The development of nerve connections in the peripheral nervous system

The structural basis for the functions of the brain is the wiring of nerve cells. The wiring is achieved by 100 billion neurons via some 100,000 billion synaptic connections. Formation of this network is a formidable task which requires intricate, but also reliable mechanisms at work. It is generally recognized that the precise connections between nerve cells are accomplished by two different sorts of mechanisms. First, various molecular guidance cues lead outgrowing axons from specific areas to particular target regions which are rather broadly defined. The second stage includes an exact, point-to-point matching between each axon and the specific target neurons. This is achieved by an intricate interaction between the involved nerve cells. In the following, the attention will primarily regard the second type of events, but some aspects on the initial outgrowth of axons and axon guidance mechanisms will also be given. A large part of the description will focus on the neuromuscular junction, since it has been extensively used as a model for the study of synapse development.