MICROANATOMY

Detailed explanation-1: -Answer and Explanation: The correct answer is (b): Depolarization occurs when sodium ions enter into the muscle fiber cell causing the membrane potential to move towards +30 mV. Cholinergic receptors embedded in sarcolemma are ligand-gated sodium channels that open when they bind acetylcholine.

Detailed explanation-2: -The depolarization, also called the rising phase, is caused when positively charged sodium ions (Na+) suddenly rush through open voltage-gated sodium channels into a neuron. As additional sodium rushes in, the membrane potential actually reverses its polarity.

Detailed explanation-3: -Sodium ions enter the muscle fiber, and an action potential rapidly spreads (or “fires”) along the entire membrane to initiate excitation-contraction coupling.

Detailed explanation-4: -In neurons, the rapid rise in potential, depolarization, is an all-or-nothing event that is initiated by the opening of sodium ion channels within the plasma membrane. The subsequent return to resting potential, repolarization, is mediated by the opening of potassium ion channels.

Detailed explanation-5: -The resting potential is the state of the membrane at a voltage of −70 mV, so the sodium cation entering the cell will cause it to become less negative. This is known as depolarization, meaning the membrane potential moves toward zero.