Final answer:
Gap junctions and desmosomes are the two membranous structures within muscle fibers that play important roles in linking excitation to contraction.
Step-by-step explanation:
The two membranous structures within muscle fibers that play important roles in linking excitation to contraction are gap junctions and desmosomes.
Gap junctions form channels between adjacent muscle fibers and allow the flow of depolarizing current, which helps in the quick transmission of action potentials and coordinated contraction of the entire muscle.
Desmosomes are cell structures that anchor the ends of muscle fibers together, preventing them from pulling apart during contraction.