Skeletal Muscle Contraction and Anatomy

Skeletal Muscle Contraction

• Skeletal muscle makes up about 40% of the body
• Cardiac and smooth muscle constitutes about 10% of the body's muscle mass
• The fundamental principles for contraction are consistent across all three muscle types

Physiological Anatomy of Skeletal Muscle

• Skeletal muscles consist of many fibers, ranging from 10 to 80 micrometers in diameter
• Each fiber comprises smaller subunits
• Most skeletal muscle fibers run the entire length of the muscle

Sarcomere

• The sarcomere's center is the M line
• The point where two actin filaments connect is the Z disk
• H bands contain thick filaments that are not overlapped by thin filaments

Sarcolemma

• The sarcolemma is a true cell membrane known as the plasmalemma
• The sarcolemma includes an outer polysaccharide material containing numerous collagen fibrils
• The surface layer of the sarcolemma merges with a tendon fiber at each end of a muscle fiber
• Tendon fibers group into bundles, creating muscle tendons that connect to bones

Myofibrils

• Myofibrils are composed of actin and myosin filaments
• Each muscle fiber contains several hundred to several thousand myofibrils
• Each myofibril consists of 1,500 myosin and 3,000 actin filaments
• Thick filaments are myosin
• Thin filaments are actin
• The interdigitation of myosin and actin produces the muscle's light and dark bands
• Light bands are actin (I bands) and are isotropic to polarized light
• Thick bands are myosin (A bands) and are anisotropic to polarized light
• The arrangement of myofibrils provides skeletal and cardiac muscle with a striated appearance
• The portion of a myofibril between two successive Z disks is a sarcomere

Titin Filamentous Molecule

• Titin maintains the side-by-side relationship between actin and myosin
• Each single titin molecule has a molecular weight of 3 million
• Titin is the largest protein molecule in the body
• Titin is filamentous, therefore springy
• Titin acts as a framework to hold actin (A) and myosin (M) filaments in place
• One end is attached to the Z disk
• One end is tethered to the M filament

Sarcoplasm

• Sarcoplasm is the intracellular fluid between myofibrils
• Sarcoplasm contains large amounts of potassium, magnesium, and phosphate, as well as mitochondria

Sarcoplasmic Reticulum

• The sarcoplasmic reticulum is a specialized endoplasmic reticulum (ER) of the skeletal muscle

Muscle Contraction

• An action potential travels along a motor nerve to its endings on muscle fibers
• Each nerve ending secretes a small amount of acetylcholine (a neurotransmitter)
• Acetylcholine acts on a muscle fiber to open acetylcholine-gated cation channels
• The opening of these channels allows large quantities of sodium ions to enter the muscle fiber, causing depolarization
• This causes the opening of voltage-gated sodium channels, initiating an action potential at the membrane
• The sarcolemma (muscle fiber membrane) conducts the action potential similarly to nerve fiber membranes
• Upon depolarization, the sarcoplasmic reticulum releases large amounts of calcium ions
• Calcium ions initiate attractive forces between actin and myosin filaments, causing them to slide alongside each other
• This creates a contractile process
• Calcium ions are pumped back into the sarcoplasmic reticulum (SR) by a calcium membrane pump
• Calcium ions remain stored in the SR until a new action potential arrives
• The removal of calcium ions from the myofibrils leads to the cessation of muscle contraction

Contractile Filaments

• Myosin filaments comprise multiple individual myosin molecules
• The myosin molecule is composed of six polypeptide chains
• Two heavy chains
• These wrap around each other to form a helix, which is referred to as the tail
• One end is folded bilaterally into a globular polypeptide which is the myosin head
• Four light chains
• These control the head function during contraction

Actin Filaments

• The actin filament's backbone is a double-stranded F-actin molecule
• Two strands are wound in a helix, similar to the myosin molecule
• Each strand consists of G-actin molecules
• An ADP molecule is attached to each G-actin molecule
• During muscle contraction, ADP serves as the active site on actin for interaction with myosin filaments

Tropomyosin

• Tropomyosin is wrapped around the sides of the actin helix
• At rest, tropomyosin sits on top of the active sites, preventing attraction between actin and myosin
• This leads to no contraction

Troponin

• Troponin is attached along tropomyosin's sides and consists of three protein subunits
• Troponin I has a strong affinity for actin
• Troponin T binds to tropomyosin
• Troponin C binds calcium ions

Additional Muscle Contraction Details

• A pure actin filament binds instantly with myosin heads unless the troponin-tropomyosin complex is present
• If the troponin-tropomyosin (T-T) complex is added, myosin and actin are unable to bind
• Therefore, the T-T complex usually inhibits or physically covers active sites on relaxed muscles' actin
• The T-T complex's effect is inhibited in the presence of large amounts of calcium (Ca) ions
• Calcium ions bind with Troponin-C, changing the troponin complex and tugging on the tropomyosin molecule
• This process uncovers the active sites of actin, allowing attraction of myosin cross-bridge heads and ultimately causing contraction

Walk-Along (Ratchet) Theory of Contraction

• The heads of two cross-bridges attach to and engage the active sites of actin
• When a head attaches to an active site, it tilts, pulling the actin filament
• Tilting of the head is known as the power stroke
• Immediately after tilting, the head breaks away
• The head binds to the adjacent active site along the actin filament
• The head tilts again to produce another power stroke

Assignment topics

• Characteristics of whole muscle contraction
• Isometric vs isotonic contraction
• Fast vs slow muscle fibers
• Summation
• Tetanization

Overview of skeletal muscle contraction, physiological anatomy, and the role of the sarcomere and sarcolemma. Skeletal muscle comprises approximately 40% of the body's mass. The fundamental principles of contraction are consistent across different muscle types.