L14: Skeletal Muscle

What is a muscle?

Whole organ that contains bundles of muscle fibers

What is a fascicle?

Bundle of muscle fibers (cells) wrapped together by connective tissue (perimysium)

What is a muscle fiber?

A single long muscle cell that can contract

What is a myofibril?

Long, rod-like contractile structure inside muscle fiber

What are myofilaments?

Thread-like protein strands

What is the organization of skeletal muscle tissue from smallest to largest?

1. Myofilament

2. Myofibril

3. Muscle fiber

4. Fascicle

5. Muscle

What connective tissue layer surrounds the entire muscle?

Epimysium

What connective tissue layer surrounds fascicles?

Perimysium

What connective tissue layer surrounds individual muscle fibers?

Endomysium

What surrounds and separates groups of muscles?

Deep fascia

What is in the presynaptic terminal of NMJ?

ACh synaptic vessicles

What is the motor end plate in the NMJ?

specialized, folded sarcolemma that contains nicotinic ACh receptors

What is sarcolemma?

Specialized cell membrane that surrounds muscle cells

Which structure within the synaptic cleft breaks down ACh?

Basal lamina

What travels down the motor neuron’s axon to presynaptic terminal?

Action potential

What opens to allow Ca2+ to enter the terminal?

Voltage-gated Ca2+ channels

What triggers synaptic vesicles to fuse with membrane?

Ca2+

What binds to nicotinic receptors?

ACh

What produces an end-plate potential?

ACh binding to nicotinic receptors

What does end-plate potential do?

Triggers muscle action potential

What binding happens at the NMJ that elicits action potential?

ACh binds to nicotinic receptors on motor end plate, opening ligand-gated channels

What happens with the ions after ACh binds?

Influx of Na+ and K+ efflux

What causes depolarization (EPP)?

Entry of positive charge

What happens in sarcolemma if threshold is reached?

Depolarization opens voltage-gates Na+ channels in adjacent sarcolemma

Where does action potential propagate?

Muscle fiber

What is excitation-contraction coupling?

Process where an action potential in sarcolemma leads to Ca2+ release from sarcoplasmic reticulum—resulting in initiation of muscle contraction

What is the resting membrane potential of muscle fibers?

-85mV

Does muscle tension last longer than electrical signal?

Yes

Where does AP propagate?

Sarcolemma and T-tubules

What enables muscle contraction?

Ca2+ released from sarcoplasmic reticulum

What causes contraction summation?

AP triggers additional force before muscle fully relaxes (multiple action potentials)

Why can contraction summation happen?

Skeletal muscle refractory period ends before contraction ends

What is sarcoplasm?

Cytoplasm of muscle fiber

What is T-tubule?

Inward fold of sarcolemma that extends into muscle fiber

What does T-tubule allow?

Rapid transmission of action potentials from surface to interior

What is sarcoplasmic reticulum?

Smooth ER that stores and releases Ca2+ for muscle contraction and takes it back up for relaxation

What is terminal cisternae?

Enlarged regions of SR beside the t-tubules

What does terminal cisternae store/release?

Ca2+

What is triad made of?

T-tubule + 2 terminal cisternae

What are the 3 structures that link excitation to contraction?

1. NMJ

2. triad

3. sarcomere

How does NMJ link excitation to contraction?

Initiates muscle action potential

How does triad link excitation to contraction?

Couples action potential in T-tubule to Ca2+ release from SR

How does SR link excitation to contraction?

Generates contractile force

Where does electrical signal travel along for calcium release?

Sarcolemma

Where does action potential travel down for calcium release?

T-tubules

What does depolarization activate in calcium release?

DHP receptors

What does activation of DHP receptors do?

Triggers opening of ryanodine receptors in SR, which releases Ca2+ in sarcoplasm

What does Ca2+ enable?

Actin-myosin interaction

What does actin-myosin interaction lead to?

Muscle contraction

What are actin filaments made of?

Globular actin (G-actin) units arranged in 2 intertwined strands

What does each actin unit have?

Myosin-binding site covered by tropomyosin at rest

What is troponin?

Small protein complex attached to tropomyosin that has 3 subunits

What are the 3 subunits of troponin?

1. Tn-C (Troponin C)

2. Tn-I (Troponin I)

3. Tn-T (Troponin T)

What does Tn-C do?

Binds Ca2+, which triggers contraction

What does Tn-I do?

Inhibits—blocks actin-myosin interaction at rest

What does Tn-T do?

Anchors troponin to tropomyosin

Tropomyosin function?

Prevents contraction when muscle is relaxed

What does myosin look like?

Has a tail that forms the filament and two heads

Which filament is thin/thick?

Actin= thin

Myosin= thick

What does myosin head do (3)?

1. Binds to actin

2. Binds + hydrolyzes ATP

3. Power stroke (pulls for force generation)

What are light chains on myosin?

Small proteins attached to the neck that help stabilize and regulate movement

What moves during power stroke?

Myosin neck

What does ATP binding do to myosin head?

Causes myosin to detach from actin (new ATP forces the head to let go of actin)

What does ATP hydrolosis do to myosin head?

Breaks ATP molecule to provide energy to pull myosin head back into position

What allows myosin heads to bind and pull?

Ca2+ exposes myosin-binding sites on actin

Where does Ca2+ bind?

Troponin C

What happens when Tn-C binds to Ca2+?

Tropomyosin shifts, which exposes the binding site

What happens after myosin binds actin?

Myosin head pulls and there is contraction

What happens to myosin at rest?

Can’t bind b/c Tropomyosin blocks myosin-binding sites on actin

When is cross-bridge formed?

Cross-bridge is formed when myosin binds to actin

How is myosin head energized?

ADP + Pi bond

When does binding occur?

When Ca2+ exposes binding sites on actin

What promotes power stroke?

Pi is released

What pulls actin toward center of sarcomere?

Myosin head pivots

When is ADP released?

When myosin head pivots

What binds to myosin head after ADP is released?

New ATP

What causes myosin head to let go of actin?

ATP binding to myosin head

What hydrolyzes ATP?

Myosin head

When does myosin detach?

ATP binding

When does myosin re-cock?

ATP hydrolosis

What do myofibrils contain?

Repeated sarcomeres arranged end-to-end

What is the smallest unit of muscle contraction?

Sarcomere

Where does sarcomere extend from?

One Z disc to next Z disc

What is the Z disc line?

Protein structure that anchors actin and marks boundary of sarcomere

What is M line?

Structural proteins that hold and align thick filaments at sarcomere center

What is Titin?

Keeps thick filaments centered and supports elastic recoil

What is H zone?

Central part of A band that contains thick filaments only

What is I band (light)?

Contains thin filaments but no thick filaments

When does muscle contraction occur?

When thin actin filaments slide past thick myosin filaments

Why do muscle fibers shorten?

Because sarcomeres shorten

What remains the same when sarcomere shortens?

Filament length remains unchanged and A band remains the same

What changes when sarcomere shortens?

I band shortens and H zone decreases

What is dystrophin?

Structural protein located under sarcolemma

What is function of dystrophin?

Protects muscle fiber

What links cytoskeletal actin to extracellular matrix?

Dystrophin

What does connection between cytoskeletal actin and ECM do?

Stabilizes sarcolemma and protects it from damage during contraction

What is duchenne muscular dystrophy caused by?

Repeated sarcolemma damage