Neuro: Somatosensation II Flashcards
What side of the body to the afferent fibres of the spinothalamic tract (anterolateral system) enter and then ascend up the spinal cord?
- They enter the spinal cord on the right and synapse with 2nd order neurons
- The 2nd order neurons cross to the left of the spinal cord and ascend up the left hand side through the anterolateral column
What side of the body to the afferent fibres of the dorsal column-medial lemniscous system enter the and then ascend up the spinal cord?
- Afferent fibres enter spinal cord on the left side of spinal cord and then ascend up the left side of the spinal cord through the dorsal column
Explain the dual aspect model of pain
- Dual aspect model of pain states that pain has a Sensory aspect to it and therefore has:
- Location
- Intensity
- Duration
- Quality
- Model also states that pain has an Affective aspect to it and this explains the unpleasantness of pain
- Affective aspect of pain also explains why pain affects our mood
A-delta and C fibres both involved in nociception (pain) but these fibres contribute to different aspects of pain, what aspects of pain does each fibre contribute to?
- A-delta fibres contribute to fast sharp first pain
- C-fibres contribute to slow dull second pain
Why do the A-delta and C-fibres contribute to different aspects of pain?
Because A-delta and C-fibres have different properties:
- A-delta fibres thinly myelinated and have medium conduction speed
- C-fibres are unmyelinated and slow conduction speed
Explain how eating a chilli leads to a burning sensation in the mouth?
- Capsaicin protein found in chillis is able to bind to TRPV1 receptor present on some thermosensitive A-delta and C fibres in the mouth
- This binding activates the TRPV1 receptor leading to influx of Ca2+ and Na+
- This influx leads to depolarisation which activates the A-delta and C-fibres
- This results in the burning sensation in the mouth when eating a chili
Pain is usually caused by tissue damage but pain signals can last long after tissue damage. Explain why this is the case?
- Once tissue damaging stimulus is removed, the inflammatory mediators released during the initial tissue injury still cause depolarisation/activation of A-delta and C-fibres
- Examples of these inflammatory mediators include Histamine and Prostaglandin
- Also, branches of peripheral axon of C-fibre can release substance P
- Substance P causes release of histamine (an inflammatory mediator) from neutrophils which results in further activation A-delta and C-fibres
The phenomenon of A-delta and C-fibres being activated long after pain stimulus is gone can account for both Hyperalgaesia and Allodynia. What are Hyperalgaesia and Allodynia?
- Hyperalgaesia - Hypersensitivity to pain
- Allodynia - When a non-painful stimulus causes pain
What is the name of the specific part of the spinal cord in which A-delta and C-fibres synapse with the 2nd order neurons?
Dorsal horn (Grey matter)
What is the specific name of the fibres that A-delta and C-fibres synapse with in the dorsal horn of the spinal cord?
Dorsal horn interneurons
Explain how the ability of the dorsal root interneurons to receive input from multiple nociceptive afferents originating in different locations of the body leads to referred pain
- A-delta and C-fibres synapse with dorsal root interneurons in the dorsal root
- However, there are also other visceral afferents that synapse with the same dorsal root interneurons
- This means that if a particular visceral afferent activates a dorsal root interneuron the brain perceives the pain as coming from where the A-delta and C-fibres that synapse with that same activated dorsal root interneuron originate from within the body
- This results in referred pain
Both afferent fibres of the dorsal column-medial lemniscal system and the spinothalamic tract (anterolateral system) synapse with thalamic neurons in the ventral posterior nuclear complex in the thalamus. Do the afferent fibres from each system converge withe same thalamic neurons?
No the afferent fibres of the DCML and the STT synapse with different thalamic neurons in the ventral posterior nuclear complex
What does the fact that stimulation of the primary somatosensory cortex leads to tactile sensation and not pain mean for pain sensation?
It means that there are other areas within the brain that contribute to pain sensation other than the primary somatosensory cortex
The spinolthalamic tract (antereolateral system) diverges at the ventral posterior nuclear complex in the thalamus. What are the names of the divisions of the spinothalamic tract?
- Lateral system
- Medial system
What brain areas do the lateral system of the spinothalamic tract go to?
- Lateral system - Afferent fibres of STT go to and synapse with the ventral posterior (VP) nuclei of the thalamus
- VP nuclei axons then project to primary and secondary somatosensory cortex
What brain areas do the medial system of the spinothalamic tract go to?
- Afferent fibres of STT go to and synapse with midline nuclei of the thalamus
- Midline nuclei axons project to anterior cingulate and insular cortex
Explain the mechanism for how endogenous opioids such as enkephalins or endorphins are able to modulate pain
- In the dorsal horn of the spinal cord, at the point where Nociceptive C fibres or A-delta fibres synapse with the dorsal root interneurons, There are also descending fibres.
- These descending fibres are able to activate local interneurons which sit between the descending fibre and the nocicepitive C or A-delta fibre
- Activation of these interneurons leads to the release of endogenous opioids from it which are then able to bind to opiate receptors on the pre-synaptic terminal of the A-delta or C-fibre
- This binding of the endogenous opioid to the opiate receptor reduces synaptic transmission from the A-delta or C-fibre leading to the dorsal horn interneuron not being activated
- This means pain sensation isn’t transmitted to higher brain areas
