18 Questions
What is the organization of myofilaments in smooth muscle?
Thick and thin filaments are arranged diagonally.
What is the key difference between smooth muscle and skeletal muscle in terms of myofilament arrangement?
Thick filaments in smooth muscle have heads along their entire length, unlike in skeletal muscle.
Which statement accurately describes the myofilaments in smooth muscle?
The ratio of thin to thick filaments is lower than in skeletal muscle.
What is a characteristic feature of the arrangement of thick filaments in smooth muscle?
Thick filaments lack heads along their length.
How do the thin and thick filaments in smooth muscle contribute to its contraction?
Thin filaments dominate the organization with the ratio being higher than in skeletal muscle.
What distinguishes the myofilament organization in smooth muscle from skeletal muscle?
Thick filaments have heads distributed along their entire length in smooth muscle.
What is the main reason for the shortening of sarcomeres during muscle contraction?
The interaction between actin and myosin molecules
What is the role of T-tubules in muscle fibers?
Conducting impulses to the deepest regions of the muscle
Which structure regulates intracellular calcium levels in muscle cells?
Sarcoplasmic reticulum
What is the function of smooth endoplasmic reticulum (SR) in muscle cells?
Regulation of intracellular calcium levels
What component pairs form perpendicular cross channels within muscle cells?
Terminal cisternae of the sarcoplasmic reticulum
Which protein molecule hydrolyzes ATP during muscle contraction?
Myosin ATPase
What gives muscle cells their striated appearance?
Alternating light (I) and dark (A) bands
Which region contains tiny protein rods that hold adjacent thick filaments together in a sarcomere?
H-zone/bare zone
What is the chief protein component of thin filaments?
Actin
Which myofilament extends the entire length of dark A-bands?
Myosin filaments
What are the two smaller, light polypeptide chains on each myosin molecule called?
Cross bridges
What are the two regulatory subunits bound to actin in thin filaments?
Tropomyosin and troponin
Study Notes
Intercalated Discs
- Appear as dark, transverse lines that occur at irregular intervals in LM
- Exhibits two regions: transverse and lateral portions in EM
Skeletal Muscle
- Multinucleate with oval nuclei beneath the sarcolemma
- Nuclei are pushed aside by long, ribbon-like organelles called myofibrils
- Displays birefringence in polarized light
- Striated appearance due to alternating light (I) and dark (A) bands
Sarcomeres
- Structural and functional units of skeletal muscle
- Responsible for the striations of skeletal muscle
- Consist of light I-bands with a midline interruption called Z-disc and dark A-bands with a lighter central area called the H-zone
Myofilaments
- Thick (myosin) filaments extend the entire length of dark A-bands
- Thin (actin) filaments are present in the light I-bands
- Thick filaments composed of myosin molecules with rod-like tails and globular heads
- Thin filaments composed of actin, tropomyosin, and troponin
Sarcomere Arrangement
- Thick and thin filaments slide past each other, causing sarcomere shortening during muscle contraction
- Sliding filament theory explains muscle contraction
Muscle Contraction
- Motor units consist of a motor neuron and all the muscle fibers it activates
- Myosin ATPase hydrolyzes ATP, activating the myosin head and "cocking" it in an extended position
- Myosin head binds to an active site on actin, enabling muscle contraction
Smooth Muscle
- Non-striated, uninucleate, and fusiform in shape
- Thin and thick filaments are present, but with a higher ratio of thin to thick filaments compared to skeletal muscle
- Thick filaments have heads along their entire length, and there is no troponin complex
- Filaments are arranged diagonally, causing contraction in a corkscrew manner
- Non-contractile intermediate filament bundles attach to dense bodies at regular intervals
- Exhibits slow, synchronized contractions due to electrical coupling with gap junctions
- Some smooth muscle cells act as pacemakers, setting the contractile pace for the whole sheet of muscles
Test your knowledge on the arrangement of filaments in a sarcomere, myofilaments banding pattern, and the structure of the sarcoplasmic reticulum. Explore topics such as transverse tubules, excitation of muscle fibers, and the role of T-tubules in muscle function.
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