Question

The generation of excitation-contraction coupling involves all the following events except
A. Generation of endplate potential
B. Release of calcium from troponin
C. Formation of cross-linkages between actin and myosin
D. Hydrolysis of ATP to ADP

Answer 1

Hint: Excitation–contraction coupling was first coined by Alexander Sandow in 1952. It describes the rapid communication between electrical events occurring in the plasma.

Complete Answer:
Excitation–contraction coupling is a physiological process that converts an electrical stimulus to a mechanical response. It is the link between the action potential generated in the sarcolemma and the start of a muscle contraction.

The various steps involved in the excitation-contraction coupling:
Step 1: Action potential extend along the sarcolemma to the T-tubules (transverse tubules)
Step 2: Calcium is released to the sarcoplasmic reticulum
Step 3: Calcium binds with actin and the blocking action of the tropomyosin is removed.
Step 4: Myosin heads attach to begin the contraction.
Step 5: Calcium is detached and the binding sites on actin turn blocked again by tropomyosin.
Step 6: Muscle relaxes.

- Excitation-contraction coupling is the machinery that links plasma membrane stimulation with cross-bridge force production. The muscle receives a neural signal and converts the signal into mechanical force after synapsing at the neuromuscular junction. The chemical signal for this process comes from the release of acetylcholine.
- ATP Provides energy for power stroke of the myosin head. And it brings about a dissociation of myosin head from actin filament. It helps muscle relaxation by pumping $Ca^{2+}$ back into the sarcoplasmic reticulum.
- The ECC mechanism represents a rapid communication between electrical actions occurring in the plasma membrane and the $Ca^{2+}$ release from the SR, which leads to skeletal muscle contraction.
- It takes place in multiple levels in hearts which involves single myocytes even to the sarcomeres. The central process for this contraction is calcium mobilization. The calcium ions in this process get bound to protein troponin and myosin binds to actin.

Hence, the correct option is B, “Release of calcium from troponin”.

Note: the motor neuron connects to a muscle at the neuromuscular junction, where a synaptic terminal forms a synaptic cleft with a motor-end plate. The neurotransmitter acetylcholine diffuses across the synaptic cleft, causing the depolarization of the sarcolemma. The depolarization of the sarcolemma stimulates the sarcoplasmic reticulum to release $Ca^{2+}$, which causes the muscle to contract.