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- Modified version of endoplasmic reticulum that wraps around myofibrils.

- It consist of Ca²⁺ which are released from longitudinal tubules

- They aid in the contraction of muscles

- Myosin – ability to create movement

- Actin – makes up the thin filaments of muscle fiber

- Long tail and round head connected by hinge (allows for a swivel mov’t)

- When 250 myosin come together they form a swiveling staircase

- Sarcomeres are pattern segment in myofibril.

- Starts and ends with z disk (thin filiments/ I band ) --> A band (mainly thick filaments but over laps with thin filiments (actin) H zone --> z disk

- T-tubules are lumen in the muscle fiber (myofibrils) that allow for action potential to reach the center at a faster rate

- Action potential would run at a slower rate b/c it would rely on diffusion of + ions from cytosol --> delaying myofibrils response

- Tropomyosin – protein polymer that wraps actin to prevent binding of myosin to actin (* cock blocker) (when muscle isn’t contracting)

- Troponin – turn on/off contraction à b/c its a Ca binding complex of 3 proteins, which positions tropomyosin

- Titin – huge elastic protein (largest, 25,000 AA)

*helps stabilize*

*returns muscle to rest place*

- Nebulin – inelastic huge protient hat attaches to z disk from thin filaments (helps align actin)

- The force created by contracting muscles

- Load

- Overlapping of actin and myosin slide pass each other (energy dependent) during contraction resulting in the sliding filament theory of contraction

- Decrease the level of Ca in cytosol

- Ca²⁺ ATPase pumps Ca²⁺ back in to the sarcoplasmic reticulumà stops Ca²⁺ from binding to troponin

- The sarcomere gets shorter b/c myosin binds to actin forming a power strike (overlapping in H zone)

• - Occurs during contraction, in which Ca binds to troponin -->
• allows tropomyosin to shift -->
• creates myosin binding site to open-->
• myosin hydrolyses ATP (releases Pi ) -->
• crossbridge tilting of 45 degrees from 90

- ATP releases Pi leaving ADP

- The myosin head moves from 90 degrees to 45 (cock blocker is removed)

- Head swivels and swings toward M line

- Process in which muscle Ψ initiates ca signals at neuromuscular junction

- --> turns on contraction – Relaxation cycle (muscle twitched)

- Release of Ach by a somatic motor neuron

- Ψ of myofibrils

- Increase in Ca²⁺ from sarcoplasmic reticulum

- Ca binds to troponin

- Single muscle contraction-relaxation cycle

- Acetylcholine

- Cant initiate there own contraction

- Are not control directly by hormones

- Motor end plate (receptors)

- These receptors allows influx of Na --> Ψ b/c more Na in cell due to ∆ membrane

- Dihydropyridine channels (DHP) (voltaged gated)

- DHP is mechanically linked to ryanodine recptors channels on SR

- Myosin ATPase – crossbridge mov’t and relase

- Ca ATPase – collect excess ca in cytosol and pumps back to SR

- Na – K ATPase – restores the amount of Na in cytosol to initial state (in order for another contraction to occur)

- Physical – muscle cant generate power input

• - Psychological – (central fatigue) long day at work
• --> tired --> party --> you rally

- Decrease in Neurotransmitter release (dosnt occur w/normal excesice)

*Ach not synthesized fast enough in axon to keep up with firing rate

*threshold at motor end plate not reachedà no Ψ in t-tubule (due to in activity of Na – K ATPase)

- depletion of glycogen stored in muscle --> leak of Ca²⁺ from SR

- increase in Pi from phosphocreatine in muscle fiber b/c Ca2+ binds to it to form Ca²⁺ phosphate (major in fatigue)

- White muscles have less myoglobin, O2 larger diameter few BV --> run out of O2 --> rely on aerobic glycolysis --> faster fatigue

- Red muscle have more myoglobin --> red color --> 02 binding pigment à more oxygen diffusion due to smaller diameter --> rely on glucose

*Better oxidative phosporylation for ATP production*

- The actin and myosin will overlap before the initial contraction begins

- Preventing crossbridge formation (mov’t of actin by myosin ATPase)

- Incomplete has action potential that are close together w/ a slight point and reaches maximum tension

- Complete tetanus also reaches maximum tension, but the Ψ does not relax between stimulus --> steady tension

( fatigue causes lost of tension even when continuous stimulus)

- We can change the rate of the Ψ in muscle fiber

- Muscles are composed of multiple MU of different type (diversity)


§ Changing the type of MU that are active

§ Changing the number of MU that are responding

- Creates force and moves load (bicep curls)

§ Primary muscles shortens

- Resisting the weight from pulling down completely

§ Muscles have a delayed muscle sorness

• - Contraction that
• create force w/o moving load (front/lateral raises)

§ Muscles doesn’t shortens b/c load is not being moved

§ Not enough force is created to move load

- b/c the sarcomere shortens more after the elastic components are stretched