Regardless of the vasomotor and biochemical mechanisms that produce headache and facial pain, there appear to be three potential pathways for the transmission of painful impulses.
(1) The painful sensations are carried centrally by sensory nerves. In the case of headache, the trigeminal nerve would be the primary transmitter; however, the glossopharyngeal, vagal, and facial nerves as well as posterior cervical nerves may also transmit painful facial impulses. The glossopharyngeal and facial nerves transmit such impulses from the region of the ear, while the posterior cervical nerves transmit them from the posterior aspect of the head and neck.
(2) There may be sympathetic sensory conduction. The existence of sympathetic sensory conduction is controversial. It is generally believed that the sympathetic neural transmission is efferent only; however, Harris (1937) cited an instructive case report of Tinel that suggested the existence of afferent sympathetic conduction as well. A woman had a section of the sensory root of the trigeminal nerve on one side for severe orbital and supraorbital pain and had a complete facial palsy postoperatively. Eight months later, following an accident, she complained of severe pain on touch as well as spontaneous and paroxysmal pains on the side of the sensory root section. The pains radiated over the entire left half of the jaws and face, scalp, occiput, and upper neck. They were elicited by eating, swallowing, etc. Cutaneous anesthesia was present as before, and the facial palsy was unchanged. Sweating, which had been abolished on the operated side of the face, became excessive over the remainder of the half of the body on the side of the operation. Harris remarked that since the sensory impression of pain could not be carried over the trigeminal or facial nerves, it must have been transmitted over the sympathetic pathways.
Surgery upon the sympathetic system has been performed for the relief of pain. The possible afferent role of the sympathetic system on transmission of painful impulses forms a questionable indication for such surgery, which is usually performed because of the influence of the sympathetic system on blood vessels through efferent impulses. Sympathetic stimuli account for vessel tone, and section of the sympathetics provides for widening of blood vessels. This response is not as pronounced when the cervical sympathetic pathway is interrupted as when the abdominal and thoracic sympathetic nerves are divided. Cervicothoracic sympathectomy has been performed for Raynaud's disease, thromboangitis obliterans, chronic infectious arthritis, and for miscellaneous conditions such as hyperhidrosis, encephalitis, etc. In 126 patients on whom sympathectomy was performed, as reported by Love and Adson (1936) of the Mayo Clinic, 18 had complained of severe headache. Of these 18 patients, 5 had had migraine, 11 suffered from unclassifiable headache, and the headaches in 2 patients were unexplained. Following operation, all patients suffering from migraine were considered improved while about half of the other patients who suffered from unclassifiable headache were said to be improved. Dandy (1931) reported cervicothoracic sympathectomy in two patients with migraine with favorable results, but such operations are no longer being performed. Craig (1933) has stated that the improvement in migraine that follows ligation of the middle meningeal artery is dependent upon the consequent interruption of sympathetic fibers. Adson (1934) performed periarterial sympathectomy of the common carotid artery, ligation and division of the external carotid, and removal of the superior cervical ganglion and upper portion of the sympathetic trunk on the side of the vascular headache. This operation failed to give lasting relief.
(3) Pain from irritation of extracephalic regions and distant viscera must be referred. Head (1896) observed that in a viscus that is innervated by the vagus nerve, the referred pain is within the face and neck, since parts of these areas represent the somatic sensory distribution corresponding to the vagus. Head observed that, in general, the higher the position of tenderness in the trunk, the farther forward is the correlated area in the head. Campbell (1914) suggested that irritation of the upper thoracic areas is associated with pain over the nose and forehead. Damage to the 7th, 8th, 9th, or 10th thoracic regions was thought to be associated with pain over the temporal, vertical, parietal, or occipital areas, respectively. Thus, disease of the upper part of the stomach, the base of the lung, or the mitral valve, tends to give rise to pain and tenderness not only in the seventh thoracic area on the trunk, but in the temporal portion of the cranium as well. Head described supraorbital pain in association with cardiac pain, and Miller (1985) has observed several such cases.
Similarly, it is possible to explain headaches that are produced by bright lights, noises, and powerful odors. Campbell (1914) emphasized that since the special senses have originated from common sensibility, and since there is a close connection between their cerebral centers, a close association might be expected to exist between cephalic centers and the sensory centers pertaining to the skin about the eyes, ears, and nose.
Additional evidence suggests that there are nervous system connections between the trigeminal ganglia and cerebral blood vessels, termed the trigeminovascular system (Moskowitz, 1984). Trigeminovascular neurons and their peripheral unmyelinated nerve fibers contain the neurotransmitter peptide, substance P. Stimulation of this system by a variety of mechanisms would cause the release of substance P, which is postulated to increase vascular permeability and dilate cerebral blood vessels. The role of this system in the generation of human vascular headache may account for the effects of hormones or other circulating substances that change the receptive field properties of trigeminal ganglion cells. Individuals prone to chemically induced headaches from ingestion of tyramine, alcohol, phenylethyamine, monosodium glutamate, nitroglycerine, wine or chocolate also experience spontaneous headaches (Raskin, 1981). Extensive studies of the reactivity of blood vessels in migraine (Wolff, 1963) and cerebral blood flow (Olesen, et al, 1981; Skyhoj-Olesen et al, 1987; O’Brien, 1971; Simard and Pauson, 1973) suggest that abnormal vasomotor responses may be present in migraine patients between, as well as during, migraine attacks.

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