thick and thin filaments do not overlap to the greatest degree so fewer cross-bridges can form.
  • As a sarcomere shortens, the zone of overlap reduces as the thin filaments reach the H zone, which is composed of myosin tails.
  • Conversely, if the sarcomere is stretched to the point at which thick and thin filaments do not overlap at all, no cross-bridges are formed and no tension is produced.

• Exercise-Induced Muscle Damage

  • Previously attributed to the accumulation of lactic acid during exercise, it is now understood that DOMS is due to structural damage in sarcomeres, particularly to the z-disks and contractile filaments.
  • Damage to the sarcomeres causes aninflux of white blood cells, leading to inflammation, which is itself associated with increased plasma enzyme concentration, myoglobinemia, and abnormal muscle structure and histology.
  • A further response to sarcomere damage is necrosis following damage to the mysium, which peaks about 48 hours following exercise.

• Muscular Atrophy and Hypertrophy

  • Schematic of filament arrangement in normal, functional sarcomeres, versus atophied sarcomeres following 17-day space flight

• Types of Muscle Contractions: Isotonic and Isometric

  • Cross-bridge cycling occurs, shortening the sarcomere, muscle fiber, and muscle.
  • Cross-bridge cycling occurs even though the sarcomere, muscle fiber, and muscle are lengthening, controlling the extension of the muscle.
  • In both instances, cross-bridge cycling is maintaining tension in the muscle; the sarcomere, muscle fibers, and muscle are not changing length.

• ATP and Muscle Contraction

  • Muscles contract in a repeated pattern of binding and releasing between the two thin and thick strands of the sarcomere.
  • When the actin is pulled approximately 10 nm toward the M-line, the sarcomere shortens and the muscle contracts.

• Myocardial Thickness and Function

  • Cardiac muscle, like skeletal muscle, is comprised of sarcomeres, the basic, contractile units of muscle.
  • Sarcomeres are composed of long, fibrous proteins that slide past each other when the muscles contract and relax.
  • Two of the important proteins found in sarcomeres are myosin, which forms the thick filament, and actin, which forms the thin filament.
  • The tissue structure of cardiac muscle contains sarcomeres that are made of myofibrils with intercalated disks, that contain cardiomyocytes and have many mitocondria.