Which hormone is common as a neurotransmitter and is involved in the signaling between nerves?

Acetylcholine is the hormone that also functions as a neurotransmitter, playing a crucial role in the signaling between nerve cells. It is primarily responsible for transmitting signals in the nervous system, particularly at neuromuscular junctions where it stimulates muscle contraction. Acetylcholine is released by neurons and binds to specific receptors on target cells, leading to various physiological effects, including muscle movement and modulation of heart rate.

Additionally, acetylcholine is involved in cognitive functions such as attention and memory within the central nervous system. Its dual role as both a neurotransmitter and a hormone makes it unique among the options provided, as it directly facilitates communication between neurons and influences numerous bodily functions.