Pain pathways in the nervous system

What is first stage of linking stimulus to a response?

• Pain sensing

• Involves nociceptors located in various tissues around the body

• Nociceptors are specialised nerve endings dedicated to the perception of pain

What is second stage of linking stimulus to a response?

Nociceptive stimulus creates signals that are transmitted from the peripheral tissues to the dorsal horn of the spinal cord via afferent neurones of the PNS

What is third stage of linking stimulus to a response?

Transmission of signals through the spinal cord to the brain via ascending tracts in the spinal cord

What is fourth stage of linking stimulus to a response?

• Transmission of signal from thalamus to somatosensory cortex where pain is consciously perceived

• Connections from thalamus to prefrontal cortex (coordination of cognitive & motor responses) & limbic system (affective or emotional experience of pain)

What are the different order neurones and its locations?

• First- & second-order neurones located at dorsal horn

• Third-order neurone located in the thalamus

What is the dorsal horn synapse?

• Critical point in pain pathway

• Enables signals transmitted to brain to be inhibited (inhibitory neurotransmitters) or amplified (excitatory neurotransmitters)

• Allows spinal level reflexes to initiate immediate protective motor responses

What is the gate control theory of pain?

• Dorsal horn synapse is a gate through which afferent pain signals have to pass in order for pain to be transmitted into the brain

• May be influenced by factors:

  • Descending nerve impulses from thalamus & cerebral cortex

  • Other local afferent sensory inputs

How can pain be classified?

Nociceptive pain

• Somatic pain

  • Superficial - skin

  • Deep - muscles, joints, bones

  • Well-localised

• Visceral pain

  • Internal organs

  • Common ailments (e.g indigestion)

  • Serious life-threatening disease (e.g MI)

  • Poorly-localised

  • Pain may be referred

  • Autonomic activation

Inflammatory pain (secondary cause of pain)

• Inflammation is the main driver for activation of the pain pathway

• Acute inflammation pain - insect stings, sunburn trauma

• Chronic inflammation pain - RA, IBD, gout, gastritis

Neuropathic pain

• Damage to nerves in the PNS or CNS

• Aetiology - trauma, ischemia, infection, disease (e.g diabetes)

• Description - typically described as ‘burning’, ‘shooting’ or ‘stabbing’

• Allodynia - pain caused by normally non-painful sensory stimuli

• Hyperalgesia - exaggerated pain in response to mildly painful stimuli