pain

WHAT IS PAIN?

Definition

Pain is:

  • an unpleasant sensory and emotional experience

  • associated with actual or potential tissue damage

Important:
Pain is:

  • physical
    AND

  • emotional/psychological


IMPORTANT CONCEPT

Pain ≠ nociception.

Nociception

Detection of harmful stimuli by sensory neurones.

Pain

The conscious experience produced by the brain.

So:

  • nociceptor activity alone does not automatically equal pain.


WHY IS PAIN IMPORTANT?

Survival value

Pain:

  • protects us

  • warns about tissue damage

  • encourages withdrawal from danger


Acute

Chronic

<3 months

>3 months

Usually clear cause

Cause often unclear

Protective

Often maladaptive

Resolves with healing

Persists after healing


CHRONIC PAIN IMPORTANT POINT

Chronic pain changes the nervous system itself.

Includes:

  • spinal cord changes

  • brain remapping

  • sensitisation

So chronic pain is NOT simply “long acute pain.”


PAIN PATHWAY

This is one of the MOST important exam sections.

Basic pathway

  1. Nociceptor activated

  2. First-order neurone carries signal

  3. Signal relays in spinal cord

  4. Second-order neurone ascends

  5. Third-order neurone projects to cortex

  6. Pain consciously perceived


DESCENDING CONTROL

The brain can also:

  • suppress
    OR

  • enhance
    pain signals.

Descending pathways travel:

  • brain → spinal cord

They modulate pain transmission.


NOCICEPTORS

Definition

Free nerve endings detecting harmful stimuli.

Respond to:

  • thermal stimuli

  • mechanical stimuli

  • chemical stimuli


TYPES OF PAIN FIBRES

C-FIBRES

Structure

  • small diameter

  • unmyelinated

  • slow conduction

Speed

≈ 0.5–2 m/s

Pain type

  • dull

  • aching

  • diffuse

Important feature

Polymodal:
respond to:

  • thermal

  • mechanical

  • chemical stimuli


Aδ FIBRES

Structure

  • larger diameter

  • myelinated

  • faster conduction

Speed

≈ 6–120 m/s

Pain type

  • sharp

  • well-localised

Important feature

Mostly respond to:

  • mechanical

  • heat stimuli

NOT usually chemical stimuli.


VERY IMPORTANT COMPARISON

C-fibres

Aδ-fibres

Unmyelinated

Myelinated

Slow

Fast

Dull pain

Sharp pain

Diffuse

Localised

Small diameter

Larger diameter


HYPERALGESIA

Definition

Increased pain response to normally painful stimuli.

Caused by:

  • sensitisation


ALLODYNIA

Definition

Pain caused by normally NON-painful stimuli.

Example:

  • light touch causing pain


PHANTOM LIMB PAIN

Definition

Pain felt in a missing limb.

Example of:

  • neuropathic pain


IMPORTANT CONCEPT

Phantom pain shows:

  • pain is generated by the nervous system/brain

  • pain does not require actual tissue presence


CORTICAL REMAPPING

After limb loss:

  • brain sensory maps reorganise

This contributes to:

  • phantom limb pain


ANALGESIA

Definition

Reduction of pain.

Analgesic

Drug that relieves pain.


SIMPLE ANALGESICS

Paracetamol

  • pain relief

  • antipyretic


NSAIDs

Example:

  • ibuprofen

Function:

  • reduce inflammation

  • reduce pain


WHO ANALGESIC LADDER

Pain treatment escalates:

  1. simple analgesics

  2. weak opioids

  3. strong opioids


OPIOIDS

Derived from:

  • opium poppy

Natural opiates:

  • morphine

  • codeine


OPIOID EFFECTS

Desired effect

  • analgesia

Side effects

  • respiratory depression

  • constipation

  • nausea

  • itching

  • euphoria

  • dependence

  • tolerance


OPIOID CLASSIFICATION

Weak opioids

  • codeine

  • tramadol

  • dihydrocodeine


Strong opioids

  • morphine

  • fentanyl

  • oxycodone

  • heroin (diamorphine)


OPIOID RECEPTORS

Three main receptor types:

\mu,\ \delta,\ \kappa

(MOP, DOP, KOP)


IMPORTANT POINT

Opioid receptors are:

GPCR

(G-protein coupled receptors)


MAIN CLINICAL TARGET

μ (mu/MOP) receptor

Produces:

  • analgesia

  • euphoria

  • respiratory depression

  • constipation

  • dependence


HOW OPIOIDS REDUCE PAIN

They:

  • reduce afferent pain transmission

  • reduce neurotransmitter release in spinal cord

Result:

  • less pain signalling reaches brain


OPIOID EPIDEMIC

Problem:

  • dependence/addiction

  • overdose risk

  • respiratory depression

Need:

  • safer analgesics


MOST IMPORTANT EXAM CONTENT

If short on time, prioritise:

1. Acute vs chronic pain

2. Pain pathway

3. C-fibres vs Aδ-fibres

4. Hyperalgesia vs allodynia

5. Phantom limb pain

6. Opioid receptors + effects

7. μ receptor effects


THE ENTIRE LECTURE IN ONE FLOW

Tissue damage →

nociceptors activated →

C-fibres/Aδ-fibres carry signal →

spinal cord →

ascending pathways →

cortex →

pain perception →

descending pathways modulate signal →

analgesics/opioids reduce transmission.