ANATOMY

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.

Detailed explanation-2: -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-3: -Upon entering a presynaptic terminal, an action potential opens Ca2+ channels, and transiently increases the local Ca2+ concentration at the presynaptic active zone. Ca2+ then triggers neurotransmitter release within a few hundred microseconds by activating synaptotagmins Ca2+.

Detailed explanation-4: -Influx of calcium ions into the presynaptic nerve terminal causes vesicles (loaded with neurotransmitters) migrate toward the presynaptic membrane. Then, the vesicle and membrane fuse, and neurotransmitter is released into the synaptic cleft by exocytosis.

Detailed explanation-5: -Depolarisation and calcium ion release. Actin and myosin cross-bridge formation. Sliding mechanism of actin and myosin filaments. Sarcomere shortening (muscle contraction)