Physiology of pain

Physiology of pain

1. Physiology of Pain

2. Objectives 1. Definition of pain 2. Types of pain 3. Body reactions or responses to pain 4. Perception of pain 5. Neural pathways for pain 6. Referred pain 8. Endogenous analgesia system 9. Gate control theory

3. Def : Pain is an unpleasant sensory and emotional experience associated with actual or potential tissue damage according to; International association for the study of pain ( IASP ). Significance: Pain is a warning signal for tissue damage. It is the prominent symptom of tissue damage Pain has a protective function. It initiates protective reflexes that; Get rid of the painful stimulus. Minimize tissue injury or damage . Pain Sensation

4. Classification of Pain

5. A) According to its mechanism or cause: Pain is classified into 3 main types; Pain Sensation

6. N.B. Neuropathic pain: according to site of injury is classified into; Central → Central pain e.g. thalamic infarct Peripheral → e.g. nerve compression, neuralgias. Mixed → e.g. post herpetic neuralgia. Character : neuropathic pain is characterized by burning, tingling, numbness, squeezing, itching, constant +/- intermittent shooting, and electric. Pain Sensation

7. B) According to its duration: Both nociceptive and neuropathic pains are classified into 2 types; Acute pain : less than 7 weeks Chronic pain : more than 7 weeks Pain Sensation

8. C) According to the quality or character : A pain it is classified into 5 types; Pain Sensation

9. Pain Sensation

10. Pain Sensation

11. Pain Sensation D) According to site of origin : 3 types;

12. Body Reactions to Pain increased heart rate Diaphoresis increased respiratory rate  blood sugar elevated blood pressure  gastric acid secretion pallor or flushing  gastric motility dilated pupils  blood flow to the viscera , kidney and skin nausea occasionally occurs

13. 13 Painful stimulus Pain sensation Pain Perception

14. 1)Types: They are morphologically similar but functionally they are specific Morphological types : are specific free nerve endings Functional types : classified according to their adequate stimulus into:- 1) Mechanical Pain Rs: Respond to strong mechanical forces, such as cutting, or pricking 2) Thermosensitive pain Rs: Respond to excessive changes in temp. (above 45°C and below 10°C). 3) Chemical Pain Rs: respond to noxious chemical stimuli. 4) Polymodal Pain Rs : Respond to a combination of mechanical, thermal, and chemical noxious stimuli Pain Receptors

15. Mechanism of stimulation: Pain Receptors Directly stimulate Pain Receptors Chemical stimuli Strong acids or Alkalies Mechanical stimuli Cutting or pricking Thermal stimuli temp. > 45 C and < 10 C Tissue damage 1 st class K ions, Histamine, Serotonin, and Bradykinin 2 nd class PGE2, leukotriens and Substance P Release of Pain Producing Compounds (PPS) Sensitize the pain Rs by lowering its threshold to stimuli

16. Tissue Damage Direct stimulators Sensitizers

17. Neural Pathways of Pain

18. Pathway: Neospinothalamic tract A) 1 st order neuron : A delta afferent fibers End in lamina I of dorsal horn of spinal cord B) 2 nd order neuron : Axons of neurons lamina I of dorsal horn of spinal cord cross the opposite side in front of central canal and ascend as neospinothalamic in spinal cord and as spinal leminiscus in brain stem End in posteroventral nucleus of thalamus (PVNT) C) 3 rd order neuron : Axons of neurons of PVNT ascend in sensory radiations End in primary somatic sensory area (area 3,1,2) Note: The chemical transmitter released at the central end of A delta fibers that carry fast pain is glutamate Fast Pain Pathway

19. Fast Pain Pathway Receptors Free nerve endings A delta Lamina I Lateral spinothalamic tract Spinal Leminiscus PVNT Sensory Radiations

20. Pathway: Paleospinothalamic tract A) 1 st order neuron : C afferent fibers End in lamina II and III (called substantia Gelatinosa of Rolandi SGR ) of dorsal horn of spinal cord B) 2 nd order neuron : Axons of neurons SGR of dorsal horn of spinal cord cross the opposite side in front of central canal and ascend in spinal cord and as; Spinoreticular tract end in RF of MO and Pons Spinotectal tract end in PAG areas of midbrain Paleospinothalamic tract end in non specific thalamic nuclei (intralaminar and midline) C) 3 rd order neuron : Axons of neurons from RF and NSTN of thalamus ascend in sensory radiations Terminate diffusely in all areas of the cerebral cortex Note: The chemical transmitter released at the central end of c fibers that carry fast pain is substance P Slow Pain Pathway

21. Receptors Free nerve endings Afferent C fibers SGR Laminae II, III Paleospinothalamic tracts Reticular formation In MO and Pons Periaqueductal gray area (PAG) in midbrain Non-specific nuclei of thalamus All cortical areas

22. Chronic (Slow ) Acute (Fast) PAIN Skin, deep tissues, and viscera Skin only Source All types Mechanical and thermal Stimulus Burning Pricking Quality One or more seconds after stimulation Within 0.1 sec after stimulation Onset Long (few minutes) Short (one second) Duration Diffuse Well –localized Localization C A-delta Afferent Paleospinothalamic tract Neospinothalamic tract Tract Thalamus Cerebral cortex Centre Substance P Glutamate Chemical trans.

23. Def: Referred pain is pain felt away from the site of its origin Radiating pain is pain appear to migrate away from the its original site Referred pain is a part of radiating pain N.B. Visceral pain usually referred, deep pain may be referred but cutaneous pain never referred Referred Pain Site of referral is determined by dermatomal rule: The pain from a viscera is referred to a somatic structure (skin or deep structure) which were developed in the same embryonic segment and supplied by the same dorsal root ganglia. Abnormal sites are due to migration of viscera.

24. Mechanism : Convergence Projection Theory: Afferents pain fibers from skin area and diseased viscera converge on the same 2 nd order neuron and finally stimulate the same cortical neuron The cortex project (feel) pain as it is coming from the skin because the sensory cortex is accustomed to receive pain from the skin Skin is the commonest site of pain Convergence may occurs also at the level of thalamus or sensory cortex Referred Pain

25. Mechanism : Convergence Projection Theory: Afferents pain fibers from skin area and diseased viscera converge on the same 2nd order neuron and finally stimulate the same cortical neuron.The cortex project (feel) pain as it is coming from the skin because the sensory cortex is accustomed to receive pain from the skin Skin is the commonest site of pain Convergence may occurs also at the level of thalamus or sensory cortex

26. Pain Control by Rubbing Painful stimuli A beta fibers Rubbing of the skin Interneuron Pain C fiber

27. Acupuncture Painful stimuli A delta fibers Acupuncture Interneuron Pain C fiber

28. There is endogenous analgesic system as proved by : Stress analgesia e.g. soldiers wounded by the heat of the battle may feel no pain until the battle is over . The pain felt from an injured area is reduced when the skin around the injured area is rubbed or stroked . Morphine injected in minute doses in experimental animals in the third ventricle of the brain produce marked analgesia . Th is Pain Control System can modulate pain sensation (mainly inhibitory) by release of endogenous opiate like (opioid) peptides such as encephalins, endorphins It cause both presynaptic and postsynaptic inhibition of nociception in the dorsal horn via enkephalin r elease which inhibits release of substance P , transmitter at pain nerve endings Pain Control System

29. 1) Def, System which control pain transmission in CNS or inhibit pain transmission i.e. endogenous analgesia system 2) Sites: Periventricular area of hypothalamus Periaqueductal area (PAG) around aqueduct of sylivus in midbrain and Pons Nucleus raphe magnus (NRM) AND Nucleus reticularis paragigantocellularis (NRPG) in lower Pons and upper medulla Pain inhibitory complex (PIC): interneuron in dorsal horn of spinal cord Endogenous Analgesia System

30. Posterior horn of spinal cord Nucleus raphe magnus Periventricular nucleus of hypothalamus Substance P Periaqueductal gray area Serotonin ++ Endorphins ++ Encephalin ++ Encephalins -- Stress 1 st order neuron of pain Ascending pathway

31. Endogenous Analgesia System