Fractures & Dislocations - Physiology Flashcards
Which nerve fibres make up nociceptors?
A-delta (Aδ) and C-fibre
Compare Aδ-fibres and C-fibres in the areas: myelin (yes/no), diameter (μm), conduction velocity (msec), location and function describe
Aδ-fibres
- Has myelin
- Diameter: Larger
- Conduction velcotiy: Faster
- Location: Afferent sensory nerves
- Function: Pain, cold temperature, touch
C fibres
- No myelin
- Diameter: Smaller
- Conduction velcotiy: Slower
- Location: Afferent sensory nerves
- Function: Pain, warm temperature, touch
Describe the relationship between diameter and condcution velcoity in fibre class A, B and C
Fibre class A has the largest diameter, then B and then C.
The larger the diameter the faster the conduction velocity
Therfore fibre class A has fastest conduction velocity, then B and C has the slowest conduction velcoity.
Describe the endogenous pain-relieving mechanism of opiods
Bind and act on opiod receptors causing the body to produce endorphins
Describe the endogenous pain-relieving mechanism of NSAIDs
They inhibit COX enzymes which prevent the production of prostaglandin E2 (PGE2) or leukotrienes (prolong inflammation)
Describe the endogenous pain-relieving mechanism of local anaesthetics
They reversibly block the conduction of action potentials by blocking Sodium channels on axon
Explain the endogenous pain-relieving mechanism of the gate control theory
If you have a C-fibre alone it will activate a projection neuron leading to a nociceptive signal - opening the gate
If activate a A-beta fibre along with a C-fibre it switches on an inhibitory internuerone. This may switch of the projection neurone and therefore preventing a nocicepptive signal - closing the gate
So concurrent activity in large-diameter (Aβ) primary afferents reduces the transmission of pain signals in small-diameter (Aδ and C) afferents
Which theory explains why rubbing the site of a painful stimulus helps alleviate the pain?
Gate control theory
a) Describe the principle that the TENS (transcutaneous electrical nerve stimulation) machine is based on
c) Explain how TENS works
a) It is based on the principle of electrical stimulation for pain relief using the gate control theory
b) It delivers high-frequency, low-intensity currents sufficient to stimulate Aβ and Aδ fibres and hence close the pain gate.
Define pain
Pain is an unpleasant sensory and emotional experience associated with or resembling actual or potential tissue damage
Define nociception
The neural process of encoding noxious stimuli (pain arising from activiation of the nociceptors)
a) What processes are involved in nociception?
b) Does nociception require consciousness?
a) Transduction, transmission and modulation of neural signals from periphery to CNS
b) No
Describe the anatomical pathways along which pain is transmitted and the physiological mechanism
- Pain is transmitter by primary sensory neruones (A-delta fibres and C fibres) enter the superficial aspect of the dorsal horn of the spinal cord
- Projection neurons (2nd order) from lamina I transmits fast fibre (Aδ) pain via the lateral spinothalamic tract (STT) and terminate in the posterior nucleus of the thalamus.
- The spinoreticular pathway transmits slow (C fibre) pain
- The trigeminal pathway is the route for pain (and temperature and crude touch sensation) of the face
Describe the transduction, transmission and processing (perception) of noxious stimuli
Transduction
- Tissue damage triggers action potential
Transmission
- Pain arises from neural activity of nociceptors
- A-delta and C fibres enter the superficial aspect of dorsal horn
- Projection neurons (2nd order) from lamina I transmit fast fibre (Aδ) pain via the lateral spinothalamic tract (STT) and terminate in the posterior nucleus of the thalamus.
- The spinoreticular pathway transmits slow (C fibre) pain
Processing - C-fibre nociceptor terminal
- ATP comes from tissue that have been damaged
- When each of these channels have been opened triggers an influx of Na+ causing an action potential, mediated by NaV channels
In response to injury, chemcials are released from leaky blood vessels and damaged tissue. Describe the role of this on nociceptors including examples of the chemicals that can be released
- In response to injury, chemicals are released from leaky blood vessels and damaged tissues
- These include ATP, PGE2, histamine, serotonins, bradykinin, and substance P
- The chemicals directly excite nociceptors or make them more sensitive to other agents
