Pathophysiology of Pain

Question 1

Define Pain

Answer 1

An unpleasant sensory experience associated with tissue damage
It is not a stimulus, it is a final interpretation of nociception in the brain

Question 2

What nerve fibres carry nociception?

Answer 2

Adelta - Small Myelinated - Fast Pain

C - Unmyelinated - Slow/delayed Pain

Question 3

What are Rexed Layers?

Answer 3

Rexed divided the spinal cord into 10 layers based on their cytoarchitecture

Question 4

What are the 3 types of neurons in the spinal cord and what rexed layer are they found?

Answer 4

Low Threshold Mechanoreceptive Neurons - Layer 3/4 - Abeta fibres

Nociceptive Specific Neurons - Layer 1/2 - Adelta/C fibres

Wide Dynamic Range (WDR) neurons - Layer 5 - respond to both noxious and non-noxious stimuli

Question 5

What tracts do pain fibres travel up?

Answer 5

Spinothalamic Tract, divided in two:

• Lateral STT carries fast and slow pain + Temp
• Anterior STT Carries crude touch

The STT arises in Rexed LAmina 2 & 5

Question 6

Where do the fibres in the Spinothalamic Tract go?

Answer 6

Ventral STT -> Medial Thalamic Nuclei -> Limbic System (emotional response to pain & desc. control)

Lateral STT -> Ventroposterior Thalamic Nuclei -> Primary Somatosensory cortex (spatial & intensity discrimation of pain)

Question 7

Explain the descending pathways of pain?

Answer 7

Periacqueductal grey (PAG) matter activates a descending noradrenergic system to the dorsal horn where it gates pain using endogenous opioids

Question 8

What are the categories of pain sensitization?

Answer 8

Peripheral
Segmental Central
Suprasegmental Central

Question 9

What are the types of peripheral pain sensitization?

Answer 9

• Hyperalgesia (Increased neuronal response to pain signals)
• Spontaneous Pain (Spontaneous activity in nociceptive nerve fibres)
• Allodynia

Question 10

What is Allodynia?

Answer 10

Sensitization of the nociceptors themselves

So the nociceptor threshold lowers causing it to trigger signals for stimuli that arnt normally considered noxious

Question 11

What is hyperalgesia?

Answer 11

Exaggerated response of the neurons to already painful stimuli
I.e. the stimulus-response curve shifts left, your perception of pain increases

Question 12

What are the types of Segmental Central Sensitization?

Answer 12

Wind up - Increased response through the 1-2nd order synapse

Classical - Opening up of new synapses

Long-Term Potentiation

Question 13

How does Wind up sensitization work?

Answer 13

Homosynaptic Activity-Dependant Progressive increase in Neuronal Response.

Over the course of the stimulus the active synapses become more responsive, this terminates with the stimulus

Question 14

How does Classical Sensitization work?

Answer 14

If the stimulus is intense enough then it opens up new synapses, thus increases the active pain fibres.

Can outlast the stimuli as maintainable by a low intensity stimulus

Question 15

How does longterm potentiation work?

Answer 15

We dont really know but it involved already activated synapses (unlike classical) and very intense stimuli

Question 16

Pain conditions such as fibromyalgia and the painful symptoms of depression are caused by what?

Answer 16

Its believed the pain is sourced from supra-segmental pain sensitization in the thalamus & Cortex

Question 17

Whats the difference between chronic and acute pain?

Answer 17

In acute pain theres usually obvious damage and the pain resolves with healing, Its serving a protective function. Nociceptive

In chronic pain it lasts beyond the period of healing, It has no protective function and actually degrades health and function. Nociceptive and/or Neuropathic

Question 18

Whats the difference between Nociceptive & Neuropathic pain?

Answer 18

Nociceptive:

• Localised to site of injury
• Throb/ache/stiff
• Responds to conventional analgesics
• May be chronic e.g. osteoarthritis

Neuropathic:

• Causes by lesion or dysfunction of somatosensory nervous system
• E.g. Neuroma caused by nerve damage, stroke or MS
• Pain found at region supplied y nerve not necessarily region of injury
• Almost always chronic

Question 19

What are the categories of pain management?

Answer 19

• Transduction
• Transmission
• Perception
• Descending Modulation

Question 20

Whats involved in pain management through Transduction?

Answer 20

NSAIDs
Ice
Rest
Local Anaesthetics

Question 21

What methods are there for pain management through transmission?

Answer 21

• Nerve blocks e.g. General Aneasthetics
• Drugs e.g. Opiods or Anticonvulsants
• Surgery e.g. Cordotomy

Question 22

How do we manage perception of pain?

Answer 22

• Cognitive Behavioral Therapy
• Distraction
• Relaxation
• Education

Also for people with missing limb pain we can use Graded Motor Imagery & Mirror Box Therapy

Question 23

How do we control pain through descending modulation?

Answer 23

• Placebos
• Drugs e.g. Opiods and antidepressants
• Surgery e.g. Spinal Cord Stimulation