Lecture 10 Slides - Nociception and Pain(1)

Page 1: Introduction to Biological Psychology and Nociception

Afferent Fibre Groups:
- Muscle Nerve:
  - Fibre Type: I (Aα)
    - Diameter: 13 – 20 μm
    - Conduction Velocity: 80 – 120 ms
- Cutaneous Nerve:
  - Fibre Type: II (Aβ)
    - Diameter: 6 – 12 μm
    - Conduction Velocity: 35 – 75 ms
Nociceptive Afferents:
- Type III (Aδ): Smallest fibres
  - Diameter: 1 – 5 μm
  - Conduction Velocity: 5 – 30 ms
- Type IV (C): Non-myelinated fibres
  - Diameter: 0.2 – 1.5 μm
  - Conduction Velocity: 0.5 – 2.0 ms

Key Components of Inflammatory Response:
- Substance P: Triggers histamine release from mast cells.
- Histamine, Bradykinin, Serotonin: Key mediators in pain and inflammation.
- Potassium: Released during cellular lesioning.
- Prostaglandins: Induce vasodilation and oedema.

Substances and Their Effects:
- Potassium: Released by damaged cells, causes activation.
- Serotonin: Derived from platelets, synthesized from tryptophan hydroxylase, induces activation.
- Bradykinin: Comes from plasma kininogen via kallikrein, activation effect.
- Histamine: Released from mast cells, effects activation.
- Prostaglandins: Synthesized from arachidonic acid in damaged cells through cyclo-oxygenase, leads to sensitization.
- Substance P: Found in primary afferent fibres, contributes to sensitization.

Spinal Cord Organization:
- Focuses on the role of the dorsal horns in nociception.

Dorsal Horn Fiber Types:
- Aa/Aß Fibres: Responsible for touch and proprioception.
- A Fibres II: Relate to sharper pain signals.
- C Fibres: Associated with slower, dull pain signals.
- Important Note: Thin dorsal root (C) fibres do not synapse directly onto spinothalamic cells; they use spinal interneurons for indirect influence.

Connections to the Thalamus:
- Lateral STT: Responsible for fast and slow pain & temperature sensations.
- Anterior STT: Processes simple touch sensations.
- Fiber Types Included: Aδ and C fibres.
- Role of Interneurons: Critical for signal integration.

Key Structures:
- Medial Lemniscus, Thalamus, Somatosensory Cortex: Primary areas of fast pain processing.
Main Targets for Fast Pain:
- Ventral Posteriolateral nucleus (VPL)
- Ventral Posteriomedial nucleus (VPM)
- Posterior nuclei (PO)

Pathways:
- Medulla, Mesencephalon, Periaqueductal Grey Area (PAG)
- Main Slow Pain Tracts: Paleospinothalamic tract, Spinomesencephalic tract, Spinoreticular tract.
Slow Pain Targets:
- Spinoreticular tract and Reticular formation
- Spinomesencephalic tract targeting the PAG
- Paleospinothalamic tract associated areas.

Thalamic Projections:
- Fast pain projections mainly terminate in lateral (ventroposterior) thalamic nuclei.
- Slow pain projections mainly in medial thalamic nuclei.
Fast Pain Targets:
- VPL, VPM, PO
Slow Pain Targets:
- NSIN and other medial/midline nuclei.

Central Pathways for Pain Transmission:
- Direct (Fast Pain) Tracts:
  - Lateral STT – subcortical targets: VPL, parietal lobe.
- Indirect (Slow Pain) Tracts:
  - Involving cortical and subcortical targets like the limbic system.
Functional Distinctions:
- Discriminative pain vs. affective-arousal components of pain.

Pathway Overview:
- Nociceptive signals travel from the peripheral nociceptors through the primary afferent Aδ and C fibres to the dorsal root ganglion then the spinal cord, before reaching the thalamus and somatosensory cortex.

Pain Systems in the Brain:
- Medial Pain System: Processes pain localization, intensity, and emotional response.
- Lateral Pain System: Focuses on sensory-discriminative functions and attention supervision.
- Important Brain Structures Involved: Thalamus, insula, and frontal cortex along with the PAG.

Case Studies:
- Ploner et al. on pain affect without sensation.
- Ramachandran et al. on synaesthesia in phantom limbs.

Key Structures:
- Periaqueductal Grey Area, Raphe Magnus nucleus also plays a role in pain modulation through descending pathways.

Pathway Overview:
- Serotonin pathways from the Raphe Magnus to the dorsal horn impact nociceptive signalling.

Mechanism:
- Inhibitory interneurons release Enkephalin to modulate pain through the inhibition of Substance P effects on dorsal horn neurones.

Key Literature:
- Barbaro et al., Kandel et al., McLean et al., Ploner et al., Ramachandran et al., Reynolds on pain relief studies and related neuroscience research.