Comprehensive description of pain pathways which covers related definitions, benefits, theories, classification and mechanism of pain with factors that affect pain and diagnosis of pain. Also covers assessment and management of pain along with brief description of ascending and descending pain pathways.

1. PAIN PATHWAYS
DR. MEGHUL CHADHA
MDS 1st YEAR
PEDIATRIC AND PREVENTIVE DENTISTRY

2. Contents
• Introduction
• Definitions
• Benefits of pain sensation
• Classification of pain
• Acute pain
• Chronic pain
• Neuropathic pain
• Musculoskeletal pain
• Referred pain
• Mechanism of pain
• Receptors

3. • Sensory Neurons
• Pain Pathways
• Neural Pain Pathways
• Pain theories
• Factors affecting pain
• Pain in orofacial region
• Pain Diagnosis
• Assessment of pain
• Pain management
• Conclusion
• References

4. Introduction
• The word pain is derived from the latin word Peone and the greek Poine
meaning penalty or punishment.
• Pain is an intensely subjective experience, and is therefore difficult to
describe.
• Prevention and management of pain is an important aspect of healthcare.
Textbook of Medical Physiology 10th edition- Guyton and Hall

5. • Nociception is the reception of noxious sensory information elicited by tissue
injury, which is transmitted to the CNS by nociceptors.
• Pain is the perception of discomfort or an agonising sensation of variable
magnitude, evoked by the stimulation of sensory nerve endings.
Textbook of Medical Physiology 10th edition- Guyton and Hall

6. Definitions
• According to International Association for the study of pain says an
unpleasant sensory and emotional experience associated with actual or
potential tissue damage, or described in terms of such damage.
• According to Monheim - An unpleasant emotional experience usually
initiated by noxious stimulus and transmitted over a specialised neural network
to the CNS where it is interpreted as such.
Kumar KH, Elavarasi P. Definition of pain and classification of pain disorders. J Adv Clin Res Insights 2016;3:87-90

7. Benefits of pain sensation
• Pain is an important sensory symptom. Though it is an unpleasant sensation,
it has protective or survival benefits such as
• Gives warning signal about the existence of a problem or threat. It also
creates awareness of injury.
• Prevents further damage by causing reflex withdrawal of the body from
the source of injury.
• Forces the person to rest or to minimize the activities thus enabling rapid
healing of injured part.
• Urges the person to take requires treatment to prevent major damage.

8. Characteristics of pain
• There are 4 characteristics of pain -
• Threshold and intensity
• Adaptation
• Localisation of pain
• Influence of the rate of damage on intensity of pain

9. Classification of Pain
• Based on source / location/ referral and duration
Acute Pain / Traumatic pain Chronic Pain
Visceral or Splenic Pain Somatic Pain
Superficial or Cutaneous Pain Deep Somatic Pain
Malignant/Cancer Pain
Non-Malignant/Benign Pain
Musculoskeletal
Pain
Neuropathic
Pain
Kumar KH, Elavarasi P. Definition of pain and classification of pain disorders. J Adv Clin Res Insights 2016;3:87-90.

10. Based on Transmission
Fast Pain Slow Pain
• Felt about 0.1 sec after a
pain stimulus is applied.
• It is described as sharp,
pricking, acute and electric
pain.
• Not felt in most deeper
tissues of the body.
• Usually begins after a
second or more and may
range from seconds to
minutes.
• Described as slow, burning,
aching, throbbing, nauseous
and chronic pain.
• Associated with tissue
destruction.
Textbook of Medical Physiology 10th edition- Guyton and Hall

11. Acute Pain
• Acute has a sudden onset, usually subsides quickly and is characterised
by sharp, localized sensations with an identifiable cause.
• Felt within 0.1 second after pain stimulus is applied.
• Examples - Sharp pain, pricking pain, electric pain

12. • Acute pain - usually characterized by increased autonomic nervous systemic
nervous system activity resulting in
‣ Psychological symptoms such as anxiety
‣ Tachypnoea
‣ Tachycardia with hypertension
‣ Pallor
‣ Diaphoresis
‣ Pupil dilation
Textbook of Medical Physiology 10th edition- Guyton and Hall

13. Viceral Pain
• Type of nociceptive pain that comes
from the internal organs.
• Unlike somatic pain it is harder to
pinpoint.
• Caused by the activation of pain
receptors in the chest, abdomen or
pelvic areas.
• In cancer patients pain is caused by
tumor infiltration, constipation,
radiation and chemotherapy.
Textbook of Medical Physiology 10th edition- Guyton and Hall

14. Somatic Pain
• Nociceptors are involved
• Often well localized
• Caused when pain receptors in tissues (skin,
muscles, skeleton, joints, and connective tissues) are
activated.
• Stimuli like force, temperature, vibration, or swelling
activate these receptors.
• Usually described as throbbing or aching
• Can be
‣ Superficial (Skin, Muscle)
‣ Deep (Joints, Tendons, Bones).
Textbook of Medical Physiology 10th edition- Guyton and Hall

15. Superficial Pain
• It is also known as cutaneous pain.
• It arises from superficial structures
such as skin and subcutaneous
tissues.
• It is a sharp with burning quality
and maybe abrupt or slow in
onset.
Deep Somatic Pain
• It originates in deep body structures
such as periosteum, muscles,
tendons, joints and blood vessels.
• Radiation of pain from original site of
injury
Textbook of Medical Physiology 10th edition- Guyton and Hall

16. Chronic Pain
• Chronic pain is arbitrarily defined as
pain lasting longer than 3 to 6 months
• Persistent or episodic pain of
duration or intensity that adversely
affects the function and well being of
the patient
• May be nociceptive, inflammatory,
neuropathic or functional in origin
• Varies from unrelenting extremely
severe pain to pain of escalating or
non - escalating nature.
Textbook of Medical Physiology 10th edition- Guyton and Hall

17. Neuropathic Pain
• Neuropathic Pain is a result of an injury or malfunction of nervous system. It is
described as
‣ Aching
‣ Throbbing
‣ Burning
‣ Shooting
‣ Stinging
‣ Tenderness / Sensitivity of skin
• It is further classified into central, peripheral and mixed
Textbook of Medical Physiology 10th edition- Guyton and Hall

18. Peripheral
Pain
• Neuropathy ( Diabetic, alcoholic or
post - chemotheraputic)
• Radiculopathy
• Traumatic nerve lesions
• Post-mastectomy, -thoracotomy, -
herniotomy syndrome ( these may
also be mixed neuropathic -
nociceptive pain syndromes )
• Acute Herpetic Neuralgia
• Distal Neuropathies
• Chemotherapeutic agents
Central Pain Mixed Pain
• Central stroke
• Spinal cord injuries
• Multiple sclerosis
• Trigeminal Neuralgia
• Subgroups of patients with chronic
back pain
• Complex regional pain syndrome
(CRPS, Sudeck’s dystrophy)
• Subgroups of patients with cancer
related pain
Textbook of Medical Physiology 10th edition- Guyton and Hall

19. Musculoskeletal Pain
• This is a type chronic non cancer pain
occurring due to musculoskeletal
disorders such as
• Rheumatoid arthritis
• Osteoarthritis
• Fibromyalgia
• Peripheral neuropathies
Textbook of Medical Physiology 10th edition- Guyton and Hall

20. Referred Pain
• It is perceived at a site adjacent to or away from the
site of origin. Deep pain or some visceral pain are
referred to other areas.
• Superficial pain is is not referred.
• Examples -
‣ Cardiac Pain - Felt at inner part of left arm and
Left shoulder
‣ Pain from testis is felt in the abdomen
‣ Pain in diaphragm is referred to shoulder
‣ Pain in gall bladder is referred to epigastric
region
Textbook of Medical Physiology 10th edition- Guyton and Hall

21. Referred Dental Pain
• Dental pain may occur as a result of extra dental causes.
• For instance, several types of headaches can refer as pain in the teeth and
jaws
• Cluster Headache - can result in toothache
• Migraine and paroxysmal hemicrania can produce pain in the maxillary molars.
Textbook of Medical Physiology 10th edition- Guyton and Hall

22. Mechanism of Pain
Dermatome Rule
• According to dermatome rule, pain is
referred to a structure, which is developed
from the same dermatome from which the
pain producing structure is developed. A
dermatome includes all the structures or
other parts of the body, which are innervated
by afferent nerve fibres of one dorsal root.
• For example, the heart and inner aspect of
left arm originate from the same dermatome.
So, the pain in the heart is referred to the left
arm.
Textbook of Medical Physiology 10th edition- Guyton and Hall

23. Receptors
• Sensory Receptors are classified as
According to stimulus source
Exteroceptors Proprioceptors Interoceptors
According to modality
Nociceptors Thermoreceptors Mechanoreceptors

24. Nociceptors / Pain Receptors
• They are sensory receptors that are activated by noxious receptors that are
activated by noxious insults to peripheral tissues
• The receptive endings of the peripheral pain fibres are free nerve endings
• These receptive endings are widely distributed in the
• Skin
• Dental Pulp
• Periosteum
• Meninges
Textbook of Medical Physiology 10th edition- Guyton and Hall

25. • Mechanosensitive nociceptors (of A∂
fibres), which are sensitive to intense
mechanical stimulation (such as pinching
with pliers) or injury to tissues.
• Temperature sensitive
(thermosenistive) nociceptors (of A∂
fibres), which are sensitive to intense
heat and cold.
• Polymodal nociceptors (of C fibres),
which are sensitive to noxious stimuli that
are mechanical, thermal or chemical in
nature. Although most nociceptors are
sensitive to one particular type of painful
stimulus, some may respond to two or
more types.
Textbook of Medical Physiology 10th edition- Guyton and Hall

26. Sensory Neurons
Sensory Neurons
Second Order
First Order Third Order

27. First Order Neurons
• These are the cells in the posterior nerve root ganglion, receive impulses from
pain receptors through dendrites.
• These impulses are transmitted through the axons to spinal cord.
• Impulses are transmitted but A∂ fibres or C fibres.

28. Aβ Fibers A∂ Fibers C Fibers
Threshold Low Medium High
Axon diameter 6-14 μm 1-6 μm 0.2 - 1 μm
Myelination Yes Thinly No
Velocity 36-90 5-36 0.2-1
Receptor types Mechanoreceptor
Mechano/Nocicept
or
Nociceptor
Receptors field Small Small Large
Quality Touch Sharp Dull

29. Second Order Neurons
• The neurons of marginal nucleus and substantial gelatinosa from the second
order neurons.
• Fibres from these neurons ascend in the form of the lateral spinothalamic
tract.
• Fibres of fast pain arise from neurons of the marginal nucleus.
• The fibres of slow pain arise from neurons of substantial gelatinosa.

30. Third Order Neurons
• The neurons of pain pathway are the neurons in thalamic nucleus, reticular
formation, tectum, grey matter around the aqueduct of sylvius.
• Axons from these neurons reach the sensory area of cerebral cortex or
hypothalamus.

31. Pain Pathways
• The ascending pathway that mediate pain consist of three different tracts
• Neospinothalamic tract
• Paleospinothalamic tract
• Archispinothalamic tract
• The first order neurons are located in the dorsal root ganglion for all three
pathways. Each pain tract originates in different spinal cord regions and
ascends to terminate in different areas in the CNS.

32. Neospinothalamic Tract
• A∂ fibres transmit mainly mechanical
and thermal pain, terminate in the
dorsal horns, cross over to the opposite
side of the cord and continue upwards
to the brain as anterolateral columns.
• Most fibres terminate in the
venterobasal or posterior nuclei of the
thalamus; few fibres terminate in the
reticular areas. Signals are also sent to
the somatosensory cortex.
• Glutamate is the neurotransmitter
secretes in the spinal cord at A∂ fivers.

33. The first order nociceptive neurons make synaptic
connections in rexed layer 1 neurons.
Axons from layer 1 neurons decussate in the anterior
white commissure, at approximately the same level they
enter the cord.
Ascend in the contralateral anterolateral quadrant.
Most of the pain filers from the lower extremity and the
body below the neck terminate in the
ventroposterolateral nucleus and ventroposteroinferior
nucleus of thalamus
Which serves as a relay station that sends the signals to
the primary cortex

34. Paleospinothalamic Tract
• The C fibres which carry slow
pain terminate in the
substantial gelatinosa of dorsal
horns in spinal cord. They also
cross over to the opposite side
and continue as anterolateral
ascending tracts

35. • The paleospinothalamic tract tract
terminates in the brain stem in one of
the following areas -
• Reticular nuclei of medulla, pons
and mesencephalon.
• Tectal area of mesencephalon
deep 10-25% of the fibres pass to
the thalamus.
• Periaqueductal gray region
surrounding the aqueduct of
sylvius.

36. First-order nociceptive neurons make synaptic
connections in Rexed layer Il (substantia
gelatinosa) and the second-order neurons make
synaptic connections in laminae IV-VIII
Most of their axons cross and ascend in the
spinal cord primarily in the anterior region and
thus called the Anterior spinal thalamic tract
(AST) and few remain uncrossed.
These fibres contain several tracts. Each of
them makes a synaptic connection in different
locations. ( Mesencephalic reticular formation,
Periaqueductal gray also called spinoreticular
tract, tectum and these fibres are known as
spinotectal or spinomedullary tract, the
centromedian parafascicular complex also
known as spinothalamic tract. )

37. The above three finer tracts are known as
paleospinothalamic tract.
The innervation of these three tracts is
bilateral because some of ascending divers
do not cross to the opposite side of the cord.
From centromedian parafascicular complex,
these fibres synapse bilaterally in the
somatosensory cortex (SC 2 - Brodman
Area)

38. Archispinothalamic Tract
• Mediates visceral and emotional
reactions to pain.
• First-order neurons are found in
Rexed layer II, which project to
neurons in Rexed layers IV and VII.
• From the latter two layers, diffuse
projections are sent to the midbrain
reticular formation and the
periaqueductal gray.
• Neurons in the midbrain reticular
formation and periaqueductal gray
then send projections to the
hypothalamus, limbic system nuclei,
PF, and CM nucleus.
Rexed layers / laminae
1.Kendroud S, Fitzgerald LA, Murray I, Hanna A. Physiology, Nociceptive Pathways [Internet]. PubMed. Treasure Island (FL): StatPearls Publishing; 2021

39. Analgesic / descending pain
pathway
• Analgesic pathway that interferes
with pain transmission is often
considered as descending pain
pathway, the ascending pain
pathway being the transmit pain
sensation to the brain.
Textbook of medical physiology 7th edition- Sembulingam

40. Neural Pain Pathways
• Pain sensation involves a series of complex interactions
between peripheral nerves & CNS
• Pain sensation is modulated by excitatory and inhibitory
neurotransmitters released in response to stimuli
• Sensation of pain is composed of 4 basic processes
• Transduction
• Transmission
• Modulation
• Perception

41. Transduction:
This is the conversion of one form of energy to
another. It occurs at a variety of stages along
the nociceptive pathway from:
•Stimulus events to chemical tissue events.
•Chemical tissue and synaptic cleft events to
•Electrical events in neurones.
•Electrical events in neurones to chemical
events at synapses.
Transmission:
•Electrical events are transmitted along
neuronal pathways, while molecules in the
synaptic cleft transmit information from one cell
surface to another.
Textbook of Medical Physiology 10th edition- Guyton and Hall

42. Modulation:
•The adjustment of events, by up- or down
regulation. This can occur at all levels of the
nociceptive pathway, from tissue, through
primary (1º) afferent neurone and dorsal
horn, to higher brain centres.
•Thus, the pain pathway as described by
Descartes has had to be adapted with time.
Perception:
•Third order neurons project the nociceptive
signal to cerebral cortex of brain. In the
cortex the brain perceive the signal as a
pain.
Textbook of Medical Physiology 10th edition- Guyton and Hall

43. Pain Theories
Pain theories are proposed to offer the possible physiologic mechanisms
involved in pain. They are as follows
• Specificity theory
• Pattern theory
• Gate control theory
Textbook of Medical Physiology 10th edition- Guyton and Hall

44. Specificity Theory
• DESCARTES 1664, MULLER
1840
• Pain occurs due to stimulation of
specific pain receptors
(nociceptors) with transmission by
nerves directly to the brain
Pattern Theory
• GOLDSCHEIDER - 1894 -
stimulus intensity and central
summation are critical
determinants of pain
• Particular patterns of nerve
impulses that evoke pain are
produced by summation of
sensory input within the dorsal
horn of spinal cord
Textbook of Medical Physiology 10th edition- Guyton and Hall

45. Gate Control Theory
• Proposed by MELZACK & WALL IN 1965
• Pain stimuli transmitted by afferent pain
fibres are blocked by gate mechanism
located at the posterior gray horn of the
spinal cord
• If the gate is open pain is felt, and if the
gate is closed pain is suppressed.
• This theory of pain takes into account the
relative in put of neural impulses along
large and small fibres, the small nerve
fibres reach the dorsal horn of spinal cord
and relay impulses to further cells which
transmit them to higher levels.
• The large nerve fibres have collateral
branches, which carry impulses to
substantia gelatinosa where they stimulate
secondary neurons.

46. Factors affecting pain
• Emotional status
• Fatigue
• Age
• Race and nationality characteristics
• Sex
• Fear and apprehension
Textbook of Medical Physiology 10th edition- Guyton and Hall

47. Pain in orofacial region
Odontogenic pain
• Refers to pain initiating from the
teeth or their supporting structures,
the mucosa, gingivae, the maxilla,
mandible or periodontal membrane.
Apical pain
• Caused by infection spreading
through the apical foramen of the
tooth into the apical periodontal
region causing inflammation (apical
periodontitis) and ultimately a dental
abscess if left untreated.

48. Pericoronitis
• Pain commonly arises from the supporting gingivae and mucosa when
infection arises from an erupting tooth (teething or pericoronitis). This is the
most common cause for the removal of third molar teeth (wisdom teeth). The
pain may be constant or intermittent, but is often evoked when biting down
with opposing maxillary teeth.

49. Pain Diagnosis
Diagnosing a pain complaint consists of these major steps
• History
• Clinical examination
• Accurately identifying the location of the extractions from which the pain
emanates
• Establishing the correct pain category that is represented in the condition
under investigation
Textbook of Medical Physiology 10th edition- Guyton and Hall

50. Assessment of pain
In the assessment of pain intensity, rating scale techniques are often used. The
most commonly used techniques are:
• Numerical Rating Scale
• Visual Analogue Scale
• McGill Pain Questionnaire
• Behavioral Rating Scale
1.Breivik H, Borchgrevink PC, Allen SM, Rosseland LA, Romundstad L, Breivik Hals EK, et al. Assessment of pain. British Journal of Anaesthesia. 2008 Jul;101(1):17–24.

51. • Numerical Rating Scale (NRS)
• Visual Analogue Scale
1.Breivik H, Borchgrevink PC, Allen SM, Rosseland LA, Romundstad L, Breivik Hals EK, et al. Assessment of pain. British Journal of Anaesthesia. 2008 Jul;101(1):17–24.

52. McGill Pain Questionnaire
• It is also known as McGill pain index, is a scale of rating pain developed at
McGill University by Melzack and Torgerson in 1971.
• It is a self-report questionnaire that allows individuals to give their doctor a
good description of the quality and intensity of pain that they are experiencing.
• It is a very widely used questionnaire
Behaviour rating Scale
• For patients unable to provide a self-report of pain, a score from 0 to 10 is
assigned based on clinical observation

53. Management of Pain
GOALS OF THERAPY
•To decrease the subjective intensity
•To reduce the duration of the pain complaints
•To decrease the potential for conversion of acute pain to chronic persistent pain
syndromes
•To decrease the physiological, psychological, & socioeconomic sequelae
associated with under treatment of pain
Textbook of Medical Physiology 10th edition- Guyton and Hall

54. Non Pharmacological Management
The non - pharmacological management involves the following approaches
• Physiotherapy
• Psychological techniques
• Stimulation therapies - Acupuncture & Transcutaneous
• Electrical Nerve Stimulation (TENS)
• Palliative care - involves the alleviation of symptoms but does not cure the
disease
Textbook of Medical Physiology 10th edition- Guyton and Hall

55. Pharmacological Management
Textbook of Medical Physiology 10th edition- Guyton and Hall

56. Surgical Procedures for the Relief of Pain
• The surgical procedures include
• Cordotomy
• Thalamotomy
• Sympathectomy
• Rhizotomy
• Frontal Lobotomy
• Transcutaneous Neural Stimulation
• Acupunture
Textbook of Medical Physiology 10th edition- Guyton and Hall

57. PAIN INHIBITING MECHANISM
It can be
• Exogenous
• Endogenous
Textbook of Medical Physiology 10th edition- Guyton and Hall

58. Endogenous method of controlling pain includes
•Removing the cause- It is a desirable methods. It is imperative that any removal
leave no permanent environmental changes in tissue, since this condition would
then be able to create the impulse, even though the original causative factor had
been eliminated.
•Blocking the pathway of pain impulses- This can be done by injecting drug
possessing local analgesic property in proximity to the nerve involved.
Thus preventing those particular fibers from conducting any impulses centrally
beyond that point.
Textbook of Medical Physiology 10th edition- Guyton and Hall

59. • Raising the pain threshold
‣ Raising pain threshold depends on the pharmacological activity of drugs
possessing analgesic properties.
‣ These drugs raise pain threshold and therefore alter pain reaction,
conceptually there are two components of pain
‣ Nociceptive
‣ Affective component.
‣ The path of nociceptive component is spinothalamic tract →Thalamus. This
component is purely physical component of pain.
Textbook of Medical Physiology 10th edition- Guyton and Hall

60. • Affective Component
‣ It is the psychological component associated with pain. The path is that some
fibres from STT to thalamus terminate in some intermediate stations in the
reticular formation of brain stem and are called spinoreticular thalamic system.
‣ Non-narcotic analgesic like aspirin can inhibit the nociceptive but not the
affective component of pain whereas opioid (Morphine) inhibit affective as well
as nociceptive components of the pain. They act centrally at cortical and sub
cortical centres, to change patient mind and his reaction towards pain.
Textbook of Medical Physiology 10th edition- Guyton and Hall

61. • Preventing pain reaction by cortical depression
‣ Eliminating pain by cortical depression is by the use of general anaesthesia.
• Using Psychosomatic Method
‣ This method affects both pain perception and pain reaction. It include audio
analgesia
Textbook of Medical Physiology 10th edition- Guyton and Hall

62. Conclusion
• Pain is bad but not feeling can be worse.
• Dental pain is multi causative in origin.
• The dentist should use multi-modalities to treat the patient.
• Nothing is more satisfying to the clinician than the successful elimination of
pain.
• The most important part of managing pain is understanding the problem and
cause of pain
• It is only through proper diagnosis that appropriate therapy can be selected

63. References
• Textbook of Medical Physiology 10th edition- Guyton and Hall
• Textbook of medical physiology 7th edition- Sembulingam
• Essentials of Medical Pharmacology 7th edition- KD Tripathi
• Kumar KH, Elavarasi P. Definition of pain and classification of pain disorders.
J Adv Clin Res Insights 2016;3:87-90.
• Tandon OP et al Neurophysiology of pain: insight to orofacial pain. Indian J
Physiol Pharmacol 2003; 47 (3) : 247-269
• Chapter 7: Pain Tracts and sources. Ann textbook for the neuroscience
• Renton T. Dental (odontogenic) pain. v o 1. 5 -no.1 - marc
h2011
• Gupta R, Mohan V, Mahay P, Yadav PK (2016) Orofacial Pain: A Review.
Dentistry 6: 367.