Regulation of Muscle Contraction

Question 1

what is excitation contraction coupling?

Answer 1

1. action potential propagates along the sarcolemma and T system
2. this causes the release of Ca2+ from the terminal cisternae so that sarcoplasmic Ca2+ concentration increases from 10-7 to 10-5 molar
3. troponin binds Ca2+ causing conformational changes which allow crossbridge formation with actin and releases of ADP and phosphate from the Myosin ATPase
4. further conformational changes move the crossbridges thereby pulling the actin filaments toward the center of the A band
5. myosin ATPase binds ATP causing dissociation of the crossbridges, hydrolysis of ATP and return to the ‘cocked’ position for further cycles
6. relaxation occurs when the sarcoplasmic Ca2+ concentration returns to normal by active transport of Ca2+ back into the sarcoplasmic reticulum where it is bound to calsequestrin

Question 2

what receptor detects the change of volage in the transverse tubule?

Answer 2

the Dihydropyridine receptors = acts as a voltage sensor which causes the ryanodine receptor in the Sarcoplasmic reticulum to open it’s Ca2+ channel

Question 3

Describe the propagation of an action potential through a muscle cell and how it leads to muscle contraction

Answer 3

1. axon terminal of somatic motor neuron releases acetylcholine at neuromuscular junction
2. net entry of Na+ through ACh receptor channel initiates a muscle action potential
3. action potentila in tubule alters the conformation of the DHP receptor
4. the DHP receptor opens Ca2+ release channels in sarcoplasmic reticulum and Ca2+ enters cytoplasm
5. Ca2+ binds to troponin allowing strong actin-myosin binding
6. myosin heads execute power stroke
7. actin filament slides toward center of sarcomere

Question 5

Does the muscle require energy to contract? Or does it require energy to relax?

Answer 5

energy is not being used to move the head along the actin filament, energy is only used to dissociate the myosin head from the actin - this isn’t intuitive - you would think that it requires energy to move inwards

Question 6

what is malignant hyperthermia?

Answer 6

a rare disorder caused by anaesthesis - if untreated rise in body temperature, muscle rigidity, damage and death

caused by abnormal ryanodine receptor reaction with agents used in anaesthesis resulting in excessive calcium entering the sarcoplasm

treated by Dantrolene - a muscle relaxant that blocks the ryanodine receptor

Question 7

is there more ATP or creatine phosphate stored in a cell for energy use?

Answer 7

there is a small store of ATP and a large store of creatine phosphate intracellularly

Question 8

does additional creatine in your diet improve your muscle performance levels?

Answer 8

dietary creatine can increase muscle creatine phosphate levels, however the evidence for the effectivenes sof this improving sports performance isn’t great

Question 9

when there isn’t oxygen available to the muscle, how does the muscle generate energy?

Answer 9

we have a very elaborate glycolytic system - so the skeletal muscle cells can also genergate ATP via anaerobic glycolysis by breaking glucose down into lactic acid and producing 2ATP with the reduction of NAD+ to NADH

Question 10

describe the chemical energy used for muscle contraction from the first second to 4 hours later

Answer 10

• first 1-2 s = ATP
• 5-8 s = ADP + phosphocreatine
• 1min = breakdown of glycogen to pyruvic and lactic acids
• oxidative metabolims - carbohydrates, fats, proteins, hours
• 2-4 hours = stored glycogen, muscle, liver

Question 11

high serum levels of what are useful indicator of muscle disease?

Answer 11

high serum levels of creatine kinase are useful indicator of muscle disease - due to muscle breakdown

Question 12

Describe how Rigor Mortis occurs

Answer 12

there is no ATP available anymore to dissociate the myosin from the actin - the crossbridge cycle will always end with the myosin head sticking to the actin- eventually after a few hours the stiffness will dissapear b/c of the breakdown of the cells - causes the myosin heads themselves to break down t

Question 13

what is an isotonic contraction?

Answer 13

tension increases until it equals the weight to be lifted (actually slightly more) - then the muscle shortens and the tension stays constant

isotonic = same tension

Question 14

what is an isometric contraction ?

Answer 14

during isometric contractions, tension is developed but the overall length of the muscle does not change (isometric = same length)

the tension will remain the same so long as there is no acceleration if it maintains a constant velocity- the tension will remain the same

*in reality, you’ll never get a purely isotonic or isometric concentric contraction -

Question 15

in an isometric contraction? What part lengthens, what part shrinks?

Answer 15

contractile element = shrinks

elastic element = lengthens

Question 16

what is an eccentric contraction?

Answer 16

isotonic contractions are either concentric or eccentric in which tension is developed but the muscle lengthens due to an external force