Final answer:
Motor neurons allow nerve impulses that initiate muscle contraction to travel along the sarcolemma, leading to muscle fiber contraction at the neuromuscular junction. So, the correct answer is option d.
Step-by-step explanation:
The correct answer to the question is D) Motor neurons. Motor neuron axons connect to muscle fibers at a neuromuscular junction, a key site where the nerve impulse initiates muscle contraction. When an action potential travels along the motor neuron to its axon terminals at the neuromuscular junction, it leads to the release of the neurotransmitter acetylcholine (ACh).
ACh binds to receptors on the sarcolemma, which is the muscle fiber's membrane, and this binding causes ligand-gated ion channels to open. The influx of cations depolarizes the sarcolemma, creating an action potential that travels along the sarcolemma, triggering the muscle contraction process. This excitation-contraction coupling is crucial for muscles to function properly.