A method, apparatus, and system for affecting neuromodulation based upon an evoking signal applied to a patient's body. An internal and/or external evoking and/or therapeutic signal is applied to a first target portion of a patient's body. Data relating to a physiological response resulting from the internal and/or external evoking and/or therapeutic signal is received. A neurotransmission characteristic of the patient's body is determined based upon the data relating to the physiological response. At least one parameter defining an electrical therapeutic signal provided by an implantable medical device is controlled based upon the determined neurotransmission characteristic to treat a disorder.