Acetylcholine
One of the main functions of acetylcholine (ACh) in the peripheral nervous system is to change the membrane permeability to sodium ions that determine whether or not a signal is sent. In the central nervous system, ACh has been associated with memory, learning, and attention. Because of this, one of the first things I thought of to represent was a troll guarding a bridge, specifically the Grumpy Old Troll from my childhood. Not only is it a memory, representing acetylcholine's function in the CNS, but the troll guarding the bridge reminds me of how ACh can regulate whether or not sodium can get into a neuron.