Central Processing of Pain Flashcards
Define pain. (3)
An unpleasant sensory and emotional experience
associated with, or resembling that associated with,
actual or potential tissue damage.
Name three general types of pain, in terms of the type of tissue damage or change that it is associated with. (3)
Give an example of each. (3)
- Neuropathic (damaged nerve)
- Inflammatory (arthritis)
- Central (stroke)
Name the six main anatomical contributors to pain processing. (6)
- Peripheral nociceptors (including free nerve endings and receptors)
- Primary afferent neurones
- Intrinsic spinal dorsal horn neurones
- Ascending projection neurones
- Higher centre neurones
- Descending neurones
Name the three main afferent fibre types which are involved in pain sensation. (3)
- A beta
- A delta
- C
Describe Ab fibres, in terms of the following:
- Diameter
- Conduction velocity
- Type of conduction (saltatory/continuous)
- Myelination
(4)
- Largest diameter
- Highest conduction velocity
- Saltatory conduction
- Heavily myelinated
Give four sensory modalities that are conveyed by A beta fibres. (4)
- Light touch
- Pressure
- Vibration
- Some pain sensations
Describe Ad fibres, in terms of the following:
- Diameter
- Conduction velocity
- Type of conduction (saltatory/continuous)
- Myelination
(4)
- Medium diameter
- High conduction velocity
- Saltatory conduction
- Myelinated (but less heavily than Ab fibres)
Give three sensory modalities that are conveyed by Ad fibres. (3)
- Touch
- Temperature
- Sharp pain
Describe C fibres, in terms of the following:
- Diameter
- Conduction velocity
- Type of conduction (saltatory/continuous)
- Myelination
(4)
- Thinnest diameter
- Slow conduction velocity
- Continuous conduction
- Not myelinated
Give four sensory modalities that are conveyed by C fibres. (4)
- Noxious temperature
- Itch
- Significant pressure
- Dull pain
Describe how C fibres are arranged within a nerve. (1)
Grouped in bundles
Name four general structures in the body that are innervated by pain fibres. (4)
- Skin
- Joints
- Muscles
- Viscera
Describe the specialised sensory nerve endings that are found on nociceptive fibres. (2)
- Free nerve endings
- Which express a number of different receptors and channels
Why is it important that the free nerve endings of primary afferent neurones express a range of different receptors and channels? (1)
So that they can respond to and encode different stimuli.
Which part of the spinal cord receives afferent (sensory) signals? (1)
Dorsal horn
Which part of the spinal cord gives rise to outgoing motor information (efferent signals)? (1)
Ventral horn
Describe the sensory input received in the superficial laminae of the dorsal horn. (1)
Which laminae are classed as superficial?
Pain specific input
The superficial laminae are I and II
Describe the sensory input received in the intermediate laminae of the dorsal horn. (1)
Which laminae are classed as intermediate?
Touch specific neurones
The intermediate laminae are III and IV
Describe the sensory input received in the deep lamina of the dorsal horn. (1)
Which lamina is classed as deep?
All sensory input (pain and touch)
The deep lamina is V
Which dorsal horn laminae do Ad fibres mostly synapse on? (1)
Lamina I and V
Which dorsal horn laminae do C fibres mostly synapse on? (1)
Lamina II and V
Which dorsal horn laminae do Ab fibres synapse on? (1)
Laminae III and IV and V
Which afferent sensory fibres synapse on lamina V of the dorsal horn? (1)
What is the role of this layer? (1)
All fibres
This layer integrates incoming sensory information
Where do peripheral nociceptors have their cell bodies? (1)
Describe the exact location. (1)
Dorsal root ganglia
Adjacent to but just outside the spinal cord
Complete the sentences relating to primary afferent fibres and the spinal cord. (2)
Sensory neurones innervate their target tissue with their ……………………….. axon.
They also innervate the dorsal horn of the spinal cord, and enter via the ……………………..
peripheral
dorsal root
True or false? Explain your answer if necessary. (1)
Different parts of each lamina (eg. medial and lateral parts) respond to different stimuli in different areas of the body.
True - this helps to accurately encode the type of stimulus detected and its location
Fill the gaps relating to nociception in the spinal cord. (4)
Primary afferent fibres synapse with ………………………., which can also be called ………………………….
These neurones are either ………………………….. or …………………………….
dorsal horn neurones
second order neurones
projection neurones
spinal interneurones
Describe the role of projection neurones in the spinal cord. (1)
Convey information to the brain
Describe the role of interneurones in the spinal cord. (1)
Which types of neurone do interneurones synapse with? (2)
Relay and integrate noxious information between the different dorsal horn laminae
- Projection neurones
- Motor neurones
Suggest three ways that dorsal horn interneurones have been classified. (3)
Which classification do we rely on the most? (1)
- Somatodendritic morphology
- Firing pattern in response to injected current
- Neurochemistry
We rely on neurochemistry the most.
Describe in general how dorsal horn interneurones can be classified using neurochemistry. (1)
Look at the neurotransmitters or receptors contained in/on the neurone (eg. with staining or IHC).
Describe the distribution of the NK1 receptor in the dorsal horn. (2)
Found throughout the spinal cord
but present at the highest concentration in lamina I.
Very briefly describe what the NK1 receptor is. (1)
Receptor for substance P
Describe why we would expect to see a higher density of NK1 receptors in the superficial laminae of the dorsal horn. (3)
NK1 is activated by substance P
Substance P is released from peptidergic C fibres
And C fibres terminate in the superficial laminae
True or false? Explain your answer if necessary. (1)
NK1 receptors are only present on laminae I and II of the dorsal horn.
False - they are found throughout the dorsal horn, including on interneurones
Complete the sentence relating to nociception and pain. (1)
The degree of nociception which occurs and pain that is felt is determined by ……………………………
*Hint: the answer is a phrase
the overall excitability of dorsal horn synapses.
Give two characteristic phenomenons that are experienced in chronic pain. (2)
- Allodynia
- Hyperalgesia
Define ‘allodynia’. (1)
Pain in response to a normally innocuous stimulus.
Define ‘hyperalgesia’. (1)
Increased pain in response to an already-noxious stimulus
Complete the sentence relating to pain. (1)
Allodynia and hyperalgesia are the result of……
*Hint: the answer is a phrase
a sensitisation of the properties of CNS neurones.
Which paper was a turning point in understanding that chronic pain may be due to changes in the CNS? (1)
Very briefly describe the main concept conveyed by this paper. (1)
Woolf (1983)
Not all pain is due to activity in nociceptors.
Woolf (1983) showed the concepts of hyperalgesia and allodynia in the nervous system following injury.
Describe how this was shown. (3)
- Burnt feet of decerebrate rats
- Showed reduced mechanical threshold using von Frey hairs
- Showed reduced response latency when immersing injured foot in hot water
Woolf (1983) showed the concept of central sensitisation in the nervous system following injury.
Describe how this was shown. (4)
- Burnt feet of decerebrate rats
- Showed increased spontaneous activity in biceps femoris efferent neurones
- Showed reduced mechanical threshold using von Frey hairs
- Showed increased response amplitude and duration in the biceps femoris efferent after a pinch
Woolf (1983) recorded responses from efferent motor nerve fibres to show central sensitisation in response to pain/injury.
Describe how recording motor responses can show central sensitisation. (3)
- Motor neurones would show ongoing and increased activity
- This is due to sustained and increased input from the spinal cord
- This shows that the spinal cord is more excited and has sensitised
Suggest a way in which hyperalgesia can be shown to occur after a noxious stimulus in experimental conditions. (2)
- Provide a noxious stimulus (or activate C fibres)
- Test mechanical threshold (eg. using von Frey hairs in rodents)
Describe the changes you would expect to see in the receptive fields of dorsal horn (second order) neurones, and their pattern of neuronal firing, after a period of prolonged C fibre activation. (2)
- Receptive fields are expanded
- Neuronal firing is enhanced
Very briefly explain what is meant by ‘peripheral sensitisation’. (1)
Sensitisation of nociceptive nerve endings
True or false? Explain your answer if necessary. (1)
Peripheral sensitisation always leads to both hyperalgesia and allodynia.
True - it does this by shifting the stimulus-response curve to lower intensities
Describe the name of the phenomenon when increased pain occurs after an injury, but at a remote site from the point of injury. (1)
Spreading hyperalgesia
Define ‘neuronal plasticity’. (2)
Changes in the properties or functions of neurones or neuronal nets
that outlast the stimulus that caused these changes.
Define ‘central sensitisation’. (2)
Increased responsiveness of nociceptive neurones in the central nervous system
to their normal or subthreshold afferent input.
Very briefly describe what is meant by ‘secondary hyperalgesia’. (2)
Enhanced pain in undamaged tissue (surrounding the site of injury)
to only mechanical stimuli.
Which part of the nervous system is sensitised in secondary hyperalgesia? (1)
CNS
Describe what is meant by ‘primary hyperalgesia’. (2)
Increased pain in an injured area
in response to both mechanical and heat stimuli.
Which part of the nervous system is responsible for primary hyperalgesia? (1)
Primary afferent fibres (peripheral nervous system)
A study by Raja (1984) investigated secondary hyperalgesia in response to an injury.
Briefly describe the methods used in this study. (3)
- Two burns made on the palms of volunteers
- Test mechanical pain threshold (von Frey hairs) at various sites
- Test normalised pain ratings to various temperatures at various sites
A study by Raja (1984) investigated secondary hyperalgesia in response to an injury.
They made burns on the palms of volunteers and then tested mechanical and thermal pain in various surrounding areas.
Explain the results you would expect to see. (3)
- Decreased mechanical pain threshold (von Frey hairs) similar in both primary and secondary zones
- Hyperalgesia to heat (increased pain to heat) observed in the area of flare (the injured zone)
- The area between the two burns was actually hypoalgesic to heat but hyperalgesic to mechanical stimuli
Suggest six different categories of chemicals/molecules which can induce central sensitisation. (6)
Suggest where these chemicals might be released from in the context of pain. (1)
- Peptides
- Proteins
- Cytokines
- Chemokines
- Prostanoids
- Neurotrophic factors
These chemicals are released from primary afferent fibres.
Suggest two general changes to spinal neuronal networks which may induce central sensitization. (2)
- Increased excitability
- Loss of inhibition
Name three possible experimental models that may be used to explain the neuronal mechanisms behind central sensitisation. (3)
- Wind-up
- LTP
- Classic heterosynaptic central sensitisation
State whether wind-up and LTP are considered homosynaptic or heterosynaptic potentiation. (2)
What is meant by the terms ‘homosynaptic’ and ‘heterosynaptic’ potentiation? (2)
Wind up and LTP are both homosynaptic.
Homosynaptic = only neurones which are directly activated show a change
Heterosynaptic = neurones which are not directly activated also show a change
Define ‘wind-up’, in terms of its characteristics. (3)
Progressive increase in action potential output
from dorsal horn neurones
elicited during a train of low-frequency C fibre inputs.
True or false? Explain your answer if necessary. (1)
During wind-up, the dorsal horn neurone increases its AP frequency as it receives ongoing input from the C fibre, and when the C fibre input stops, APs stop being produced.
False - the neurone continues to fire after the stimulation has ceased
Very briefly describe how wind-up would be measured under experimental conditions. (4)
- Animal anaesthetised
- Single dorsal horn synapse isolated and electrode inserted (usually a wide dynamic range neurone is used)
- Stimulus applied to foot
- APs from the dorsal horn neurone measured
0.2Hz stimulations of C fibres are not sufficient to produce wind-up.
Explain why this is. (3)
- 0.2Hz stimuli cause steady neuronal discharges (eg. discharge as the stimulus is applied)
- The membrane has sufficient time to return to baseline prior to the next stimulus
- There is no sustained depolarisation
0.5Hz stimulations of C fibres are sufficient to produce wind-up.
Explain the molecular mechanisms behind this. (6)
- At this frequency, trains of action potentials stimulate SubP and CGRP to be released from C fibres
- These neuropeptides elicit a sustained and cumulative increase in membrane depolarisation
- Because the termination of neuropeptides in the synapse is relatively slow so they hang around for a longer time
- The membrane doesn’t have time to completely return to baseline potential between stimuli (due to neuropeptides)
- Mg block on the NMDA receptor is removed
- This allows calcium into the neurone and boosts the responses in a non-linear fashion
A key event in wind-up, is the membrane not being able to repolarise properly between action potentials.
If this is so, how are more action potentials produced without the voltage-gated sodium channels being able to reset? (2)
- The membrane potential does repolarise enough to reset the VGNaCs so more APs can be produced
- But it remains just depolarised enough to remove the NMDA Mg block, which is another key mechanism in wind-up
For approximately how long after the stimulus has ceased are responses in the dorsal horn neurones still facilitated in wind-up? (1)
Usually within 10-20 seconds
At approximately what membrane potential is the Mg block removed from the NMDA receptor. (1)
At membrane potentials higher than about -30mV
Describe the changes that occur in the neurone due to opening of the NMDA receptor during wind-up. (2)
- Calcium enters the neurone
- Calcium causes structural and functional changes in the neurone to increase synaptic efficacy
Suggest two issues with the wind-up model being used as a mechanism of chronic pain. (2)
- Chronic pain has to last >3 months, but wind-up tends to last seconds-minutes
- It cannot be tested in humans, it can only be proven in anaesthetised animals because you have to isolate a single synapse
True or false? Explain your answer if necessary. (1)
Peripheral sensitisation often leads to central sensitisation.
True - this may be because peripheral sensitisation leads to more pain signals being delivered to the CNS
When investigating wind-up, which lamina of the dorsal horn is usually used? (1)
Explain why this is. (1)
Lamina V
Because recordings are taken from WDR neurones, which are integrative neurones in lamina V.
What type of peripheral stimulus is used to activate the C fibres when investigating wind-up? (1)
Electrical stimulus (low frequency)
Out of the three possible models of central sensitisation, which is the simplest way to describe the potentiation of signals? (1)
LTP
Briefly describe the general principle of LTP. (2)
Brief, high-frequency pre-synaptic stimuli from primary afferent fibres
leads to AMPA-mediated potentiation of postsynaptic responses in second order neurones.
In LTP, how do AMPA receptors mediate changes in the second order neurone? (4)
- AMPA receptors depolarise membrane
- NMDA receptors can open
- Calcium flows into cell
- Calcium induces cellular changes
Discuss the evidence that LTP may lead to central sensitisation. (1)
LTP of spinal synapses has been described in laboratory studies in animals
Describe how the intracellular LTP mechanisms are different in spinal LTP and hippocampal LTP. (1)
They are the same
Give three drawbacks of using the LTP model to explain chronic pain and central sensitisation. (3)
- LTP is homosynaptic which means that only affected synapses are potentiated
- LTP requires a stimulus of about 100Hz, and primary afferent fibres do not fire at these high frequencies under normal physiological conditions
- LTP is difficult/impossible to measure in man
Fill the gaps relating to classical heterosynaptic central sensitisation. (3)
The original description of central sensitisation referred to ………………………. onset, ………………………..-dependent increase of …………………… that outlasted the stimulus for tens of minutes.
This is classical heterosynaptic central sensitisation.
immediate
activity
excitability
Describe the general mechanism underlying classical heterosynaptic central sensitisation. (3)
10-20second nociceptor driven conditioning stimulus
leads to normally subthreshold stimuli beginning to activate dorsal horn neurones
due to an increase in synaptic efficacy.
True or false? Explain your answer if necessary. (1)
In classic heterosynaptic central sensitisation, synapses not directly affected by the conditioning stimulus can also be potentiated, such as Ab fibre synapses.
True - this is what is meant by heterosynaptic
Describe why classical heterosynaptic central sensitisation is the most popular model of central sensitisation. (2)
Low frequency stimulation (like what is seen physiologically) can cause heterosynaptic sensitisation
so this form of potentiation is the most prominent form in the dorsal horn.
A conditioning stimulus (5 minutes of 500msec trains of 500uA) was applied to the sciatic nerve.
Describe how you would expect this stimulus to affect mechanical thresholds after it has been applied. (2)
State the name of the process that causes this change in threshold. (1)
- Decrease in mechanical threshold (hyperalgesia)
- Present for 48hrs post stimulus
The process is called classical heterosynaptic central sensitisation
Describe the three ways in which classical heterosynaptic central sensitisation is manifested. (3)
- Reduction in threshold due to recruitment of Ab fibres
- Increased responsiveness of dorsal horn neurones
- Expansion of receptive fields
Fill the gaps relating to classical heterosynaptic central sensitisation. (2)
Classical sensitisation of this type is the result of engaging multiple intracellular …………………………… that were effectively …………………. prior to the conditioning stimulus.
signalling cascades
dormant
Name the ion that plays a key role in classical heterosynaptic central sensitisation. (1)
How does this ion lead to central sensitisation? (1)
Calcium
leads to changes in expression of key molecules and signalling pathways in the second order neurone.
Suggest three molecules which may be released from primary afferent fibres which may play a role in classical heterosynaptic central sensitisation. (3)
- Glutamate
- Substance P
- BDNF
Describe what is meant by ‘transcriptional-dependent central sensitisaion’. (1)
Looking at expressing key molecules to make neurones more or less excitable.
Describe how the GABAa receptor affects a cell’s membrane potential. (3)
Pumps chloride into cell
so is hyperpolarising
and is inhibitory.
Describe the effect that GABAa receptors would usually have on the excitability of the spinal cord dorsal horn. (1)
Would increase inhibition (reduce excitability)
Why does the GABAa receptor usually pump chloride into the cell rather than out of the cell? (1)
Because chloride is more concentrated on the outside of the cell
Which ion pump is important for maintaining the chloride concentrations outside of cells? (1)
KCC2 (potassium-chloride co-transporter)
Describe the normal role of the KCC2 pump. (2)
Pumps chloride from inside the cell to outside the cell.
Describe the effect of neuropathic pain on expression of KCC2 pumps on neurones, and the effects of this. (4)
- In neuropathic pain there are fewer KCC2 pumps
- So chloride cannot be pumped out of the cell
- The concentration gradient of chloride is reduced
- So when the GABAa receptor is activated chloride now flows out of the cell and depolarisation and excitation occur
Describe how you would expect a peripheral nerve injury to affect mechanical withdrawal threshold. (1)
Cause a significant reduction
An experiment measured mPSPs from lamina I neurones in both naive and peripheral nerve injury rats.
They tried blocking the mPSPs with both strychnine and bicuculline.
What effect would you expect to see in naive rats and explain what this means. (2)
mPSPs abolished with strychnine
Strychnine is a glycine blocker so suggests that the mPSPs were caused by glycine
An experiment measured mPSPs from lamina I neurones in both naive and peripheral nerve injury rats.
They tried blocking the mPSPs with both strychnine and bicuculline.
What effect would you expect to see in peripheral nerve injury rats and explain what this means. (3)
mPSPs partially blocked by strychnine
but fully blocked with strychnine and bicuculline
Bicuculline blocks GABAa receptors, so GABA must be contributing to the mPSPs in peripheral nerve injury rats
Describe how you could measure dorsal horn KKC2 expression levels in rats with a peripheral nerve injury. (2)
Describe what you would expect to see. (1)
- Western blot for the protein
- With actin as a control measure
Would expect to see significantly lower levels of KCC2 in the ipsilateral dorsal horn to the nerve injury.
What effect would knocking down the KCC2 protein with anti-sense probes have on the withdrawal threshold of naive rats? (1)
What control measure would be used in this experiment? (1)
Significant reduction in withdrawal threshold
Scrambled probes with base pairs in wrong order so will not bind
Describe a simple, non-invasive way that the excitability of the spinal cord can be investigated. (1)
By looking at monosynaptic reflex arcs and measuring the reflex response
Give three changes you would expect to see when measuring reflexes if there is increased spinal cord excitability. (3)
- Increased response duration
- Increased response magnitude
- Lower threshold to elicit response
Define ‘descending control’ in reference to pain. (3)
The way in which the brain (or components of the brain)
modulate the excitability of the spinal dorsal horn
to increase or decrease nociception and therefore impact pain perception.
Who proposed the gate theory of pain modulation? (1)
Melzack and Wall (1965)
True or false? Explain your answer if necessary. (1)
A role for supraspinal sites in modulating nociception is a very new idea, first proposed in 2000.
False - a role for supraspinal sites in modulating nociception has been postulated since the early 20th century.
In the gate theory of nociception modulation, describe the proposed relationship between the G cells and T cells. (2)
G (gate) cells inhibit T (transmission cells).
The T cells would usually produce pain, or another response when activated by A and C fibres (nociceptive or non-nociceptive).
Describe the gate theory of pain control, in terms of G cells, T cells, sensory fibres, and central control. (6)
- G cell usually acts to inhibit T cell
- When Ad and C fibres (noxious) are activated, the T cell is stimulated
- Ad and C fibres also inhibit the G cell, so G cell inhibition is removed resulting in more pain/response from the T cell
- Providing an innocuous stimulus via Aa and Ab cells activates the G cell
- So the G cell can now inhibit the response from the T cell
- The innocuous stimulus also activates the central control systems, which influence the whole process
Briefly describe the general idea behind the gate-control theory of pain. (1)
Providing an innocuous stimulus (ie. rubbing it better) improves pain.
True or false? Explain your answer if necessary. (1)
The gate-control theory of pain suggested that the brain has a role in controlling nociception however did not specify how.
True
Very briefly describe how we can determine whether pain is central or peripheral. (1)
By lesioning specific areas of the nervous system and seeing if pain is still present
Describe a specific experimental method that would allow us to determine whether pain is central or peripheral. (5)
- Apply QX-314 (membrane impermeable local anaesthetic which blocks VGNaCs)
- Also apply capsaicin (to open the TRPV1 receptor)
- QX-314 gains access to neurone through TRPV1 channel
- This allows specific inactivation of TRPV1 expressing C fibres
- If the pain is still present it is central, if pain is no longer present it was peripheral
An experiment injected MIA into a joint and measured alterations in weight bearing as a sign of pain.
Describe the results seen on days 7, 14, and 28 when QX-314 + capsaicin was also added. (3)
Briefly explain the mechanism by which QX-314 + capsaicin affected weight bearing. (1)
Make a conclusion. (1)
On day 7, alterations in weight bearing were reversed.
On day 14, alterations in weight bearing were reversed.
On day 28, alterations in weight bearing were still present.
QX-314 + capsaicin inactivated TRPV1-expressing nociceptors.
These results suggest that at days 7 and 14, the pain was due to activity in peripheral nociceptors, and at day 28, the pain was due to activity in the CNS.
The earliest evidence for a selective and specific descending pain modulatory system was provided by Mayer and Price (1976).
What was their experimental finding regarding descending pain modulatory systems? (1)
Electrically stimulating the PAG of rats elicited stimulation-produced analgesia.
Fill the gaps relating to descending pain control mechanisms. (5)
In experiments, animals remain alert and active, however following both …………………. stimulation and …………………. stimulation together, pain behaviours such as …………………….. and ………………….. are ……………………….
A similar pattern of responses can be observed in humans.
These were some of the earliest experiments investigating descending pain modulatory systems.
noxious
PAG
vocalisation
escape behaviours
absent
Stimulating the periaqueductal grey produces analgesia.
Give two reasons why is this not a standard pain treatment in humans? (2)
- It is invasive
- The PAG is also responsible for other functions such as autonomic control, so PAG stimulation causes unwanted effects in humans
Suggest two roles of the periaqueductal grey in humans. (2)
- Pain modulation
- Autonomic control
Describe the location of the PAG in humans. (2)
In the midbrain
surrounding the cerebral aqueduct.
Fill the gaps relating to the PAG and pain. (4)
The PAG ………………. inputs from the …………………….. and ……………………. with ascending inputs from the ………………………………
integrates
limbic forebrain
diencephalon
dorsal horn of the spinal cord
Name the six major inputs into the PAG. (6)
- Amygdala
- Prefrontal cortex (anterior cingulate and insular cortices)
- Nucleus accumbens
- Hypothalamus
- Locus coeruleus
- Dorsal horn of the spinal cord
Fill the gaps relating to analgesia and the PAG. (5)
Microinjection of ………….. or ………………… stimulating the amygdala results in analgesia, which can be reversed by ……………….. injection into the ………………. to deactivate it.
Another brain area which shows the same effect is …………………………..
Describe what is meant by this statement. (1)
opioids
electrically
lidocaine
PAG
certain hypothalamic areas
Certain brain areas can produce analgesia, however this is reliant on the periaqueductal grey to transmit the signals to the spinal cord.
Stimulating certain brain regions (eg. amygdala and hypothalamus) can result in analgesia.
Describe the general mechanism behind this, including which other brain area is involved. (3)
These brain areas feed into the PAG
so stimulating amygdala/hypothalamus results in excitation of the PAG
and therefore ultimately results in descending inhibition.
Fill the gaps relating to the periaqueductal grey. (3)
The PAG is …………………… and …………………….. heterogenous.
This means that it contains …………………………….
cytoarchitecturally
neurochemically
many different types of neurones
Which area of the PAG is central to descending control? (1)
Ventrolateral PAG
Fill the gaps relating to the PAG. (3)
The ventrolateral PAG projects to the ……………………. and the …………………….. and is central to …………………..
rostral ventral medulla (RVM)
dorsal lateral and ventrolateral pontine tegmentum
pain control
Fill the gaps relating to the PAG. (2)
The dorsolateral PAG projects to the ………………………, a region which is critical for ……………………..
ventrolateral medulla
autonomic control
Fill the gaps relating to the PAG. (3)
The rostral PAG projects particularly to the …………………. and ……………………, which are areas involved in ……………………………….
medial thalamus
orbital frontal cortex
ascending nociceptive pathways
True or false? Explain your answer if necessary. (1)
All regions of the dorsal horn contain neurones that project to all regions of the PAG (dorsal horn input to the PAG is not segregated).
False - specific regions of the PAG receive input from specific regions of the spinal dorsal horn
Second order neurones in the spinal dorsal horn which are activated by C fibres project preferentially to which area of the PAG? (1)
ventrolateral PAG
Second order neurones in the spinal dorsal horn which are activated by larger A fibres project preferentially to which area of the PAG? (1)
dorsolateral PAG
In experiments investigating dorsal horn projections to the PAG, heat ramps are often used to selectively activate C or A fibres.
Describe the heat ramp stimulus used to activate C fibres. (1)
Heat increases at slow rates
In experiments investigating dorsal horn projections to the PAG, heat ramps are often used to selectively activate C or A fibres.
Describe the heat ramp stimulus used to activate A fibres. (1)
Heat increases at fast rates
Fill the gaps relating to the PAG and analgesia. (1)
Activating the PAG predominantly inhibits ………….-fibre evoked activity in the dorsal horn.
C
In experimental conditions, DLH can be added to the PAG to manipulate its activity.
Would DLH stimulate or inhibit the PAG? (1)
Give the full name of DLH. (1)
Describe the mechanism of action of DLH. (1)
Stimulate
DL-Homocysteic acid
Activates all glutamate receptors
A heat ramp was applied to the skin, and DLH was added to the PAG.
Describe the effects of DLH on heat ramp-mediated firing in the dorsal horn - both slow and fast heat ramp stimuli. (2)
Slow heat ramp stimulus activity (C fibre activity) is abolished with DLH.
Fast heat ramp activity (A fibre activity) was reduced but not abolished with DLH.
DLH was added to the PAG in an experiment.
Describe the resultant change in thermal threshold of dorsal horn neurones responding to both A and C fibres. (2)
Dorsal horn neurones responding to both A and C fibres showed increased thresholds.
True or false? Explain your answer if necessary. (1)
The PAG receives a very dense afferent projection from the spinal dorsal horn, and the reciprocal efferent projection is equally as dense.
False - the reciprocal efferent projection is sparse
Efferent projections from the PAG to the spinal dorsal horn are very sparse.
If this is so, how does the PAG work to modulate dorsal horn excitability? (3)
The PAG has its actions indirectly
via a projection arising in the vlPAG
to the nucleus raphe magnus in the rostral ventral medulla (RVM).
The rostral ventral medulla is a general brain area, rather than a defined structure.
Describe the structures that the RVM incorporates. (2)
The midline nucleus raphe magnus
and the adjacent reticular formation.
Does electrically/pharmacologically stimulating the RVM facilitate or inhibit dorsal horn excitability? (1)
Can do both depending on the stimulus etc
What is the major input to the RVM? (1)
PAG
Give five chemicals/neurotransmitters that may facilitate the chemical connection between the PAG and the RVM. (5)
Which of the chemicals make few connections, and which make more connections? (5)
GABA - few
Substance P - few
Enkephalin - few
Neurotensin - more
Serotonin - more
True or false? Explain your answer if necessary. (1)
The chemical connection between the PAG and the RVM is very complex and not yet very well understood.
True
When stimulating the RVM, dorsal horn excitability can both increase and decrease depending on how the RVM is stimulated.
How can we determine the resultant effect on the dorsal horn when stimulating the RVM? (1)
Measured using reflexes
An experiment aimed to determine the effects of stimulating the RVM in rats. They used reflexes to see the excitability of the dorsal horn.
Describe how this experiment would have been carried out. (6)
- Lightly anaesthetise the rat so that reflexes are still present
- Implant stimulator into RVM
- Measure EMG activity in response to mechanical stimulation of hindpaw with von Frey hairs
- Use a range of hairs (eg. two subthreshold, one threshold, and one suprathreshold)
- Plot EMG response to vFHs of different mechanical pressures and find area under curve
- Can compare area under curve with and without RVM stimulation
An experiment stimulated the RVM at different amplitudes (5-200uA) and recorded EMG activity in response to a range a von Frey hair stimuli. They plotted EMG activity and calculated the area under the curve.
How would you expect the AUC to change at low frequency stimulation of the RVM? (1)
Does this suggest that the RVM has facilitated or inhibited dorsal horn excitability? (1)
AUC increases (more EMG activity and larger muscle response)
Facilitated
An experiment stimulated the RVM at different amplitudes (5-200uA) and recorded EMG activity in response to a range a von Frey hair stimuli. They plotted EMG activity and calculated the area under the curve.
How would you expect the AUC to change at high frequency stimulation of the RVM? (1)
Does this suggest that the RVM has facilitated or inhibited dorsal horn excitability? (1)
AUC decreases (less EMG activity and smaller muscle response)
Inhibited
Describe the mechanism by which low frequency stimulation of the RVM is thought to facilitate DH excitability, and high frequency stimulation is thought to inhibit DH excitability. (4)
Lower amplitudes are thought to activate larger cells
larger cells are thought to be facilitatory (thought to be the ON cells).
At higher amplitudes the smaller cells are recruited
smaller cells are thought to be inhibitory (thought to be the OFF cells).
Describe the effect of lesioning or reversibly inactivating the RVM on the analgesic actions of the PAG. (1)
Abolishes PAG analgesia
Suggest two ways the RVM can be lesioned or inactivated. (2)
Very briefly describe how these two methods work. (2)
Lidocaine - local anaesthetic (blocks VGNaCs)
Kainate - causes excitotoxicity
Describe the change in mechanical threshold expected after the RVM has been inactivated. (1)
Describe why this change occurs. (1)
Threshold will decrease after the RVM has been inactivated.
In the resting state, the RVM provides tonic inhibition to the spinal cord which is removed if it is inactivated.
True or false? Explain your answer if necessary. (1)
There are 5 distinct populations of pain-modulating neurones in the RVM in terms of physiological properties.
False - there are 3 distinct populations
Name the 3 types of pain-modulating neurones found in the RVM. (3)
- ON cells
- OFF cells
- Neutral cells
Describe the tonic activity, and the activity in response to a painful stimulus of ON cells in the RVM. (2)
- Tonically inactive
- Fire in response to a painful stimulus
Describe the tonic activity, and the activity in response to a painful stimulus of OFF cells in the RVM. (2)
- Tonically active
- Cease firing in response to a painful stimulus
Describe the tonic activity, and the activity in response to a painful stimulus of neutral cells in the RVM. (2)
- Tonically active
- No consistent change in response to pain
Which population of cells in the RVM are thought to facilitate pain? (1)
ON cells
Which population of cells in the RVM are thought to inhibit pain? (1)
OFF cells
Fill the gaps relating to cell populations in the RVM. (3)
Both ON and OFF cells project to these lamina/e of the dorsal horn: ………………………….
Activity of both cell types is modulated by manipulations of the ………………
ON and OFF cells are considered to be central to mediating the effects of …………………………..
I, II, and V
PAG
PAG activation
Which cell type in the RVM stops firing in response to a painful stimulus? (1)
OFF cells
Which cell type in the RVM starts firing in response to a painful stimulus? (1)
ON cells
Describe how ON cells and OFF cells in the RVM are functionally connected. (4)
- OFF cell connected to ON cell via GABAergic interneurone
- Ongoing (tonic) activity in the OFF cells excites the interneurone
- The interneurone then releases GABA to the ON cell
- So the ON cell is inhibited
True or false? Explain your answer if necessary.
When a painful stimulus is applied, changes in the RVM occur more quickly than the reflex reaction.
True - changes in the RVM precede the reflex reaction, however it’s not really understood how because the information has to travel longer to get to the brainstem than it does to synapse on the reflex motor neurone
In an experiment, rats were injected with carrageenan in either the foot or knee. Thermal withdrawal thresholds on the ipsilateral hindpaw were measured before and after injections.
Some animals had their RVM intact, and others had their RVM lesioned.
Give two molecules that were used to lesion the RVM and describe how they work. (4)
Ibotenic acid - excitotoxin
APV - glutamate antagonist
In an experiment, rats were injected with carrageenan in either the foot or knee. Thermal withdrawal thresholds on the ipsilateral hindpaw were measured before and after injections.
Describe why, when they were assessing pain, the experimenters injected the inflammatory agent in the paw or the knee as a variable. (2)
When they injected it into paw and then measured paw withdrawal threshold, it shows primary hyperalgesia.
When they injected it into knee and then measured paw withdrawal threshold, it shows secondary hyperalgesia.
In an experiment, rats were injected with carrageenan in either the foot or knee. Thermal withdrawal thresholds on the ipsilateral hindpaw were measured before and after injections.
Some animals had their RVM intact, and others had their RVM lesioned.
Describe the results seen. (2)
Make a brief conclusion. (1)
For primary hyperalgesia (paw inflammation and paw thresholds measured), lesioning the RVM had no effect on hyperalgesia.
For secondary hyperalgesia (knee inflammation and paw thresholds measured), lesioning the RVM reduced secondary hyperalgesia.
Removing the RVM removes secondary hyperalgesia, so the RVM must be important for central sensitisation via its facilitatory mechanisms.
Name three anatomically different descending pain modulatory systems. (3)
- PAG/RVM
- Locus coeruleus and NA
- Hypothalamus and DA
The locus coeruleus projects what neurotransmitter to the dorsal horn to modulate pain? (1)
Give an alternative name for the locus coeruleus. (1)
Noradrenaline
A7 cell group
In which laminae of the spinal dorsal horn are NA-containing terminals from the LC found? (1)
In which lamina are they particularly concentrated? (1)
NA-containing terminals are distributed in all laminae of the dorsal horn.
However are particularly concentrated in lamina I.
True or false? Explain your answer if necessary. (1)
Noradrenergic terminals projecting from the locus coeruleus to the spinal dorsal horn establish axo-somatic synapses only.
False - they establish axosomatic, axodendritic, and axoaxonic synapses
Fill the gaps relating to descending pain control facilitated by noradrenaline. (5)
NA produces a direct inhibition of ……………….. neurones such as those in the ……………………………., also known as lamina II.
These neurones can be excitatory or inhibitory, but the overall effect of inhibiting these neurones is ………………………….
It is also known that a substantial number of NA terminals make synaptic connections with …………………. that extend their axons to the …………………., indicating the direct control of the output of ascending projection neurones as well as the modification of sensory transmission among interneurones.
inter
substantia gelatinosa
reduced signal transmission in lamina II
projection neurones
thalamus
Does the descending noradrenergic system from the locus coeruleus to the spinal dorsal horn facilitate or inhibit nociception? (1)
Inhibit
Describe the different effects that the descending NA system can have on dorsal horn interneurones. (2)
NA may inhibit excitatory interneurones
NA may excite inhibitory interneurones
Describe the effect of NA on the inhibitory interneurones in the spinal dorsal horn. (1)
NA has this effect by binding to which receptor? (1)
- NA stimulates inhibitory interneurones
- By binding to a1 receptors
Describe the effect of NA on the second order dorsal horn neurones. (1)
NA has this effect by binding to which receptor? (1)
- NA can either depolarise or hyperpolarise second order neurones
- By binding to a1 receptors (depolarisation) or a2 receptors (hyperpolarisation)
Are a1 receptors generally excitatory or inhibitory? (1)
Excitatory
Are a2 receptors generally excitatory or inhibitory? (1)
Inhibitory
Describe the effect of NA on the primary afferent fibres in the spinal dorsal horn (both Ad and C fibres). (1)
NA has this effect by binding to which receptor? (1)
- NA inhibits the nociceptive primary afferent fibres
- NA binds to a2 receptors
True or false? Explain your answer if necessary. (1)
Although a dopaminergic system is known to modulate pain processing in the spinal cord dorsal horn, the DH is actually devoid of DA cell bodies.
True - signals must come from higher centres
Which parts of the dorsal horn do descending dopaminergic projections innervate? (2)
Dorsal horn
Lamina X
Name three brain structures which may give rise to descending dopaminergic projections to the spinal cord. (3)
Two of these structures play a very minor role. Which structure is the principle source of descending DAergic pathways? (1)
- Substantia nigra pars compacta
- PVN of the hypothalamus
- Posterior periventricular (A11) region of the hypothalamus (PRINCIPLE SOURCE)
What is the effect of activating D2 receptors on neuronal activity? (1)
Suppression of neuronal activity
What is the effect of activating D1 receptors on neuronal activity? (1)
Enhancement of cellular excitability
Complete the sentence relating to descending dopaminergic pain modulatory systems. (1)
The effect of dopamine on different dorsal horn neurones is determined by ……………………………… that the neurone possesses.
the subtype of dopamine receptor
What is the overall effect of the descending dopaminergic system on nociception? (1)
Can be either facilitatory or inhibitory
Describe which receptor (D1 or D2) you would expect dopamine to be binding to on the following dorsal horn neurones, if it was providing descending inhibition. (3)
- Primary afferent fibre
- Projection neurone
- Inhibitory interneurone
- D2
- D2
- D1
Describe which receptor (D1 or D2) you would expect dopamine to be binding to on the following dorsal horn neurones, if it was providing descending facilitation. (3)
- Primary afferent fibre
- Projection neurone
- Inhibitory interneurone
- D1
- D1
- D2 (potentially)
An experiment aimed to show the effects of dopamine on hyperalgesic priming.
They used 6-OHDA + desipramine as an active drug.
What is the effect of this drug (ie, why did they use it?)? (1)
Depletes dopamine from the dorsal horn.
An experiment aimed to show the effects of dopamine on hyperalgesic priming.
They used 6-OHDA + desipramine as an active drug, and measured thresholds.
Describe the results seen with 6-OHDA + desipramine. (1)
Write a short conclusion. (1)
Threshold did not lower after IL-6 and PGE2, which cause pain.
Depleting DA in the dorsal horn prevents hyperalgesic priming, so DA is required to develop hyperalgesia and for the chronification of pain.
Are opioids better to use in acute or chronic pain? (1)
Acute pain
Complete the sentence relating to opioids and pain. (1)
Opioid drugs powerfully inhibit nociception by …………………………….
Hint: the answer is three words
inhibiting neuronal firing.
Name the three types of opioid receptor. (3)
u (mu opioid receptor, MOR)
k (kappa opioid receptor, KOR)
d (delta opioid receptor, DOR)
A large part of the opioid’s analgesic actions are mediated in which two brain regions? (2)
- PAG
- RVM
Do opioids act by depolarising or hyperpolarising the postsynaptic neurone? (1)
Hyperpolarising
True or false? Explain your answer if necessary. (1)
Opioid receptors are metabotropic and mediate their effects via Gq G proteins.
False - they are metabotropic, but mediate their effects via Gi proteins
Give two intracellular effects of activating the MOR. (2)
What is the overall result of these effects? (1)
- Decreasing cAMP levels
- Increasing K conductances
Overall, these effects inhibit neuronal firing.
Which cell population/s in the RVM express the MOR? (1)
Only ON cells
How does the fact that only ON cells in the RVM contain MORs help us to determine the function of these cells? (3)
Opioids acting on the MOR inhibit neuronal firing.
And opioids are known analgesic drugs.
So ON cells must facilitate pain, if opioids inhibit ON cells and provide pain relief.
Suggest a way that we could view/image ON cells in the nucleus raphe magnus. (1)
Explain why this method would work. (1)
Stain for mu opioid receptors.
This would work because only ON cells contain MORs in the RVM.
An experiment aimed to investigate the role that the RVM plays in mediating opioid analgesia.
EMG recordings were made before and after a microinjection of a drug into the RVM.
They used a drug called DAMGO. How does this drug work? (1)
It is a mu-opioid receptor agonist
An experiment aimed to investigate the role that the RVM plays in mediating opioid analgesia.
EMG recordings were made before and after a microinjection of a drug into the RVM.
Describe the effect of injecting DAMGO on the EMG response to noxious stimulation in both adult and preadolescent rats. (2)
Is this effect dose-dependent or dose-independent? (1)
- Decreases EMG activity in adults
- Increases EMG activity in preadolescents
This effect is dose-dependent
Name five types of endogenous opioid peptides that play a role in RVM functioning. (5)
- Enkephalins
- Endorphins
- Dynorphin
- Endomorphin
- Nociceptin
Fill the gaps relating to opioids and descending pain modulatory networks. (3)
Opioid peptides can be found throughout the component supraspinal nuclei of the pain pathway.
They form a ……………………. between these centres, connecting the brain areas involved in …………….. through a constant …………………
functional link
pain
opioid tone
Opioids are thought to form a functional link between different brain areas associated with descending pain modulating networks.
Describe how this can be demonstrated experimentally. (3)
Stimulate activity in one brain centre
then block the actions of opioids via naloxone in another brain centre
and look at the overall outcome.
Opioid receptor antagonists can be used to investigate functional links between different brain areas involved in pain modulation.
Name an opioid receptor antagonist that could be used for this purpose. (1)
CTAP
Opioid receptor antagonists can be used to investigate functional links between different brain areas involved in pain modulation.
Describe how this could be done, including the drugs that can be used. (3)
Describe the results seen. (2)
- Inject DAMGO (MOR agonist) into basolateral amygdala
- Inject CTAP (MOR antagonist) into PAG
- Measure reflexes (eg. tail flick latency) with just DAMGO and with DAMGO+CTAP
With just DAMGO, tail flick latency increased (slower reflex, less pain).
CTAP treatment attenuated this increased latency (reflex was quicker, DAMGO could no longer provide pain relief).
To investigate the effects of opioids in the RVM, MOR-containing neurones can be selectively lesioned.
Describe and explain how this would be done. (4)
- Use dermorphin-saporin
- Dermorphin is a MOR agonist (used to activate receptor)
- Receptor is internalised along with Derm-SAP
- Saporin is a cytotoxin which kills cells
To investigate the effects of opioids in the RVM, MOR-containing neurones were selectively lesioned.
Spinal nerve ligation neuropathic surgery was then performed.
Describe the results seen with the MOR-containing neurones intact, and after they were lesioned. (3)
Write a short conclusion. (1)
All groups developed hyperalgesia after the SNL surgery (reduced paw withdrawal threshold).
However MOR-intact groups maintained this hyperalgesia (central sensitisation).
MOR-lesioned groups could not maintain the hyperalgesia (no central sensitisation).
This suggests that the RVM, particularly the ON cells, is involved in the maintenance of central sensitisation.
Name two brain areas involved in pain where cholecystokinin immunoreactive nerve terminals are present. (2)
- Ventrolateral PAG
- Rostral ventral medulla
Fill the gaps relating to CCK and pain. (4)
CCK acting via the ………………. receptor acts as a ……………………. of opioid analgesia.
This means that CCK has the ………………….. effect compared to opioids, and it ………………………. neuronal firing.
CCK2
functional antagonist
opposite
activates
Describe the predicted effect of CCK2 antagonists on pain. (1)
We could assume that CCK2 antagonists should provide analgesia.
Name three regions of the nervous system where the actions of CCK on pain have been demonstrated. (3)
- Spinal cord
- Amygdala
- RVM
Which cell type in the RVM would you expect to respond to CCK injection? (1)
Explain your answer. (1)
ON cells
Because CCK is a functional antagonist of opioids and only ON cells contain opioid receptors.
An experiment compared firing rates of ON cells, OFF cells, and neutral cells in the RVM, both before and after CCK injection.
Describe the results. (3)
Only ON cells show a significant response to CCK injection.
Their firing rate increased post CCK.
The firing rate of all other cell types remained the same.
Does CCK facilitate or inhibit pain? (1)
Facilitate
Fill the gaps relating to CCK2 antagonists and pain. (3)
CCK2 antagonists are …………………… and ……………….. the actions of opioids at the MOR.
They have also been shown to reduce this particular problem relating to opioid analgesia: ………………………….
analgesic
potentiate
opioidergic tolerance
There have been attempts to give CCK2 antagonists along with a reduced dose of opioids to provide analgesia.
Give a potential benefit of this drug regime. (1)
Give a reason why this regime was not successful. (1)
- Avoiding opioid side effects
- However the CCK2 antagonists themselves caused side effects
Fill the gaps relating to CCK and pain. (2)
Overall, there is strong evidence for CCK acting in the ………………… axis, and that it contributes significantly to the development of ………………………………….
Hint: the last answer is a phrase
PAG-RVM
tonic enhanced spinal cord pain transmission
Are cannabinoids thought to facilitate or inhibit pain sensations? (1)
Inhibit
True or false? Explain your answer if necessary. (1)
Cannabinoids are thought to mediate their analgesic actions only in the spinal cord.
False - they mediate their analgesic actions at sites throughout the pain pathway
Name a brain region involved in pain where CB1 receptors are densely expressed. (1)
vlPAG
Fill the gaps relating to cannabinoids and pain. (3)
Microinjection of CB1 agonists into the ……………… or …………… produces …………………………..
PAG
RVM
dose-dependent analgesia
Describe the mechanism of action of CB1 agonists on neurones, describing their pre and post synaptic effects. (2)
- Negligible post-synaptic effects
- Mediate their actions presynaptically by blocking synaptic currents
CB1 agonists can be used to investigate the effects of cannabinoids on pain.
Suggest a CB1 agonist that could be used. (1)
Give its general effect on pain. (1)
WIN
antinociception
CB1 agonists can be used to investigate the effects of cannabinoids on pain.
WIN (a CB1 agonist) was injected into the RVM. Tail flick latencies were recorded and expressed as a percentage of the maximum possible effect.
Describe the effect that WIN had on tail flicks. (3)
WIN produced a the highest effect on tail flick latencies
so the tail flicks were slower
showing antinociception.
True or false? Explain your answer if necessary. (1)
Serotonergic neurones comprise about 0.5% of the total RVM population.
False - they comprise about 20% of the total RVM population
The cells in the RVM which contain serotonin all appear to be which type of cell? (1)
Neutral cells
Explain how the following conditions affect serotonergic neurones in the RVM. (3)
- Opioids
- Electrical stimulation of the PAG
- Electrical stimulation of the RVM
Opioids do not affect RVM serotonergic neurones
PAG stimulation does not affect RVM serotonergic neurones
RVM stimulation releases serotonin to the spinal cord
True or false? Explain your answer if necessary. (1)
Afferent input to serotonergic RVM neurones appears to be different to afferent input to ON and OFF cells.
True:
- All serotonergic RVM neurones appear to be neutral cells
- Serotonergic neurones do not respond to opioids or PAG stimulation
Fill the gaps relating to serotonin and pain. (4)
There is considerable evidence for serotonin playing an important role in nociception.
Electrically stimulating the ……………… releases serotonin in the …………………., and the ………………….. (analgesia/hyperalgesia) produced by this is blocked by …………………………………
RVM
spinal cord CSF
analgesia
intrathecal serotonin antagonists
Briefly describe the effect of serotonin on firing of dorsal horn neurones in response to noxious stimulation. (1)
Inhibits firing
Briefly describe a way in which serotonin can affect opioid-mediated analgesia. (1)
Systemic opioids mediated analgesia can be transiently reduced by depleting spinal serotonin levels.
Which brain area is the exclusive source of spinal serotonin? (1)
RVM
Fill the gaps relating to serotonin and pain. (2)
It has been proposed that the RVM ………………. releases serotonin into the dorsal horn, which modulates ………………………….
tonically
the actions of other descending pain modulatory systems
Give a reason why research into the role of serotonin in pain processing in the spinal cord is difficult. (1)
The multiplicity of receptors for endogenous serotonin.
