To understand how REM sleep establishes neurological function, we must understand how neural connections are formed and maintained. Before regular sleep cycles are charted in a developing infant, unpatterned electrical neurological activity can be observed. This seemingly random, unordered firing of neurons is actually instrumental in the growth of developing axons.
To review the neurobiology: each neuron, or cell of the nervous system, receives and transmits information using electrical and chemical signals. Neurons receive information either from sensory receptors--such as those which sense touch, heat, or pressure, for example--or from other neurons. Synaptic connections between two nerve cells allow neuronal communication: they allow us to perceive the outside world, store memories, make decisions, and learn. Neurons receive information at their dendrites, "choose" whether or not to propagate information to other neurons at the axon hillock, and send information via electrical signal to other neurons using long, wire-like axons.
During neural development, which includes establishing the infrastructure and function of the cells of the nervous system and their connections, axons must grow outwards from their origins at neuron cell bodies and establish connections with other neurons or tissues. This process is called pathfinding, and it is the source of the non-patterned neural firing observed in a growing fetus. This non-patterned firing is characteristic of nervous system development and is therefore paramount in the formation of synaptic connections between neurons.
