Physiology: pain (not relevant for finals) Flashcards
Define what is pain ?
An unpleasant sensory and emotional experience, associated with actual tissue damage or described in terms of such damage
What are the 3 main forms of pain ?
- Nociceptive pain - adaptive
- Inflammatory pain - adaptive
- Pathological pain - maladaptive
What are nociceptors?
Nociceptors are primary afferent neurones innervating peripheral tissues – activated only by mechanical, thermal, or chemical stimuli that are noxious. (only provoked by intense stimuli)
Why is pain essential to the protection of the organism ?
Pain motivates the individual to withdraw from damaging situations, to protect a damaged body part while it heals, and to avoid similar experiences in the future.
Describe nociceptive pain
- Is adaptive – serves as an early warning system to detect and minimise contact with damaging stimuli (noxious events)
- Is high threshold – provoked only by intense stimuli that activate nociceptors
Describe inflammatory pain
- Is adaptive and protective – caused by activation of the immune system in injury, or infection
- Causes pain hypersensitivity (heightened sensitivity to noxious stimuli) and allodynia (innocuous stimuli now elicit pain)
Describe what pathological pain is
Is maladaptive with no protective function - results from abnormal nervous system function
What are the 2 different types of nociceptors ?
- Ad(delta)-fibres
- C-fibres
Describe the action of Ad (delta) nociceptor fibres
- Ad-fibres are mechanical/thermal nociceptors that respond to noxious mechanical and thermal stimuli.
- Mediate ‘first’, or fast, pain
Describe the action of C-fibre nociceptors
- Ad-fibres are mechanical/thermal nociceptors that respond to noxious mechanical and thermal stimuli.
- Mediate ‘first’, or fast, pain
Which nociceptors detect first pain and which detect second pain ?
- Ad (delta) fibres detect first pain
- C-fibres detect second pain
Describe the nociceptive pathway from a peripheral site to the dorsal horn of the spinal cord
- Stimulus (mechanical, thermal or chemical) opens ion channels in nerve terminal to elicit depolarising receptor potential
- Amplitude of generator potential in proportion to intensity of stimulus
- Local current flow triggers ‘all or none’ AP’s at a frequency proportional to the amplitude of the receptor potential
Describe the the projection pathways from the spinal cord to the brain
Nociceptive C and Ad(delta) fibres terminate Nociceptive C- and Aδ-fibres mostly terminate superficially in laminae I and II
Second order neurones then ascend the spinal cord in the anterolateral system comprising mainly:
- The spinothalamic tract (STT)
- The spinoreticular tract (SRT)
Describe the spinothalmic tract (STT) and the nociceptors it follows on from
- Projection neurones originating from lamina I (fast fibre Aδ pain) terminate in posterior nucleus of the thalamus
- Projection neurones originating from lamina V (WDR neurones) terminate in posterior and ventroposterior nucleus of the thalamus
- Pain perception (location, intensity) requires simultaneous firing in both pathways
Describe the spinorecticular tract (SRT) and the nociceptors it follows on from
- Largely transmits slow C-fibre pain
- Makes extensive connections with reticular nuclei in the brainstem [e.g. periaqueductal grey (PAG) and parabrachial nucleus (PBN)]
- Involved in autonomic responses to pain, arousal, emotional responses, fear of pain
Outline the development of neurogenic inflammation
Define what is meant by hyperalgesia
Abnormally heightened sensitivity to pain.
Define what is meant by allodynia
Refers to central pain sensitization (increased response of neurons) following normally non-painful, often repetitive, stimulation.
What are the 2 main different functions of C-fibres ?
They have both efferent (away from) and afferent (towards) functions
- Afferent – Transmit nociceptive information to the CNS
- Efferent – Release pro-inflammatory mediators contributing to neurogenic inflammation
What are the transmitters causing Neurotransmission Between the Primary Afferent and Second Order Neurone in the Dorsal Horn?
- Primary transmitter is glutamate
- Peptides (substance P and CGRP) also participate
