Sensory Pathways: Nociception

Question 1

What are the pain receptors called?

Answer 1

Nociceptors

Question 2

Describe 4 features of nociceptors.

Answer 2

Polymodal: different types of nociceptor respond to different stimuli
Free nerve endings
High threshold: higher activation threshold than touch receptors
Slow adapting: good because it means you are constantly reminded of the presence of a potentially harmful stimulus

Question 3

What are the 2 main types of sensory neurone that carry information on pain, temperature and crude touch? State some characteristics of each.

Answer 3

AD: Large, Fast conducting + adapting, Produce pain fast
C-fibre: Small, produces a dull, aching pain, remind you of the injury so that you guard this part of the body, SLOW conducting (UNMYELINATED)

Question 4

Compare the receptive fields of nociception to those of touch.

Answer 4

Receptive fields for nociception are much LARGER

Don’t need to be able to localise pain as well as touch.

Question 5

Describe the method of coding intensity in nociception.

Answer 5

Increase in frequency of impulses

Question 6

Describe the passage of the central pathway, which carries information about pain and temperature.

Answer 6

1st order neurone enters spinal cord, synapses in the dorsal horn with a 2nd order neurone.
2nd order neurone decussates immediately, travels up the white matter of spinothalamic tract on contralateral side.
In thalamus, synapses with a 3rd order neurone, which then goes to the primary somatosensory cortex.

Question 7

Describe the somatotopic arrangement of the fibres in the spinothalamic tract.

Answer 7

Lower fibres = Lateral
Higher fibres = Medial
(Opposite of dorsal columns)

Question 8

Which thalamic nucleus relays sensory information from below the neck?

Answer 8

Ventral Posterior-lateral nucleus

Question 9

Where does decussation occur in the pain and temperature pathway?

Answer 9

At the same level as the info. coming into the spinal cord

Question 10

Which nerve carries nociceptive information from the face?

Answer 10

Trigeminal Nerve (CN V)

Question 11

Where does the trigeminal nerve enter the brainstem?

Answer 11

Pons

Question 12

Describe the passage of the trigeminal nerve from entry into the brainstem.

Answer 12

Enters trigeminal ganglion in the pons, then moves downwards along the trigeminal nucleus.
Synapses in lower part of the trigeminal nucleus in the medulla.
2nd order neurone then decussates + joins the medial end of the spinothalamic tract.

Question 13

Describe the division of the trigeminal nucleus.

Answer 13

Trigeminal nucleus = column of grey matter, runs from midbrain to medulla.
Divided into areas where each area serves a different modality.

Question 14

Which thalamic nucleus relays pain information from the face?

Answer 14

Ventral Posteromedial

Question 15

Which part of the trigeminal nucleus does the first order nociceptive neurones from the face synapse in?

Answer 15

Spinal Trigeminal Nucleus

Question 16

What is the role of the primary somatosensory cortex in processing the nociceptive stimulus?

Answer 16

Register LOCATION + INTENSITY of the stimulus

Question 17

What are the two pathways that can reduce the amount of pain that you feel?

Answer 17

Central + Peripheral Inhibition Pathways

Question 18

What is the focus of the central inhibition pathway?

Answer 18

Periaqueductal Grey Matter

Question 19

Describe the arrangement and function of the central inhibition pathway.

Answer 19

Increased brain activity increases impulses going down the central inhibition pathway, which goes to the dorsal horn at every level.
These descending axons synapse with an interneurone + activate the interneurone.
Interneurone synapses with 1st + 2nd order nociceptive neurones + release ENKEPHALIN, which is inhibitory.
Enkephalin release reduces amount of info. going up the spinothalamic tract hence you feel less pain.

Question 20

What type of molecule is Enkephalin?

Answer 20

Opioid

Morphine mimics the action of this central inhibition system.

Question 21

Other than a first order nociceptive neurone, what else has input into the second order nociceptive neurone?

Answer 21

Non-nociceptive neurones
Axons of non-nociceptive touch neurones go into the dorsal horns but also have collaterals capable of activating an inhibitory interneurone, which can reduce activity of the projecting neurone + hence reduce the activity going up the spinothalamic tract.

Question 22

Describe the arrangement and function of the peripheral inhibition pathway. (Gate control theory)

Answer 22

Stimulation of touch receptors in same area as pain sensation leads to increased activity of the non-nociceptive touch neurones meaning there is increased activation of the inhibitory interneurone + hence reducing activity going up spinothalamic tract.

Question 23

State 2 examples of loss of pain sensation.

Answer 23

Syringomyelia
Charcot Joints: due to peripheral neuropathy, don’t realise inappropriate/ excessive use of joints, leads to joint deformities

Question 24

How can you get exacerbation of pain?

Answer 24

In chronic pain, certain peripheral nerves coming into spinal cord will be carrying high levels of input for a long time.
Cells in dorsal horn can lower their sensitivity or their synapses will change, which means info going into the spinothalamic tract is increased thereby increasing the level of chronic pain.

Question 25

How does the spinothalamic pathway differ for pain and temperature to crude touch?

Answer 25

Pain + temperature: Ascend in lateral spinothalamic tract

Crude touch: Ascends in anterior spinothalamic tract

Question 26

What would result from an anterior spinal cord lesion?

Answer 26

Ischemic damage to anterior part of spinal cord
Spinothalamic tract damage causes loss of pain + temp. sensation below level of lesion
Retained light touch, vibration + 2 point discrimination due to intact dorsal columns

Question 27

What are pain pathways composed of?

Answer 27

Sensory component (lateral spinothalamic tract)
Emotional component (spinoreticular tract)

Question 28

What is involved in the cerebral signature for pain, as seen on a functional MRI?

Answer 28

Cortex (SI, SII, Insula cortex, anterior cingulate cortex, prefrontal cortex)
Amygdala
Cerebellum
Brainstem

Question 29

How can serotonin and noradrenaline influence pain experienced?

Answer 29

Inhibitory effect on spinothalamic tract

Question 30

What is the difference between nociceptive pain and neuropathic pain?

Answer 30

Nociceptive: Noxious stimulation of a nociceptor (somatic or viscera)
Neuopathic: Lesion or disease of the somatosensory system

Question 31

What is peripheral and central sensitisation?

Answer 31

Peripheral: increased sensitivity (decreases threshold to peripheral stimuli at site of injury)
Central: increased sensitivity (decreases threshold to peripheral stimuli at an adjacent site to the injury), expansion of receptive field + spontaneous pain

Question 32

Define Allodynia

Answer 32

pain due to a stimulus that does not normally provoke pain

Question 33

Define Hyperalgesia

Answer 33

increased pain from a stimulus that normally provokes pain