Pain Transmission Flashcards
Nociceptive pain can be caused by two types of pain. What are these and describe them.
Visceral and Somatic.
Visceral - internal pain e.g. stomach cramps, chronic pain, can be pathological.
Somatic - acute pain, like stabbing/throbbing pain, protective mechanism.
What is analgesia?
Absence/reduction of pain.
What is hyperalgesia?
Increased sensitivity to painful stimuli.
What is allodynia?
Pain in response to no apparent stimuli - can be linked to environmental, psychological or genetic factors.
What is neuralgia?
Neuronal pain without stimulation of nociceptors caused by ion gate malfuntions.
What is nostalgia?
Psychological pain where individual longs for happiness from past.
Describe A-type axons.
These are myelinated axons. They have faster transmission speeds and responsible for initial sharp pain perceived at time of injury.
Describe C-type axons.
These are unmyelinated axons. Have slower transmission speeds. Responsible for longer lasting dull pain.
What are the main pain transmitting neurones and what are their key physiological differences?
Neurones typically fall into 3 primary categories named A, B and C where the the differences relate to their physical features and signal conduction properties.
How does diameter of axon relate to transmission speed?
Large dimeter = faster transmission/ faster electrical impulse.
What class of axons are pain transporting neurones?
A-delta and C-fibres = both activated by nociceptors.
What are afferent neurons?
Neurons/impulses which travel towards the CNS = a sensory neuron.
What are nociceptors?
Class of receptors that stimulate pain fibres.
What are efferent neurons?
Neurons/impulses which travel away from the CNS = motor neuron.
Where is the cell body located?
Located in a specialised region just outside of the spinal cord called dorsal root ganglia.
What is the effect of having long and myelinated neurons?
Increases the transmission propagation speed along the axons and into the central nervous system.
What fibres are classed as A-alpha?
Efferent and Afferent fibres.
- they have the largest diameters so are the faster transporter of APs
What are the two pain transporting neurons?
A-delta and C-fibres = activated by nociceptors.
Where are nociceptors located?
Found at free nerve endings of the afferent neurons.
How are nociceptors activated?
Activated by noxious stimuli and have a high activation threshold.
Why is it important that nociceptors have a high activation threshold?
To prevent nociceptors from firing all the time.
What does polymodal mean in relation to nociceptors?
Means that nociceptors can be activated by many stimuli.
Describe the role of the spinal cord with regards to transmission of signals.
Transiting from the peripheral nervous system to the central nervous system, the spinal cord is the site of transmission towards the brain.
Pain signals ae transmitted along the spinothalamic tract by afferent neurons. How does Ad and C fibres travel?
A-delta fibres are transmitted along the neospinothalamic tract and the C-fibres are tranmistted along the palaeospinothalamic tract.
The site of first synapse within the brain is in the Thalamus. What is the thalamus?
Thalamus acts as a relay centre in the brain where the neurons propagate out into the distal regions of the cortex and the frontal cortex where pain is perceived and a response to the painful stimuli occurs.
What are the other regions in the brain which related to pain percption?
The cortex and limbic system.
What is the white matter called?
Substantia alba
What is the grey matter called?
Substantia grisea
What is the black matter called?
Substantia nigra
Describe white matter.
Substantia alba is myelinated - has increased nerve transmission propagation speed. This region is found on the outside of spinal cord
Describe grey matter.
Substantia grisea is unmyelinated. It is found in the centre (inner part) of the spinal cord and the outer part of the cerebrum. It contains the neuronal cell bodies and capillaries.
Describe black matter.
Called black matter as dopamine signaling neurons which makeup a small component of the brain is located in the substantia nigra. These dopamine signaling neurons secrete melanin which gives skin pigment.
Where is white and grey matter located in the brain?
Grey matter found in the peripheral regions (outer) and white matter at the centre (interior) of the brain.
- myelinated material transmits the neurons towards the synapse endings found on the exterior region of the brain.
What is the dorsal root ganglion?
This is where the cell bodies of the afferent neurons are found.
Where do the afferent nerve fibres synapse when entering the spinal cord?
Synapse within the dorsal horn of the grey matter.
What are the regions called in the grey matter? And how many are there?
They are called laminae.
There are 6 laminae within the dorsal horn and 4 within the ventral regions.
Which laminae do the A-delta fibres synapse onto?
Laminaes 1, 2 or 5
Which laminae do the C fibres synapse onto?
Laminaes 1 or 2
What is laminaes does the substantia gelatinosa contain?
Contains both 1 and 2
- synapsing sites for A-delta and C fibres.
What is the importance of the substantia gelatinosa containing laminae 1 and 2?
If you want to inhibit neuronal transmission in laminae 1 or 2, substantia gelatinosa can be inhibited. This can stop pain propagation along the neuron, up the CNS and into brain - block pain perception.
What neurotransmitter substance does Ad and C fibres release when synapsing onto the substantia gelatinosa?
Ad releases glutamate.
C releases substance P (neuropeptide)
What does the white mater consist of?
Ascending and descending neural tracts.
Pain transmitted through spinothalamic tracts.
Ad travel contralaterally through the lateral neospinothalamic tract.
C travel mostly contralaterally through the anterior palaeospinothalamic tract.
What does decussates mean?
Term to describe the fibres crossing over the spinal cord.
What contralateral mean?
Means that the fibres cross over the spinal cord - all fibres cross through the anterior white commissure to the opposite side of the grey matter.
What does ‘mostly contralateral’ mean in terms of C fibres?
Most C fibres decussate but a few will ascend towards the brain on the same side of the spinal cord = ipsilateral.
Describe the functions of the thalamus.
It relays information between the cerebral cortex and sub-cortical centres.
All sensory info passes through the cortex apart from olfactory.
Regulates sleep.
What does subcortical mean?
Means all regions below the brain.
What is the main sub centre?
Hypothalamus.
Sub centres of the brain include hypothalamus. What roles does it have?
Hormonal regulation (regulated from the pituitary) Fluid retention and core body temperature control
Describe double pain sensation.
A single stimuli is capable of generating two sequential pain sensations.
1st pain sensation is rapid, carried by the Ad fibres and the 2nd pan sensation is slow carried by the C fibres.
What does the inhibition of the substantia gelatinosa cause?
Causes an inhibition of signal transduction which results in analgesia.
What is the result of nociceptive signal transduction?
Pain.
Describe A-Beta mechanoreceptor fibres.
These have a lower threshold/sensitivity level than pain neurons (Ad/C fibres). AB also synapse in the dorsal horn like Ad/C but are myelinated more hence increased transmission speed.
Mechanoreceptors synapse in the midbrain whereas Ad/C fibres synapse in the thalamus which propagates towards cortex to perceive pain.
What happens when both sets of neurons, A-Beta and Ad/C fibres are stimulated?
The faster signals of A-Beta will inhibit the transmission of the pain signals along the Ad/C fibres.
Describe inhibitory interneuron.
Inhibitory interneuron is normally in its active state which prevents the propagation of pain along these fibres.
If you stimulate large myelinated fibres, that keeps the inhibitory interneuron active and the inhibitory interneurons prevent propagation along the CNS.
The small nerve fibres = Ad and C fibres. If you don’t stimulate the large fibres, you are helping to switch off the inhibitory interneuron effect.
Describe the periaqueductal grey (PAG).
It is part of the mesencephalon (midbrain). It has a key role in the propagation and modulation of pain and sympathetic responses. It is also involved in descending signals.
How does descending inhibitory circuits control pain transmission?
- PAG is activated by opioids.
- This then activated nucleus raphe magnus (NRM).
- The input descends via neuronal fibres which terminate in the substantia gelatinosa (SG).
- This pathway has a negative effect on the transmission of pain -> analgesia.
What effect do opioids have on PAG, NRM and SG?
Opioids have a stimulatory effect on NRM and PAG and an inhibitory effect on the SG as they inhibit the release of NT substance.
-Both pathways have the same ultimate effect = inhibition of signal transduction to the brain = analgesia.
