4.6 Pain Pathways

Question 1

Describe the pathway taken when a painful stimulus is administered to the sole of foot

Answer 1

Sensory Nociceptors

Local Inflammation

Peripheral

Central

Other

Question 2

Sensory Nociceptors

Answer 2

Detect painful stimulus
Primary nociceptors 
free unmyelinated nerve endings
have threshold
exceeded
=
Initiate AP

Local tissue inflammation
Lowers threshold of receptors
Activates quiescent nociceptors in adjacent tissue

Question 3

Peripheral

Answer 3

Primary afferent to SC
x 2 possible routes

1. Sharp fast immediate pain
   myelinated Ad fibres
   2-5um
   15-30m/sec
2. Burning diffuse unpleasant pain
   Unmyelinated C fibres
   0.5-1.2um
   0.5-2m/sec

Question 4

Peripheral

Answer 4

1. Cell bodies neurons
   lie in DRG
2. Synapse w/ second order interneurons in Dorsal horn
   SC
3. Ad fibres synapse in lam 1+5
4. C fibres in substantia gelatinosa
   Lam1+2
5. Some projection to inhibitory interneurons w/ SC
6. Fibres decussate to contralat side of Cord

Question 5

Central

Answer 5

1. Fibres ascend

lateral spinothalamic tract

2. Located - ventrolateral quadrant of SC
   - > project to thalamus
3. Small component of C fibres
   ascend in cord
   Spinoreticular Tract
4. Thalamus:
   discrimination of modality occurs
   according to it synapses

Question 6

Central pathways

Answer 6

1. Fast pain spinothalamic tract

2. Slow pain spinothalamic tract

Question 7

Fast pain spinothalamic tract

Answer 7

Axons terminate in 3 main areas

1. VPLN
   Ventral posterolateral nucleus
2. VPMN
   Ventral Posteromedial Nucleus
3. PON
   Posterior Nucleus

Collectively know as ventroposterior Nucleus

A/W conscious pain perception +
discrimination of differing modalities

Question 8

Slow Pain Spinothalamic Tract

Answer 8

1. Axons synapse w/ non specific intralaminar nuclei
   ILN
   Thalamus and reticular formation in brainstem
2. Axons a/w affective quality of pain
   unpleasantness and fear further injury

Spinoreticular pathway joins areas

Question 9

Slow Pain Spinothalamic Tract

Answer 9

VP nucleus
projects third order neurons to cortical areas
a.w pain perception

Displays somatotrophic organisation
+
Third order neuros project to specific area primary sensory cortex in parietal lobe

Question 10

Slow Pain Spinothalamic Tract

Answer 10

Slow pain impulses within intralaminar nuclei
Multiple projections

1) Anterior part of cingulate gyrus
Emotion

2) Amygdala
memory and emotion

3) Hypothalamus
Emotion and vascular response to emotion

Question 11

Other pathways involved

Answer 11

1. Dorsal Column
   Transmits visceral nociception to thalamus
2. Spinomesencephalic pathway
   Projects to PAG and superior colliculus
   linking central areas
   involved in affect of pain perception
3. Spinohypothalamic pathway

Question 12

3. Spinohypothalamic pathway

Answer 12

Passes direct to midbrain
without synapsing to convey emotional pain information

NT involved
1 Excitatory 
Glutamate
aspartate
AMPA
Calcitoin G RP
Sub P

2 Dorsal horn receptors
NMDA
Opioid
Adenosine
Norad

Question 13

Descending inhibition

Answer 13

Projections from ascending pathways
responsible for descending inhibition
pain transmission
at spinal cord level

1 PAG
+
Periventricular grey projection
-> nucleus raphe magnus

2. Enkephalin NT
   Activate serotoninergic descending neurons
   activate inhibitory interneurons in cord
3. Locus Coeruleus
   Direct projection to cord
   Using NA to activate inhib neurons