Physiology of pain

Physiology of pain

1. Introduction :

Pain is an unpleasant subjective expression that remains one of the main reasons for patients to consult a dentist.

         It is possible to distinguish several types of pain according to their evolutionary profile:

Acute pain 

Inflammatory pain

Chronic pain

Visceral pain

Cancer pain 

2. Definition :

” Pain is an unpleasant sensory or emotional experience associated with actual or potential tissue damage, or described in terms implying such damage .” 

• Pain has a dual dimension: sensory and psychological . 
• The sensory dimension represents the neurological component at the origin of the painful sensation. The brain thus has a discriminative function allowing it to distinguish with the greatest precision the intensity of the pain, the nature of the aggression, its duration and its location. 
• The psychological dimension is a real individual variant. It represents the affective, emotional, cognitive or behavioral response to a painful aggression.

3. Physiology:

The majority of nociceptive messages from the orofacial region travel along one of the three branches of nerve V, and travel from the periphery to the nerve centers following these steps:

1. Transduction;
   1. Stimulation of nociceptors:

• mechanical: Is done by the deformation of the nerve membrane of the fibers. 
• thermal: Hot: the pain is triggered from 43°C, Cold: the pain appears below 
• chemical: fiber activation thresholds are between pH 6.9 and pH 6.1 

2. Pain receptors or Nociceptors:

• Nociceptive messages are generated at the free endings of nerve fibers. 
• They constitute plexiform arborizations in the cutaneous, muscular, articular and pulpo-dentin tissues.

Physiology of pain

3.2. Transmission; 

3.2.1. Nociceptive nerve fibers:

Fiber type	Aδ	C
Diameter	1 – 5 μm	0.3 – 1.5 μm
Myelin sheath	+		–
Conduction velocity	5-40 m/s	1-2 m/s
Stimulus	mechanical	Mechanical, Chemical, Thermal
Type of pain	Fast	Slow

3.2.2. transmission from the periphery to the brainstem:

• The peripheral trigeminal fibers run towards the brainstem and gather within the trigeminal sensory root which includes two nuclei: the main nucleus, the spinal nucleus. 
• Aδ fibers of cutaneous origin project to the three subnuclei of the spinal nucleus, whereas those originating from deep tissues only project to the caudal subnucleus, and C fibers project exclusively to the caudal subnucleus. 

3.2.3. transmission from the brainstem to the thalamus:

• The nociceptive message from the trigeminal sensory complex is then transmitted to the thalamus (the Ventro-Postero-Medial nucleus), which is responsible for the sensory-discriminative aspect of pain.

3.2.4. transmission from the thalamus to the cerebral cortex:

• From the Thalamus, nociceptive messages are projected to the frontal and parietal areas of the cortex which participate in the integration of different aspects: sensory-discriminative and affective-emotional. 

3.3. Pain modulation : 

• Pain does not result from the passive transfer of nociceptive information detected in the periphery and conveyed to the pain centers, but from an active processing of information by the peripheral and central nervous systems which allows a response to be produced adapted to physiological conditions. 

3.3. 1. The “Gate control” door control:

• Tactile messages carried by large caliber myelinated fibers (Aβ) are transmitted faster than those of pain (Aδ and C) and will prevent the passage of nociceptive messages transmitted more slowly.

3.3. 2. Neurochemical modulation:

• GABA and glycine are the main inhibitory neurotransmitters . 
• Excitatory synaptic transmission is largely mediated by glutamate 

4. Classifications: 

4.1. Classifications by mechanism:

1. Pain due to excess nociception: 

• Resulting from overstimulation of the fibers carrying the nociceptive message from the periphery to the nerve centers. 
• These pains respond well to analgesics.

2. Inflammatory pain: 

• These are pains triggered by inflammation resulting from tissue damage, stimulating nociceptive fibers in a lasting and repeated manner. 
• These pains respond to analgesics and anti-inflammatories. 

3. Neuropathic pain:

• Are due to changes in the processes of transmission and/or control of the pain message following a central nervous lesion (e.g.: neurological diseases: multiple sclerosis, epilepsy) or peripheral lesion (e.g.: essential facial neuralgia). 

• Neuropathic pain is characterized by a component: paroxysmal, spontaneous or provoked …
• It is generally associated with strange sensations described by the patient in the form of burning, tingling, prickling, numbness, etc. 
• These types of pain are not sensitive to analgesics, they respond to centrally acting drugs which are certain antidepressants and antiepileptics. 

4. Psychogenic pain: 

• The diagnosis is psychiatric. 
• Mental suffering can be expressed through a painful experience that is very real for the patient. 
• They should not be confused with the psychological repercussions of symptomatic pain (anxiety, depression). 

5. Idiopathic pain: 

• The exact cause of which is poorly understood and which seems to be linked to different factors (neuropathic, psychological, emotional, etc.). 

Physiology of pain

4.2. Temporal classifications: 

4.2.1. Acute pain / symptom pain:

Represents the symptom of an injury and its alarm signal, it is a sharp, immediate, and generally brief pain. 

2. Chronic pain / disease pain:

At this stage, pain represents the main part of the patient’s illness; it is prolonged pain over a period of more than three months. 

3. Topographic classifications: 
4. Primary/localized pain:

The site of origin of the pain coincides with the injured tissue. 

Anesthesia at the painful site disappears the pain 

2. Secondary pain:

• Referred/Referred : Pain is felt in the innervation territory of an injured nerve. 
• Referred : pain felt in an area different from the innervation area of ​​the injured nerve

5. Pain in practice:

PAIN ASSESSMENT: WHY?

• Identify the nature and extent of the problem 
• Make the appropriate therapeutic decision 
• Check the given effectiveness of a treatment 
• The nature of the pain: acute or chronic, spontaneous or provoked. 
• Duration of pain: in minutes, hours, days 
• Frequency of occurrence: every day, several times a week, etc. 
• Triggering/aggravating factors: Cold, chewing, etc. 
• Relieving factors: heat, painkillers, etc. 
• Associated symptoms and signs: nasal obstruction, watery eyes, redness, dizziness, etc.

Physiology of pain

6. Conclusion:

Pain between physiology and pathology 

Both physiological, being an alarm of the human body in the face of any aggression, however it also constitutes a pathological entity which requires adequate management. 

Physiology of pain

  Cracked teeth can be healed with modern techniques.
Gum disease can be prevented with proper brushing.
Dental implants integrate with the bone for a long-lasting solution.
Yellowed teeth can be brightened with professional whitening.
Dental X-rays reveal problems that are invisible to the naked eye.
Sensitive teeth benefit from specific toothpastes.
A diet low in sugar protects against cavities.
 

Physiology of pain