The vagus nerve is one of 12 pairs of cranial nerves that originate in the brain and is part of the autonomic nervous system. The nerve, when stimulated, activates neurons in the locus coeruleus — the primary source of the neuromodulator norepinephrine in the brain. Vagus nerve stimulation (VNS) is the use of a device to send gentle electrical signals to the brain and activate the vagus nerve.