In part 1 we looked at the trigeminal nerve (cranial nerve V) as being the key nerve relating to headaches and migraine. This week we will look at where this information goes and how it is controlled.
Walking along a path you misjudge ducking under a tree and a low hanging branch scratches you on the forehead. Branches of the trigeminal nerve sensing pressure and movement under the skin are the quickest to react – sending information into your brainstem. Here the peripheral nerve ends, and passes its information onto a number of central nerves by releasing chemicals (neurotransmitter) across a small gap between nerves called a synapse. This meeting of the peripheral nerve with central nerves happens in a nucleus. The trigeminal nerve shares its nucleus with the upper three nerves from the cervical spine and as such the area is referred to as the trigemino-cervical nucleus (TCN). On top of that three other cranial nerves send pain input from all muscles of the face and scalp (via the facial nerve), the sensory fibres from the middle ear (via the Glossopharyngeal nerve) and sensation from the outer ear and the dural lining of the brain (via vagus nerve). This means that this little area in the brainstem is responsible for relaying all of the sensory information from the face, head and neck into the brain.
Back to our scratched head; some of the central nerves activated will cause an immediate response via a reflex loop - it happens without your control or without having to think. This may cause your eye to close (nociceptive blink reflex) and your head to turn away (trigemino-cervical reflex) to help protect you from further damage.
A spilt second later the information travels along another central nerve that relays that information to the brain. You now become “aware” that something has struck you. Another fraction of a second later the feeling of pain is registered (it travels along slower nerves) You then make an appropriate response that is mediated through the peripheral nerves i.e. look for the object that struck you, reach up to check your wound.
Nerve impulses arriving in the trigemino-cervical nucleus have multiple interconnections that can activate many different parts of the brain including memory and emotion. Lets illustrate the interconnections in the brain with another example. While sitting outside a plant’s branch is only just touching your ear. Your first reaction may be to swipe it away. Those with particular phobias may leap from their chair, maybe even screaming out, brushing anxiously at their ear thinking a spider is crawling on them.
No pain, and no “real” threat, but sensory information from the head and neck is treated differently (due to the importance of the brain and brainstem), and is often interpreted through memories and emotions, so our reaction to very minor sensations or “inputs” can be quite extreme.
We need to be able to control the “volume” of noise that the nerves are making. Most information coming into the TCN does not normally need to be “sensed” as a threat, and in fact is often not sensed at all. Information about movement from the muscles and joints in the neck, air movement from breathing, or touch sensation from the tongue moving in the mouth is not something that we need to be aware of. The brain has the ability to control the volume on information it receives, much like when you put on a woolen jumper – prickly for 30 seconds or so then you don’t notice it. It is no less prickly but your brain has turned down the volume.
“Volume” control in the brainstem is achieved using a neurotransmitter, or a chemical, much the same as we described above with the nerves forming a chemical junction or synapse. These neurotransmitters either excite (turn up) or depress (turn down) the activity of the nerves. We have different neurotransmitters controlling different areas so we don’t depress all nerves in all areas. The activity in the trigeminal nucleus is regulated by a neurotransmitter called serotonin. In fact serotonin has a significant role throughout the brain in areas related to mood, sleep, appetite, and has been associated with illnesses such as depression.
Put simply, Serotonin’s role in the brainstem is to set the threshold for pain. Any stimulus that goes above the set threshold will trigger a pain response in the trigeminal system – a headache.
There is very good evidence from multiple sources – reflex studies, PET scan studies (looking at activity of nerves) and from pharmacological studies (the Triptan group of drugs) that the Trigemino-cervical nucleus is “Headache” or “Migraine” central and is overactive.
In the next installment of “insights” I will outline what the abnormal trigeminal system looks like and why it has caused a major rethink in terms of the possible pain generators in headache and migraine.
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Roger O'Toole is the Director and Senior Clinician of the Melbourne Headache Clinic and has over 10 years experience as a physiotherapist.