The firing of an action potential, and consequently the release of neurotransmitters, occurs by this gating mechanism. In synaptic gating, in order for an action potential to occur, there must be more than one input to produce a single output in the neuron being gated. The interaction between these sets of neurons creates a biological AND gate. The neuron being gated is bistable and must be brought to the up state before it can fire an action potential. When this bistable neuron is in the up state, the gate is open. A gatekeeper neuron is responsible for stimulating the bistable neuron by shifting it from a down state to an up state and thus, opening the gate. Once the gate is open, an excitatory neuron can cause the bistable neuron to further depolarize and reach threshold causing and action potential to occur. If the gatekeeper does not shift the bistable neuron from down to up, the excitatory neuron will not be able to fire an action potential in the bistable neuron. Both the gatekeeper neuron and excitatory neuron are necessary to fire an action potential in the bistable neuron, but neither is sufficient to do so alone.