A method of forming an access opening through a psoas muscle to a patient's spine includes laterally inserting a stimulating dilator into the psoas muscle. The stimulating dilator has a stimulation channel formed in an outer surface thereof. An electrical pulse is transmitted into the stimulating dilator to locate a position of a nerve in the patient's psoas muscle. The stimulating dilator is laterally inserted through the psoas muscle and toward the patient's spine in a way that avoids the nerve. A stimulating probe is inserted into the stimulation channel along the outer surface of the stimulating dilator while transmitting an electrical pulse into the stimulating probe to verify the position of the nerve.