Lec 10 - Contractile Apparatus

What are the three types of muscle cells

Skeletal, cardiac, and smooth muscle.

What contractile epithelial-like cell type is also discussed with muscles

Myoepithelial cells, which aid secretion in ducts and are not true muscle.

Which muscle types are striated

Skeletal and cardiac muscle are striated.

Which muscle type is nonstriated

Smooth muscle is nonstriated.

Which muscle is voluntary

Skeletal muscle is voluntary and activated by motor neurons.

Which muscle types are involuntary

Cardiac and smooth muscle are involuntary.

What is unitary (single‑unit) smooth muscle

Densely packed smooth muscle cells that contract as a single unit, e.g., in GI tract motility.

Where are myoepithelial cells found and what do they do

Around ducts in glands to help expel secretions.

What are skeletal muscle precursor cells called

Myoblasts.

How do skeletal myofibers become multinucleated

Fusion of myoblasts to form a syncytium.

What is a syncytium in skeletal muscle

A large cytoplasmic mass containing many nuclei formed by myoblast fusion.

What intermediate cell appears during skeletal myogenesis before full striation

Myotube.

How do cardiomyocytes become binucleate without fusion

Karyokinesis without cytokinesis (division of nuclei without cytoplasm division).

How many nuclei do smooth muscle and myoepithelial cells typically have

One nucleus.

What connective tissue surrounds individual muscle fibers

Endomysium.

What connective tissue surrounds a fascicle (bundle of mucle fibers sourround by connective tissue)

Perimysium.

What connective tissue surrounds the entire muscle

Epimysium.

Define myofilaments.

Longitudinal assemblies of actin (thin) and myosin (thick).

Define a myofibril.

A bundle of myofilaments arranged in repeating sarcomeres.

Define a myofiber.

A muscle cell containing many myofibrils and peripheral nuclei.

What is the sarcolemma

The specialized plasma membrane of a muscle fiber.

What is sarcoplasm

The cytoplasm of a muscle cell.

What organelles surround myofibrils in skeletal muscle

Mitochondria and sarcoplasmic reticulum.

What are T‑tubules (transverse tubules)

Invaginations of the sarcolemma that carry depolarization deep into the fiber.

What is a triad in skeletal muscle

One T‑tubule flanked by two terminal cisternae of sarcoplasmic reticulum.

What is the sarcoplasmic reticulum (SR) in muscle

Specialized endoplasmic reticulum storing and releasing Ca2+.

What is a sarcomere bounded by

Two Z discs.

What defines the I band

Light band with actin thin filaments only (no myosin thick filaments).

What defines the A band

Dark band containing the entire length of a thick myosin filament.

What is the H zone

A region within the A band with myosin but no actin thin filaments.

What is the M line

The midpoint of the sarcomere with myosin tails and elastic protein connections.

What anchors actin filaments in the sarcomere

Z disc anchors the barbed (plus) ends of actin filaments.

Which actin end points toward the M line

The pointed (minus) end.

Which filaments are called thin and thick

Thin are actin; thick are myosin.

What are the two molecular rulers in sarcomeres

Titin and nebulin.

What does titin do

An elastic protein providing a blueprint for sarcomere assembly and extensibility.

What does nebulin do

Acts as a ruler for thin filament length.

What caps the barbed end of actin at the Z disc

CapZ.

What caps the pointed end of actin in sarcomeres

Tropomodulin.

What crosslinks actin at the Z disc

Alpha‑actinin.

Who proposed the sliding filament model and when

Huxley & Niedergerke and Huxley & Hanson in 1954.

What happens to Z discs during contraction

They move closer together as sarcomeres shorten.

What happens to the A band during contraction

A band width stays the same.

What happens to the I band during contraction

I band narrows and can nearly disappear.

What happens to the H zone during strong contraction

H zone can diminish or disappear in extreme contraction.

What physically slides in the sarcomere

Thin actin filaments slide past thick myosin filaments toward the M line.

What are the components of myosin II

Two heavy chains and two pairs of light chains (essential and regulatory).

What interacts with actin on myosin II

The globular head domains of the heavy chains.

What forms the myosin II tail

Heavy chain alpha‑helical coiled‑coil dimer.

What supports the myosin neck and is regulated by phosphorylation

Light chains (ELC and RLC).

Are there muscle and non‑muscle forms of myosin II

Yes, distinct isoforms exist in muscle and non‑muscle cells.

What happens when ATP binds to myosin bound to actin

Myosin releases actin.

What does ATP hydrolysis do to the myosin head

Cocks the head, enabling weak binding to a new actin site.

What triggers tight actin–myosin binding and initiates the power stroke

Release of inorganic phosphate (Pi).

What is released during/after the power stroke to complete the step

ADP is released following Pi, completing the stroke.

What is the rigor state

Myosin tightly bound to actin with no nucleotide present.

What enzymatic activity powers myosin movement

Myosin ATPase converts ATP energy into mechanical work.

Clinical: Why do muscles become stiff in rigor mortis

Lack of ATP prevents myosin release from actin, causing persistent cross‑bridge attachment.

Clinical: Which mechanism is impaired in rigor mortis

Release of myosin from actin.

Clinical: In a biopsy pair, which image represents cramping muscle

The one with shortened sarcomeres and reduced/absent I bands and H zones.

Clinical: What class of drugs can reduce uncontrolled skeletal muscle contractions in the case vignette

• commonly used for conditions like muscle spasms, spasticity from neurological disorders, or during surgery to induce muscle relaxation.

Calcium channel blockers.

What protein complex blocks myosin binding sites on actin at rest

Tropomyosin with the troponin complex (TnT, TnI, TnC).

Which troponin binds Ca2+

Troponin C (TnC).

What happens when Ca2+ binds troponin C

The complex shifts, moving tropomyosin to expose myosin‑binding sites on actin.

Is myosin light chain phosphorylation required for skeletal muscle activation

No, it is not required for skeletal muscle motor activation.

What is the key Ca2+ event in skeletal/cardiac contraction

SR Ca2+ release displaces the tropomyosin–troponin complex from actin.

What neurotransmitter is released at the motor end plate

Acetylcholine (ACh).

What receptor does ACh bind on the muscle cell

Nicotinic ACh receptor (ligand‑gated ion channel).

What ion flux occurs when AChR opens at the neuromuscular junction

Na+ influx depolarizes the sarcolemma.

What receptor in the T‑tubule senses depolarization in skeletal muscle

Dihydropyridine (DHP) receptor in the T‑tubule membrane.

What SR channel releases Ca2+ into the cytosol

Ryanodine receptor (RyR).

What is the primary source of cytosolic Ca2+ for contraction

The sarcoplasmic reticulum via RyR channels.

What is the functional unit coordinating PM and SR signaling in skeletal muscle

The triad: T‑tubule plus two terminal cisternae.

Where are Ca2+ release channels located relative to myofibrils

In terminal cisternae of SR closely apposed to T‑tubules near each sarcomere.

What initiates relaxation in skeletal muscle

Cessation of action potentials.

How is cytosolic Ca2+ reduced to end contraction

SERCA pumps Ca2+ back into the SR.

What happens to troponin–tropomyosin as Ca2+ falls

Tropomyosin re‑covers actin binding sites, preventing cross‑bridge formation.

What Ca2+ sensor activates MLCK in smooth muscle

Calmodulin.

What does MLCK phosphorylate to activate contraction in smooth muscle

Myosin regulatory light chains.

Is MLCK‑mediated light chain phosphorylation essential in skeletal muscle

No, it is not essential for skeletal muscle activation.

What structures are examples of contractile assemblies in non‑muscle cells

Stress fibers and adhesion belts.

What is a hallmark structural feature of cardiac muscle cells

Intercalated discs.

What junctions are found in intercalated discs

Desmosomes, adherens junctions, and gap junctions.

What do desmosomes in cardiac muscle provide

Structural integrity by mechanically linking cells.

What do gap junctions in cardiac muscle provide

Electrical coupling for synchronized contraction.

How many nuclei do cardiomyocytes usually have

One or two nuclei.

How can binucleation occur in cardiac cells without fusion

Karyokinesis without cytokinesis.

Clinical: What pathology can result from disrupted desmosomes in intercalated discs

Cardiomyopathy.

Where are nuclei located in skeletal myofibers by light microscopy

Peripherally at the cell’s edge.

What electron‑dense structures appear darker under EM

Myosin thick filaments.

What does a myofibril show in longitudinal section

Repeating dark (A) and light (I) bands bounded by Z discs.

What indicates the center of a sarcomere in EM

The M line.

Clinical: What mechanism do calcium channel blockers target to reduce cramping

They reduce Ca2+ signaling that precipitates SR Ca2+ release and contraction.

Clinical: Which band remains unchanged width during contraction

The A band.

Clinical: Which bands/zones decrease during contraction

The I band and often the H zone.

Clinical: What molecular event directly prevents myosin–actin interaction at rest

Tropomyosin covering myosin‑binding sites on actin.

Clinical: Why does ATP depletion increase stiffness even with normal Ca2+

ATP is required for myosin detachment; without it cross‑bridges stay locked.

What is the key event enabling actin–myosin binding in skeletal/cardiac

Ca2+‑dependent displacement of troponin–tropomyosin from actin.

What pump resets cytosolic Ca2+ to end contraction

SERCA in the SR membrane.

What anatomical feature ensures rapid EC coupling near each sarcomere

The triad aligning T‑tubules with SR terminal cisternae.