lecture 10 MUSCLE TISSUE

A comprehensive set of practice flashcards covering muscle organization, NMJ events, sarcomere structure, sliding filament theory, cross-bridge cycling, neural control, muscle fiber types, and differences among skeletal, cardiac, and smooth muscle.

Describe the organization of skeletal muscle from largest to smallest components.

Epimysium → muscle fascicles → perimysium → muscle fibers (cells) → endomysium → myofibrils → sarcomeres → myofilaments (actin & myosin).

What connective tissue surrounds a muscle fascicle?

Perimysium.

What connective tissue surrounds a single muscle fiber (cell)?

Endomysium.

What connective tissue surrounds the entire muscle?

Epimysium.

What is a myofibril?

A bundle of myofilaments within a muscle fiber that contains sarcomeres.

What is a sarcomere?

The contractile unit of skeletal muscle from one Z line to the next, containing I band, A band, H zone, M line; made of thin (actin) and thick (myosin) filaments; regulated by troponin and tropomyosin; requires Ca2+ and ATP.

Name the key proteins and ions involved in the cross-bridge cycle.

Actin, myosin, troponin, tropomyosin, titin, Nebulin; Ca2+; ATP.

How does Ca2+ regulate the actin binding sites on the thin filament?

Ca2+ binds to troponin, causing tropomyosin to move away from the myosin-binding sites on actin.

Describe the steps of the cross-bridge cycle.

Ca2+ exposes active sites on actin; myosin head binds actin forming a cross-bridge; power stroke occurs with ADP and Pi release; ATP binds to detach the cross-bridge; ATP is hydrolyzed to reset the myosin head; cycle repeats while Ca2+ and ATP are present.

What is the sliding filament model?

Filaments slide past one another to shorten the sarcomere; I and H bands shrink while the A band remains the same.

What happens at the neuromuscular junction during activation?

An action potential triggers release of acetylcholine (ACh) into the synaptic cleft; ACh binds receptors on the motor end plate, opening Na+ channels and depolarizing the sarcolemma.

What neurotransmitter is released at the NMJ?

Acetylcholine (ACh).

What is a motor unit?

One motor neuron and all the muscle fibers it innervates; the size of the motor unit varies with the required precision of movement.

What are the three types of skeletal muscle fibers and their abbreviations?

Slow oxidative (SO, Type I), Fast oxidative glycolytic (FOG, Type IIa), Fast glycolytic (FG, Type IIx).

Describe Slow Oxidative (Type I) fibers.

Slow contractions, rely on aerobic respiration, many mitochondria, red in color, fatigue-resistant, small diameter.

Describe Fast Oxidative Glycolytic (Type IIa) fibers.

Fast contractions; primarily aerobic but can use glycolysis; intermediate fatigue resistance; more mitochondria than FG; red/pink; moderate diameter.

Describe Fast Glycolytic (Type IIx) fibers.

Fast contractions; mainly anaerobic glycolysis; few mitochondria; white; fatigue quickly; large diameter.

How do skeletal, cardiac, and smooth muscle differ in structure and control?

Skeletal: multi-nucleated, striated, voluntary control; Cardiac: striated, usually one nucleus, intercalated discs, involuntary; Smooth: non-striated, one nucleus, involuntary.

What are intercalated discs and what do they contain?

Intercalated discs contain desmosomes and gap junctions; desmosomes anchor cells; gap junctions allow direct electrical coupling between cells.

What is notable about cardiac muscle contraction and mitochondria compared to skeletal muscle?

Cardiac muscle has many mitochondria; contractions last 10 to 15 times longer than skeletal muscle.

What is the skeletal muscle triad?

T-tubule with two terminal cisternae of the sarcoplasmic reticulum.

What initiates excitation-contraction coupling in skeletal muscle?

A motor neuron action potential releases ACh at the NMJ, triggering sarcolemma depolarization, propagating via T-tubules to the sarcoplasmic reticulum to release Ca2+ and start the cross-bridge cycle.

What is the role of calcium in contraction?

Ca2+ binds to troponin, moving tropomyosin away from actin’s myosin-binding sites, enabling cross-bridge formation.

What is force production in muscle proportional to, and what are two ways to increase it?

Proportional to the number of cross-bridges (number of active fibers); increased by recruiting more fibers and increasing the frequency of action potentials (summation/tetanus).

What is the outermost connective tissue layer that surrounds the entire skeletal muscle?

Epimysium.

What bundles of muscle fibers are located immediately deep to the epimysium?

Muscle fascicles.

What connective tissue layer surrounds each muscle fascicle?

Perimysium.

What are the individual cells that make up a muscle fascicle?

Muscle fibers (cells).

What delicate connective tissue layer surrounds a single muscle fiber (cell)?

Endomysium.

What are the contractile organelles found within a muscle fiber, composed of bundles of myofilaments that contain sarcomeres?

Myofibrils.

What is the basic contractile unit of skeletal muscle, extending from one Z line to the next, containing I band, A band, H zone, M line; made of thin (actin) and thick (myosin) filaments; regulated by troponin and tropomyosin; requires Ca2+ and ATP?

Sarcomere.

What are the primary protein filaments (actin and myosin) that compose the sarcomeres and are responsible for muscle contraction?

Myofilaments.

Name the key proteins and ions involved in the cross-bridge cycle.

Actin, myosin, troponin, tropomyosin, titin, Nebulin; Ca2+; ATP.

How does Ca2+ regulate the actin binding sites on the thin filament?

Ca2+ binds to troponin, causing tropomyosin to move away from the myosin-binding sites on actin.

Describe the steps of the cross-bridge cycle.

Ca2+ exposes active sites on actin; myosin head binds actin forming a cross-bridge; power stroke occurs with ADP and Pi release; ATP binds to detach the cross-bridge; ATP is hydrolyzed to reset the myosin head; cycle repeats while Ca2+ and ATP are present.

What is the sliding filament model?

Filaments slide past one another to shorten the sarcomere; I and H bands shrink while the A band remains the same.

What happens at the neuromuscular junction during activation?

An action potential triggers release of acetylcholine (ACh) into the synaptic cleft; ACh binds receptors on the motor end plate, opening Na+ channels and depolarizing the sarcolemma.

What neurotransmitter is released at the NMJ?

Acetylcholine (ACh).

What is a motor unit?

One motor neuron and all the muscle fibers it innervates; the size of the motor unit varies with the required precision of movement.

What are the three types of skeletal muscle fibers and their abbreviations?

Slow oxidative (SO, Type I), Fast oxidative glycolytic (FOG, Type IIa), Fast glycolytic (FG, Type IIx).

Describe Slow Oxidative (Type I) fibers.

Slow contractions, rely on aerobic respiration, many mitochondria, red in color, fatigue-resistant, small diameter.

Describe Fast Oxidative Glycolytic (Type IIa) fibers.

Fast contractions; primarily aerobic but can use glycolysis; intermediate fatigue resistance; more mitochondria than FG; red/pink; moderate diameter.

Describe Fast Glycolytic (Type IIx) fibers.

Fast contractions; mainly anaerobic glycolysis; few mitochondria; white; fatigue quickly; large diameter.

How do skeletal, cardiac, and smooth muscle differ in structure and control?

Skeletal: multi-nucleated, striated, voluntary control; Cardiac: striated, usually one nucleus, intercalated discs, involuntary; Smooth: non-striated, one nucleus, involuntary.

What are intercalated discs and what do they contain?

Intercalated discs contain desmosomes and gap junctions; desmosomes anchor cells; gap junctions allow direct electrical coupling between cells.

What is notable about cardiac muscle contraction and mitochondria compared to skeletal muscle?

Cardiac muscle has many mitochondria; contractions last 10 to 15 times longer than skeletal muscle.

What is the skeletal muscle triad?

T-tubule with two terminal cisternae of the sarcoplasmic reticulum.

What initiates excitation-contraction coupling in skeletal muscle?

A motor neuron action potential releases ACh at the NMJ, triggering sarcolemma depolarization, propagating via T-tubules to the sarcoplasmic reticulum to release Ca2+ and start the cross-bridge cycle.

What is the role of calcium in contraction?

Ca2+ binds to troponin, moving tropomyosin away from actin’s myosin-binding sites, enabling cross-bridge formation.

What is force production in muscle proportional to, and what are two ways to increase it?

Proportional to the number of cross-bridges (number of active fibers); increased by recruiting more fibers and increasing the frequency of action potentials (summation/tetanus).

Organization of a Muscle (Skeletal)

Skeletal muscle is organized hierarchically: Epimysium surrounds the whole muscle, Perimysium surrounds fascicles, Endomysium surrounds individual muscle fibers (cells), which contain Myofibrils, composed of repeating Sarcomeres made of Actin and Myosin myofilaments.

Neuromuscular Junction (NMJ)

The NMJ is the synapse between a motor neuron and a muscle fiber where an action potential triggers the release of acetylcholine (ACh). ACh binds to receptors on the sarcolemma, causing Na+ influx and initiating a muscle action potential that leads to Ca2+ release from the sarcoplasmic reticulum, thus initiating contraction.

Sarcomere

The sarcomere is the basic contractile unit of striated muscle, extending from Z line to Z line. It contains Thin filaments (actin) and Thick filaments (myosin), regulated by troponin and tropomyosin, with distinct bands (I band, A band, H zone) and lines (Z line, M line).

Sliding Filament Model

During muscle contraction, thin filaments (actin) slide past thick filaments (myosin) toward the center of the sarcomere. The filaments themselves do not change length, but the I band and H zone shorten/disappear, Z lines move closer, and overlap between actin and myosin increases, leading to overall sarcomere shortening.

Skeletal Muscle Fiber Types

Skeletal muscle fibers are classified by contraction speed and metabolism: Slow Oxidative (Type I) are slow, aerobic, fatigue-resistant (red); Fast Oxidative Glycolytic (Type IIa) are fast, mixed metabolism, intermediate fatigue (intermediate); Fast Glycolytic (Type IIx) are fast, anaerobic, fatigue quickly (white).

Differences in Muscle Types

Skeletal muscle is voluntary, striated, multinucleated, with rapid force; Cardiac muscle is involuntary, striated, single nucleus, with intercalated discs, rhythmic, long-lasting contractions; Smooth muscle is involuntary, non-striated, single nucleus, slow and prolonged contractions, found in hollow organs.