Anticholinesterases (anti-AChE) are a class of chemicals which can take the place of ACh at the active site of AChE, significantly reducing the neurotransmitter's ability to bind with AChE and essentially limiting the rate at which ACh can be broken down. Since there is no pre-synaptic re-uptake mechanism to transport ACh back into the presynaptic cell and since ACh cannot readily dissolve into the surrounding medium, the presence of AChE inhibitors result in an excess build-up of ACh at the synapse. This build-up leads to an increased level of ACh activity at post-synaptic receptor molecules and, subsequently, a continuous firing of action potentials.