Sensory Pathways: Nociception

Question 1

What are the pain receptors called?

Answer 1

Nociceptors

Question 2

Can very high intensity stimulation of mechanoreceptors cause the feeling of pain?

Answer 2

No – only nociceptors can cause pain sensation

Question 3

Describe some features of nociceptors.

Answer 3

Polymodal – different types of nociceptor respond to different stimuli
Free nerve endings – usually just free axonal endings of neurones
High threshold – higher activation threshold than touch receptors
Slow adapting – this is good because it means you are constantly reminded of the presence of a potentially harmful stimulus

Question 4

What are the two main types of sensory neurone that carries sensory information? State some characteristics of each.

Answer 4

A
 Large  
 Fast conducting  
 Fast adapting  
 Produces pain fast  
C-fibre  
 Smaller
 Produces a dull, aching pain
 It reminds you of the injury so that you guard this part of the body 
 SLOW conducting -UNMYELINATED

Question 5

Compare the receptive fields of nociception to those of touch.

Answer 5

Receptive fields for nociception are much LARGER because the nociceptive pathway is phylogenetically older than touch and you don’t need to be able to localise pain as well as touch.

Question 6

Describe the method of coding intensity in nociception.

Answer 6

Same as touch – increase in frequency of impulses

Question 7

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

Answer 7

First order neurone enters the spinal cord and synapses in the dorsal horn with a second order neurone.
Second order neurone decussates immediately and travels up the white matter of the spinothalamic tract.
It then goes up to the thalamus where it synapses with a third order neurone, which then goes to the primary somatosensory cortex.

Question 8

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

Answer 8

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

Question 9

Which thalamic nucleus relays sensory information from below the neck?

Answer 9

Ventral Postero-lateral

Question 10

Where does decussation occur in the pain and temperature pathway?

Answer 10

At the same level as the information coming into the spinal cord

Question 11

Which nerve carries nociceptive information from the face?

Answer 11

Trigeminal Nerve (CN V)

Question 12

Where does this nerve enter the brainstem?

Answer 12

Pons

Question 13

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

Answer 13

It enters the trigeminal ganglion in the pons and then it moves DOWNWARDS along the trigeminal nucleus.
It then synapses in the lower part of the trigeminal nucleus in the medulla.
The second order neurone then decussates and joins the medial end of the spinothalamic tract.

Question 14

Describe the division of the trigeminal nucleus.

Answer 14

The trigeminal nucleus is a column of grey matter that runs from midbrain to medulla.
It is divided into areas where each area serves a different modality.

Question 15

Which thalamic nucleus relays pain information from the face?

Answer 15

Ventral Posteromedial

Question 16

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

Answer 16

Spinal Trigeminal Nucleus

Question 17

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

Answer 17

It registers the LOCATION and INTENSITY of the stimulus

Question 18

As the spinothalamic tract projects towards the primary somatosensory cortex, it gives off collateral branches. Whichstructures do these branches go to?

Answer 18

Brainstem (reticular formation)
Thalamus (intralaminar nuclei) Hypothalamus
Limbic structures

Question 19

What is the point of these collateral connections?

Answer 19

The connections to the reticular formation and intralaminar nuclei allow the spinothalamic tract to increase your level of arousal to make sure that you are aware of potentially harmful situations.

Question 20

Which CNS structures are involved in signalling the unpleasantness of a stimulus?

Answer 20

Limbic structures and hypothalamus

Question 21

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

Answer 21

Central and Peripheral Inhibition Pathways

Question 22

What is the focus of the central inhibition pathway?

Answer 22

Periaqueductal Grey Matter

Question 23

Describe the arrangement and function of the central inhibition pathway.

Answer 23

Increased brain activity will increase the impulses going down the central inhibition pathway, which goes to the dorsal horn at every level.
These descending axons synapse with an interneurone and activate the interneurone.
The interneurone synapses with the first and second order nociceptive neurones and release ENKEPHALIN, which is inhibitory.
So enkephalin release will reduce the amount of information going down the spinothalamic tract hence you feel less pain.

Question 24

What type of molecule is Enkephalin?

Answer 24

Opioid

Morphine mimics the action of this central inhibition system.

Question 25

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

Answer 25

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

Question 26

Describe the arrangement and function of the peripheral inhibitionpathway.

Answer 26

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

Question 27

State two examples of loss of pain sensation.

Answer 27

Syringomyelia
Charcot Joints – due to peripheral neuropathy, you don’t realise that you are using your joints inappropriately or excessively – this leads to joint deformities

Question 28

How can you get exacerbation of pain?

Answer 28

Wind up in the dorsal horn
If someone has chronic pain then certain peripheral nerves coming into the spinal cord will be carrying high levels of input for a long time.
The cells in the dorsal horn can lower their sensitivity or their synapses will change, which means that the information going into the spinothalamic tract is increased so this can actually increase the level of chronic pain.