Sensory Pathways: Nociception Flashcards
What are the pain receptors called?
Nociceptors
Can very high intensity stimulation of mechanoreceptors cause the feeling of pain?
No – only nociceptors can cause pain sensation
Describe some features of nociceptors.
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
What are the two main types of sensory neurone that carries sensory information? State some characteristics of each.
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
Compare the receptive fields of nociception to those of touch.
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.
Describe the method of coding intensity in nociception.
Same as touch – increase in frequency of impulses
Describe the passage of the central pathway, which carries information about pain and temperature.
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.
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 Postero-lateral
Where does decussation occur in the pain and temperature pathway?
At the same level as the information coming into the spinal cord
Which nerve carries nociceptive information from the face?
Trigeminal Nerve (CN V)
Where does this nerve enter the brainstem?
Pons
Describe the passage of this nerve from entry into the brainstem.
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.
Describe the division of the trigeminal nucleus.
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.
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?
It registers the LOCATION and INTENSITY of the stimulus
As the spinothalamic tract projects towards the primary somatosensory cortex, it gives off collateral branches. Whichstructures do these branches go to?
Brainstem (reticular formation)
Thalamus (intralaminar nuclei) Hypothalamus
Limbic structures
What is the point of these collateral connections?
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.
Which CNS structures are involved in signalling the unpleasantness of a stimulus?
Limbic structures and hypothalamus
What are the two pathways that can reduce the amount of pain that you feel?
Central and 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 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.
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 inputinto the second order nociceptive neurone?
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.
Describe the arrangement and function of the peripheral inhibitionpathway.
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.
State two examples of loss of pain sensation.
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
How can you get exacerbation of pain?
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.
What are the types of pain fibre?
Aδ fibers mediate sharp, intense or first pain
Type 1: noxious mechanical
Type 2: noxious heat
C-fibres mediate dull, aching or second pain
Noxious thermal, mechanical and chemical stimuli (polymodal)
What’s the first synapse in the pain pathway?
Glutamate release at the dorsal horn
Postsynaptic calcium influx
Uptake into the presynaptic membrane by N-type VGCC
What are the main pain pathways?
Lateral spinothalamic tract (sensory component) Spinorecticular tract (emotional component)
What are the peripheral controls of pain?
Gate Control Theory - Inhibition of primary afferent inputs before they are transmitted to the brain through ascending pathways.
Aβ-fibers (mechanoreceptors) cause inhibition of c fibre pain projecting neurones
What are the central controls of pain?
Periaqueductal grey - rostral ventral medial medulla axis (PAG-RVM axis)
the release of serotonin/noradrenaline to inhibit nociceptive processing in the dorsal horn
What is allodynia?
Allodynia: pain due to a stimulus that does not normally provoke pain
What is Hyperalgesia?
Hyperalgesia: increased pain from a stimulus that normally provokes pain
Primary
Secondary
What are the types of sensitisation?
Peripheral - 1st order sensory neurone becomes sensitised. Reduced the threshold to peripheral stimuli at the site of injury
Central - 2nd order neurones become more sensitised. Expansion of the receptive field
decreased threshold to peripheral stimuli at an adjacent site to the injury
Spontaneous pain
