 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.