Which protein in the sarcolemma specifically serves as a receptor for acetylcholine?

The protein in the sarcolemma that specifically serves as a receptor for acetylcholine is the ACh receptor. This receptor is integral for neuromuscular transmission, as it binds acetylcholine, a neurotransmitter released from motor neurons at the neuromuscular junction. When acetylcholine binds to its receptor on the sarcolemma, it causes ion channels to open, leading to depolarization of the muscle membrane and ultimately triggering muscle contraction.

Myosin and actin are contractile proteins within the muscle fibers responsible for muscle contraction through the sliding filament mechanism. While they play critical roles in muscle function, they do not serve as receptors for acetylcholine.

Acetylcholinesterase, on the other hand, is an enzyme that breaks down acetylcholine in the synaptic cleft, thus terminating the signal for muscle contraction. While it plays an important role in neuromuscular transmission and the regulation of acetylcholine levels, it is not a receptor.

The specificity of the ACh receptor for acetylcholine is crucial for effective muscle signaling, making it the correct answer in this context.