Skeletal Muscle Physiology Quiz

Skeletal Muscle Physiology

• Skeletal muscle is attached to the skeleton and moves bones at joints when it contracts.
• Skeletal muscles are made of many multinucleated muscle cells, called muscle fibers.
• Each muscle fiber has thousands of contractile filaments called myofilaments.
• Myofilaments are of two types: thin (actin) and thick (myosin).
• Actin filaments have binding sites for myosin.
• Myosin filaments have heads that bind to actin.

Learning Objectives

• Outline the functional structure of skeletal muscle.
• Outline the structure of the myoneural junction.
• Describe excitation-contraction coupling.
• Define a simple muscle twitch.
• Describe isotonic and isometric skeletal muscle contractions.

Gross Anatomy of Skeletal Muscle

• Muscle fibers are composed of myofibrils.
• Myofibrils are arranged in repeating units called sarcomeres.
• Sarcomeres are bounded by Z lines.
• Each sarcomere has a central dark band (A band) and two peripheral light bands (I bands).
• The A band contains thick filaments (myosin).
• The I bands contain thin filaments (actin).
• The H zone is the lighter region in the middle of the A band.
• The bare zone is a region in the H zone devoid of thin filaments.

Proteins of the Thin Filaments

• Actin is the structural protein of the thin filament.
• Actin has attachment sites for myosin.
• Tropomyosin blocks myosin-binding sites on actin.
• Troponin is composed of three subunits: troponin-T (binds to tropomyosin), troponin-I (binds to actin and inhibits contraction), and troponin-C (binds to calcium).
• Under resting conditions, calcium is not bound to troponin, preventing actin-myosin interaction.
• Binding of calcium to troponin-C causes the troponin-tropomyosin complex to move, exposing the myosin-binding site on actin.

Proteins of the Thick Filaments

• Myosin has ATPase activity.
• Splitting ATP puts myosin in a high-energy state, increasing its affinity for actin.
• Myosin binding to actin initiates the power stroke.
• If the force generated by the power stroke is sufficient to move the load, the muscle shortens (isotonic contraction).
• If the force generated is not sufficient to move the load, the muscle stays the same length (isometric contraction).

Neuromuscular Transmission

• The junction between the nerve and muscle is called the myoneural junction.
• The myoneural junction consists of:
  • Nerve terminal with acetylcholine granules.
  • Space between the nerve terminal and muscle membrane.
  • Skeletal muscle membrane.

Events Occurring at the Myoneural Junction

1. Release of acetylcholine from motor nerve terminals into the space between the nerve and muscle.
2. Acetylcholine crosses the space and binds to specific receptors on the skeletal muscle membrane, increasing membrane permeability to sodium.
3. Action potential is generated and spreads along the skeletal muscle membrane.
4. Depolarization of the muscle membrane causes calcium release from intracellular stores into the sarcoplasm.
5. Calcium causes contractile filaments to slide over each other, leading to muscle shortening.
6. Calcium is pumped back into intracellular stores, leading to muscle relaxation.

Regulation of Cytosolic Calcium

• The sarcoplasmic reticulum (SR) has a high concentration of calcium.
• The SR creates a strong electrochemical gradient for calcium to diffuse into the cytosol.
• Key receptors involved in calcium flux: dihydropyridine (DHP) and ryanodine (RyR).
• DHP is a voltage-gated calcium channel.
• RyR is a calcium channel on the SR membrane.
• In the resting state, RyR is blocked by DHP, preventing calcium from diffusing into the cytosol.

Sequence of Skeletal Muscle Contraction

1. Skeletal muscle action potential is initiated at the neuromuscular junction.
2. The action potential travels down the T-tubule.
3. The voltage change causes a conformational shift in DHP, removing its block of RyR.
4. Removal of the DHP block allows calcium to diffuse into the cytosol.
5. The rise in cytosolic calcium opens more RyR channels (calcium-induced calcium release).
6. Calcium binds to troponin-C, initiating cross-bridge cycling.
7. Calcium is pumped back into the SR by SERCA.
8. The fall in cytosolic calcium causes tropomyosin to cover actin's binding sites for myosin, and the muscle relaxes.

Mechanism of Relaxation

• The effect of acetylcholine is terminated by the enzyme cholinesterase at the neuromuscular junction.
• Calcium is actively pumped back into the SR.
• Myosin-binding sites on actin are covered again.

Summation and Recruitment

• Under normal circumstances, enough calcium is released by a single action potential to completely saturate troponin-C binding sites, which means that all available cross-bridges are activated.
• Force is increased by summation and recruitment.
• Summation occurs when multiple action potentials are generated before the muscle has completely relaxed, adding the force from the previous action potentials.
• Recruitment occurs by activating more motor units, each containing more muscle fibers, by engaging more muscle fibers, to cause stronger force production.

Isometric and Isotonic Contractions

• Isometric contraction: The muscle does not shorten, but the force developed is maximal.
• Isotonic contraction: The muscle shortens, but the force developed is not maximal.

Test your knowledge on the physiology of skeletal muscle. This quiz covers the structure and function of skeletal muscles, including muscle fibers, myofilaments, and the process of excitation-contraction coupling. Explore key concepts related to muscle contractions and the gross anatomy of skeletal muscle.