Muscle Structure & Contraction: Chapter 8

• Topic 8 is about muscle structure and contraction, referring to Chapter 8.

Skeletal Muscle

• The human body contains over 600 skeletal muscles
• Skeletal muscle accounts for 40-50% of total body mass
• Skeletal muscle functions to produce force for locomotion and breathing
• Skeletal muscle functions to produce force for postural support
• Skeletal muscle functions in heat production during cold stress

Connective Tissue Covering Skeletal Muscle

• Epimysium is the outer layer
• Perimysium is connective tissue that surrounds bundles of muscle fibers called fascicles
• Endomysium is connective tissue that surrounds each muscle fiber within the fascicles
• The basement membrane is protective tissue around every fiber under the endomysium
• Sarcolemma is a cell membrane of muscle cells

Satellite Cells

• Located above the sarcolemma but below the basement membrane
• Play a role in muscle growth and repair
• The addition of nuclei increases capacity for protein synthesis
• Important for adaptation to strength training
• Important for adaptation to myonuclear domain

Microstructure of Skeletal Muscle

• Includes the myofibril, satellite cells, and skeletal muscle fiber
• Includes Z line and sarcomere

Microstructure of Skeletal Muscle

• Includes the H zone, M line, Z line, I band, and A band
• Myosin filaments are thick
• Actin filaments are thin

Sarcoplasmic Reticulum and Transverse Tubules

• Includes the sarcolemma, triad of the reticulum, terminal cisternae, transverse tubule, myofibrils, A band, I band, sarcoplasmic reticulum, mitochondria, Z line, and the Nucleus

Neuromuscular Junction

• Includes mitochondria, synaptic vesicles, synaptic cleft, folded sarcolemma, motor end plate, motor neuron fiber, nerve fiber branches, muscle fiber nucleus, and myofibril of muscle fiber

Neuromuscular Junction

• The neuromuscular junction: Area where the motor neuron and muscle fiber connect
• A motor unit is a motor neuron and all the fibers it innervates
• Motor end plate: pocket formed around the motor neuron by the sarcolemma
• The neuromuscular cleft is a gap between the neuron and muscle fiber
• Acetylcholine is released from the motor neuron
• Acetylcholine causes end-plate potential (EPP)
• EPP results in depolarization of the muscle fiber

Muscular Contraction

• Muscular contraction is complex, involving proteins and energy systems
• The end result is actin sliding over the myosin, which causes the muscle to shorten and develop tension
• The sliding filament theory explains muscular contraction

Sliding Filament Theory

• Muscle fibers contract by a shortening of their myofibrils because actin slides over myosin
• The distance between the Z lines reduces
• Cross-bridges between actin and myosin form
• Myosin heads point towards the actin, bind, and pull the actin towards the center causing shortening and force generation

Sliding Filament Theory

• Includes sarcomere, A band, Z line, actin filaments, and myosin filaments

Energy for Contraction

• Energy is required at several steps of the process
• ATP breakdown is done via ATPase on the myosin head
• Energy release energizes the myosin cross-bridge, resulting in actin being pulled towards the center
• A single contraction cycle, or power stroke, of all the cross-bridges in a muscle would only shorten the muscle ~1%, some muscles can shorten up to 60%, shortening must be done many times

Excitation

• Process begins when a nerve impulse arrives at the neuromuscular junction
• Depolarization goes down the transverse tubules deep into the muscle fiber
• Results in the release of Ca++ from the SR

Contraction

• Released Ca++ diffuses into the muscle and binds to troponin
• Troponin lies directly on tropomyosin in the grooves of the actin double strand
• In a relaxed muscle, cross-bridges cannot form because tropomyosin covers binding sites

Contraction

• Stored Ca++ released from the lateral sac of the SR binds to troponin which causes a position change in tropomyosin that reveals the binding sites for cross-bridge formation
• Binding starts energy release leading which causes shortening/contraction
• When a "fresh" ATP attaches, the actin-myosin cross-bridge breaks
• The ATPase hydrolyzes ATP again and provides energy for a new cocking of the myosin head where the cross-bridge reattaches at a new site to generate another power stroke
• Repeats as long as there is Ca available to bind to troponin

Steps of Skeletal Muscle Contraction

• Nerve impulse reaches the neuromuscular junction
• Depolarization sweeps through the transverse (T) tubules
• Ca++ is released from SR
• Ca++ binds to troponin
• Tropomyosin rolls and reveals the site of cross-bridge formation
• ATP binds to myosin, energy is released, and myosin is cocked and energized
• Myosin binds to actin when tropomyosin is gone
• Pi is released, Myosin rotates toward midline and shortening occurs
• ADP is released and myosin remains bound to actin until another ATP binds to myosin
• ATP is broken down by myosin ATPase and shortening occurs

Relaxation

• Signal to stop contraction is the absence of a nerve impulse
• Absence of nerve impulses triggers an energy-requiring Ca++ pump within the SR to resequestor Ca++ back into the SR
• Removing Ca++ from troponin will cause tropomyosin to cover binding sites on actin

Steps of Skeletal Muscle Contraction

• Nerve impulse is gone triggering the re-sequestration of Ca++ into the SR
• Once Ca++ is pulled back into the SR the contraction is over and myosin can't bind anymore because tropomyosin is in the way

Muscle Excitation, Contraction, and Relaxation

• Muscle excitation involves the muscle action potential being propagated along the muscle plasma membrane
• Ca2+ is released during Contraction from the lateral sac of SR and leads to Ca2+ binding to troponin.
• The removal of Ca++ from troponin restores the tropomyosin blocking action during relaxation.
• Cross-bridges move to facilitate the event for the thick filament to bind with actin