Session 6 - Pain Pathways

Question 1

What is pain?

Answer 1

A complex, unpleasant awareness of a sensation (modified by experience, expectation, immediate context, culture etc

Question 2

What is the stimulus threshold for pain?

Answer 2

o The range where tissue damage occurs, e.g. temperature activated nociceptive pathways between 44-460C
oThe same in everyone

Question 3

What is pain tolerance?

Answer 3

o Variable reaction to a painful stimulus

Question 4

What modifies pain tolerance? (4)

Answer 4

Environment
Situation
Psychological/Emotional factors
Increases with age

Question 5

Give the four stages of nociception

Answer 5

o Transduction – Activation of Nociceptors by a stimulus
o Transmission – Relay of action potentials along nociceptive fibres to CNS
o Modulation – By other peripheral nerves or CNS mechanisms
o Perception – The interpretation by the brain of the sensation as painful

Question 6

How is nociception activated?

Answer 6

Noxious thermal, chemical or mechanical stimuli can trigger firing of primary afferent fibres through the activation of Nociceptors (pain-specific receptors, mostly polymodal) in the peripheral tissues.

Question 7

How is pain transmitted?

Answer 7

Transmission of pain information from the periphery to the dorsal horn of the spinal cord is via A Fibres (Sharp Stabbing Pain) and C-Fibres (Dull Nagging Pain).

Question 8

How is pain information modulated?

Answer 8

Pain Information can be inhibited or amplified by a combination of local (spinal) neuronal circuits and descending tracts from high brain centres. This constitutes the ‘Gate-Control Mechanism’.

Question 9

How is perception of pain information moderated?

Answer 9

Thalamocortical projections carry information on location, intensity and nature of pain. An emotional response is then made via the limbic system, and a stress response is made via the hypothalamus.

Question 10

Outline the gate control mechanism of pain awareness

Answer 10

o Primary afferent fibres synapse in Lamina I, II and V of spinal cord dorsal horn
o Transmitter peptides are involved in ascending pain pathways
 Substance P, Calcitonin, Bradykinin, Glutamate, Nitric Oxide
o The activity of the dorsal horn relay neurons is modulated by several inhibitory inputs. These include:
 Local inhibitory interneurons, which release opioid peptides
 Descending inhibitory noradrenergic fibres from the locus ceruleus area of the brainstem
 Activated by opioid peptides
 Descending inhibitory serotonergic fibres from the Nucleus Raphe Magnus and Periadueductal grey areas of the brainstem
 Activated by opioid peptides

Question 11

What do visceral receptors respond to?

Answer 11

o Muscle contraction and distension
o Rapid stretching of capsules of viscus organs
o Ischaemia of smooth muscle
o Compression or stretching of ligaments
o Chemical irritation

Question 12

What do visceral receptors not respond to?

Answer 12

o Cutting
o Pinching
o Burning.

Question 13

Why is referred pain felt in an area away from the painful stimulus?

Answer 13

Visceral receptors converge on spinal cord 2nd order neurones shared by somatic nociceptive fibres (lamina V). Therefore the brain perceives the pain as coming from that spinal nerve’s dermatome.

Question 14

How is pain information passed on?

Answer 14

The onward passage of pain information is via the Spinothalamic Tract, to the higher centres of the brain. Here, there is coordination of the cognitive and emotional aspects of pain and control of appropriate reactions.

Question 15

How is pain sensation modulated?

Answer 15

Opioid peptide release in both the spinal cord and brainstem can reduce the activity of the dorsal horn relay neurons and cause analgesia, known as ‘shutting the gate’

Question 16

Define analgesia

Answer 16

Inability to perceive pain when tissue damage is occurring

Question 17

What is local pain modulation?

Answer 17

o Local inhibitory interneurons, which release opioid peptides

Question 18

Where is central pain modulation centred?

Answer 18

Fibres from the Periaqueductal Grey Matter (PAG) of the mid-brain regulate descending pain modulation pathways. The PAG consists of a collection of cells highly sensitive to opiod neuropeptides (enkephalins, endorphins, dynorphin) and direct stimulation of this area can have an analgesic effect.
o Descending inhibitory noradrenergic fibres from the locus ceruleus area of the brainstem
 Activated by opioid peptides
o Descending inhibitory serotonergic fibres from the Nucleus Raphe Magnus and Periadueductal grey areas of the brainstem
 Activated by opioid peptides
 End on cells in the Substantia Gelatinosa, causing the release of Enkephalin, which modulates the activation of ascending pain pathways

Question 19

What happens in damage to the thalamus?

Answer 19

The experience of pain in areas of the body can be due to neural damage to the posterior Thalamus, e.g. by blockage of the posterolateral branch of the posterior cerebral artery.
Symptoms:

Question 20

Give four symptoms of thalamic damage in terms of pain

Answer 20

o Loss of sensation from contralateral side
o Followed by extreme pain, regardless of stimulus type
o Pain is insensitive to opioids (lesion above their point of action)
o Pain is sensitive to anti-epileptic drugs

Question 21

What is phantom limb pain?

Answer 21

Some patients retain sensations (sometimes painful) of their limbs after amputation. The sensations are not responsive to morphine, suggesting they arise centrally.

Question 22

What is peripheral nerve pain?

Answer 22

Arises from peripheral nerve lesions such as in peripheral neuropathy, e.g. in diabetes. This type of pain is normally localised in the territory of the affected nerve.

Question 23

Outline migraine

Answer 23

Migrates cause moderate to severe headaches, associated with nausea, vomiting and photophobia. Patient may perceive an aura prior to onset, a visual, sensory, language or motor disturbance. The pain is central, as it is unresponsive to morphine.