The formation of synaptic connections is an essential property of nervous system development. Synapses are formed between neurons and also with targets that are not part of the nervous system, e.g. muscle. Axon terminals, under the direction of a variety of extracellular cues, grow towards particular targets. Once they arrive at the target, they stop growing and the growth cone changes to form a synapse. As with axon growth, the formation of the synapse is dependent on an interaction between the target and the growth cone. But unlike axon growth, the exchange of chemical signals also operates from neuron to target; the presynaptic terminal releases neurotransmitter and other signals which influence the target. Once formed, the synapse continues in a state of continual change, able to alter its size, its precise location on the target and the extent to which it communicates with the target. These processes of change are essential to allow for adjustment as the animal grows and to allow the accretion of experience, usually known as learning.