47. Contraction of Striated Muscle 2024

What is the sliding filament theory of muscle contraction?

It states that thick and thin filaments slide past each other without shortening

What do the myosin crossbridges do during contraction?

Attach cyclically to actin and generate force by pulling thin filaments

What is the function of ATP in crossbridge cycling?

ATP provides the energy for myosin to detach and reattach to actin

Which band shortens during muscle contraction?

I-band and H-zone

Which band remains the same length during contraction?

A-band

What is the Z-line?

The boundary of the sarcomere to which thin filaments are anchored

What is titin?

An elastic protein that extends from M-line to Z-line, contributing to passive elasticity

What is nebulin believed to do?

Act as a molecular ruler for thin filament length

What defines the H-zone?

Region in A-band with no thin-thick filament overlap

What is the sarcolemma?

The plasma membrane surrounding a muscle fiber

What are T-tubules?

Invaginations of the sarcolemma that conduct action potentials into the muscle fiber

What is the sarcoplasmic reticulum (SR)?

A membrane system that stores and releases calcium for contraction

What forms a triad in skeletal muscle?

A T-tubule flanked by two terminal cisternae of the SR

How is the excitation signal carried into the muscle interior?

Via T-tubules propagating an action potential

What does depolarization of the T-tubule cause in skeletal muscle?

Direct opening of Ca²⁺ release channels (RyR1) on the SR

What triggers calcium release in cardiac muscle?

Influx of extracellular Ca²⁺ via DHPR opens RyR2 on SR

What type of calcium release occurs in skeletal muscle?

Mechanically-gated release via voltage-sensor coupling

What type of calcium release occurs in cardiac muscle?

Calcium-induced calcium release

What protein resequesters calcium into the SR after contraction?

SR Ca²⁺-ATPase (SERCA pump)

What is the main component of thick filaments?

Myosin

What is the main component of thin filaments?

Actin

What part of myosin forms the crossbridges?

The globular heads

What is the structure of actin filaments?

Two helical chains of actin subunits

What activates myosin's ATPase activity?

Binding to actin

What is the first step in the crossbridge cycle?

ATP binds to myosin, causing detachment from actin

What happens after ATP hydrolysis by myosin?

The crossbridge becomes energized and reorients

What allows myosin to bind actin again?

Presence of Ca²⁺ exposes binding sites on actin

What causes the power stroke in the crossbridge cycle?

Release of ADP and Pi from myosin

What causes myosin to detach from actin again?

Binding of a new ATP molecule

What is the rate-limiting step in the cycle?

Dissociation of ADP and Pi

How does actin accelerate myosin's ATPase cycle?

It speeds up dissociation of ADP and Pi from myosin

What triggers contraction in skeletal and cardiac muscle?

Increase in sarcoplasmic Ca²⁺ concentration

What protein complex regulates actin-myosin interaction in skeletal and cardiac muscle?

Troponin-tropomyosin complex

What does the troponin-tropomyosin complex do in the absence of calcium?

Inhibits actomyosin ATPase by preventing myosin from binding actin

What are the three subunits of troponin?

TnI, TnC, TnT

What is the function of TnI?

Inhibits ATPase activity of actomyosin

What is the function of TnC?

Binds calcium and relieves inhibition by TnI

What is the function of TnT?

Links TnI and TnC to tropomyosin

Where are troponin and tropomyosin located?

On thin filaments

How does tropomyosin inhibit contraction?

It blocks myosin binding sites on actin when Ca²⁺ is low

What happens to tropomyosin when Ca²⁺ binds to troponin?

It shifts position to expose myosin binding sites on actin

How many actin monomers are regulated by each troponin-tropomyosin unit?

Seven

What shape is tropomyosin?

Coiled-coil protein made of two α-helices

What occurs when myosin binding sites are exposed on actin?

Crossbridge cycling and contraction proceed

What happens to the position of tropomyosin when Ca²⁺ is added?

It moves from a blocking to an exposing position on actin

What disease can result from sarcomeric protein mutations?

Familial Hypertrophic Cardiomyopathy

What happens to sarcomeric proteins in certain genetic mutations?

They develop point mutations that can lead to cardiomyopathies

What is a common sarcomere-related disease affecting young adults?

Familial Hypertrophic Cardiomyopathy

What percent of individuals are affected by sarcomeric protein mutations?

Approximately 1 in 250

What regulatory strategy could potentially treat sarcomeric mutation-related diseases?

Modulating thin filament Ca²⁺ sensitivity or controlling myosin crossbridge recruitment

What triggers thin filament movement in muscle contraction?

Calcium binding to troponin, shifting tropomyosin

What molecular event stops contraction and promotes relaxation?

Resequestration of Ca²⁺ into the sarcoplasmic reticulum via Ca²⁺-ATPase

How is myosin ATPase activity related to contraction?

It provides energy for crossbridge cycling

How does actin influence myosin ATPase activity?

Actin accelerates the rate-limiting step of the ATPase cycle