Somatosensation II Flashcards
What are the two major pathways of the somatosensory system?
→ Dorsal column
→ spinothalamic tract
What does the dorsal column receive inputs from?
→ Large diameter myelinated afferents : tactile
What does the spinothalamic tract receive inputs from?
→ small diameter
→ thinly myelinated
→ coarse touch, nociception and temperature sensitivity
What happens if there is a cut on one half of the spinal cord?
→ You interrupt all the ascending axons on one side
What are the two aspects of pain?
→ Sensory discriminative
→ Affective motivational
What are nociceptors?
→ Neurons specialized for the detection of painful stimuli
What are the 3 types of nociceptors?
→ Mechanical
→ Chemosensory
→ Thermal
When does temperature become painful?
→ 45 degrees or higher
What is the difference between non nociceptive and nociceptive thermal receptors?
→ non nociceptive plateau above a certain range
→ nociceptors don’t response until the temperature approaches the range that is deemed painful
What is the TRPV1 receptor involved in?
→ Transduction of noxious heat
What kind of a receptor is TRPV1?
→ Ion channel that allows both Na+ and Ca2+ to pass
What can activate TRPV1 and why?
→ Capsaicin
→ it is lipid soluble
What does capsaicin binding to TRPV1 cause?
→ Activation of thermal nociceptor
What is the conduction of the A delta fibres like?
→ Slowest of the myelinated fibres
→ Conduct moderately compared to C fibres
How do C fibres conduct?
→ Unmyelinated
→ Slowly
What pain does the A delta fibre mediate?
→ Mediates the sharp first pain
What pain does the C fibre mediate?
→ second dull burning pain
What blocks the smallest fibres?
→ low dose anaesthetic
What blocks larger axons?
→ Pressure
What maintains the sensitivity of the C fibres after the original stimulus?
→ Prostaglandins
→ Cytokines
→ Signaling molecules
Why is there inflammation?
→ Tissue damage
What is hyperalgesia?
→ Increased sensitivity to pain
What is allodynia?
→ Something painless like gentle touch will be painful
What do peripheral neurons release?
→ Substance P which reinforces the release of inflammatory molecules
Describe the pathway from an injured right hand to the somatosensory cortex
→ Cells are damaged and release PGE
→ Sensory nerve fibres send the impulse to the dorsal horn (1ST ORDER NEURONS)
→ The 1st order neurons synapse with 2nd order neurons in the dorsal horn
→ The 2nd order neuron crosses over contralaterally and enters the spinothalamic tract
→ they eventually come to lie alongside the medial lemniscus
→ 2nd order neuron continues moving up to the thalamus where it synapses with a 3rd order neuron
→ 3rd order neuron sends impulse to somatosensory cortex
Where do C fibres terminate?
→ Exclusively in superficial layers
Where do A delta fibres terminate?
→ in both superficial and deep
Why is angina not felt in the heart?
→ Afferents from cardiac muscle are converging on dorsal horn interneurons on the cervical segments
What are the pathways of the spinothalamic and the dorsal column like in relation to each other?
→ Parallel
What is necessary for the localisation of pain?
→ primary somatosensory cortex
Where do the lateral and the medial systems of the anterolateral system diverge?
→Diverge at the level of the thalamus
Where is the cingulate gyrus?
→ Just above the corpus callosum
Where do the lateral system 2nd order neurons project to?
→ ventral posterior nucleus of the thalamus
→ primary and secondary somatosensory cortex
Where do the medial system 2nd order neurons project to?
→ Midline nuclei of the thalamus
→ Anterior cingulate and insular cortex
What type of pain does the lateral system mediate?
→ Sensory discriminative
What type of pain does the medial system mediate?
→ Affective motivational
What are the effects of a lobotomy?
→ Behaviour is more docile
→ loss of sensitivity to pain
What procedure is done for intractable pain?
→ Anterior cingulotomy
→Unpleasantness/motivational can be dissociated
What are 2 endogenous opioids?
→ Enkephalins
→ Endorphins
Describe how the descending pathways modulate pain
→ 5-HT neurons project to the place where the 1st order neurons synapse with 2nd order neurons
→ they release NA and 5-HT which inhibits the release of substance P from the 1st order neuron
→ They stimulate interneurons
→ Interneurons inhibit the 2nd order neurons and release enkephalins
Why does phantom limb pain occur?
→ The somatosensory part of the cortex that corresponds to the limb is still there
What is chronic pain defined as?
→ >3 months
What is neuropathic pain attributable to?
→ Nerve damage
Compare the lateral and medial pain systems of the anterior lateral system
→ Lateral: Sensory-discriminative
Project via specific somatosensory thalamic nuclei
→medial: Affective-motivational
Project to different cortical areas via (non-specific) midline thalamic nuclei
How is the periaqueductal grey involved in descending pain modulation?
→Enkephalins are found in midbrain, in the periaqueductal.
→Sends descending projections to structures associated with neuromodulator cells like noradrenaline and serotonin.
→Theses projects to dorsal horn of the spinal cord where A-beta, delta and C fibres
→Stimulation of the midbrain periaqueductal grey induce profound analgesia.
→Activates Raphe neurones which are serotoninergic.
→They project down to spinal cord and activate local spinal cord interneuron(opioid). →They pre-synaptically inhibit incoming C-fibres by binding to opioid receptors on the C-fibre.
How does the STT and DCML run?
→in parallel
What is S1 necessary for?
localization of pain, but stimulation of S1 gives rise to referred tactile, not painful, sensations
Describe the dorsal horn interneurons
→Located in superficial and deep layers of dorsal horn
→Synaptic input from C- and A-δ fibres
→Axons cross and ascend in anterolateral white matter
What happens when a person first touches a really hot radiator.
→ sharp increase in A-delta fibre action potentials
→increase in C-fibre firing
→ upon continuous exposure, A-delta doesn’t fire but c-fibre fires.
→then A-delta fires but C-fibre stops firing
