Lecture 13 - 3rd Asessment - Pain

Pain-I: Physiological mechanisms - Pain is an unpleasant sensory experience associated with tissue damage, it is a protective mechanism - Fast pain, sharp, pricking, acute (< .1 sec) e.g., needle Mainly in cutaneous tissue (skin) - Slow pain, dull, aching, chronic (> 1 sec) e.g., continuous pressure on tissue Mainly in deep tissues, muscle, joint, organs

Pain receptors (nociceptors) are free nerve endings They are sensitive to noxious (painful) stimuli They may be mechano, thermo, or chemosensitive i.e. pain is caused by mechanical, thermal, chemical stimuli Nociceptors are tonic, nonadapting receptors They keep the brain aware of damage

- Substances found in injured tissue: Bradykinin, protease, they elicit pain - Ischemia: reduced blood flow, anaerobic metabolism Lactic acid elicits pain - Muscle spasm: can cause pain in two ways: 1) Mechanical stretch, 2) Ischemia, elicit pain Spasm due to low Ca or excessive sympathetic activity

Pain transmission Anterolateral (lateral spinothalamic tract): Fast - sharp pain pathway: Myelinated A-delta (Group III) fibers, 30 m/sec Slow - chronic pain pathway: Unmyelinated Type C (Group IV) fibers, 2 m/sec

Primary S.S. cortex : Accurate intensity & location of pain Damaged primary S.S. cortex: Pain perception remains, crude localization Because posterior parietal cx receives input from Th.

Visceral sensations - From internal organs: Stomach, heart, liver, appendix, kidneys, … etc - Modalities : Pain, burning, pressure - Stimuli : Stretch, spasm, acids, ischemia

Referred visceral pain - Felt in area covering organ OR in area remote from organ - Appendix : lower R. abdomen, umbilicus - Heart : upper thorax, left side of neck, shoulder, left arm - Kidneys : Anterior abdominal wall although they are posterior

Parietal pain pathway - Fibers enter Sp. cord at the same segment of the covering tissue - Pain is referred to the covering tissue - Sharp pain, A-delta fibers

Visceral pain pathway - Fibers enter Sp. cord at a different segment of the covering tissue - Pain is referred to a remote tissue - Slow pain, type C fibers

Parietal pain pathway - Fibers enter Sp. cord at the same segment of the covering tissue - Pain is referred to covering tissue - Sharp pain, A-delta fibers

Visceral pain pathway - Fibers enter Sp. cord at a different segment of the covering tissue - Pain is referred to a remote tissue - Slow pain, type C fibers

Hyperalgesia ↑ Sensitivity to pain due to a painful stimulus, it may be: - Peripheral: due to injury, burn, sunburn, or inflammation caused by products like bradykinin, proteases, & prostaglandins - Central due to: - a lesion in spinal cord (e.g., a tumor affecting anterolateral system) - a vascular lesion in the thalamus (thalamic syndrome): Th.facilitation:degeneration in VPL→facilitation in posterior nuclei Analgesia: loss of pain (blocked by local anesthetics like procaine)

Herpes zoster (Shingles) Herpes virus infects DRG Extreme pain in the dermatome

Brown-sequard syndrome Left or right half cord lesion -

Motor : lost on same side

-

Light touch, vibration, position : lost on same side (dorsal column)

-

Pain & temperature : lost on opposite side (anterolateral)