HEADACHES: CRANIAL NERVES
There are twelve cranial nerves, the nerves that arise from the brain itself, but all the pain impulses in the head are carried by only two of these, the fifth and ninth cranial nerves, which also serve most of the other sensations felt in the head and face. Pain can also be produced by stimulating the tenth and eleventh cranial nerves, as well as nerves coming from the spinal cord in the upper part of the neck; this pain is felt at the back and top of the head.
The fifth or trigeminal nerve arises from an area in the upper part of the hind-brain and its control centre (nucleus) lies deep in the spinal cord in the neck. In this part of the nucleus the areas of the face are represented in a concentric way, so that damage to the upper part can cause tingling or loss of sensation round the lips. The main trunk of the nerve splits into three branches in an area of the skull near to the front of the upper part of the jaw bone. The upper branch goes to the area of face above the eye and forehead, the middle branch to the cheek, whilst the lower one goes to the lower jaw. There is, of course, an identical nerve on the other side of the face. When a dentist anaesthetizes half the jaw, it is a branch of this nerve that he is blocking.
Much of the knowledge regarding these pain pathways was ascertained by experiments on volunteers undergoing operations, since such a detailed analysis could not be obtained by work on animals; this speaks volumes for the co-operation and interest of the volunteers. Many of those who suffer from headaches are keenly interested in co-operating in research. The methods employed in these experiments varied, e.g. mild electric shocks and traction, but they were not the sort of stimuli that cause headaches in everyday life.
The sites within the head which give rise to pain are related to blood vessels, and the relationship between these and the structures they supply are of paramount importance.
Blood vessels, especially arteries, have a muscular coat which enables them to change their diameter. These changes both alter the amount of, as well as the resistance to, blood flow. The smallest blood vessels (capillaries) form a network to supply organs with oxygen and nutrients, without which body tissues die. Blood flow to the tissues can also be regulated by opening or closing parts of the capillary system.
Pain due to stretching of blood vessels is the explanation of all vascular headaches, including migraine. The pulsation felt over the temple during an attack is due to dilatation of blood vessels and pressure on an artery, because it prevents or limits pulsations, lessens the pain. This is the way that an ice pack, by producing constriction of blood vessels, relieves an attack.
Before the headache phase of migraine, which is due to widening of blood vessels (vasodilatation) outside the skull, there is a narrowing of blood vessels (vasoconstriction) inside the head. Why this first phase occurs is not known but, during a migrainous attack, there may be a very low cerebral blood flow (similar to that following a stroke caused by blockage of a blood vessel).
The decrease in blood flow to the brain accounts for the visual symptoms and other odd sensations before an attack. During or soon after the decrease of flow in the brain, there is a dilatation of the branches from the extra-cranial carotid artery and this marks the start of the headache. The throbbing pain is made worse by the release of chemical substances (kinins and histamine). Nausea supervenes and this also occurs when the vessels are dilated passively. This simplistic explanation of the sequence of events in a severe attack of migraine gives a picture of the complex changes involved.
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