Pain Pathways and Neural Mechanism of Pain (3.1 Part B)

Question 1

What are sensory neurons?

Answer 1

• Pain sensory neurons –> nociceptive nerve endings
• Dendron –> pre nerve ending (peripheral branch) before cell body (soma) in the autonomic ganglion
• Axon –> afer soma and synapse at dorsal horn in spinal cord
• Transduction of impulse along dendron
• Conduction of impulse along the axon

Question 2

Describe the 6 steps of the pain pathway

Answer 2

1. Nociceptors: transduction along dendron and further conduction along axon
2. At spinal cord mediators released signal threshold reached –> electrical impulse transmission along spinothalamic tract to brain –> integrate perception of pain
3. In thalamus and cerebral cortex
4. Descending inhibitory pathways –> modulation of pain
5. At dorsal horn in spinal cord –> gate control of pain (endorphins block pain)

Question 3

What are the phases of nociceptive pain?

Answer 3

• Transduction
• Conduction
• Transmission
• Perception
• Modulation

Question 4

What are the differences between the Aδ and C afferent neurons?

Ascending pathway

Answer 4

Aδ (fast)

• Pain localization, withdrawal reflexes
• Intense, sharp, stinging pain
• Neospinothalamic pathway

C (slow)

• Autonomic reflexes
• Pain memory
• Pain discomfort
• Dull burning, aching pain
• Paleospinothalamic pathway

Question 5

What is the relationship between descending pathways and endorphin response?

Answer 5

• Endorphin receptors are located close to known pain receptors in the periphery and ascending and descending pain pathways

Question 6

Describe the afferent and descending pathways in rleations to inhibitory pathways

Answer 6

Afferent pathways –> inhibitory control

• Bradykinin, 5-HT, prostaglandins

Descending inhibitory pathways

• NA, 5HT, ENKEPHAILINS, GABA, GLYCINE

Question 7

What is the difference between NSAIDs and Opiods in the way they work on the CNS

Answer 7

NSAIDs

• Block peripheral generation of the nociceptive impulses
• inhibit production of PGs
• reduce sensitivity of sensory nociceptive nerve endings to SP

Opiods

• Act on spinal cord & limbic system
• Stimulate descending inhibitory pathways
• Inhibit transmission at dorsal horn
• minimal peripheral actions

Question 8

Compare Opioid vs Non-Opioid analgesics in the way they work?

Opiods: Mu (μ), Kappa (κ) , Delta (δ)

Answer 8

Opioids

• Activate inhibitory systems (descending pathway)
• Atypical drugs agonist-antagonist

Salicylates

• Inhibit prostaglandin synthesis
• NSAIDs and COX-2 inhibitors

Question 9

How does local anaesthetic (LA) work?

Answer 9

• preferentially blocks small nociceptive fibers

Fibre type

> A (Delta): pain, temperature

> B: preganglionic autonomic

> C: dorsal horn (pain) and postganglionic

Diameter gets smaller from A to C (more sensitivity to (LA)

small nerve fibres are preferentially blocked because of their high surface-area to volume ratio