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28 Cards in this Set

  • Front
  • Back
what are the two main "players" in skeletal muscle contraction?
Myosin and Actin
What 3 things regulate skeletal muscle?
What is the function of the alkali light chain on the myosin heavy chain?
alkali stabilizes myosin head
regulates myosin ATPase activity
What holds myosin in place?
(anchored at z and m lines)

-largest protein identified in nature
-fx in control of myosin assembly, muscle elasticity, and generation of passive tension
What holds actin in place?

-helps keep actin aligned and determines length of sarcomere
In the absence of ATP, what is the state of skeletal muscle?
What are the 5 steps of muscle contraction?
1. ATP binds myosin head: actin & myosin dissociate
2. ATP hydrolysis (myosin ATPase)
3. Cross-bridge formation: myosin binds actin at new site (if enough Ca2+ present in cytosol)
4. Release of P from myosin: power stroke occurs
5. ADP release
(muscle will contract as long as Ca2+ is present, need repetitive action potentials for this)
Rigor mortis is caused by what?
loss of ATP which is required to release myosin actin cross bridge leading to rigidity
muscle membrane potential is....

the activation threshold for the membrane is......

(larger stimulus required than for neuron AP)
How does a muscle action potential differ from a neural action potential (3 ways)?
-resting potential is more neg
-duration of action potential is longer
-velocity of conduction is slower
How does excitation of skeletal muscle occur?
-an action potential in the motor-neuron in the anterior horn of the spinal cord releases ACh into neuromuscular junction/motor-end plate
-ACh binds a nicotinic receptor on an ion channel and the membrane of the skeletal muscle fiber becomes permeable to Na+ and K+ leading to depolarization
-local end-plate potential is created
How is a signal terminated (2 steps) ?
1. hydrolysis of ACh by ACHe and re-uptake into pre-synaptic terminal
2. diffusion of ACh away from synapse
AChE hydrolyzes Ach into what 2 things?
choline and acetate
(2-step process, choline then acetate released)

*choline used for re-synthesis of ACh
DHRP (dihydropyridine) is on the (SR/T-Tubule). What is mechanically coupled to DHPR?
DHRP is a voltage sensor "L-type" Ca2+ channel on the t-tubule
(4 DHRPs make a tetrad on T tubule faces)

RyR is mechanically coupled to the DHPR
RyR (ryanodine receptor) is on the (SR/T-tubule). What happens to RyR when DHPR undergoes a conformational change?
RyR is on the SR
(each RyR has 4 subunits that each bind a DHPR)

confirmational change on DHPR opens the RyR (Ca2+ release channel), it is "unplugged"
(when Ca2+ is higher in SR it allows it to move out into cystol)
What are the 3 steps of calcium release from SR?
1. membrane depolarization opens DHRP
2. DHRP causes RyR to open
3. Ca2+ exits SR via RyR and activates troponin C, leading to muscle contraction
for muscle relaxation to occur what must happen?
Calcium must be removed from cystol and pumped into SR via SERCA pumps (uses ATP)

(Calcium binds calsequestrin w/i SR)
If an electrical stimulus is applied to a muscle, but no Calcium is present in cystol, will contraction still occur?
yes, DHPR and RyR can initiate contraction with an electrical impulse, releasing calcium into cytosol, initial calcium presence not necessary
Where are the 3 major sites that drugs effect to inhibit synaptic (neuromuscular) transmission?
give an example of each
1. neuronal side of synapse
2. within synaptic cleft
3. on the postsynaptic membrane
At the neuronal side of synapse what are 4 ways to effect signal transmission?
1. block voltage-dependent Na+ channel
2. block voltage-gated K+ channel
3. bind and inhibit Ca2+ channel on nerve terminal
4. cleave key proteins involved in exocytosis
Within the synaptic cleft, what are 2 ways to effect signal transmission?
1. produce inactive AChE
2. inhibit enzyme
At the postsynaptic membrane what are 3 ways to effect signal transmission?
1. block voltage-dependent Na+ channel
2. bind nicotinic AChR
3. inhibit AChR receptor
(AChR agonist & antagonist)
What 2 regulatory proteins are associated with F-actin?
Troponin and tropomyosin
In relaxed state, tropomyosin covers ____________________
myosin binding site on actin
What are the 3 units of the troponin complex and what do they do?
Troponin T (TnT)- binds tropomyosin
Troponin I (TnI)- blocks myosin binding site on F-actin filament (inhibitory site)
Troponin C (TnC)- has 4 Ca2+ binding sites that regulate actin-myosin interaction (regulatory site)
What is the triad made up of?
1 T-tubule (penetrates deep into muscle)
2 sarcoplasmic reticulum cisternae (Ca2+ storehouse)
(one on each side of T tubule)
Troponin C (TnC) has 4 Ca2+ binding sites, 2 high affinity and 2 low affinity, when the low affinity sites are bound what happens?
TnI is moved away from actin
Tnt is pushed away from myosin binding site
--> this initiates muscle contraction via cross bridge cycling
how does an action potential lead to contraction?
action potential--> increased calicum--> muscle contraction