Sensory pathways; nociception

Question 1

Explain how nociceptive receptors differ from those for touch and proprioception/features of nociceptors

Answer 1

Simple free nerve endings
High threshold and slow adapting fibres
Large receptive fields, low density
Small diameter
Aδ (IV) and C (IV) types
Polymodal; different types of nociceptors respond to different pain stimuli

Question 2

Difference between nociceptive stimulus and interpretation

Answer 2

A nociceptive stimulus can be measured where pain is subjective and subject to other contextual influences

Question 3

What are some examples of the ion channels which are stimulated by heat, cold, ph<7, intense pressure?

Answer 3

Heat: TRPV1
Cold: TRPM8
pH<7: Acid sensing channels
Intense pressure: K+ channels

Question 4

What is the commonest cutaneous receptor?

Answer 4

Polymodal C fibre

Question 5

What is the commonest skeletal muscle receptor?

Answer 5

Chemoreceptor

Question 6

Which pathway carries visceral pain?

Answer 6

Peripheral: autonomic nerves
Central: Spinothalamic and dorsal columns

Question 7

What is referred pain?

Answer 7

Occurs at sites of the body wall whose innervation enters the spinal cord at the same level as the organ

Question 8

What are the features of visceral pain?

Answer 8

Poorly localised
Often referred to the body wall at the same spinal level
Characteristic midline pain (epigastric)
Visceral hyperalgesia may occur

Question 9

Describe the features of the two sensory neurones which carry nociceptive information

Answer 9

Ad: large, fast adapting, fast conducting
C: smaller, slow conducting (unmyelinated), produces dull aching pain

Question 10

Which neurotransmitters modulate nociceptive information?

Answer 10

Bradykinins, histamine, prostaglandins

Question 11

Describe the pathway of how nociceptive information from the body and face travels to ‘higher centres’

Answer 11

Axon from primary sensory neurone synapses in dorsal column
Second order sensory neurone axon decussates and will ascend in the spinothalamic tract
[Info from limbs is lateral. Info from arms is medial.]
Primary sensory neurones from the face travels from trigeminal nerve fibres in the pons down to the spinal trigeminal nucleus in the medulla. Synapses here with second order neurone and cross to join spinothalamic tract.
Second order neurones travel to ventral posterolateral or venture posteromedial nucleus and tertiary sensory neurones relay info to primary somatosensory cortex

Question 12

What is the use of SI when there is a nociceptive stimulus?

Answer 12

It detects the location and intensity of the stimulus.

Question 13

What additional collateral branches are given off by the spinothalamic tract?

Answer 13

Brainstem (via reticular formation)
Thalamus (via intralaminar nuclei)
Hypothalamus
Limbic system

Question 14

What is the purpose of collateral inputs from the spinothalamic tract?

Answer 14

By activating the reticular activating system, increases level of arousal so you will be more aware of danger.
The hypothalamus and limbic system will correlate to the emotional aspects of the stimulus.

Question 15

What is central (descending) inhibition?

Answer 15

Neurones in the periaqueductal grey matter (surrounds the cerebral aqueduct) when there is high activity in the brain will send impulses down the reticular formation of the brainstem.
The descending neurones synapse with interneurones which release ENKEPHALIN which dampens down the activity between the first and second order neurones.

Question 16

What is peripheral inhibition (gate control theory)?

Answer 16

C-fibres (primary nociceptive neurone) enters the dorsal column to synapse with a projection neurone to cross over and join the spinothalamic tract.
Non-nociceptive fibres (e.g. Aa, Ab mechanoceptors) also enter the dorsal columns and have collateral branches that can activate the inhibitory interneurones to inhibit the secondary nociceptive neurone.
Rubbing the area increases the intensity of the stimulus and less activation of second nociceptor neurone.

Question 17

What causes loss of pain sensation?

Answer 17

Damage to the spinothalamic tract and location of the lesion

Question 18

What are two examples of nociceptive disruption?

Answer 18

Syringomyelia; cyst forms in the cervical region and disrupts fibres from crossing the midline
Charcot joints; hereditary, lack of development of nociceptors, loss of perception of joint pain

Question 19

What are two examples of nociceptive dysfunction?

Answer 19

Wind up in dorsal horn: chronic stimulation leads to lowering of pain threshold which causes feelings of continuous pain due to AP from non-noxious stimuli
Thalamic syndrome: pain hypersensitivity

Question 20

How would you treat neuropathic pain?

Answer 20

Capsaicin patches
Binds to TRPV1 nerve endings
Direct toxicity on mitochondria which reduces hypersensitivity

Question 21

What is the definition of allodynia?

Answer 21

Pain due to stimulus that does not normally provoke pain

Question 22

Define hyperalgesia

Answer 22

Increased pain from a stimulus that usually does cause pain

Question 23

Define sensitisation

Answer 23

Increased responsiveness of nociceptive neurons to their normal input

Question 24

Define hypoalgesia

Answer 24

Diminished pain in response to a normally painful stimulus

Question 25

Define hyperpathia

Answer 25

Painful syndrome; abnormally increased pain response to a stimulus, especially one that is repetitive, as well as in increased threshold

Question 26

Define paraesthesia

Answer 26

Abnormal sensation (can be spontaneous or evoked)

Question 27

Define dysaethesia

Answer 27

Unpleasant abnormal sensation (can be spontaneous or evoked)