Final answer:
The neurotransmitter that causes muscle fibers to contract at the neuromuscular junction is acetylcholine (ACh). It is released from motor neuron terminals when stimulated by an action potential, binds to receptors on the muscle fiber, and initiates a chain of events leading to muscle contraction.
Step-by-step explanation:
Neuromuscular Junction and Muscle Contraction
The neurotransmitter released by effector neurons to cause muscle fibers to contract is acetylcholine (ACh). When an action potential—the nerve impulse—travels down the axon of a motor neuron, it arrives at the axon terminal within the neuromuscular junction, a specialized synapse between a neuron and muscle fiber. This triggers the release of ACh from synaptic vesicles into the synaptic cleft between the neuron and muscle fiber. ACh then binds to receptors on the muscle fiber's membrane, initiating depolarization and a subsequent muscle contraction.
Acetylcholine acts on the sarcolemma (the muscle fiber's plasma membrane), and when it binds to the ACh receptors, ligand-gated ion channels open, allowing for an influx of sodium ions. This change in ion concentration alters the electrical charge across the sarcolemma, creating an action potential that travels along the muscle fiber. The action potential eventually leads to the release of calcium inside the muscle cell, initiating the sliding of myosin and actin filaments, which is the basis for muscle fiber contraction. The process continues until the neurotransmitter is broken down by the enzyme acetylcholinesterase, causing an end to the action potential and, therefore, muscle contraction.