Question

Number the events below 1 – 7 to represent the correct sequence of events in skeletal muscle contraction and relaxation

___Ca2+ binds to troponin; tropomyosin moves, exposing the active site of actin

___Acetylcholine (ACh) triggers an end-plate potential in the motor end plate.

___ The motor neuron stops releasing ACh and Acetylcholinesterase degrades the ACh in the synaptic cleft

___An Action potential in the sarcolemma travels down the T-Tubules

___ Ca2+ is released from the sarcoplasmic reticulum into the cytosol

___ The cytosolic concentration of Ca2+ returns to the resting level, the active sites of actin are blocked, and the muscle fiber relaxes

___ Actin and myosin bind, and myosin undergoes a power stroke. ATP detaches actin and myosin, and the cycle repeats resulting in contraction of the muscle fiber

Answer 1

1) Acetylcholine (ACh) triggers an end-plate potential in the motor end plate.

2) An Action potential in the sarcolemma travels down the T-Tubules

3) Ca2+ is released from the sarcoplasmic reticulum into the cytosol

4) Ca2+ binds to troponin; tropomyosin moves, exposing the active site of actin

5) Actin and myosin bind, and myosin undergoes a power stroke. ATP detaches actin and myosin, and the cycle repeats resulting in contraction of the muscle fiber

6) The motor neuron stops releasing ACh and Acetylcholinesterase degrades the ACh in the synaptic cleft

7) The cytosolic concentration of Ca2+ returns to the resting level, the active sites of actin are blocked, and the muscle fiber relaxes

mechanism of muscle contraction

The voltage-gated ion channels of the sarcolemma next to the end plate open in response to the end plate potential. These voltage-gated channels are sodium and potassium specific and only allow one through. This wave of ion movements creates the action potential that spreads from the motor end plate in all directions.

Excitation–contraction coupling occurs when depolarization of skeletal muscle cell results in a muscle action potential, which spreads across the cell surface and into the muscle fiber's network of T-tubules, thereby depolarizing the inner portion of the muscle fiber. Depolarization of the inner portions activates dihydropyridine receptors in the terminal cisternae, which are in close proximity to ryanodine receptors in the adjacent sarcoplasmic reticulum

As the ryanodine receptors open, Ca2+ is released from the sarcoplasmic reticulum into the local junctional space, which then diffuses into the bulk cytoplasm to cause a calcium spark.

The Ca2+ released into the cytosol binds to Troponin C by the actin filaments, to allow crossbridge cycling .

Relaxation occurs when stimulation of the nerve stops. Calcium is then pumped back into the sarcoplasmic reticulum breaking the link between actin and myosin. Actin and myosin return to their unbound state causing the muscle to relax.