Pain Pathway

(NB: On smartphones, better read in landscape mode.)

• Pain is defined as an unpleasant and emotional experience associated with or without actual tissue damage.
• Pain sensation has two components: 1. Fast pain 2. Slow pain
• Fast pain is the first sensation whenever a pain stimulus is applied. It is experienced as a bright, sharp, and localized pain sensation
• Fast pain is followed by slow pain, which is experienced as a dull, diffused and unpleasant pain.
• Pain sensation from various parts of body is carried to brain by different pathways which are:
• Pain sensation in face is by Trigeminal nerve.
• Chief sensation present in viscera which is poorly localized(often referred), unpleasant and associated with nausea and autonomic symptoms.
• Quality of pain varies such as gripping, shooting, aching, etc.
• E.g. –
  • Ischemia,
  • Chemical stimuli like HCl of stomach
  • Spasm of hollow organs like intestines
  • Over distention of hollow organs like bladder
• Sympathetic(Thoracolumbar) Nerves – for thoracic and abdominal nerves.
• Vagus & Glossopharyngeal- For oesophagus, trachea, and pharynx.
• From deeper structures of pelvic region by sacral parasympathetic nerves
• Receptors– Free nerve endings distributed through the body.
• First Order Neurons-
  • Cells in posterior nerve root ganglia receiving impulses of pain sensation from pain receptors through their dendrites & transmitted to spinal cord through their axons.
    • Fast Pain Fibres-
      • In Peripheral nerves- Fast pain signals are transmitted from Aδ fibres at velocities between 6 and 30 m/s to dorsal root ganglion and then enter the spinal cord at dorsal root of spinal nerve (formed by axons of cells of dorsal root ganglion).
      • In Spinal Cord- Aδ fibres ascend or descend for one or two segments in the tract of Lissauer lying immediately posterior to the dorsal horn and then terminate into neurons of lamina I or the Marginal nucleus.
    • Slow Pain Fibres-
      • In Peripheral Nerves- Impulses are carried by slow conducting unmyelinated fibres at velocities ranging from 0.5 to 2 m/s to the dorsal root ganglion and then enter the spinal cord at dorsal root of spinal nerve (formed by axons of cells of dorsal root ganglion).
      • In Spinal Cord:- The C fibres terminate in the laminae II and III of the dorsal horn, together known as the Substantia Gelatinosa of Rolando.
• Second Order Neurons-
  • Neurons of marginal nucleus and Substantia Gelatinosa of Rolando form the second order neurons.
    • Fast pain Fibres-
      • In the spinal cord- Neurons of marginal nucleus give rise to fibres which cross to the opposite side of the spinal cord through anterior grey commissure and ascend to the brain in anterolateral columns as neospinothalamic tract.
      • In the brain stem- Few fibres of the neospinothalamic tract  terminate in the reticular formation(reticular activating system) but most of them ascend upwards.
      • In the thalamus– Most fibres project in the Ventral Postero-Lateral Nucleus (VPL)   of thalamus while some terminate in the ascending reticular activating system of brain.
    • Slow Pain Fibres-
      • In Spinal Cord– Fibres from Substantia Gelatinosa of  Rolando cross the midline near their level of origin and run along with the fibres of fast pain as paleospinothalamic fibres.
      • In Brain Stem– These fibres terminate very widely. 20% terminate in the VPL nucleus of thalamus. Remaining 80% terminate in any of the following regions:-
        • Reticular formation(mainly)
        • Superior colliculus(tectum)
        • Periaqueductal Grey (PAG) region around the aqueduct of Sylvius.
• Third Order Neurons-
  • These are neurons arising from-
    • Thalamic Nucleus
    • Reticular Formation-
    • Superior Colliculus
    • PAG region
  • Axons from these regions project on to the primary sensory cortex.
  • Fibres from reticular formation proceed rostrally to:-
    • Intralaminar nuclei of thalamus- Relays signals to all parts of brain.
    • Posterior nuclei of thalamus
    • Part of Hypothalamus- concerned with arousal mechanism due to pain stimulus.
• Centers for pain sensation- The phenomenon by which noxious stimuli reach consciousness. It has two components-
  • Nociceptive-
    • Occurs at subcortical level, i.e., at thalamus and subcortical level.
  • Affective-
    • Involves spinothalamic tract-limbic system pathway

NB: Include a proper diagram from your standard textbook mandatorily depicting the two pathways of fast and slow pain