actin

tropomyosin

troponin (3 subunits)

• 6 subunits
• one pair of heavy chains
• two pairs of light chains
• two globular heads
  • light chain
  • amino part of heavy chain

Binding sites within globular heads

actin

site that hydrolyzes ATP

• multinucleated
• long, thin, and cylindrical
• striated (regularity of fibers making up actin/myosin contractile system)

• attach to skeletal system by tendons
• made of many individual muscle cells called muscle fibers aranged in parallel fashion to maximize their effectiveness

• skeletal muscle made of muscle fibers
• muscle fibers made of myofibrils
• myofibrils contain SR
• SR contains terminal cisterns

1. motor unit fires → A spread to nerve terminal at neuromuscular junction
2. presynaptic release of ACh
3. causes activation of nicotinic receptors, causing depolarizing end plate potential (EPP)
4. increase in sodium and potassium condunctances generate AP at end plate
5. AP spread thru muscle fibers
6. spread of depolarization along T tubule system
   1. causes configuration change in DHPR  (loc.- T tublules) leading its activation
7. DHPR interact with and allow RYR to open
   1. allow calcium to flow down gradient into cytoplasm from the SR, especially terminal cisterns
8. calcium bind to TnC → uncover myosin binding site on actin
   1. cause time lag between increase in calcium and muscle twitch
9. ATP will hydrolyze within hinge of myosin head and head binds to actin
   1. cause thin filament to slide along thick filament to create muscle fiber contraction

1. cytosolic calcium pumped back into SR via SERCA (Ca/ATP transporter), and sequestered by calsequestrin (low affinity binding protein)
   1. if blocked, causes contracture/sustained contraction
2. decrease in calcium, so it cant bind to TnC
3. no more cross linking

ATP DEPENDENT, like contraction

• constant muscle length
• generates force
• example: muscle conract during standing still or holding a heavy load steady

• force constant
• decrease length
• example: lifting up heavy objects

ONLY TYPE OF CONTRACTION THAT CAN DO WORK, BECAUSE THERE IS DISPLACEMENT OF OBJECT.

contraction where frequency of action potentials is high enough to produce a continuous maximal contraction in the muscle

• total
• passive
• active (cant be measured, so must find the other two tensions to solve problem)

At what point is passive tension in muscle fiber the highest?

Describe the point at which active muscle tension and muscle contraction velocity is highest.

• increases with increasing length/preload to the point at which most cross bridges can form (aka normal rest length)
• if it strecteches too much, the active tension will go down

• slow twitch
• red muscle (no myoglobin)
• function- responsible for prolonged conractions

• fast twitch
• white muscle
• fatigue more rapidly
• depend on glycolytic metabolism

• no gap junctions
• each innervated by en passant bouton

• longer lasting than skeletal muscle
• more discrete, localized

norepi.

epi.

Mechanism of action of angiotension II receptor

angiotensin II stimulates increase in PLC → increases in calcium

• increase norepi, epi leading to stimulation of alpha-1 adrenergic receptor → increase IP3 leading to increase in calcium
• increase angiotensin II stimulatin Ang II receptor → increase PLC leading to increase calcium
• depolarization open L type calcium channel

Cellular mechanism of action of single unit smooth muscle once calcium increase occurs

1. four calciums bind to calmodulin
2. activates MLCK
3. leads to phosphorylation of myosin light chain
   1. this will cause it to form cross bridge with actin → contraction
   2. it could be attacked by myosin phophotase, removing the phosphate, leading to latch bridge (tonic tension)

1. diffuse into cell
2. increase cGMP
3. increases myosin phophatase activity
   1. remove phosphate from myosin light chain leading to latch bridge/tonic tension

1. Stimulated by epinepherine
   leading to increase of cAMP
2. causes phosphorylation of MLCK
3. leading to its inactivation
4. causing relaxation

smooth muscle of intestines

smooth muscle of bv's

• muscarinic receptor activation via ACh
  • GI- decrease cAMP levels (M2 subtype) and activates PLC increasing calcium leading to contraction
  • blood vessel- M3 subtype receptor increase nitric oxide leading to relaxing of smooth muscle to decrease blood pressure

• electric connection via gap junction (functional synctium)
• form large sheets of muscle
  • location- GI tract, bladder, uterus

What external stimulus will activate single unit smooth muscle?