Pain & Sensory Pathways

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Pain Pathways

  • Surgery produces injury to tissue with release of:
  • Histamine
  • Inflammatory mediators - eg. peptides, lipids, neurotransmitters and neurotrophins
  • These factors activate peripheral nociceptors which initiate transduction and transmission of nociceptive information to the CNS
  • Also causes neurogenic inflammation - where release of neurotransmitters in the periphery causes vasodilatation and plasma extravasation
  • Most common nociceptor is the C‐fibre polymodal nociceptor, which responds to a broad range of physical and chemical stimulation
  • Peripheral nociceptor signals are transmitted via and C nerve fibres to the dorsal horn of the spinal cord, where integration with descending input occurs
  • In the dorsal horn, complex modulating occurs which determines further transmission:
  • Some impulses pass to the ventral and ventrolateral horns to initiate spinal reflex responses which can cause increased skeletal muscle tone, phrenic nerve inhibition, decreased GI motility
  • Some pass to higher centres via the spinothalamic and spinoreticular tracts, inducing suprasegmental/cortical responses - producing the perception and affective component of pain
  • Continuous release of inflammatory mediators sensitizes functional nociceptors and activates dormant nociceptors
  • Sensitisation of peripheral nociceptors can occur:
  • Decreased activation threshold
  • Increased rate of discharge with activation
  • Increased rate of basal discharge
  • Intense noxious input from periphery can cause central sensitisation and hyperexcitability

Sensory Pathways

  • Activation of sensory receptors is converted into nerve impulses and this sensory input is then conveyed via the fibers of the cranial or spinal nerves to their respective relay nuclei in the central nervous
  • The sensory information is then further processed as it progresses, via the ascending sensory systems (pathways) to the cerebral cortex or to the cerebellum.
  • Sensory information is also relayed to other parts of the CNS where it may function to elicit a reflex response, or may be integrated into pattern-generating circuitry.
  • The ascending sensory pathways are classified according to the functional components (modalities) they carry as well as by their anatomical localization.
  • The two functional categories are:
  • The general somatic afferent (GSA) system - transmits sensory information such as touch, pressure, vibration, pain, temperature, stretch, and position sense from somatic structures
  • The general visceral afferent (GVA) system, which transmits sensory information such as pressure, pain, and other visceral sensation from visceral structures.
  • The three anatomical sensory categories are:
  • The anterolateral system - ALS
  • Primary neurons enter the spinal cord and then ascend one to two levels before synapsing in the substantia gelatinosa
  • Includes the spinothalamic, spinoreticular, spinomesencephalic, spinotectal and spinohypothalamic tracts
  • Relays mainly pain and temperature sensation
  • Also relays nondiscriminative touch, pressure and proprioceptive sensation
  • The dorsal column-medial lemniscal pathway
  • Relays fine tactile sense, vibration sense and position sense
  • Primary axons entering below T6 travel in the fasciculus gracilis of the dorsal column, whereas those above T6 travel in the fasciculus cuneatus which is lateral
  • The primary axon ascends to the lower medulla, leaving its fasiculus and synapses with a secondary neuron in the nucleus gracilis or nucleus cuneatus
  • Axons then leave the nucleus, passing anteriorly and medially as the internal arcuate fibres, which then travel as the contralateral medial lemniscus
  • Secondary axons from the medial lemniscus terminate in the Ventral Posterolateral Nucleus of the thalamus, synapsing with the tertiary neurons
  • These then ascend via the posterior limb of the internal capsule and end in the primary sensory cortex
  • The above 2 pathways use three different neurons to get information from sensory receptors at the periphery to the cerebral cortex. These neurons are designated primary, secondary and tertiary sensory neurons.
  • In both pathways, primary sensory neuron cell bodies are found in the dorsal root ganglia, and their central axons project into the spinal cord.
  • The Spinocerebellar tracts to the cerebellum
  • Relay primary proprioceptive information