36. Muscle Tissue 2024

What are the three types of muscle tissue?

Skeletal, cardiac, and smooth.

Which muscle types appear striated under the microscope?

Skeletal and cardiac.

Which muscle type lacks visible striations?

Smooth muscle.

What is the cytoplasm of muscle cells called?

Sarcoplasm.

What is the smooth ER of muscle cells called?

Sarcoplasmic reticulum.

What is the muscle cell membrane called?

Sarcolemma.

What is a muscle fiber?

A multinucleated syncytium of a skeletal muscle cell.

What structures make up a gross skeletal muscle?

Many fascicles.

What are fascicles?

Bundles of muscle fibers.

What are myofibrils?

Unbranched rods containing sarcomeres that span the length of the fiber.

What is a sarcomere?

The contractile unit of striated muscle from Z-line to Z-line.

What two filaments make up myofibrils?

Thin actin and thick myosin filaments.

What connective tissue surrounds the entire named muscle?

Epimysium.

What surrounds each fascicle?

Perimysium.

What surrounds each individual muscle fiber?

Endomysium.

What fiber type is best for visualizing structural features of muscle?

Skeletal muscle.

Where are the nuclei located in skeletal muscle?

Peripherally, adjacent to the sarcolemma.

What occupies 80% of the sarcoplasm in skeletal muscle?

Myofibrils.

What do A-bands represent?

Dark bands where thick myosin filaments are located.

What do I-bands represent?

Light bands with thin actin filaments.

What is the H-band?

Lighter zone within the A-band with only thick filaments.

What is the M-line?

Middle of the H-band anchoring myosin filaments.

What proteins stabilize the sarcomere structure?

Tropomyosin, titin, desmin, alpha-actinin, dystrophin.

Do myofilament lengths change during contraction?

No, filaments slide past each other.

Where are periodic striations visible?

Only in longitudinal sections.

What causes the granular appearance in transverse skeletal sections?

Myofibrils.

What is the triad in skeletal muscle?

One T-tubule plus two terminal cisternae of SR.

Where is the triad located in skeletal muscle?

At the A-I band junction.

What do voltage-sensitive proteins in T-tubules trigger?

Calcium release from SR cisternae.

What does calcium release initiate in muscle fibers?

Actin-myosin interaction and contraction.

What embryonic cells form skeletal muscle fibers?

Mesoderm-derived myoblasts.

What do fused myoblasts form?

Multinucleated myotubes.

Where do myofibrils first form in developing muscle?

In the periphery of myotubes.

Where do mature nuclei localize in skeletal muscle?

Peripherally, beneath the sarcolemma.

What are satellite cells?

Stem cells in the basal lamina of adult muscle fibers.

Does postnatal growth increase the number of muscle fibers?

No, only fiber size increases.

Which muscle fiber type is slow-twitch and oxidative?

Type I (red fibers).

What features define Type I fibers?

High myoglobin, abundant mitochondria, fatigue resistance.

Which fiber type is fast-twitch and glycolytic?

Type II (white fibers).

What features define Type II fibers?

Rapid, forceful contraction; quick fatigue; few mitochondria.

What are intermediate fibers?

Type II fibers with properties of both type I and II.

What causes hypertrophy of muscle fibers?

Repeated resistance overload.

What causes disuse atrophy?

Muscle underuse, tendon injury, or denervation.

What allows muscle fiber regeneration after injury?

Intact satellite cells and external lamina.

What is sarcopenia?

Age-related muscle mass and strength decline.

What kind of filaments are present in cardiac muscle?

Same actin and myosin arrangement as skeletal muscle.

Are cross-striations visible in cardiac muscle?

Yes, they are visible in histologic sections.

How many nuclei does a typical cardiac myocyte have?

One (occasionally two), centrally located.

What is the sarcoplasmic cone?

Cytoplasmic area around the nucleus in cardiac muscle.

How do cardiac muscle cells form connections?

Via interdigitating processes and intercalated discs.

What is an intercalated disc?

A specialized junctional complex joining cardiac cells.

How do intercalated discs appear under the light microscope?

As dark transverse lines between cells.

What are the three junction types in intercalated discs?

Fascia adherens, desmosomes, and gap junctions.

What is the role of fascia adherens in cardiac muscle?

Anchor actin filaments from sarcomeres at cell ends.

What do desmosomes (macula adherens) do in cardiac muscle?

Bind cells together to resist mechanical stress.

What is the function of gap junctions in cardiac muscle?

Electrically couple cells for synchronous contraction.

Where are gap junctions located in cardiac muscle?

Along the parallel steps of intercalated discs.

How does the T-tubule system in cardiac muscle compare to skeletal?

Larger diameter and fewer in number.

Where are diads located in cardiac muscle?

At the Z-lines.

What is a diad?

One T-tubule and one terminal cistern of SR.

How developed is the SR in cardiac muscle?

Less developed than in skeletal muscle.

How does cardiac muscle generate contraction?

Spontaneously, with autonomic modulation.

What regulates heart rate and force of contraction?

The autonomic nervous system.

What organelle is more numerous and larger in cardiac than skeletal muscle?

Mitochondria.

Where are mitochondria located in cardiac muscle?

Between myofibrils.

What is the shape of smooth muscle cells?

Spindle-shaped (tapered at both ends).

How many nuclei do smooth muscle cells contain?

One, centrally located.

Are striations present in smooth muscle?

No, they are non-striated.

Where is smooth muscle found?

Walls of blood vessels, airways, and GI organs.

What nervous system controls smooth muscle?

Autonomic nervous system.

What type of filaments dominate smooth muscle cytoplasm?

Thin actin filaments.

Where do actin filaments insert in smooth muscle?

Dense bodies.

What are dense bodies?

Electron-dense structures that anchor actin filaments.

Where are dense bodies located?

Throughout the cytoplasm and at the sarcolemma.

What proteins are found in dense bodies?

Alpha-actinin.

What do dense bodies functionally resemble?

Z-lines in striated muscle.

What intermediate filaments insert into dense bodies?

Desmin or vimentin.