Skeletal Muscle Contraction and Anatomy

Skeletal Muscle Contraction

• Skeletal muscle makes up about 40% of the body's mass
• Cardiac and smooth muscle account for 10% of body mass
• Basic contraction principles apply to all three muscle types

Physiologic Anatomy of Skeletal Muscle

• Skeletal muscles contain numerous fibers ranging from 10-80 micrometers in diameter
• Each fiber comprises successively smaller subunits
• Most skeletal muscle fibers extend the entire length of the muscle

Sarcomere

• The M line is located at the center
• The Z disk connects two actin filaments.
• H bands contain thick filaments not overlapped by thin filaments

Sarcolemma

• The sarcolemma consists of a true cell membrane known as the plasmalemma
• The sarcolemma also features an outer coat of polysaccharide material containing collagen fibrils
• At a muscle fiber's end, the sarcolemma's surface layer fuses with a tendon fiber
• Tendon fibers bundle together to form muscle tendons which then insert into bones

Myofibrils

• Myofibrils are composed of actin and myosin filaments
• Each muscle fiber contains several hundred to several thousand myofibrils
• A myofibril consists of approximately 1,500 myosin and 3,000 actin filaments
• Thick filaments consist of myosin
• Thin filaments consist of actin
• Myosin and actin interdigitation creates alternating light and dark bands
• Actin composes light bands (I bands) and is isotropic to polarized light
• Myosin composes thick bands (A bands) and is anisotropic to polarized light
• They give skeletal and cardiac muscle a striated appearance
• A sarcomere is the portion 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 molecular weight of 3 million, making it the largest protein molecule in the body
• Titin is filamentous and springy which acts as a framework and holds actin and myosin filaments in place
• One titin end is attached to the Z disk
• One titin end is tethered to the M filament

Sarcoplasm

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

Sarcoplasmic Reticulum

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

Muscle Contraction

• An action potential travels along a motor nerve to its endings on muscle fibers
• The nerve endings secrete a small amount of acetylcholine (a neurotransmitter)
• Acetylcholine acts on the muscle fiber, opening acetylcholine-gated cation channels
• Channel opening allows large quantities of sodium ions to diffuse into the muscle fiber membrane, causing depolarization
• This opens voltage-gated sodium channels, initiating an action potential in the membrane
• The action potential travels along the muscle fiber membrane in the same manner as in nerve fiber membranes
• The action potential depolarizes the muscle membrane and causes the sarcoplasmic reticulum to release stored calcium ions
• Calcium ions initiate attractive forces between actin and myosin filaments, leading to the filaments sliding alongside each other, which is a contractile process
• Calcium ions are pumped back to the sarcoplasmic reticulum by a calcium membrane pump after a fraction of a second, remaining stored until a new action potential arrives
• The removal of calcium ions from the myofibrils ceases muscle contraction

Contractile Filaments

• Myosin filaments are composed of multiple myosin molecules
• A myosin molecule has 6 polypeptide chains
• Two heavy chains wrap spirally to form a helix- tail
• One end of each heavy chain is folded bilaterally into a globular polypeptide, forming a myosin head
• There are also 4 light chains
• The function of each head is to control muscle contraction

Actin Filaments

• The backbone of each actin filament is a double-stranded F-actin molecule
• Two strands are wound in a helix, similarly to myosin
• Each strand consists of G-actin molecules
• One molecule of ADP is attached to each G-actin molecule
• ADP are active site on actin, when myosin filaments interact to cause muscle contraction

Tropomyosin

• Tropomyosin is wrapped around the sides of the f-actin helix
• Tropomyosin lies on top of the active sites in a resting state to prevent attraction between actin and myosin
• No contraction occurs

Troponin

• Troponin are small proteins attached along the sides of tropomyosin
• Troponin is a complex of 3 loosely bound protein subunits with roles in muscle contraction
  • Troponin I has a strong affinity for actin
  • Troponin T binds to tropomyosin
  • Troponin C binds calcium ions

Muscle Contraction (Actin and Myosin)

• A pure actin filament without the troponin-tropomyosin complex binds instantly with myosin filament heads
• When the troponin-tropomyosin (T-T) complex is added to actin, myosin and actin cannot bind
• Active sites on relaxed muscle actin are inhibited or physically covered by the T-T complex
• In the presence of large amounts of calcium ions, the T-T complex effect is inhibited
• Calcium ions bind with Troponin-C, which triggers a change in the troponin complex
• The tropomyosin molecule is then tugged on
• This uncovers the active sites of the actin, which causes attractions to activate in myosin heads, resulting in contraction

Walk-Along (Ratchet) Theory of Contraction

• Two cross-bridge heads attach to and engage from actin's active sites
• When a head attaches to an active site, the head tilts toward the arm, dragging the actin filament
• The tilt of the head is known as the power stroke
• The head breaks immediately after tilting
• The head binds to the next active site along the actin filament
• The head then tilts again to produce another power stroke

Assignment topics

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

Overview of skeletal muscle contraction, including physiologic anatomy and the roles of the sarcolemma, sarcomere and myofibrils. Skeletal muscle comprises approximately 40% of the body's mass. Basic contraction principles apply to all three muscle types.