Introduction
Dental patients most often complain of the severity of tooth pain especially when it is decayed or in situations of pulpal exposure and trauma that is addressed by the dentist through endodontic treatment such as root canal treatment. Read this article to know the nociceptive pathway and receptivity of the pulp and its expression in pathologies such as tooth pain, the types of axon fibers that innervate the dental pulp, and the diagnosis of pulpal health by pulp testing in dentistry.
What Is Pulpal Pain?
Toothache or dental pain is indeed a severe clinicopathologic entity and several scientific research and clinical trials have been conducted to study and analyze the nociceptive mechanism of the dental pulp which emerges through the root canal of the tooth. Tooth pain is most often a severe pain that can result due to the following:
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An undesirable stimulation either of the tooth pulp or dentin (dentinal hypersensitivity also correlates or has clinical symptoms close to or similar to dental pulpal pain).
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Carious lesions of the tooth as well as regressive alterations of the teeth are responsible for causing pain or either dentinal hypersensitivity (exposed dentinal tubules being the most common cause for sensitivity).
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Pulpal infections.
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Gingival recession and gingivitis (inflammation of the gingiva) lead to referred dental pain.
The dentin or the second tooth layer after the first enamel layer of the tooth is not as richly innervated as the dental pulp which is the third layer and the main source of innervation, blood, and nerve supply to the tooth structure. The nerve fibers penetrating the tooth extend only for a short distance into the dentinal layer and in fact do not even extend further up to the dentino-enamel junction which is the superficial layer of the tooth. Within the dentin and its eventual nerve extensions from the pulp, the nerve fibers travel along the dentinal tubules located in the odontoblast processes.
What Is Pulpal Nociception?
The tooth pulp nociceptive pathway is by its innervation through the neurons that are located along with the receptive cell bodies within the trigeminal as well as spinal nuclei. The central terminations of both the trigeminal and spinal nuclei form the principal roots of nociception in the trigeminal ganglion. The maxillary and mandibular branches of the trigeminal nerve (fifth cranial nerve) divide further into alveolar branches that supply the tooth. Several morphological studies have been conducted in the past and over the recent decades which demonstrate the following:
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There are two kinds of axons that supply and may be found to innervate the tooth completely. The myelinated and the unmyelinated axons range from 0.2 to 15 micrometers.
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The nociceptive transmission of the tooth is by the presence of the A-delta and C fibers that innervate the tooth pulp. These fibers with varying conduction velocities can be activated upon electric stimulation of the tooth pulp. These fibers enter at the root apex of the tooth entering through into the pulp thus innervating it. These fibers innervating the pulp region vary in branching according to electron microscopic studies.
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The ratio according to research is 1:3 for the radicular pulp to the coronal pulp portion (as the coronal pulp portion shows extensive branching towards the crown tip more in comparison).
How Do Axons Innervate the Dental Pulp?
Innervating axons can be grouped into two:
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Sensory fibers that are of an afferent nature.
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Motor fibers of an autonomic nature.
These sensory fibers also contain a neuroactive substance that is expressed through all the smaller diameter fibers. This neuroactive substance is a ‘calcitonin gene-related peptide’ that has been shown through immunohistology studies. It is the calcitonin gene-related peptide fibers that are of sensory function that course through the pulp and then get further distributed through the dentinal tubules of the pulp.
The fibers of sensory or motor function are further traditionally sub-grouped or categorized based on their functions and size by immunocytochemical staining:
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Cholinergic Fibers: These are either the large diameter or the small diameter fibers of the dental pulp axons that are mainly afferent.
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Small Diameter Fibers: These are the substance P (SP, a neuropeptide expressed by the central nervous system) containing afferent nature fibers that are usually of very small diameter.
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Peptide-Containing Fibers: These are sympathetic and unmyelinated axons that contain either norepinephrine or vasoactive peptide fibers.
What Challenges Occur While Testing the Health of the Pulp?
A number of factors may hinder the testing of clinical pulp nociception or receptivity including the diagnostic challenges faced by pathologists in the histologic analysis of the pulp. The pulp that is encased within the enamel and dentin layers needs recording through electrophysiological responses to pulp nociceptors. Pulp sensitivity tests currently in endodontic practice use a principle similar to tooth crown stimulation (to detect the vital or active response of a live tooth) which is a very common procedure for diagnosis. Stimulation of the pulp can be through electric, thermal, chemical, or mechanical pulp testing. Most scientific research that analyzes the pulp nociceptors and their response to trauma and tooth impact is studied mostly by electric pulp testing because of the ease of application and operator control. False-positive and false-negative responses are very common errors faced by dentists or endodontists in teeth that have been traumatized or necrotized. Also in certain immunocompromised patients, chronic alcoholics, and individuals suffering from psychiatric disorders, the extent of pulpal inflammation tends to be extremely high irrespective of the vitality status of the tooth which becomes a common cause for such errors because of the altered pathophysiology state in the vascular pulp innervation. Hence this can be a diagnostic challenge for the operator.
Conclusion
The understanding of the pulpal nociceptors and the innervation through dental axons to the coronal and radicular pulp by the dentist or endodontist is crucial to diagnosing pulpal health issues. Timely dental management by endodontic treatment for pulpal pain is not only essential to relieve the patient's pain but also in preventing further spread of infection to the jaws.
