The muscle fiber is surrounded by a connective tissue basement membrane that continues into the synaptic cleft, sending extensions into the junctional folds. The basement membrane contains a large amount of collagen as well as most of the acetylcholinesterase (AChE) in the neuromuscular junction (see Plate 2-11). It also contains other important molecules that help guide the growth of the nerve terminal during development and regeneration, determine the locations of the presynaptic active zones, and induce the clumping of acetylcholine receptors opposite the synaptic vesicle.