Craniofacial pain, or pain of the head and face, deals with sensory nerve fibers that are present in the skin/teeth/gums and lips. These afferent (sensory) nerves project from the trigeminal nerve (fifth cranial nerve). The trigeminal nerve is responsible for sensation in the face. The trigeminal nerve breaks into three sections which innervate the eyes, upper jaw, and the lower jaw.
Dental pain comes in a variety of forms. Pain can be thermal, mechanical, or chemical. It usually arises when nerve terminals are somewhat exposed or not protect by tissue that was once intact. Most people know that enamel is the hard, protective layer on the teeth. If enamel wears away due to erosion the layer beneath the enamel is exposed. This layer is called dentin. Dentin is composed of avascular tubules. The dentin is also pain sensitive.
Fluid changes within the dentin tubules cause the nerve in the tooth to send a signal. In other words the nerve fires and painful sensations are felt. Neuroscientists have been working on identifying the direct molecular mechanisms that cause consistent firing and lead to chronic pain.
Many ion channels that lead to nociception (pain) have been identified. Some of the most prominent are TRPV1, TRPA1, and NMDA. Different stimuli cause the opening of these channels and add to the ‘firing’ of a nerve signal. Injury of the tissue in the mouth or on the face causes these channels to open and the nerve signal to fire. In damaged nerves, there is an upregulation or an increase in the number of channels present on the nerve. More channels, means more frequent firing and an increase in the pain felt by the person.
Referred pain, or pain felt in a location that is not at the site of injury is often felt at different areas of the mouth. I am currently working with researchers to identify the specific mechanism that pain ‘spreads’ to other tissues. Many scientists call this spreading ‘intra-ganglionic spreading of nociceptive signals.’
We believe the spreading is caused in the trigemic ganglion. The ganglion is a mass of nerve cells bodies whose individual fibers project to the teeth. When one nerve (afferent fiber) is damaged this signal travels to the trigemic ganglion. In the ganglion, we believe that different cytokines (chemical hormones) are released by the damaged neuron and cause an upregulation of TRP channels in other nerves. Thus, pain is sensed at other areas of the mouth or face.