L7: Pathophysiology of pain

Question 1

What is pain?

Answer 1

Unexplained sensory & emotional experience associated with actual/potential tissue damage or described in terms of such damage

Question 2

Types of pain

Answer 2

Nociceptive pain

Neuropathic pain

Question 3

Nociceptive pain

Answer 3

"Good pain"
Protective
- Alerts body to problem
- Protects from further injury
- Aids healing --> forces body to rest

Question 4

Neuropathic pain

Answer 4

Problem with pain-sensing system

• Experience pain in absence of stimuli
• Harmful

Question 5

Nociception & noxious stimuli

Answer 5

Process of encoding noxious stimuli to neural signals

Stimuli:

• Thermal
• Mechanical eg. pinching
• Chemical eg. irritants, acid

Question 6

Four steps of nociception

Answer 6

1. Transduction
2. Transmission
3. Modulation
4. Pain

Question 7

Transduction

Answer 7

Converting one form of energy (chemical, mechanical, thermal) to nerve signals (action potential)

Occurs at free nerve ending at nociceptors

Question 8

Nociceptors

Answer 8

Free nerve endings that sense noxious stimuli –> where transduction occurs

Question 9

How does transduction work?

Answer 9

Noxious stimuli sends energy signal (chemical, mechanical, thermal) to free nerve endings

Receptor changes shape, increases ion flow across membrane –> nerve signal as action potential

Action potential travels in nerve fibre ==> TRANSMISSION

Question 10

What are the two main types of nerve fibres involved in transmission?

Answer 10

Aδ

• First & Fast –> sharp pain
• Myelinated –> increase AP speed
• Mainly mechanical (touch, pressure; sometimes temperature)
• Specific pain –> can tell where in the body the pain is

C

• Second & Slow –> aching pain
• Unmyelinated –> decreased AP speed
• Thermal, mechanical, chemical
• Non-localised pain –> eg. if eye poked, cannot tell where it was poked

Question 11

How does transmission work?

Answer 11

Sensory nerves have receptors in periphery (transduction), cell body in dorsal root ganglion, other nerve process to spinal cord –> makes synaptic contact with another nerve

Second nerve potentiates up to brain

Question 12

What are the two main pathways in transmission?

Answer 12

Spinothalamic tract

• Aδ fibres –> fast
• Periphery –> thalamus –> frontal lobe
• Awareness of pain

Spinoreticular tract

• C fibres –> slow
• Periphery –> brainstem –> thalamus –> frontal lobe
• Conveys pain

Question 13

How does transmission occur in nerves?

Answer 13

Between nerves = synapse = neurotransmitter

Electrical –> chemical –> electrical

Spinal neural circuits in spinal cord synapse can alter response of second nerve to neurotransmitters

Question 14

What is pain modulation?

Answer 14

Difference between objective & subjective measures of injury & pain

eg. similar injury –> some people hurt more than others
eg. different context; pain not felt despite injury –> battlefield, intense sports, endorphin rush when getting tattoo

Question 15

What modulates pain?

Answer 15

Endogenous opioid system

Question 16

How does the endogenous opioid system work?

Answer 16

Internally produced molecules with opioid-like action --> regulate transmission of nociceptive signals
- Enkephalins
- Endorphins
- Dynorphins
(- Serotonin (5HT))
(- Adrenaline)

Released from cortex in frontal lobe, down to spinal cord synapses

Act on opioid receptors µ, δ, κ to regulate (turn off) synaptic/nociceptive pain transmission

Question 17

What is neuropathic pain?

Answer 17

Pain in the absence of noxious stimuli, even if it was there in the beginning

Causes abnormal pain

• Allodynia –> sensitisation to non-painful stimuli (rubbing over papercut on finger. Usually wouldn’t hurt)
• Hyperalgesia –> overreaction to painful stimuli (small papercut causes so much pain)