Lecture 7.1 G.6 Muscle Fiber Structure and Sarcomere

• Each muscle fiber contains hundreds to thousands of myofibrils.
• Myofibrils are composed of actin and myosin filaments.
• Myosin filaments are thick and appear dark, forming the A bands.
• Actin filaments are thin and appear light, forming the I bands.

Sarcomere

• The sarcomere is the functional unit of muscle contraction.
• It exists between two Z discs.
• Titin connects Z discs to myosin at the M line and it is springy.
• Sarcoplasm is the fluid between myofibrils, containing potassium, magnesium, and phosphate.
• The sarcoplasmic reticulum regulates calcium storage, release, and uptake.

Myosin Filaments

• Myosin filaments consist of a tail, hinge, and head.
• Myosin molecule tails bundle together to form the body of the filament.
• A portion of the tail extends to form the arm.
• The arm and head together form the cross bridge.
• There are two hinges, one to create the arm and one to attach the head.
• Each myosin filament is twisted, with cross bridges extending in all directions.
• One ADP molecule is attached and is believed to be the active site for cross-bridging.
• Actin is strongly attached to the Z discs.
• Tropomyosin wraps around actin, covering active sites until contraction.
• Troponin is attached to tropomyosin and has a high affinity for calcium, initiating contraction.

Walk-Along Theory

• An action potential triggers calcium release from the sarcoplasmic reticulum.
• Calcium causes myosin heads to attach to active sites on actin.
• The cross bridge tilts, pulling the actin filament along (power stroke).
• The head detaches, returns to the extended position, and attaches to another active site.
• Neural stimulation releases acetylcholine and causes contraction.

Energy During Contraction

• ATP is cleaved to ADP during contraction (Fenn effect).
• ATP is utilized to extend the myosin head, storing energy for the power stroke.
• ATP is needed for the walk-along mechanism.
• ATP is required for pumping calcium into the sarcoplasmic reticulum.
• ATP is needed for pumping sodium and potassium ions for action potentials.

ATP Formation

• ATP is formed through: phosphocreatine, glycolysis, and oxidative metabolism.
• Phosphocreatine has a higher energy phosphate bond than ATP, but only small amounts are stored in the muscle.
• Stored ATP and phosphocreatine support contraction for 5-8 seconds.
• Glycolysis breaks down glycogen to pyruvic and lactic acid.
• Glycolysis can occur without oxygen and sustains contraction for about a minute until the buildup of end products stops the contraction.
• Oxidative metabolism combines oxygen with glycolysis end products and other food products.
• Oxidative metabolism provides more than 95% of the energy used by muscles.

Muscle Contractions

• Isometric contractions maintain the same muscle length like planks.
• Isotonic contractions shorten the muscle while tension remains constant like crunches.

Fast Versus Slow Fibers

• Every muscle contains a mix of fast and slow fibers.
• Slow fibers have more myoglobin, a reddish appearance, a high amount of capillaries, and many of mitochondria for extended contraction.
• Slow fibers are small and less strong.
• Fast muscle fibers are larger for greater strength.
• Fast muscle fibers have an extensive sarcoplasmic reticulum for rapid calcium release.
• They also contain large amounts of glycolytic enzymes for glycolysis.
• Fast muscle fibers have less blood supply and fewer mitochondria.

Innervation

• Each motor neuron innervates multiple muscle fibers, forming a motor unit.
• Fine motor control requires more nerve fibers.
• Large muscles have several hundred fibers per motor unit.

Summation

• Summation combines individual contractions for overall strength.
• Increasing the number of motor units contracting simultaneously increases contraction.
• Increasing the frequency of contraction increases contraction.
• The size principle states that smaller motor units contract first, then larger units as signal strength increases.
• Tetanization occurs when contractions fuse and appear smooth and continuous at a critical frequency.

Muscle Properties

• Maximum strength of contraction is 3-4 kg/cm².
• The staircase effect describes the rapid increase in muscle strength after a period of rest.
• Muscle tone is maintained by a low rate of nerve impulses.
• Muscle fatigue is related to glycogen depletion.

Muscle Remodeling

• Hypertrophy results from increased numbers of actin and myosin filaments and glycolytic enzymes.
• Stretching muscles adds new sarcomeres.
• Shortened muscles will lose sarcomeres.
• Denervation causes atrophy.
• Re-innervation can restore function, but with less capability.

Skeletal Muscle Contraction (Video Summary)

• Skeletal muscle cells contain myofibrils, which are made of myofilaments.
• Myofilaments are divided into sarcomeres where contractions take place.
• Thick filaments are composed of myosin and thin filaments contain actin and tropomyosin.
• The interaction of thick and thin filaments is the cross-bridge cycle.
• A myosin head bound by ATP detaches from the actin fiber.
• The tropomyosin blocks myosin from binding to the actin active site.
• Calcium concentration jumps in the muscle cell with stimulation.
• The troponin complex undergoes a change opening the binding site.
• Myosin hydrolyzes ATP rotating back to its cocked position.
• Phosphate dissociates from myosin, triggering the power stroke.
• The myosin head pulls the actin along.
• ADP leaves the cross bridge.
• If ATP binds to the myosin head the cycle starts over.

Explore the structure of muscle fibers, myofibrils, and the sarcomere. Learn about actin and myosin filaments, A and I bands, and the role of the sarcoplasmic reticulum. Understand the composition and function of myosin filaments and cross-bridges.