6.2 Nociception

Question 1

Nociceptors are the receptors which detect noxious stimuli and transmit the sensation perceived as pain by the brain. What are the properties of nociceptors?

Answer 1

1. Polymodal: Range of nociceptors which can respond to different types of stimuli (chemoreceptors to histamine, thermoreceptors to changes in temperature
2. Free nerve endings: No complex accessory structures attached to them
3. High activation threshold: Generally less sensitive than touch and proprioception
4. Slow-adapting: Responds to potentially harmful stimulus (allows continuous awareness as long as it is present)

Question 2

which fiber detects sharp pain?

Answer 2

Aδ

Question 3

Which fiber detects duller aching pain?

Answer 3

C

Question 4

Spinothalamic tract (Body)
1. Primary sensory neurone (cell body in \_\_\_\_\_\_\_\_\_\_\_\_\_\_) → dorsal horn → synapses with secondary neurone (in superficial area of pathway) 

2. Secondary axon crosses over midline of spinal cord → turns upwards to form spinothalamic tract in ______________ → _____________
3. Tertiary axon carries information to appropriate part of cortex
   * Input from more superior regions in the body essentially go through the same pathway, and join up to gradually increase the width of the spinothalamic tract as it approaches the brainstem

Answer 4

dorsal root ganglion;

anterolateral white matter;

thalamus (VPL)

Question 5

Spinothalamic tract (Face)

1. Impulses from trigeminal nerve (cell body in trigeminal ganglion) → part of the trigeminal nucleus in ____ → pass downwards to _______________
2. Secondary axon crosses over the midline → joins up with the medial part of the spinothalamic tract (passes through brainstem) → __________
3. Tertiary axon carries information to appropriate part of cortex

Answer 5

pons;

spinal part of the trigeminal nucleus in the medulla;

thalamus (VPM)

Question 6

The sensory nuclei for the trigeminal nerve (CN V) are collectively a long column of cells from the midbrain to the medulla → divided into different parts (deals with different modalities):

1) Mesencephalic
- Location: __________
- Modality: _____________

2) Principal sensory
- Location: Pons
- Modality: _____________
- Touch information comes into pons → direct relay of information → decussation at ____________

3) Spinal
- Location: __________
- Modality: _____________
- Pain information comes into pons → travels down to medulla → decussation at __________

Answer 6

Midbrain;

Reflex proprioception;

Light touch, proprioception of jaw;

pontine level;

Deep touch, pain, temperature;

medullary level

Question 7

Where does information from the body decussates in the spinothalamic tract?

Answer 7

spinal cord

Question 8

Where does information from the face decussates in the spinothalamic tract?

Answer 8

medulla

Question 9

Spinothalamic tract Somatopy throughout pathway: Axons are placed in the right position to relay through the correct thalamic nuclei (to travel to right area of primary somatosensory cortex) → localisation of nociceptive stimulus:
• Information from lowest part of body: axon goes to furthest ____________
• Information from upper part of body: goes to progressively more
__________ positions
• Information from face: joins onto most ______________ end of tract

Answer 9

lateral part of the developing spinothalamic tract;

medial;

medial

Question 10

What is the function of the primary somatosensory cortex (S1) cortex?

Answer 10

Receives nociceptive information → localises the nociceptive stimulus and registers the stimulus intensity

Question 11

What is the function of the secondary somatosensory cortex (S2) cortex?

Answer 11

Analyses the information at a higher level

Question 12

What is the function of the posterior parietal cortex (S2) cortex?

Answer 12

Analyses the information at a higher level

Question 13

The spinothalamic tract projects to other destinations (apart from sending nociceptive information to the S1 cortex) via collateral branches. What other pathways are there?

Answer 13

• Reticular formation of brainstem: Affective pain pathway (changes mood/emotions)
• Periaqueductal grey of midbrain: Central inhibition pathway

Question 14

Stimulation of the brainstem reticular formation affects the reticular activating system (RAS):
• RAS is an important ascending pathway controlling __________________
o Alters/modulates the activity of the cerebral cortex → the higher the level of activity in the RAS, the more aware the patient is
• Potentially harmful stimulus increases level of arousal by increasing input to the RAS
• RAS projects to the __________________ → collectively signal unpleasantness of the stimulus

Answer 14

the level of awareness/arousal;

thalamus, hypothalamus, and limbic structures (both cortical and subcortical)

Question 15

The periaqueductal grey contains cells which project down through the _____________ to the __________________:
• Influences the secondary neurone (projects across the midline to form the spinothalamic tract) in the nociceptive pathway
• Receive input from the spinothalamic tract and wide-ranging input from ______________:
o More activity into the periaqueductal grey → more likely cells there will fire
• Sends descending inputs through the reticular formation to inhibit neurones sending information into spinothalamic pathway
• Periaqueductal grey reduces amount of nociceptive information passing up to the brain by _________________ → reduces probability of perceiving pain

Answer 15

reticular formation;

dorsal horn of the spinal cord at all levels;

various cortical areas and the hypothalamus;

inhibiting spinothalamic neurones

Question 16

The peripheral inhibitory mechanism acts at the same place as the central inhibition (dorsal horn of the spinal cord) → gate mechanism
- Open: Signals coming in through the ____________ but no/few signals coming through the ____________ → activation leads to excitation of the projection neurone → full level of excitation into spinothalamic tract (perceived as pain in the brain)
- Partially closed: Activity in _____________ fibres → competition between the two due to opposing effects:
• If stimulus of non-nociceptive mechanoreceptor is strong: excitation of inhibitory interneurone → inhibitory effect on projection neurone → overwhelms excitatory effect by the nociceptive receptor

Answer 16

C fibres;

mechanoreceptor fibres;

both non-nociceptive and nociceptiv

Question 17

Transcutaneous local electrical stimulation: applied to the body to stimulate ________________ to compete with pain coming in through nociceptive fibres –> temporarily reduces the perception of pain (e.g. during birth)

Answer 17

non nociceptive mechanoreceptors

Question 18

Syringomyelia: Space opens up in the middle of the spinal cord around the central canal (usually in the _______________):
• Secondary axons carrying nociceptive information (from the arms) cross over the midline → disrupted by syrinx
• No sense of harmful stimuli on hands/arms → present with cuts/burns without knowing how they got them

Answer 18

cervical region

Question 19

Charcot joints: Uncomfortable joints resulting from not having the correct nociceptive feedback from the receptors within the joints
• Using joints too often/wrongly leads to ______________________
• Often due to a genetic mutation

Answer 19

swollen joints and possibly immobility

Question 20

Thalamic syndrome: Due to ________________ → deprives part of the thalamus of blood supply → ________________ pain

Answer 20

cerebrovascular accident;

generalised, non-localised

Question 21

Windup in dorsal horn: Develops in the dorsal horn as a result of chronic pain due to an ___________________
• High intensity input into a particular part of the spinal cord over a prolonged period → cells in the dorsal horn become more sensitive (threshold lowered)
• May continue to fire even when the original physical source of nociceptive input has been removed

Answer 21

increase in the number of NMDA glutamate receptors