Final answer:
Muscle contraction is initiated by electrical impulses transmitted from the brain to motor neurons, which release the neurotransmitter ACh at the neuromuscular junction, leading to the contraction of muscle fibers.
Step-by-step explanation:
The signals responsible for initiating muscle contraction are electrical impulses. This process begins when the brain sends electrochemical signals through the somatic nervous system to motor neurons that innervate muscle fibers. The connection between a motor neuron axon terminal and a muscle fiber occurs at a neuromuscular junction site. Here, an electrical impulse triggers the release of the neurotransmitter acetylcholine (ACh). ACh crosses the synaptic cleft to bind to receptors on the muscle fiber, causing sodium ions to enter the fiber. This depolarization leads to an action potential that sparks muscle contraction through the release of calcium ions from the sarcoplasmic reticulum, which then initiates the contraction of muscle fibers sustained by ATP.