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Questions and Answers
What is the shape of skeletal muscle cells?
What is the shape of skeletal muscle cells?
What surrounds the sarcolemma of each muscle fiber?
What surrounds the sarcolemma of each muscle fiber?
What are the contractile units of myofibrils composed of?
What are the contractile units of myofibrils composed of?
What are the thin filaments associated with in sarcomeres?
What are the thin filaments associated with in sarcomeres?
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What separates sarcomeres?
What separates sarcomeres?
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What surrounds groups of fibers called fascicles?
What surrounds groups of fibers called fascicles?
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What is the function of the sarcoplasmic reticulum in muscle cells?
What is the function of the sarcoplasmic reticulum in muscle cells?
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What is the structure that forms a triad with the sarcoplasmic reticulum and triggers Ca2+ release?
What is the structure that forms a triad with the sarcoplasmic reticulum and triggers Ca2+ release?
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What type of filaments are found in the A band of a sarcomere?
What type of filaments are found in the A band of a sarcomere?
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What is the function of the triad in muscle cells?
What is the function of the triad in muscle cells?
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Where are mitochondria located in muscle cells?
Where are mitochondria located in muscle cells?
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What is the role of Ca2+ in muscle contraction?
What is the role of Ca2+ in muscle contraction?
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What is the result of the myosin heads pivoting with ATP hydrolysis?
What is the result of the myosin heads pivoting with ATP hydrolysis?
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What is the term for the synapses of motor axons with skeletal muscle?
What is the term for the synapses of motor axons with skeletal muscle?
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What happens to the muscle fiber when the membrane depolarization ends?
What happens to the muscle fiber when the membrane depolarization ends?
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What is the term for a group of muscle fibers innervated by branches of the same motor axon?
What is the term for a group of muscle fibers innervated by branches of the same motor axon?
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What type of muscle fibers are classified based on their physiological properties?
What type of muscle fibers are classified based on their physiological properties?
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What is the main difference between skeletal muscle fibers and cardiac muscle fibers?
What is the main difference between skeletal muscle fibers and cardiac muscle fibers?
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What is the function of muscle spindles?
What is the function of muscle spindles?
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What is responsible for regulating the rate of cardiac muscle contraction?
What is responsible for regulating the rate of cardiac muscle contraction?
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What is the structural organization of sarcomeres in cardiac muscle similar to?
What is the structural organization of sarcomeres in cardiac muscle similar to?
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What is the main reason for rapid regeneration in smooth muscle?
What is the main reason for rapid regeneration in smooth muscle?
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What is unique about the sarcoplasmic reticulum in smooth muscle fibers?
What is unique about the sarcoplasmic reticulum in smooth muscle fibers?
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Which protein is involved in controlling the sliding filaments in smooth muscle?
Which protein is involved in controlling the sliding filaments in smooth muscle?
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What is a key difference between cardiac muscle and skeletal muscle in terms of regeneration?
What is a key difference between cardiac muscle and skeletal muscle in terms of regeneration?
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What is the function of α-actinin in smooth muscle fibers?
What is the function of α-actinin in smooth muscle fibers?
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What is a characteristic of smooth muscle fibers?
What is a characteristic of smooth muscle fibers?
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Study Notes
Types of Muscle Tissue
- There are three main types of muscle tissue: skeletal, cardiac, and smooth.
Skeletal Muscle Characteristics
- Skeletal muscle cells are long, multinucleated fibers with diameters up to 100 μm.
- Each fiber has a sarcolemma surrounded by an external lamina and thin connective tissue called endomysium, which contains capillaries.
Organization of Skeletal Muscle Fibers
- Groups of fibers are called fascicles, which are surrounded by perimysium.
- All fascicles are enclosed within a dense connective tissue called epimysium.
- Each muscle fiber is filled with myofibrils, which are composed of thousands of thick myosin filaments and thin actin filaments.
- Myofibrils are highly organized into contractile units called sarcomeres.
Sarcomere Structure
- Within sarcomeres, thick and thin filaments interdigitate.
- Globular myosin heads project from the thick filaments toward the F-actin filaments.
- F-actin filaments are associated with tropomyosin and troponin.
- Sarcomeres are separated by Z discs that bisect the light-staining I bands.
- I bands contain mainly thin filaments attached to α-actinin in the Z disc.
Sarcomere Structure
- Dark-staining A band contains thick myosin filaments
- Alternating light and dark bands create microscopic striations along fibers
Sarcoplasmic Reticulum
- Located between parallel myofibrils in sarcoplasm
- Composed of mitochondria and cisternae of smooth ER
- Specialized for Ca2+ sequestration and release
Transverse Tubule System
- Deep invagination of sarcolemma, called a transverse or T tubule
- Two terminal cisterns of SR contact T tubule at each sarcomere, forming a triad
- Triad triggers Ca2+ release when sarcolemma is depolarized
Mechanism of Contraction
- Ca2+ binding to troponin causes a shape change in tropomyosin, allowing myosin heads to bind to actin subunits and form crossbridges between thick and thin filaments.
- Myosin heads pivot with ATP hydrolysis, pulling thin filaments along thick filaments and shortening the sarcomere.
- The contraction cycle involves repeated attachment, pivoting, detachment, and return of myosin heads, causing filaments to slide past each other and shorten the sarcomere.
- When membrane depolarization ends, Ca2+ is sequestered, ending contraction and allowing sarcomeres to lengthen again as the muscle relaxes.
Motor Unit and Neuromuscular Junction
- Synapses of motor axons with skeletal muscle are called motor end plates (MEPs), neuromuscular junctions (NMJs), or myoneural junctions.
- The neurotransmitter at the neuromuscular junction is acetylcholine.
- A motor axon may form many terminal branches, each ending on an MEP of a muscle fiber.
- All fibers innervated by branches of a single axon comprise a motor unit.
Muscle Spindles & Tendon Organs
- Muscle spindles and tendon organs are sensory proprioceptors that consist of sensory axons wrapped around intrafusal fibers in small specialized fascicles or around myotendinous collagen bundles, respectively.
Muscle Fiber Types
- Skeletal muscles contain three main types of fibers:
- Slow, oxidative (type I) fibers
- Fast, intermediate oxidative-glycolytic (type IIa) fibers
- Fast, glycolytic (type IIb) fibers
Cardiac Muscle
- Cardiac muscle fibers are striated, cylindrical cells with one or two central nuclei.
- Each cardiac muscle fiber is linked by adherent and gap junctions at prominent intercalated discs.
- Sarcomeres of cardiac muscle are organized and function similarly to those of skeletal muscle.
- Contraction of cardiac muscle is intrinsic at nodes of impulse-generating pacemaker muscle fibers.
- Autonomic nerves regulate the rate of cardiac muscle contraction.
Smooth Muscle Characteristics
- Smooth muscle fibers are individual, small, fusiform cells connected by numerous gap junctions.
- Thin and thick filaments in smooth muscle fibers do not form sarcomeres, and no striations are present.
- Thin actin filaments attach to α-actinin in dense bodies throughout the sarcoplasm and near the sarcolemma.
- Contraction of smooth muscle cells causes individual shortening.
Muscle Fiber Structure
- Sarcoplasmic reticulum is less well-organized in smooth muscle fibers.
- There is no transverse tubule system in smooth muscle fibers.
Muscle Contraction Regulation
- Troponin is absent in smooth muscle.
- Myosin light-chain kinase (MLCK) and calmodulin regulate the sliding filaments in smooth muscle.
Muscle Tissue Regeneration
- Skeletal muscle has a population of reserve muscle satellite cells, enabling regeneration through proliferation, fusion, and new muscle fiber formation.
- Cardiac muscle lacks satellite cells and has limited regenerative capacity.
- Smooth muscle regeneration occurs rapidly due to its relatively small, less differentiated cells, which can resume mitotic activity after injury.
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Description
Learn about the three major types of muscle tissues: skeletal, cardiac, and smooth muscle. Understand the structure and organization of skeletal muscle fibers.
