Introduction to the Physiology of Pain

Question 1

What is transduction?

Answer 1

Translation of noxious stimulus into electrical activity at the peripheral nociceptor

Question 2

What is transmission?

Answer 2

Propagation of a pain signal as nerve impulses travel through the nervous system

Question 3

What is modulation?

Answer 3

Modification/hindering of pain transmission in the nervous system e.g. by inhibitory neurotransmitters like endogenous opioids

Question 4

What is perception?

Answer 4

Conscious experience of pain, causes physiological and behavioural responses

Question 5

Where does pain begin?

Answer 5

The activation of nociceptors

Question 6

What are nociceptors? How are they activated?

Answer 6

-Specific primary (first order) neurones that relay information to second order neurones in the CNS by chemical synaptic transmission sensory
-Innervate peripheral tissues
-Normally activated by intense noxious stimuli (mechanical, thermal or chemical)

Question 7

Where are second order neurones located? What do they comprise of?

Answer 7

Second order neurones ascend the spinal cord in the anterolateral system (terminate in the thalamus) comprising mainly:
- The spinothalamic tract
- The spinoreticular tract

Question 8

What is the spinothalamic tract (STT) involved in?

Answer 8

Involved in pain perception (location, intensity)

Question 9

The spinoreticular tract (SRT) involved in?

Answer 9

Involved in autonomic responses to pain, arousal, emotional responses, fear of pain

Question 10

What do third order neurones do?

Answer 10

Relay sensory information from the thalamus to the primary sensory cortex

Question 11

What are Aδ-fibres? What do they respond to?

Answer 11

• Mechanical/thermal nociceptors that are thinly myelinated
• Respond to noxious/mechanical and thermal stimuli; mediate ‘first’, or fast, pain

Question 12

What are C-fibres? What do they respond to?

Answer 12

• Unmyelinated nociceptors
• Collectively respond to all noxious stimuli (polymodal); mediate ‘second’ or slow pain

Question 13

Describe the difference in character of first and second pain

Answer 13

• First: lancinating, stabbing, pricking
• Second: burning, throbbing, cramping, aching

Question 14

Name 4 different classifications of pain + examples

Answer 14

• Mechanisms (nociceptive, inflammatory, pathological)
• Time course (acute, chronic, breakthrough pain)
• Severity (mild, moderate, or severe)
• Source of origin (somatic or visceral)

Question 15

What is nociceptive pain?

Answer 15

The normal response to injury of tissues by noxious stimuli provoked by intense stimulation of nociceptors by noxious stimuli (mechanical, chemical, thermal)

Question 16

Is nociceptive pain adaptive?

Answer 16

Yes, it functions as an early warning physiological protective system to detect and avoid noxious stimuli

Question 17

What is inflammatory pain caused by?

Answer 17

Caused by activation of the immune system by tissue injury or infection, in response to mediators released at the site of inflammation by leukocytes, vascular endothelium and tissue resident mast cells

Question 18

What does inflammatory pain cause? Why does this happen?

Answer 18

Heightened pain sensitivity to:
-Noxious stimuli (hyperalgesia)
-Innocuous stimuli (allodynia)

Question 19

Is inflammatory pain adaptive?

Answer 19

Yes, it promotes repair until healing occurs

Question 20

What are the two types of pathological pain?

Answer 20

-Neuropathic
-Dysfunctional

Question 21

What is neuropathic pain caused by +examples?

Answer 21

Damage to neural tissue (compression neuropathies, peripheral neuropathies, central pain (following stroke or spinal injury), posthereptic neuralgia, trigeminal neuralgia, phantom limb)

Question 22

How can neuropathic pain be felt?

Answer 22

• Burning, shooting, numbness, pins and needles
  
  • May be less localised

Question 23

What is dysfunctional pain caused by +examples?

Answer 23

No identifiable damage or inflammation (fibromyalgia, IBS, tension headache, temporomandibular joint disease, interstitial cystitis)

Question 24

Is pathological pain adaptive? How is it treated usually?

Answer 24

No, it is maladaptive (not protective)
- Simple analgesics usually not very effective in pathological pain
- Sometimes treated by drugs not originally developed for pain e.g. antidepressants or antieleptics

Question 25

Where does referred pain usually develop?

Answer 25

-Develops in one part of the body but felt in another structure away from the place of its development

• This leads to pain in an area of the skin which is distant from the internal organ where the pain originates

Question 26

Where is referred pain usually felt?

Answer 26

Deep pain or visceral pain can be felt as referred pain; pain originating in superficial structures is not usually referred

Question 27

What is referred pain caused by and what does it lead to?

Answer 27

Caused by convergence of nociceptive visceral and skin afferents upon the same spinothalamic level which leads to pain in an area of the skin which is distant from the internal organ where the pain originates