Somatosensory System II: Pain and Temperature

Somatosensory System II: Overview

• Focuses on pain, temperature, analgesia, referred pain, the anterolateral system (ALS), the anterior trigemino-thalamic system, cord and CNS syndromes, and case studies

Pain, Temperature, and Analgesia

• Non-discriminative touch is carried by the anterior ALS
• Thermal sensation and nociception (pain) are carried by the lateral ALS
• Mechanoreceptors and free nerve endings are involved in these sensations
• Ad and C fibers are primary pain and temperature neurons
• Crude touch, pain, and temperature discrimination involves the ALS
• ALS handles information from upper and lower extremities, projecting to the VPL then somatosensory cortex
• ATT handles information from the face and head, projecting to the VPM then somatosensory cortex
• Negative perception/response to crude touch, pain, and temperature involves: reticular nuclei, peri-aqueductal gray, intralaminar nuclei of thalamus, insular and cingulate cortices

Mechanoreceptors and Nociceptors

• Mechanoreceptors for crude touch are high-threshold and have large receptive fields, responding to rough stimuli with or without tissue damage
• Mechano-nociceptors respond to pinch, puncture, and tissue damage
• Nociceptors for pain are "free" nerve endings
• Nociceptive mediators include H+, K+ ions, venoms, serotonin, histamine, bradykinin, prostaglandins, substance P, and CGRP peptides

Thermoreceptors and Ion Channels

• Thermoreceptors respond to cool, cold, warm, and hot temperatures
• Changes elicit the largest response
• Pain and temperature receptors are distinct, with different thresholds
• Extreme temperatures activate thermo-nociceptors
• Nociceptive and thermosensitive ion channels include TRP and ASIC channels

"Painful" Terms

• Allodynia: increased sensitization, painful response to innocuous stimuli in receptive field past the damaged area
• Hyperalgesia: increased response to painful stimuli
• Hyperesthesia: increased sensitivity overall
• Anesthesia: loss of pain sensation
• Hypesthesia: reduced pain sensation
• Paresthesia: positive, altered sensation (numbness, prickling, tingling)
• Dysesthesia: altered sensation (paresthesia) that is distinctly unpleasant
• Neuralgia: non-nociceptive, nerve pain (trigeminal neuralgia)
• Analgesia: relief of pain, achieved through epidural analgesia (opioids, local anesthetics), stimulation, or transcutaneous electrical nerve stimulation (TENS)
• Shingles (herpes zoster virus) can cause hyperesthesia and post-herpetic neuralgia, with paresthesia in V1/V2 divisions

Central Control of Pain

• Ascending information travels via DRG/ALS, with rapid III (Ad) fibers for sharp pain and slow IV (C) fibers for dull pain
• Descending information is modulated via peri-aqueductal gray and raphe nuclei
• Amines (serotonin, norepinephrine) and endogenous opiate peptides (enkephalin) suppress pain transmission mediated by nociceptive DRG neurons (substance P, glutamate) at the spinal cord level

Gate Theory of Central Pain

• C fibers inhibit inhibitory INTN and excite ALS INTN
• Aβ mechanoreceptors excite inhibitory INTN
• Descending inputs excite inhibitory INTN
• Strong pain is carried only by C fibers
• Weak pain is weak with mechanoreceptors
• Weak pain is also present with descending inputs
• Trans-cutaneous electrical neural stimulation activates Aβ to relieve pain
• Shaking a limb has a similar effect
• Myelotomy is used to treat intractable intestinal pain in cancer patients by cutting the anterior white commissure (AWC)

Dermatomes and Referred Pain

• Central overlap occurs in dermatome skin regions
• Dermatomes represent skin areas centrally, organized by trigeminal divisions (V1, V2, V3) and spinal roots
• Shingles is caused by reactivation of varicella-zoster virus, affecting dermatomes
• Fingertip and trigeminal regions exhibit less dermatomal overlap, indicating narrower receptive fields
• 'Visceral' nociceptors and skin nociceptors share common dorsal horn target interneurons
• Activation of visceral nociceptors is localized as dermal pain (e.g., angina pectoris)

Anterolateral System (ALS)

• DRG fibers enter the 'posterolateral fasciculus' and Lissauer's tract over several spinal segments
• Synapse in the dorsal horn with decussation at the anterior white commissure (AWC)
• somatotropic organization in ALS - VPL, somatosensory cortex
• Targets for negative perception and response include reticular formation, intralaminar nuclei, tectum, peri-aqueductal gray matter, and cingulate cortices
• ALS deals with touch, thermal sense and pain

Anterolateral System Pathways

• Spinothalamic pathway is responsible for localization and perception
• Spinoreticular pathway mediates arousal
• Spinomesencephalic pathway mediates response to pain

Vascular Syndromes

• Damage to anterior spinal artery will affect both corticospinal tracts and the anterolateral system
• Damage to the posterior spinal arteries will affect dorsal column medial lemniscus

Vascular Supply

• Spinal cord supplied by anterior spinal artery, posterior spinal arteries
• Medulla supplied by anterior spinal artery and posterior inferior cerebellar artery (PICA)
• Thalamus supplied by thalamogeniculate artery, a branch of the posterior cerebral artery
• Cortex supplied by middle cerebral artery (MCA) and anterior cerebral artery (ACA)

Wallenberg Syndrome

• Also known as lateral medullary syndrome
• Typically the result of occlusion of PICA
• Leads to Hemi-analgesia (loss of pain sensation) and Hemi-thermoanesthesia (loss of temperature sensation)
• Results in contralateral loss of pain in the body and ipsilateral loss of pain in the face and head

Spinal Cord Hemisection

• Known as Brown-Sequard Syndrome
• Results in;
  • Ipsilateral loss of motor control (post-decussation, descending)
  • Ipsilateral loss of touch / proprioception (pre-decussation, ascending)
  • Contralateral loss of pain / temperature (post-decussation, ascending)

Case Studies: Spinal Cord Lesions

• Spinal meningioma T9, left side, causes:
  • Ipsilateral deficits in lower motor neurons, DCML, and
  • Contralateral ALS deficits
• Sudden onset numbness in right face/arm/leg due to lacunar infarct in thalamus, affecting VPL (DCML) and VPM (trigeminal)
• Decreased sensation in right body arm and face, right arm numbness, aphasia and blurred vision, caused by a broad infarct covering somatosensory cortex, parieto-occipital region and Wernicke's area

Lateral Pontine or Medulla Lesions

• Lateral Pontine or Medulla: contralateral loss of pain and temperature sensation in the body (ALS) and ipsilateral loss of pain and temperature sensation in the face (ATT)

Medial Medulla Lesions

• Contralateral loss of touch and proprioception in the body (DCML)

Other Cord Sydromes

• Central cord syndrome: weakness and loss of somatosensory function below the lesion
• Anterior cord syndrome: weakness and loss of pain and temperature sensation below the lesion
• Posterior cord syndrome: loss of touch and proprioception below the lesion