Section 3: Parts of the muscle fiber (cell)

Microfibrils

Densely packed, rodlike elements in a muscle fiber

80% of cell volume

sarcomeres

contractile units containing myofilaments (myosin and actin)

What do sarcomeres exhibit

striations - perfectly aligned repeating series of dark a bands and light i bands

What are the two filaments inside of a sarcomere

thick myosin filament and thin actin filament

“A” band

dark region

Entire myosin filament with overlapping actin filament

“i” band

light region

actin filament only → not including actin that overlaps with myosin (so basically no myosin)

H zone

light region with only myosin → disregard mysoin that overlapps with actin (so basically no actin)

Z line / Z disc

coin shaped sheet of proteins on the midline of light I bands → anchors thin filaments

What connects microfibrils to each other

Z - line / z disc

Sarcomere runs from ____ to ____

z-line to z-line

Sliding Filament Theory of
Muscle Contraction

when skeleton muscle fibers contract, myosin heads binds to actin to begin sliding

sliding → cross bridges between actin and myosin form and break to pull actin (thin filament) towards sarcomere center

Does the myosin or actin shrink during contraction

No

4 steps of the Sliding Filament Theory of
Muscle Contraction

what happens to each → h zone , i band, z line , a band

1. H band and i band get smaller

2. Overlapping zones get smaller → more overlapping

3. Z line gets closer together

4. A band stays constant

Sarcoplasmic reticulum

ER of the muscle fiber (cell) → surrounds each microfibril → stores and release Ca2+

What does the sarcoplasmic reticulum contain

pairs of terminal cisternae forming perpendicular cross channels

sacs that store Ca2+

Transverse (T) tubules

continuation of sarcolemma (plasma membrane) that penetrates the cell’s interior

Triad

encircles each sarcomere

t tubules + 2 terminal cisternae of the Sarcoplasmic reticulum

Function of t tubules

conduct electrical impulses deep into the muscle fiber → in each sarcomere

Function of the triad

Convert action potential to release Ca2+ for contraction

Motor neuron
↓
Muscle membrane (sarcolemma)
↓
T tubule
↓
SR
↓
Calcium released
↓
Contraction