Shen is the most spiritual manifestation of Qi, and resides in the heart. It is a state of pure consciousness. Zhi is willpower. Po, on the other hand, is the animal (reptilian?) aspect of our mind. Giovanni Maciocia calls it the "corporeal soul".
According to Maciocia, autism is a sign of Shen deficiency. Panic disorder, on the other hand, can lead to an unsettled Shen. When shock hits the heart, Shen moves out. Thoughts become ceaseless, and Qi grows chaotic.
Kurt Cobain, on the other hand, had a little too much Shen, as can be glimpsed in the intensity of his eyes.