Thank you for writing to ATD.
When you are having sex or during an erection, the parasympathetic system is active. Also, the scrotal skin contains muscles which help tighten the scrotal skin. These muscles are innervated by nerves that have a connection to the spinal cord.
Thus, the going up or shrinking up of the scrotum that you felt happens as a reflex reaction to the stroking of the skin on the side of the thigh. When you stroke the thigh, for example, an impulse goes to the spinal cord and then via the interneuron to the effector nerve which contracts the muscles in the scrotum.
Also, the parasympathetic system could play a role in this.
Hope this answered your question.
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