MUSCLE PHYSIOLOGY

Detailed explanation-1: -The correct answer is (c): calcium ions enter the synaptic bulb. Triggering the contraction of skeletal muscle fibers involves the stimulation of the fiber by its motor neuron. At rest, the motor neuron stores acetylcholine in synaptic vesicles.

Detailed explanation-2: -The release of acetylcholine occurs when an action potential is relayed and reaches the axon terminus in which depolarization causes voltage-gated calcium channels to open and conduct an influx of calcium, which will allow the vesicles containing acetylcholine for release into the synaptic cleft.

Detailed explanation-3: -Skeletal muscle contraction begins first at the neuromuscular junction, which is the synapse between a motoneuron and a muscle fiber. Propagation of action potentials to the motoneuron and subsequent depolarization results in the opening of voltage-gated calcium (Ca2+) channels of the presynaptic membrane.

Detailed explanation-4: -As a result of the action potential, the chemical transmitter acetylcholine (ACh) is released into the synaptic cleft.

Detailed explanation-5: -(A) Neurotransmitter (acetylcholine, ACh) released from nerve endings binds to receptors (AChRs) on the muscle surface. The ensuing depolarization causes sodium channels to open, which elicits an action potential that propagates along the cell.