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What is pain?
an unpleasant experience consisting of sensory and emotional components associated with actual or potential tissue damage
Nociceptive (stimulation of peripheral nerve fibres)
Neuropathic (affects somatosensory "feeling" system). Pain from normally non-painful stimuli (allodynia).
Inflammation pain - tissue damage/infection - chemical mediators
[psychogenic (physical pain caused by mental, emotional or behavioural factors)]
Give a brief outline of the pain pathway
activation of nociceptors, afferent sensory fibres to dorsal horn of spinal cord, replay to brain via ascending spinothalamic tracts
What are Nociceptors?
What can directly stimulate them? What can be a negative result of this?
sensory receptors that detect noxious stimuli. They are free nerve endings of primary afferent A(delta) and C fibres.
Inflammatory mediators (serotonin, prostaglandins, cytokines and H+, K+) are released from damaged tissues – can stimulate nociceptors directly.
Also reduce activation threshold of nociceptors – primary sensitisation. Can lead to hyperalgesia
Types of nociceptive afferent neurones
C-fibres, A(delta) fibres and A(beta) fibres
What are C-fibres?
Main type Unmyelinated. Slow conduction rate (0.5-2 m/s).
Polymodal - respond to thermal, mechanical and chemical stimuli. Very small diameter axons. Dull pain.
What are – A(delta) fibres ?
– lightly myelinated, fast (5-30m/s), small diameter axons, sharp localised pain, responsible for initial reflex response to acute pain
What are A(beta) fibres ?
carry non-noxious stimuli. Highly myelinated, large diameter – rapid conduction. Low activation threshold and respond to light touch – transmit non-noxious stimuli
Explain the pathway from Nociceptive Afferent Neurones to brain
What are neurones involved?
• A(delta) and C fibres synapse with secondary afferent neurones in dorsal horn of spinal cord.
• Transmit info to nociceptive-specific neurones in Rexed lamina I and II – mainly short inhibitor interneurons
• Primary afferent terminals release excitatory nts – glutamate and substance P (a tachykinin from capsaicin)
• Interactions in dorsal horn between afferent, interneurons and descending modulatory pathways – determine activity of secondary afferent neurones. •
Glycine and GABA – inhibitory interneurons
What are the two ascending tracts in spinal cord – pathways for nociceptive signals to brain
• Spinothamalic tract – 20 afferent neurones decussate within a few segments of level of entry into spinal cord and ascend in contralateral spinothalamic tract to nuclei within the thalamus. Third order neurones then ascend to terminate in the somatosensory cortex.
• Spinoreticular tract – fibres also decussate and ascend the contralateral cord to reach the brainstem reticular formation, before projecting to the thalamus and hypothalamus. Emotional aspect of pain
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