Muscle Ultrastructure and Sarcomere Quiz

Muscle Ultrastructure

• The sarcomere is the basic repeating unit of muscle. It's a complex arrangement of many protein-protein interactions.
• Each muscle fibre is enclosed by a specialized cell membrane called the sarcolemma.
• Muscle fibres contain smaller myofibrils embedded in the cytoplasm, known as sarcoplasm.
• Sarcoplasm is rich in glycogen, ATP, Creatine Phosphate, and glycolytic enzymes.
• Transverse tubules (T-tubules) are membrane folds extending from the plasma membrane. They transmit electrical signals.
• The Sarcoplasmic Reticulum (SR) surrounds myofibrils and stores calcium.

Muscle Sarcomere

• The sarcomere is the basic contractile unit of a muscle fiber.
• It is composed of two main protein filaments: actin (thin filaments) and myosin (thick filaments).
• The arrangement of these filaments is responsible for the striated appearance of skeletal muscle under a microscope.
• Key components include:
  • Z-line: The boundary of the sarcomere.
  • I-band: The light band, which contains only thin filaments.
  • A-band: The dark band, containing both thick and thin filaments.
  • H-zone: The central region of the A-band, containing only thick filaments.
  • M-line: The center of the sarcomere, where thick filaments are linked.

Molecular Structure of Thin Filament (Actin)

• Composed of three components:
  • F-actin (filamentous-actin): A double-stranded protein molecule wound into a double helix.
  • Tropomyosin: A double-stranded α-helical protein molecule lying in the groove of the F-actin helix.
  • Troponin: A complex of three globular proteins (TnC, TnI, TnT) attached periodically to the tropomyosin strand.

Thin Filament Arrangement

• Tropomyosin and troponin regulate actin.
• Tropomyosin covers the myosin binding sites on actin in the resting state.
• Troponin subunits have specific functions:
  • TnT binds to tropomyosin.
  • TnI binds to F-actin.
  • TnC binds to calcium ions (Ca2+).
• When Ca2+ is released, it binds to TnC, causing tropomyosin to move away from the myosin binding site, allowing myosin to bind to actin.

Myosin Filaments (Thick Filaments)

• Composed of approximately 400 myosin polypeptides, with 200 on each side of the M-line.
• Each myosin molecule consists of:
  • A compact head region.
  • A long tail region formed by two α-helices.
• The tails pack together to form the thick central portion of the myosin filament.
• The heads extend outwards, forming "cross bridges" with actin filaments.
• Most of the tail is called light meromyosin.
• The head and a portion of the tail are composed of heavy meromyosin.

Molecular Structure Of Myosin

• Myosin molecules consist of two heavy chains and four light chains.
• The C-terminal parts of the myosin heavy chains (MHC) twist together to form the coiled-coil α-helical rod-shaped tail domain.
• The N-terminal parts of the heavy chains form the two myosin heads.
• Each head acts as an enzyme, binding to ATP and hydrolyzing it.
• The myosin head stores energy in the form of ADP and inorganic phosphate (Pi).

Sliding Filament Theory

• Proposed by A.F. Huxley and H.E. Huxley in 1950 to explain muscle contraction.
• Myosin filaments use ATP energy to "walk" along actin filaments using their cross bridges.
• Contraction is due to the sliding of actin and myosin filaments past each other.
• The movement of cross bridges generates the force of contraction.
• This pulls actin filaments closer together.

Sliding Filament Theory Steps

1. Nerve signals stimulate calcium release from the sarcoplasmic reticulum.
2. Calcium binds to troponin C, causing tropomyosin to shift position, exposing the actin binding sites.
3. Myosin heads bind to exposed actin binding sites, forming cross bridges.
4. ADP is released, initiating the "power stroke" where the thin filament is pulled towards the sarcomere midline.
5. New ATP binds to the myosin head, separating the actin-myosin cross bridge. ATP is then hydrolyzed to ADP and Pi, transferring energy to the myosin head.
6. This cycle continues until the nerve signal stops and calcium is reabsorbed.

Muscle Sarcomere Ultrastructure During Contraction

• The A-band remains the same length.
• The I-band shortens.
• The H-zone is reduced or disappears.

Ratchet Mechanism of Contraction

• Describes the biochemical and biophysical events during contraction.
• Converts ATP energy into the physical work of actin and myosin filament displacement.
• Involves the formation of cross bridges between the myosin head and actin binding sites.

Powerstroke and Cross-bridge Action

• Myosin head binds to actin.
• The myosin head "contracts" due to a conformational change, pulling the actin filament.
• The actin-myosin interaction breaks.
• The myosin head re-attaches further along the actin filament.

Ratchet Mechanism Steps

1. Actin is uncovered when Ca2+ binds to troponin. The Myosin-ADP-Pi complex binds to actin.
2. Pi is released, triggering the powerstroke. The myosin head changes conformation.
3. ATP replaces ADP, binding to the actomyosin complex, leading to actin release. The myosin head returns to its resting conformation.
4. ATP is hydrolyzed to ADP-Pi, returning to the resting state.

Excitation-Contraction Coupling (ECC)

• Describes the rapid communication between electrical events in the muscle fiber's plasma membrane and calcium release from the sarcoplasmic reticulum, resulting in contraction.
• A nerve impulse triggers acetylcholine (ACh) release at the neuromuscular junction (NMJ).
• ACh binds to receptors, depolarizing the sarcolemma, and the depolarization propagates through the T-tubule system.
• This triggers Ca2+ release from the sarcoplasmic reticulum, leading to muscle contraction.
• Ca2+ is pumped back into the sarcoplasmic reticulum, ending contraction.

Muscle Relaxants

• Two types of muscle relaxants used in anesthesia:
  • Depolarizing muscle relaxants: Cause initial muscle contraction, but prevent further contractions.
    • Example: Suxamethonium.
  • Non-depolarizing muscle relaxants: Prevent muscle contraction.
    • Example: Not specified in the text.

Test your knowledge on the ultrastructure of muscle and the specifics of the sarcomere. This quiz covers critical elements like myofibrils, sarcoplasm, and the role of filaments in muscle contraction. Dive deep into the anatomy of muscle fibers and discover their fascinating details!