Properties of Muscle Contraction

Question 1

3 steps of excitation-contraction coupling

Answer 1

1. excitation - motoneuron excites the muscle fibre; electro-chemical-electro transmission
2. contraction - cross bridge cycling -> sliding of the myofilaments; shortens sarcomere -> muscle produces force/tension
3. relaxation - re-uptake of Ca2+ in internal store; cross bridge de-attachment; return to resting state

Question 2

step 1. excitation - 6 steps of the neuromuscular junction

Answer 2

1. depolarisation of axon terminal -> voltage-gated Ca2+ channels open -> Ca2+ enters axon terminal
2. Ca2+ triggers neurotransmitter (ACh) release from vesicles into the synaptic cleft
3. ACh diffuse across the synaptic cleft
4. ACh binds to its ACh-receptor (chemically-gated Na+ ion channel) on the motor end plate of the myofiber
5. Na+ enters the myofiber -> membrane potential increases from -70 to -60mV -> depolarisation of the myofiber
6. the action potential propagates along the sarcolemma of the myofiber

Question 3

step 1. excitation - 6 steps of the Ca2+ release from the sarcoplasmic reticulum

Answer 3

1. action potential propagates along the sarcolemma of the myofiber
2. the sarcolemma dives into the myofiber -> t-tubule
3. the membrane of the t-tubule is within close proximity to the membrane of the sarcoplasmic reticulum (SR), the internal Ca2+ store
4. when the action potential arrives at the t-tubule, it initiates Ca2+ release from the SR
5. Ca2+ in the sarcoplasm diffuses to the myofilaments
6. this initiates cross bridge cycling - the sarcomere shortens - muscle contraction

Question 4

step 3. relaxation - 2 steps of the Ca2+ re-uptake in the sarcoplasmic reticulum

Answer 4

1. Ca2+ in the sarcoplasm diffuses away from the myofilaments and the Ca2+ is taken back up into the sarcoplasmic reticulum (SR). this consumes ATP (energy)
2. this terminates cross bridge cycling - sarcomere lengthens - muscle relaxation

Question 5

step 2. cross bridge cycle - the sarcomere; what is it? filaments? contraction?

Answer 5

what is a sarcomere?
contractile unit of a myofiber
made up fo filaments and associated proteins
spans from z-line to z-line
filaments:
actin - thin filament (with troponin complex)
myosin - thick filament (with myosin heads)
contraction:
z-lines move closer together, actin-myosin slide into each other
sliding filaments
sarcomere shortens