Somatic Sensations II Pain, Headache and Thermal Sensations

1. Somatic Sensations IIPain, Headache and Thermal Sensations Prof. Dr. Bayram Yılmaz Yeditepe University Faculty of Medicine Department of Physiology

2. PAIN “An unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage.”

3. Pain … • Pain is a protective mechanism • Types of pain and their qualities • Fast pain (felt 0.1 sec after stimulus • Slow pain (felt after 1 sec or more) • Fast-sharp pain is not felt in most deeper tisues • Slow pain is usually associated with tissue destruction

4. Pain Receptors and Their stimulation • Pain receptors are free nerve endings • They are widespread in superficial layers of the skin as well as in certain internal tissues • Most other deep tissues are only sparsely supplied with pain endings • But, any widespread tissue damage can summate to cause slow-chronic-aching type of pain in these areas

5. Pain Receptors and Their stimulation • Three types of stimuli excite pain receptors • Mechanical, thermal and chemical • Chemical types of pain: • Bradykinin, serotonin, histamine, K ions, acids, proteolytic enzymes and prostaglandins • Nonadapting nature of pain receptors • Slow pain and nonadaptation • Hyperalgesia

6. Pain Mediators

7. Rate of tissue damage as a stimulus for pain • Heat above 45 oC and tissue damage • Special importance of chemical pain during tissue damage • Stimulation of free nerve endings by Bradykinin and K ions • Tissue ischemia as a cause of pain • Muscle spasm as a cause of pain • Direct effect of muscle spasm in stimulating mechanosensitive pain receptors

8. Rate of tissue damage as a stimulus for pain

9. Dual pathways for transmission of pain signals • A fast pain pathway • A slow chronic pain pathway • Peripheral pain fibers • Fast (Ad) fibers • Slow C fibers

10. A Ab C Cross Section of Peripheral Nerve: Ad and C Fibers & Ab A fibers Alpha’s: biggest and fastest. 200 yd/sec Motorneurons Beta’s: touch, vibration, light pressure Delta’s: pain fibers, slowest & smallest myelinated, 20 yd/sec C fibers One class unmyelinated, 1 yd/sec ‘A’ = myelinated; ‘C’ = unmyelinated

11. Dual pathways in the cord and brain stem • Neospinothalamic tract for fast pain: The fast type Ad fibers transmit mechanical and acute thermal pain • They terminate in lamina I of dorsal horns • They give rise to long fibers that cross immediately to the opposite side of the cord and turn upward in the anterolateral columns • A few fibers terminate in the reticular areas of the brain stem • Most pass all the way up to the thalamus, terminating in the ventrobasal complex along with DC-MLS tract

12. Dual pathways in the cord and brain stem • Capability of the nervous system to localize fast pain in the body • When tactile receptors that excite the DC-MLS are simultaneously stimulated, localization can be nearly exact • Glutamate, the probable neurotransmitter of the type Ad fast pain fibers • Glutamate is the neurotransmitter substance in the spinal cord

13. Dual pathways in the cord and brain stem • Paleospinothalamic pathway for slow-chronic pain • This pathway transmits pain only from the peripheral slow-chronic type C pain fibers (some in Ad as well) • Lamina II and III (substantia gelatinosa) • Crossing in the anterior commissure and turning up to the brain in the anterolateral pathway • Substance P, the probable slow-chronic neurotransmitter of type C nerve endings

14. Dual pathways in the cord and brain stem

15. Dual pathways in the cord and brain stem

16. Projection of paleospinothalamic tract to brain stem and thalamus • Paleospinothalamic pathway terminates widely in the brain stem, some pass to the thalamus • Termination in the brainstem in 3 areas: • 1) Reticular nuclei of the medulla, pons and mesencephalon • 2) Tectal area of the mesencephalon • 3) Periaqueductal gray region • From the brainstem areas, multiple short-fiber run through thalamus and into certain portions of the hypothalamus and basal brain regions

17. Projection of paleospinothalamic tract • Very poor localization of slow chronic pain in this pathway • Function of the reticular formation, thalamus and cerebral cortex in the appreciation of pain • Complete removal of the somatosensory cortex and perception of pain • Special capability of pain signals to arouse overall brain excitability

18. Projection of paleospinothalamic tract • Electrical stimulation of the reticular formation and intralaminar nuclei of the thalamus has a strong arousal effect on the CNS • Surgical interruption of pain pathways (cordotomy in the thoracic region of the spinal cord for pain relief)

19. Congenital Insensitivity to Pain

20. Congenital Insensitivity to Pain

21. Pain suppression (Analgesia) system in the brain and spinal cord • Pain sensation and perception can vary tremendously • The analgesia system consists of three components: • 1) Periaqueductal gray and preventricular areas of the mesencephalon and upper pons surrounding the aqueduct of Sylvius, neurons send signals to • 2) Raphe magnus and nucleus reticularis paragigantocellularis (near medulla). Second order-neurons transmitted down to dorsolateral columns • 3) A pain inhibitory complex located in the dorsal horns of the spinal cord. At this point, analgesia signals can block the pain before it is relayed to the brain

22. Pain suppression (Analgesia) system in the brain and spinal cord

23. Midbrain structures may modulate or control dorsal horn transmission of ascending tracts DESCENDING CONTROL

24. DESCENDING CONTROL

25. Pain suppression (Analgesia) system in the brain and spinal cord • Several transmitter substances are involved in the analgesia system • Enkephalin and Serotonin • Many nerve fibers derived from the PADG and PeVN secrete enkephalin and serotonin • Serotonin causes local cord neurons to release enkephalin too • Enkephalin causes both pre- and post-synaptic inhibition of incoming type C and Ad fibers in the dorsal horn

26. Beta endorphin, dynorphin, and enkephalins (EOPs) Act as natural opiates, reducing our perception of pain Bind to the same receptors as opiates and morphine Mu, kappa and delta receptors Diffuse projections in the brain Endogenous Opioid Peptides

27. Mu - -endorfin P hysical dependence E uphoria A nalgesia (supraspinal) R espiratory depression Opioid Receptors Mu, Kappa and Delta • Kappa - Dynorphins • Sedation • Analgesia (spinal) • Miosis • Delta- Enkephalins • analgesia (spinal & supraspinal) • release of growth hormone   

28. Acupuncture can work through these same pathways to release endogenous opioids.

29. Inhibition of pian transmission by simultaneous tactile sensory signalsGate Control Theory • Gate - located in the dorsal horn of the spinal cord • Smaller, slower nerves carry pain impulses • Larger, faster n. fibers carry other sensations • Impulses from faster fibers arriving @ gate 1st inhibit pain impulses (acupuncture/pressure, cold, heat, chem. skin irritation). Brain Pain Gate (T cells/ SG) Heat, Cold, Mechanical

30. The gate control theory of pain modulation

31. Special Aspects of Pain • Referred pain: • Nerve impulses from deeper parts of the body (viscera and muscles) may induce pain localised to superficial regions due to the summation of impulses in the relay stations. • Temporalis muscle pain may come from the upper teeth.

33. Visceral Afferents and Referred Pain dorsal root ganglion Visceral sensory nerves • run with sympathetic & parasympathetic nerves • cell bodies in dorsal root ganglion • nerve ending in viscera Somatic sensation: • conscious, sharp, well-localized • touch, pain, temperature, pressure, proprioception Visceral sensation: • often unconscious; if conscious: dull, poorly-localized • distension, blood gas, blood pressure, cramping, irritants

34. Visceral Afferents and Referred Pain Referred Pain: • Pain originating in a visceral structure perceived as being from an area of skin innervated by the same segmental level as the visceral afferent • Results from convergence of somatic & visceral afferents on the same segmental level of the spinal cord • “Cross-talk” in the dorsal horn convergence & “cross-talk” somatic afferent visceral afferent

35. Referred Pain

36. Visceral Pain • The viscera have sensory receptors for no other modalities of sensation besides pain • Localized type of visceral pain does not cause severe pain • Diffuse stimulation of pain nerve endings cause severe pain • Causes of true visceral pain • Ischemia • Chemical stimuli • Spasm of a hollow organ • Overdistention of a hollow viscus (organ) • Insensitive viscera: liver parenchyma vs liver capsule • Lung alveoli vs bronchi and parietal pleura

37. Parietal Pain Caused by Visceral Disease • Parietal surfaces (peritoneum, pleura or pericardium) are supplied with extensive pain innervation from the peripheral spinal nerves • A knife incision through the parietal peritoneum is very painful, whereas similar cut through the visceral peritoneum or gut is not very painful • Pain from different viscera is difficult to localize • Brain – existence of the internal organs • Sensations through true visceral pathway and parietal pathway

38. Visceral Pain and Parietal Pain • Visceral pain is usually localized in the dermatomal segment from which the visceral organ originated in the embryo • Stomach originated from the 7th – 9th thoracic segments

39. Some Clinical Abnormalities of Pain and Other Somatic Sensations • Hyperalgesia can be due to • (a) excessive sensitivity of the pain receptors (primary hyperalgesia) • (b) facilitation of sensory transmission (secondary hyperalgesia) • Sunburned skin – pain receptors – stimulation by chemical factors (histamine etc)

40. Hyperalgesia: From injury/infection/inflammation of: *Body tissues *Peripheral nerves *Spinal cord *Brain Characterized by: * Decreased pain threshold * Enhanced pain from supra-threshold stimuli * Spontaneous pain

41. Some Clinical Abnormalities of Pain and Other Somatic Sensations • Herpes Zoster (Varicella zoster virus) • It is also called Shingles • Infection of the dorsal root ganglion neurons • Carriage of virus by cytoplasmic flow through neorunal axons to their cutaneous origins (dermatomes)

42. Some Clinical Abnormalities of Pain and Other Somatic Sensations • Tic Douloureux: Lancinating pain occasionally occurs in some people over one side of the face in the sensory distribution area of the V and IX nerves • Trigeminal neuralgia • Glossopharyngeal neuralgia • It is set off by exceedingly sensitive trigger areas on the surface of the face, in the mouth or inside the throat • by mechanoreceptive stimulus rather than a pain stimulus • The pain of tic douloureux can usually be blocked by surgically cutting the peripheral nerve from the hypersensitive area

43. Phantom pain: Pain can also start from deafferented areas of the skin, amputated limbs or extracted teeth by the hyperactivity of the neurons in the spinal cord or trigeminal nucleus. Special Aspects of Pain

44. Headache • A type of pain referred to the surface of the head from deep structures • Headache of Intracranial Origin • Pain sensitive areas in cranial vault • Tugging on the venous sinuses, damaging the tentorium, stretching the dura at the base of the brain can cause intense pain • Trauma or stretching of the meningeal vessels of the meninges

45. Headache • Areas of the head to which intracranial headache is referred • Stimulation of the receptors above tentorium causes referred pain to the front half of the head in the surface, Vth cranial nerve • Pain impulses from beneath the tentorium enter through the glossopharyngeal, vagal and second cervical nerves • Subtentorial pain stimuli cause occipital headache referred to the posterior part of the head

46. Headache

47. Types of Intracranial Headache • Headache of meningitis • Headache caused by low cerebrospinal fluid pressure • Migraine headache • Abnormal vascular phenomena • Prolonged emotion or tension causes reflex spasm of some of the arteries • Vasospasm causes ischemia • More migraine headache in women tha man • Exact mechanism is unknown • Prodomal sensations (nausea, loss of vision, hallucinations etc) • Alcoholic headache • Headache caused by constipation

48. Extracranial Types of Headache • Headache resulting from muscle spasm • Headache caused by irritation of nasal and accessory nasal structures • Irritation or infection of mucous membranes of the nose and nasal sinuses • Summation of stimulus from nerve endings • Headache caused by eye disorders • Tonic contraction of eye muscles • Exposure of eyes to excessive irradiation

49. Thermal Sensations • Thermal receptors and their excitation • Cold, warmth and pain receptors • Cold and warmth receptors are located immediately under the skin at discrete separated spots • There are more cold receptors than warmth receptors • Transmission through Ad nerve fibers

50. Stimulation of Thermal Receptors