Which of the following is NOT a function of acetylcholine?

Acetylcholine is a crucial neurotransmitter involved in various physiological processes, primarily in the nervous system and muscular function. It plays a significant role in muscle contraction by transmitting signals from motor neurons to muscle fibers at the neuromuscular junction. When acetylcholine is released, it binds to receptors on the muscle cell membrane, leading to muscle activation.

In the context of signal transmission in the nervous system, acetylcholine facilitates communication between nerve cells and is integral to processes such as memory and learning in the brain. It is also essential for autonomic functions, influencing heart rate and other involuntary functions through its action on various neural pathways.

The key distinction with the correct choice being that acetylcholine does not function as a hormone. While hormones are chemical messengers secreted into the bloodstream by glands and affect distant target organs, acetylcholine operates locally at synapses and neuromuscular junctions, rather than circulating systemically. This specificity highlights its role in synaptic transmission rather than as a systemic hormone, making it clear why ‘acting as a hormone’ is not consistent with acetylcholine's functions.