1. An implantable medical system comprising: an implantable medical device (IMD) operative to provide a first electrical signal; an electrode coupleable to the IMD, the electrode operative to deliver the first electrical signal from the IMD to a neural structure to cause afferent nerve impulses or afferent and efferent nerve impulses to propagate along the neural structure away from the electrode; a sensor coupleable to the IMD, the sensor operative to sense a physiological parameter, wherein the physiological parameter includes at least one of a neurotransmitter breakdown product parameter, a neuropeptide parameter, or an amino acid parameter; and a controller operative to: change a signal parameter of the first electrical signal based upon the physiological parameter to generate a second electrical signal; apply the second electrical signal to the neural structure, wherein the signal parameter includes at least one of a pulse width, a frequency, a polarity, or an amplitude, and wherein the first and second electrical signals are applied to the neural structure to treat depression; and in response to the applied first and second electrical signals, determine a severity of the depression based on the sensed physiological parameter.