Sensory Pathways: Nociception Flashcards
What are the pain receptors called?
Nociceptors
Describe 4 features of nociceptors.
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
What are the 2 main types of sensory neurone that carry information on pain, temperature and crude touch? State some characteristics of each.
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)
Compare the receptive fields of nociception to those of touch.
Receptive fields for nociception are much LARGER
Don’t need to be able to localise pain as well as touch.
Describe the method of coding intensity in nociception.
Increase in frequency of impulses
Describe the passage of the central pathway, which carries information about pain and temperature.
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.
Describe the somatotopic arrangement of the fibres in the spinothalamic tract.
Lower fibres = Lateral
Higher fibres = Medial
(Opposite of dorsal columns)
Which thalamic nucleus relays sensory information from below the neck?
Ventral Posterior-lateral nucleus
Where does decussation occur in the pain and temperature pathway?
At the same level as the info. coming into the spinal cord
Which nerve carries nociceptive information from the face?
Trigeminal Nerve (CN V)
Where does the trigeminal nerve enter the brainstem?
Pons
Describe the passage of the trigeminal nerve from entry into the brainstem.
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.
Describe the division of the trigeminal nucleus.
Trigeminal nucleus = column of grey matter, runs from midbrain to medulla.
Divided into areas where each area serves a different modality.
Which thalamic nucleus relays pain information from the face?
Ventral Posteromedial
Which part of the trigeminal nucleus does the first order nociceptive neurones from the face synapse in?
Spinal Trigeminal Nucleus
What is the role of the primary somatosensory cortex in processing the nociceptive stimulus?
Register LOCATION + INTENSITY of the stimulus
What are the two pathways that can reduce the amount of pain that you feel?
Central + Peripheral Inhibition Pathways
What is the focus of the central inhibition pathway?
Periaqueductal Grey Matter
Describe the arrangement and function of the central inhibition pathway.
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.
What type of molecule is Enkephalin?
Opioid
Morphine mimics the action of this central inhibition system.
Other than a first order nociceptive neurone, what else has input into the second order nociceptive neurone?
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.
Describe the arrangement and function of the peripheral inhibition pathway. (Gate control theory)
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.
State 2 examples of loss of pain sensation.
Syringomyelia
Charcot Joints: due to peripheral neuropathy, don’t realise inappropriate/ excessive use of joints, leads to joint deformities
How can you get exacerbation of pain?
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.
How does the spinothalamic pathway differ for pain and temperature to crude touch?
Pain + temperature: Ascend in lateral spinothalamic tract
Crude touch: Ascends in anterior spinothalamic tract
What would result from an anterior spinal cord lesion?
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
What are pain pathways composed of?
Sensory component (lateral spinothalamic tract) Emotional component (spinoreticular tract)
What is involved in the cerebral signature for pain, as seen on a functional MRI?
Cortex (SI, SII, Insula cortex, anterior cingulate cortex, prefrontal cortex)
Amygdala
Cerebellum
Brainstem
How can serotonin and noradrenaline influence pain experienced?
Inhibitory effect on spinothalamic tract
What is the difference between nociceptive pain and neuropathic pain?
Nociceptive: Noxious stimulation of a nociceptor (somatic or viscera)
Neuopathic: Lesion or disease of the somatosensory system
What is peripheral and central sensitisation?
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
Define Allodynia
pain due to a stimulus that does not normally provoke pain
Define Hyperalgesia
increased pain from a stimulus that normally provokes pain
