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Several areas of the head can hurt during a headache, including the network of nerves which extend over the surfaces of the head. The muscles of the head, and the blood vessels adjacent to the brain are also often painful because they contain delicate nerve fibers. |
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| Headaches and the pain associate with them are
most simply understood via the Trigeminal Pathway (cranial nerve V). Other associated
cranial nerves include VII, IX, X, and spinal nerves C-2, and C-3. Therefore, the bones
and tissues of the brain do not hurt because they do not contain pain sensitive fibers. Headaches are can be thought of as a neurovascular disorder because the problems are said to lie in the neural innervation and cranial circulation. This headache explanation involving the vascularization of the brain is called the vascular theory. |
 Figure 1. Trigeminal system. The afferent connections of the trigeminal system. |
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 Figure 2. Trigeminal nerve. The trigeminal nerve root. |
In the vascular theory the constriction of
peripheral arteries and the dilation of cerebral blood vessels lead to a headache. (See
the Headache Movie.) The vasoconstiction leads to a loss
of cerebral oxygen in the blood, therefore, the compensatory mechanism is vasodilation. The other theory of headache pain is the spreading depression of cortical electrical activity. This is theory relies on a pattern of reduced blood flow corresponding an electrical cortical response to noxious stimuli. |
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| The actual pain associated with a headache is realized due to the activation of free nerve ending nociceptive primary afferent fibers. These fibers are activated releasing mediators, particularly prostaglandin, which then leads to a release of Substance P and an activation of the pain receptor. The activated receptors conduct the signal of pain along the A and C fibers of the trigeminal pathway. |  Figure 3. Caudal portion of the trigeminal nucleus. The caudal, descending portion of the trigeminal nucleus is responsible for a majority of sensory processing. WDR represents wide-dynamic range neurons, and NC represents nociceptive afferents. |
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 Figure 4. Trigeminal Nerves. Schematic of the trigeminal nerves. |
Pain sensation in the trigeminal pathway
enters the brain stem to synapse in the trigeminal nucleus. The trigeminal nucleus,
particularly the most caudal regions, pain is discriminated and processed as a specific
stimulation. The pain associated with vasodilation as an effect of as an effect of the trigeminoparasympathetic reflex, or as the abnormal firing of associated with spreading depression is all support by positron emission tomography studies showing pain in the trigeminal system. |
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| Serotonin (5-hydroxytryptamine, 5-HT) is a
neurotransmitter implicated in many disorders of the central nervous system. It has been
shown to play a role in headaches. Serotonin can constrict blood vessels leading to a the
stimulation of pain receptors. Many serotonergic receptors have been located in cranial blood vessels. Excess serotonin has also been shown to lead to headache symptoms. Therefore, serotonin manipulating drugs are being thoroughly considered and studied for headache sufferers. |
 Figure 5. Trigeminal System with brain stem. Schematic of the primary components of the trigeminal system including the rostral medulla and the connecting fibers. |
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Now that you understand the basic anatomy of the pain behind a headache, why not have some fun with it. So kick back, grab some popcorn, and watch the premier headache movie on the web.
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