Muscle Structure & Contraction Chapter 8

• Topic is Muscle Structure & Contraction, further information in Chapter 8.

Skeletal Muscle

• The human body contains over 600 skeletal muscles.
• Skeletal muscle makes up 40-50% of total body mass.
• Skeletal muscle functions include force production for locomotion and breathing.
• Further functions include force production for postural support and heat production during cold stress.

Connective Tissue Coverings

• Epimysium is the outer layer of connective tissue.
• Perimysium is connective tissue surrounding bundles of muscle fibers known as fascicles.
• Endomysium is connective tissue surrounding each muscle fiber within the fascicles.
• The basement membrane is protective tissue around every fiber under the endomysium.
• Sarcolemma is the cell membrane of muscle cells.

Satellite Cells

• Satellite cells are above the sarcolemma and below the basement membrane.
• Satellite cells play a role in muscle growth and repair.
• The addition of nuclei increases capacity for protein synthesis.
• This is an Adaptation to strength training, within the myonuclear domain.

Microstructure of Skeletal Muscle

• Muscle fibers contract by shortening myofibrils due to actin sliding over myosin.
• This reduces the distance between Z lines and forms cross-bridges between actin and myosin.
• Myosin heads point towards actin, bind, then pull actin towards the center.
• This results in shortening and the generation of force.

Sarcoplasmic Reticulum and Transverse Tubules

• The triad of the reticulum has terminal cisternae and transverse tubules.

Neuromuscular Junction

• The neuromuscular junction is between a motor neuron and muscle fiber.
• The motor unit consists of a motor neuron and all the fibers it innervates.
• The motor end plate is a pocket around the motor neuron by the sarcolemma.
• There is a short gap between the neuron and muscle fiber called the neuromuscular cleft.
• Acetylcholine released from the motor neuron causes end-plate potential (EPP).
• This results in the depolarizing of the muscle fiber.

Muscular Contraction

• Muscular contraction is a complex process involving proteins and energy systems.
• The end result is actin sliding over myosin, causing the muscle to shorten and develop tension.
• The sliding filament theory explains this process.

Sliding Filament Theory

• Muscle fibers contract by myofibrils shortening due to actin sliding over myosin.
• This reduces distance between Z lines, forming cross-bridges between actin and myosin.
• Myosin heads point, bind, and pull actin towards the center, shortening the fiber.

Energy for Contraction

• Energy is required at several steps of the contraction process.
• ATP breakdown is done via ATPase on the myosin head.
• This energy release energizes the myosin cross-bridge, pulling actin towards the center.
• A single contraction cycle, or power stroke, only shortens the muscle ~1%.
• Muscles can shorten up to 60%, so this must be repeated many times.

Excitation

• The process begins when a nerve impulse arrives at the neuromuscular junction.
• Depolarization goes down transverse tubules deep into the muscle fiber.
• This causes the release of Ca++ from the SR.

Contraction

• Released Ca++ diffuses into the muscle and binds to troponin.
• Troponin is on tropomyosin, lying in the grooves of the double strand of actin.
• In a relaxed muscle, tropomyosin prevents cross-bridge formation.
• Release of stored Ca++ from the lateral sac of SR binds to troponin.
• Causes a position change in tropomyosin that reveals binding sites for cross-bridge formation.
• Binding initiates energy release, causing shortening.
• A new ATP breaks the actin-myosin cross-bridge.
• ATPase hydrolyzes ATP, providing energy for a new "cocking" of the myosin head.
• The cross-bridge reattaches to a new site, generating another power stroke.
• This repeats as long as Ca is available to bind troponin.

Relaxation

• The signal to stop contraction is the absence of a nerve impulse.
• Absence of signal triggers an energy-requiring Ca++ pump within the SR, resequestoring Ca++ back into the SR.
• This removes Ca++ from troponin, causing tropomyosin to roll back and cover binding sites on actin.

Steps of Skeletal Muscle Contraction

• Nerve impulse reaches the neuromuscular junction.
• Depolarization sweeps through the transverse (T) tubules.
• Ca++ is released from SR and binds to troponin.
• Tropomyosin rolls and reveals the site of cross-bridge formation.
• ATP binds to myosin, energy is released, and myosin gets cocked and energized.
• Without tropomyosin in the way, myosin binds to actin.
• Pi is released, myosin rotates toward the midline
• ADP is released, myosin remains bound unless "new" ATP binds and breaks the bridge.
• ATP is broken down by myosin ATPase and myosin is recocked for further shortening.
• When depleted, the nerve impulse triggers the re-sequestration of Ca++ into the SR.
• The contraction is over and myosin can't bind because tropomyosin is in the way.