Neurotransmitter diffusion is controlled by membrane potential, as this is the means of transmission along an axon. On the action potential reaching the pre-synaptic terminal, calcium ions influx into the terminal, causing vesicles containing the neurotransmitter (acetylcholine, epinephrine, 5-HT, …) fuse to the membrane, releasing their contents into the synaptic cleft. The neurotransmitter then diffuses across the cleft, binding to a complementary receptor on the post-synaptic neuron. This receptor is a ligand-gated ion channel, allowing the influx of ions and the formation of a new action potential. This process allows summation of neuronal responses, where multiple action potentials converge. This allows the initiation of an action potential in a neuron with a higher threshold value, and is present in the rod cells of eye (allowing vision in low-light conditions).