Pain pathway

Includes - definition of Pain, Components of Pain, Neuron, Nerve fibers, Pain receptors, Pain Pathway, Pain theories, Applied Aspect, References

1. PAIN PATHWAY PRESENTED BY: DR GAYATRI MEHROTRA

2. DEFINITION OF PAIN The International Association For The Study Of Pain: Pain is “an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage.” Monheim : “ An unpleasant emotional experience usually initiated by noxious stimulus and transmitted over a specialized neural network to the CNS where it is interpreted as such.”

3. COMPONENTS OF PAIN Pain sensation has two components: Fast pain Slow pain Pain stimulus is applied Bright,sharp ,localized pain produced( fast pain) Followed by dull,diffused,unpleasant pain(slow pain)

4. Neuron: the basic functional unit

5. synapse The junction between two neurons is called synapse . The main function of the synapse is to transmit the impulses.

6. Transmission Of Impulse

7. Nerve fibers Fiber type Function Diameter (um) Velocity (m/s) 1) A-alpha Motor, muscle proprioception 12 to 20 70 to 120 2) A-beta Touch, pressure, proprioception 5 to 12 30 to 70 3) A-gamma Touch , motor proprioception 5 to 12 30 to 70 4) A-delta Pain, temperature, pressure proprioception 1 to 4 12 to 30 5) B Preganglionic autonomic activity 1 to 3 14.8 6) C Pain,temp,pressure , postganglionic activity 0.5 to 1 2

8. PAIN RECEPTORS: NOCICEPTORS A nerve ending that responds to noxious stimuli that can actually or potentially produce tissue damage. Receptors for fast pain are sensitive to mechanical or thermal stimuli of noxious strength. Receptors for slow pain are sensitive not only to mechanical or thermal stimuli but also to a wide variety of chemicals associated with inflammation.

9. Since the pain receptors respond to a wide variety of stimuli, they are called polymodal . Impulses from nociceptors are transmitted via two fiber types: Myelinated A delta fibers Unmyelinated C fibers

10. A delta - fibers C -fibers 1) Myelinated 1) Unmyelinated 2) 2-5 um in diameter 2) 0.4-1.2 um in diameter 3) Rate of 12-30 m/s 3) Rate of 0.5-2 m/s 4) Threshold is medium 4) Threshold is high 5)Carry fast pain sensation 5) Carry slow pain sensation

11. SENSORY NEURONS First order neuron Second order neuron Third order neuron

12. First order neuron First order neurons are the cells in the posterior nerve root ganglia. These neurons receive impulses from the pain receptors through their dendrites and their axons reach the spinal cord. A-delta fibers then synapse with marginal cells in the posterior gray horn. C type fibers synapse with substantia gelatinosa

13. Second order neuron Marginal cells and the cells of substantia gelatinosa form the second order neurons. Fibers of marginal cells Fibers of substantia gelatinosa Form neospinothalamic tract Form paleospinothalamic tract Terminate in posterolateral ventral nucleus of thalamus

14. Third order neuron The third order neurons of pain pathway are the neurons of thalamic nucleus, reticular formation, tectum and gray matter around aqueduct of Sylvius

15. Pain pathway

16. On entering the spinal cord, the pain signals take two pathways to the brain , through Neospinothalamic tract Paleospinothalamic tract

17. Neospinothalamic tract A delta fibers transmit fast pain Terminate in lamina marginalis of dorsal horn Excite second order neurons of neospinothalamic tract Cross to opposite side of cord Turn upward passing to the brain

18. It is believed that glutamate is the neurotransmitter substance secreted in the spinal cord at the A-delta pain nerve endings. This is one of the most widely used excitatory transmitters in the central nervous system, usually having a duration of action lasting for only a few milliseconds.

19. Paleospinothalamic tract C type fibers transmit slow pain Terminate in lamina II and III( substantia gelatinosa ) Join the fibers from the fast pain pathway Cross to opposite side of cord Then upward to the brain

20. Type C fibers entering the spinal cord release both glutamate transmitter and substance p transmitter. It has been suggested that the "double" pain sensation, one feels after a pinprick might result partly from the fact that the glutamate transmitter gives a faster pain sensation, whereas the substance P transmitter gives a more lagging sensation.

21. Pain pathway of maxillofacial region The fifth cranial nerve or Trigeminal nerve is the principal sensory nerve of the head region. Any stimulus in area of trigeminal nerve is received by both myelinated and nonmyelinated fibers and conducted as an impulse along the afferent fibers of ophthalmic, maxillary and mandibular branches into semilunar or gasserian ganglion .

22. Pain theories Specificity theory: Descartes in 1644 - pain is conceived as a straight channel from skin to brain. Pattern theory: In 1894, Goldscheider was the first to propose that stimulus intensity and central summation are the critical determinants of pain.

23. Gate control theory: Proposed by Melzack and Wall in 1965. It postulates the following: Information is transmitted to CNS by small peripheral nerves. Cells in spinal cord which are excited by these injury signals are also facilitated or inhibited by other large peripheral nerves that also carry information. Descending control systems modulate the excitability of cells that transmit information about injury.

24. Applied physiology Hemisection of spinal cord: Brown- Sequard syndrome:

25. Hyperalgesia : A pain pathway sometimes becomes excessively excitable that give rise to hyperalgesia , which means hypersensitivity to pain. causes : 1) excessive sensitivity of the pain receptors 2) facilitation of sensory transmission

26. Thalamic syndrome: In this syndrome, there is damage to posterior thalamic nuclei , usually caused by obstruction of branch of posterior cerebral artery. Patients with this syndrome have attacks of prolonged, severe and extremely unpleasant pain.

27. Herpes zoster ( shingles ): Occasionally a herpes virus infects a dorsal root ganglion which causes severe pain in the dermatomal segment normally subserved by the ganglion , thus eliciting a segmental type of pain that circles halfway around the body.

28. Trigeminal neuralgia: Lancinating pain occurs in some people over one side of the face in the sensory distribution area of trigeminal nerve Pain feels like sudden electric shock and it may appear for only a few seconds at a time or may be almost continuous.

29. Wallenberg syndrome: A stroke usually affects only one side of the body. If a stroke causes loss of sensation, the deficit will be lateralized to the right side or the left side of the body. The only exceptions to this rule are certain spinal cord lesions and the medullary syndromes, of which Wallenberg syndrome is the most famous example. In Wallenberg syndrome, a stroke causes loss of pain/temperature sensation from one side of the face and the other side of the body.

30. references Essentials of medical physiology 3 edition - K Sembulingam , Prema Semulingam Ganongs review of medical physiology 23 edition- Kim E Barrett, Susan M Barman, Scott Boitano , Heddwen L Brooks Guyton and Hall textbook of medical physiology 12 edition- Monheims Local anesthesia and pain control in dental practice Human anatomy vol 3- B D Chaurasia Burkets oral medicine 11 edition- Greenberg, Glick,ship

31. questions What are nociceptors ? What is dermatomal rule? Types of pain? What is pain perception? Theories of pain?

32. What is saltatory conduction? What is pyschogenic pain?

33. Thank you