Complex sentence: For many years it was believed that the axon hillock was the usual site of action potential initiation, however, it is now thought that the earliest site of action potential initiation is found just adjeacent, in the initial (unmyelinated) segment of the axon.
Split into simpler sentences: For many years it was believed that the axon hillock was the usual site of action potential initiation. It is now thought that the earliest site of action potential initiation is found just adjeacent, in the initial (unmyelinated) segment of the axon.