The nervus intermedius (NI) appears to be the main conduit of cluster headache (CH). Parasympatheticimpulses via the NI account for the associated symptoms of ipsilateral tearing and redness of the eye,congestion of the nose and rhinorrhea. Vasodilatory stimuli to the internal carotid artery may result inpartial Horner's syndrome and set off a cascade of events causing pain, similar to the changes in theexternal carotid arterial tree during migraine. The pain of CH may also be carried by afferent impulsesaccompanying motor fibers of the deep facial muscles.The central nervous system (CNS) mechanisms responsible for the circannual recurrence of CH may beexcitatory with parasympathetic impulses channeled through the NI. Conversely, inhibitory CNSmechanisms may be invoked to explain long remissions, if CH were to be due to hypersensitivity of the NIas caused by the subtle injury of a cross compressing blood vessel.