Physiologic Anatomy of Skeletal Muscle

• The body consists of approximately 40% skeletal muscle, 10% cardiac and smooth muscle.
• Basic principles of contraction apply to all three muscle types.

Physiologic Anatomy of Skeletal Muscle

• Skeletal muscles contain numerous fibers ranging from 10 to 80 micrometers in diameter.
• Each fiber comprises successively smaller subunits.
• In most skeletal muscles, each fiber extends the entire length of the muscle.

Sarcomere

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

Sarcolemma

• Consists of a true cell membrane known as the plasmalemma.
• Has an outer coat of polysaccharide material with collagen fibrils.
• At each muscle fiber end, the sarcolemma's surface layer fuses with a tendon fiber.
• Tendon fibers collect into bundles that form the muscle tendons, which insert into bones.

Myofibrils

• Myofibrils consist of actin and myosin filaments.
• Each muscle fiber contains several hundred to thousands of myofibrils.
• There are approximately 1,500 myosin and 3,000 actin filaments in each myofibril.
• Myosin are thick filaments
• Actin are thin filaments.
• Myosin and actin interdigitate, which is responsible for the alternating light and dark bands, giving skeletal and cardiac muscle a striated appearance.
• Light bands (I bands) contain actin and are isotropic to polarized light.
• Thick bands (A bands) contain myosin and are anisotropic to polarized light.
• A sarcomere is the section of a myofibril between two successive Z disks.

Titin Filamentous Molecule

• Titin maintains the side-by-side relationship of actin and myosin.
• Each titin molecule has a high molecular weight of 3 million.
• Titin is the largest protein molecule in the body and is filamentous, thus springy.
• It acts as a framework, holding A & M filaments in place, with one end attached to the Z disk and the other tethered to the M filament.

Sarcoplasm

• It is the intracellular fluid between myofibrils.
• Sarcoplasm contains substantial amounts of potassium, magnesium and phosphate.
• Sarcoplasm contains mitochondria.

Sarcoplasmic Reticulum

• Specialized endoplasmic reticulum (ER) of the skeletal muscle.

Muscle Contraction

• An action potential travels along a motor nerve to its endings on muscle fibers.
• At each ending, the nerve secretes a small amount of acetylcholine (a neurotransmitter).
• Acetylcholine opens acetylcholine-gated cation channels in a local area of the muscle fiber,.
• Consequently, large quantities of sodium ions diffuse to the interior of the muscle fiber membrane causing it to depolarize.
• This depolarization opens voltage-gated sodium channels, which initiate an action potential at the membrane.
• The action potential travels along the muscle fiber membrane similarly to how it travels in nerve fiber membranes.
• The action potential depolarizes the muscle membrane, causing the sarcoplasmic reticulum to release large amounts of calcium ions stored in the reticulum.
• Calcium ions initiate attractive forces between the actin and myosin filaments, leading them to slide alongside each other, which is a contractile process.
• After a fraction of a second, calcium ions are pumped back to the SR by a calcium membrane pump.
• They remain stored in the SR until a new action potential comes along.
• Removal of calcium ions from the myofibrils causes muscle contraction to cease.

Contractile Filaments

• Myosin filaments consist of multiple myosin molecules.
• A myosin molecule has 6 polypeptide chains, including 2 heavy chains that wrap spirally around each other to form a helix (the tail) .
• One end of each of the heavy chains is folded bilaterally into a globular polypeptide, forming a myosin head.
• It also has 4 light chains that control the function of the head during muscle contraction.

Actin Filaments

• The backbone of the actin filament is a double-stranded F-actin molecule.
• The 2 strands are wound in a helix similarly to the myosin molecule.
• Each strand consists of G-actin molecules.
• One molecule of ADP is attached to each G-actin molecule.
• ADP are the active sites on actin that interact with myosin filaments to cause muscle contraction.

Tropomyosin

• It's wrapped around the sides of the F-actin helix.
• In a resting state, tropomyosin lies on top of the active sites of actin so that attraction between actin and myosin cannot occur, preventing contraction.

Troponin

• Troponin are small proteins attached along the sides of tropomyosin.
• They are complexes of 3 loosely bound protein subunits that play roles in muscle contraction.
• Troponin I has a strong affinity for actin.
• Troponin T is for tropomyosin.
• Troponin C is for calcium ions.

Muscle Contraction

• A pure actin filament, lacking a troponin-tropomyosin complex, binds instantly with the heads of myosin filament.
• Binding between myosin and actin does not occur if the T-T complex is added to the actin filament.
• Active sites on normal actin in relaxed muscles are inhibited or physically covered by the T-T complex.
• The effect of the T-T complex is inhibited in the presence of large amounts of calcium ions.
• Calcium ions bind with Troponin-C, causing the troponin complex to change and tug on the tropomyosin molecule.
• This uncovers the active sites of the actin, allowing attraction of myosin cross-bridge heads, causing contraction.

Walk Along Theory (Ratchet) of Contraction

• Cross-bridge heads attach to and engage with the active sites of actin.
• When a head attaches to an active site, it tilts toward the arm, dragging the actin filament along.
• The tilt of the head is known as the power stroke.
• Immediately after tilting, the head breaks away.
• Then, the head binds to the next active site along the actin filament and tilts again to produce a power stroke.

Assignment Topics

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

Explore the anatomy of skeletal muscle, including muscle fibers, sarcomeres, and the sarcolemma. Understand the basic principles of muscle contraction common to skeletal, cardiac, and smooth muscle. Learn about the structure and function of myofibrils, actin, and myosin filaments.