GQ: Skeletal Muscles

Distinguish the following structures in a skeletal muscle and indicate the membrane surrounding each structure

Muscle bundle is composed of fascicles and is surrounded by epimysium. Fascicle is a bundle of muscle fibers surrounded by perimysium. Myofiber (muscle fiber) is an individual muscle cell surrounded by endomysium and its membrane is the sarcolemma. Myofibril is the contractile structure inside the muscle fiber and is not surrounded by connective tissue. The skeletal muscle cell is the myofiber.

What are T-tubules and what function do they play in muscle contraction?

T-tubules are invaginations of the sarcolemma that carry the action potential deep into the muscle fiber, allowing rapid signal transmission and triggering calcium release from the sarcoplasmic reticulum.

Discuss the parts of a typical sarcomere

A sarcomere extends from Z line to Z line. Actin is anchored to the Z line, while myosin is located at the center forming the A band. The H zone contains only myosin, and the M line stabilizes myosin. Titin acts as a spring connecting structures. Actin and myosin interact to produce contraction by sliding, while titin maintains elasticity and resting tension.

What are the components of the troponin complex and their roles?

Troponin C binds calcium ions, Troponin T binds to tropomyosin, and Troponin I has an inhibitory role. Calcium binding to Troponin C causes movement of tropomyosin, exposing actin binding sites.

Describe the mechanism of skeletal muscle stimulation

An action potential reaches the motor neuron terminal causing acetylcholine release. Acetylcholine binds to receptors, allowing sodium entry and depolarization. The signal travels along the sarcolemma and T-tubules, triggering calcium release from the sarcoplasmic reticulum. Calcium binds to troponin, exposing actin sites, allowing myosin to bind and initiate contraction. Multiple fibers contracting result in whole muscle contraction.

What is the role of calcium ions in skeletal muscle contraction?

Calcium binds to troponin C to expose actin binding sites. It is released from the sarcoplasmic reticulum and later pumped back using ATP. If calcium remains, contraction continues; if calcium is low, contraction is weak or absent.

Aside from calcium ions, which other ions are important?

Sodium is important for depolarization, potassium for repolarization, and magnesium assists in ATP breakdown and energy transfer.

Explain the walk-along theory

The walk-along theory describes myosin repeatedly binding and pulling actin, appearing as if it walks along the filament. It differs from the sliding filament theory, which emphasizes that filaments slide past each other without changing position.

What are the assumptions of the walk-along theory?

Force is proportional to cross-bridge formation, cross-bridge formation depends on contraction speed, and force required to stretch muscle depends on cross bridges. Actin does not completely slide because excessive overlap reduces binding sites.

How is sliding related to tension?

Tension increases with cross-bridge formation and is highest at optimal overlap. It is not linear because too little or too much overlap reduces cross bridges, decreasing tension.

Describe the phases of a skeletal muscle twitch

The latent period (~5 ms) involves calcium release. The contraction phase involves cross-bridge formation and increasing tension. The relaxation phase occurs when calcium is pumped back and tension decreases.

What is excitation-contraction coupling?

It is the process linking electrical stimulation to contraction. The delay occurs due to time needed for calcium release and cross-bridge initiation.

Can a muscle fiber be stimulated again before relaxation?

Yes, this causes summation. High-frequency stimulation leads to tetanic contraction. Continued stimulation increases force but cannot be sustained due to fatigue and ATP depletion.

Fiber summation vs frequency summation

Fiber summation recruits more motor units, while frequency summation increases stimulation rate. Greater cross-sectional area increases force due to more fibers and improved mechanical advantage.

Isotonic vs isometric contraction

Isotonic contraction involves change in muscle length and movement. Isometric contraction produces tension without changing length because the load exceeds force.

How can muscles produce greater force with shorter duration?

Greater force is produced by more cross bridges and fast fibers, which contract quickly but fatigue faster. Factors include fiber type and energy supply.

Why do muscular contractions require energy?

Energy is required for cross-bridge cycling, detachment of myosin, calcium pumping, and ion transport. ATP is supplied by phosphocreatine, glycolysis, and oxidative metabolism.

Discuss the Fenn effect

The Fenn effect states that energy use increases with work done, especially in concentric contractions where greater load requires more energy.

Relationship between force, duration, and velocity

As velocity increases, force decreases. Maximum power occurs at intermediate velocity. Greater force requires slower contraction.

Role of elastic fibers

Elastic components like titin and tendons store and transmit force, maintain tension, and contribute to elasticity.

How is force related to power?

Power equals force times velocity. Maximum power occurs at intermediate velocity. Activities like cycling maintain this balance, unlike weightlifting.

How does muscle fiber type affect contraction?

Type I fibers are slow and fatigue-resistant for endurance. Type II fibers are fast and produce greater force but fatigue quickly.

Discuss lever systems

Lever systems affect force and movement efficiency. Muscle attachment relative to joints determines mechanical advantage. Third-class levers are most common.

How does fascicle arrangement influence force?

Arrangement affects force by determining fiber number and orientation. More fibers increase force and ability to overcome load.

Endurance vs resistance training

Endurance training improves fatigue resistance and oxidative metabolism. Resistance training increases muscle size and force.

Differentiate fatigue, spasm, and cramp

Fatigue is reduced contraction ability due to ATP or ion imbalance. Spasm is a sudden involuntary contraction. Cramp is a painful sustained contraction due to lactic acid or ion imbalance.

Without T-tubules?

Contraction would be slow and not uniform because signals would not reach deep muscle regions efficiently.

What causes striations?

Striations are caused by the arrangement of actin and myosin forming A bands and I bands.

Without ATP?

Muscles cannot detach cross bridges or pump calcium back, causing permanent contraction (rigor mortis).

What causes twitching in dead animals?

Residual calcium and remaining ATP allow brief contractions until ATP is depleted.

Why do muscles atrophy when denervated?

Loss of nerve stimulation reduces activity, leading to decreased protein synthesis and muscle shrinkage.