In Vivo Electrophysiology

in vivo electrophysiology

• measuring of neuronal physiological characteristics in living organism

• intracellular and extracellular recordings

• many studies are invasive, some are on anaesthetized animals or post-surgery animals

Characteristics of pain in humans

• hyperalgesia → increased pain response to previously painful stimulus

  • primary → increased sensitivity at site on injury

  • secondary → altered sensitivity at un-injured site

• Allodynia → pain response to innocuous stimulus

• decreased use of injured tissue

Pain pathway

• high threshold nociceptors activated (mechanical, thermal or chemical) → primary afferent nociceptors

• fed to spinal cord (process and modulation) , to thalamus and then to cortical areas for sensory and emotional pain areas (processing and perception)

• spinal pathways have descending inhibitory and facilitatory influences in brain stem

• Brain areas:

  • brainstem: changes BP, respiration and feedback to spinal cord

  • Thalamus: integration and modulation

  • Somatosensory cortex: conscious localisation and recognition of pain

  • Cingulate cortex and amygdala: emotional response

  • Hypothalamus: stress and neuroendocrine responses

Measuring pain

• visual analogue scales (colours red→ green, faces happy → sad, scoring pain on a scale)

• Quantitative sensory testing (QST) → assesses pain threshold by measuring how long for something to be felt → measures spinal cord excitability

• in model animals

  • non-evoked pain behaviours

    • flinching, licking, reduced burrowing

  • reflexed → eg. moving of limb from stimulus (evoked)

  • facial grimace scales

    • eye, nose, ear and whisker changes

• Problems:

  • experimenter bias

  • interpretation of behaviour

  • spontaneous pain (not externally caused) vs evoked pain

  • effects of drugs (eg. sedation)

peripheral nerve recordings

• more pain = more action potentials

single-unit electrophysiology in the spinal cord

• done while animals are anaesthetized as equipment is bulky

  • vertebrae removed to expose spinal cord and tungsten electrode into lamina 5 (layers of dorsal horn)

• single units in deep dorsal horn as most stable place for electrode and the layer receives information from both painful and non-painful stimuli (wide dynamic range neurones)

  • analgesics reduce activity here

single-unit electrophysiology in the brain

• recordings from single neurones

• anaesthetized and hole drilled into skill at specific coordinates

  • stereotaxic coordinates determined relative to bregma

  • craniotomy using dental drill

  • electrode inserted

Electromyography (EMG)

• compromise between electrophysiology and behavioural studies

• measures excitability in dorsal horn using same pathways as behaviour but less subjectively

• size of response = size of pain

multi-unit electrophysiology

• can use electrode in dorsal horn that records from 16 locations

  • more data so better as not just one neurone

  • can locate in spinal cord and record how fast responses are

  • network response