MUSCLE PHYSIOLOGY

Detailed explanation-1: -As the membrane depolarizes, another set of ion channels called voltage-gated sodium channels are triggered to open. 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-2: -The increase in the Na+ influx leads to a further depolarization. A positive feedback cycle rapidly moves the membrane potential toward its peak value, which is close but not equal to the Na+ equilibrium potential. Two processes which contribute to repolarization at the peak of the action potential are then engaged.

Detailed explanation-3: -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.

Detailed explanation-4: -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-5: -Activation of the nAChR leads to an influx of cations (sodium and calcium) that causes depolarization of the muscle cell membrane. This depolarization in turn activates a high density of voltage-gated sodium channels on the muscle membrane, eliciting an action potential.