Pain pathway

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

1. Pain pathway DR PARTH/ SR NEUROSURGERY

2. IASP DEFINITION “Pain is an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage .” ASPECTS OF PAIN- affective-motivational sensory-discriminative aspects of pain Types of pain- Nociceptive-response to injury Inflammatory- upregulation of inflammatory mediators Neuropathic-caused by nerve injury

3. PATHWAY

4. NOCICEPTORS N ociceptive neurons are different from low-threshold tactile afferents in terms of physiology, morphology, and neurochemistry. PRO-NOCICEPTIVE substances/neuropeptides- substance P , CGRP , and neuropeptide Y. ‘Plasticity in response’ to tissue conditions- alterations of neuronal phenotype. enhanced responsiveness during inflammation. or in response to damage to the nerve itself. ‘This is an important contributor to H yperalgesia and abnormal pain states’

5. 1 st order neurons-Small myelinated(A delta) and unmyelinated primary afferents(c) that travel through the dorsal root to synapse in the dorsal horn. S uperficially (in laminae I and II). D eeply (in laminae V, VI, and VII), A round the central canal

6. Second-order neurons(lamina 2 and 5,6)-are usually divided into two classes: Wide dynamic range (wdr) neurons - receive convergent input from both nociceptive and nonnociceptive primary afferents Low thresholds-within the innocuous range. Code stimulus intensity through the noxious range(c/f tactile aff .). Distributed S omatotopically within the dorsal horn. Large receptive fields-precludes localization. Common in the deeper dorsal horn Fn-allow normally nonpainful stimuli such as touch to give rise to a sensation of pain under certain conditions

7. Nociceptive-specific . Respond exclusively to noxious stimuli(a delta mechanoreceptors or by both A delta and C nociceptors .) Receptive fields are small, which indicates an important role in stimulus localization . They are concentrated in the more superficial layers-lamina 1& 2.

8. Nociceptive specific pathways in spinal cord. Spinothalamic pathway Affective-motivational(MEDIAL THALAMUS) Sensory-discriminative(LATERAL THALAMUS) aspects of pain Spinoreticular pathway Spinomesencephalic Conscious sensation Arousal, Autonomic and motor responses to noxious input. Recruitment of descending control systems Spinoparabrachial

9. Spinoparabrachial pathway consists of projections of neurons primarily in lamina i and relays information to the amygdala and hypothalamus, as well as to the midbrain periaqueductal gray matter (PAG) and caudal ventrolateral medulla. Spinohypothalamic - autonomic and reflex responses to nociception. Spinotectal –initiating eye movement to painful stimuli.

10. 3rd order neuron Cell body in thalamus, ascend ipsilaterally to project to somatosensory cortex, and other higher centres .

11. SUPRASPINAL RECEPTIVE STATIONS- “ PAIN MATRIX” Brainstem Lateral thalamic nuclei Medial thalamic nuclei Primary somatosensory Cortex Secondary somatosensory Cortex Insula Anterior cingulate cortex Prefrontal cortex

12. thalamus LATERAL SYSTEM - include the ventral posterior medial (VPM) nucleus ventral posterior lateral (VPL) nucleus ventral posterior inferior (VPI) nucleus . Posterior Part of the Ventral Medial Nucleus MEDIAL SYSTEM Intralaminar nuclei. Ventral caudal part of the medial dorsal nucleus VENTROCAUDAL (VC) NUCLEUS .

13. Somatosensory Cortex

14. SOMATOSENSORY CORTEX PRIMARY(S1) Nociception of discriminating location and intensity. Si activation has been linked specifically to pain intensity Sustained noxious stimulation -produce a decreased cortical signal in this region

15. SECONDARY(S2) Unambiguous role in cortically mediated nociception. Recognition of Painful and thermal stimuli, Pain-related learning, and integration of tactile and nociceptive information. Thalamic input to the sii comes largely from the vpi Have large receptive fields, with contralateral or bilateral activation.

16. INSULA Central structure in pain matrix. Anterior insula- nociceptive information. Posterior insula-tactile processing. Insula codes for intensity of the stimulus. Lesions of insula- Increased pain tolerance Loss of affective quality of pain while able to detect intensity thresholds.

17. Anterior cingulate cortex Affective and motivational aspects of pain. ‘Emotional distress of pain and in selection of responses to painful stimuli’ Gets stimulated in response to noxious stimulus almost in parallel to s2. Input to the acc comes from medial thalamic nuclei, the mediodorsal nuclei, and the parafascicular nuclei. Lesioning -therapeutic cingulotomy ! Pain ratings were improved only modestly, but the pain was considered less bothersome or distressing

18. PREFRONTAL CORTEX higher cognitive function and endogenous modulation of pain. medial prefrontal cortex & dorsolateral prefrontal cortex FUNCTION- Executive function, attention, and execution of high-order tasks. The dorsolateral prefrontal cortex is specifically involved in the placebo response, in which pain sensations are modulated by expectation.

19. Role of Dorsal Column Pathway in Visceral Pain Observation-midline myelotomy relieved pain in patients with cancer involving pelvic visceral structures !! significant projection ascending ipsilaterally through the dorsal columns and transmitting information to the ventroposterolateral nucleus of the thalamus may not contribute to pain sensation under normal conditions, but it could become sensitized by visceral inflammation.

20. DESCENDING MODULATORY SYSTEMS PAG/RVM system Employs endogenous opioids as neurotransmitters-in the PAG and RVM --contribute to the pain-modulating function of this system. Supports opioid analgesia-these regions are required for the analgesic actions of systemically administered opioid. Opioids act directly on inhibitory neurons that normally inhibit the pain-inhibiting output neurons. Opioids thus activate descending inhibition through disinhibition Bidirectional control.

21. Dorsal reticular nucleus in the caudal medulla (which mediates effects of counterirritation) Noradrenergic systems in the dorsolateral pontine tegmentum

22. Neural Basis for Bidirectional Control Two populations of rostral ventromedial medulla (RVM) neurons, ON-cells and OFF-cells , provide the neural basis for bidirectional control of spinal processing by the RVM

23. Craniofacial pain pathway

24. CRANIOFACIAL PAIN SYNDROMES CEPHALIC NEURALGIAS Trigeminal neuralgia vascular compression MS Glossopharyngeal neuralgia : pain usually in base of tongue and adjacent pharynx.- PICA Geniculate neuralgia : otalgia – nervus inermedius-somatosensory branch of facial n. Tic convulsif : geniculate neuralgia with hemifacial spasm-AICA- Occipital neuralgia

25. Herpes zoster : characteristic vesicles and crusting usually follow pain, most often in distribution of V1 Post herpetic neuralgia ( R amsay-hunt syndrome) Supraorbital neuralgia ( s.o.n ) Trigeminal neuropathic pain (AKA trigeminal deafferentation pain): injuries from sinus or dental surgery, head trauma. OPTHALMOPLEGIAS OTALGIAS VASCULAR PAIN SYNDROMES.

26. CLINICAL IMPLICATIONS OF PATHWAYS

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