When the axons are differentiated and myelinated, they must bind to the appropriate target cells. This process, in which synapses are created is called synaptogenesis.

Picking a location where target cells are located is done in three steps.

1. Pathway selection; axons go to the left or right hemisphere, you see this happen in the optic chiasm.
2. Target selection; axons bind to a particular part of the thalamus (in the visual pathway, the axons all go to the LGN).
3. ‘Address’ selection; the axons bind to specific target cells (this is how for example a retinotopic map is created).

The formation of synapses between the axon and the target membrane occurs through interaction between proteins at the tip of the axon and the dendrite of the target cell. As substances are released from the axon, postsynaptic receptors for this substance are brought to the tip of the dendrite. Because the axon and the dendrite now have receptors that are close together, they can communicate well.

During brain development, many more synapses are formed than will eventually be used. This is useful because children often fall. When a few synapses break down in such a fall, the leftover ones can compensate for this.

After the brain has developed, synapses are also broken down. This process is called programmed cell death (or also ‘apoptosis’). The target cells produce a limited number of neurotrophins. These are substances that are important for cell survival. Axons connected to the same target neuron ‘fight’ to obtain this substance. The axons that have the strongest connection to the target neuron get the most neurotrophins back, and will survive in this way. The less strong connections will die and disappear from the brain along with their nucleus and dendrites.

Author: Myrthe Princen (translated by Melanie Smekal)