CSA 2 Flashcards
What is pain?
Unpleasant sensory experience associated with tissue damage, accompanied with an emotional response.
Why do we feel pain?
Warning sign– avoid harmful situations, prevent further injury or death, signal to rest
What are the possible categories of sensations of pain?
Sharp stab, throbbing, burning, deep ache, freezing, Itch
What are the 3 classifications of pain?
Nociceptive (normal functioning of nociceptors), Inflammatory (pain in response to inflammation), Neuropathic (in response to injury to nervous system)
What are nociceptors?
Primary sensory neurons that detect pain.
What are the classifications of afferent nerve fibres? Describe nerve structure, use, and conduction speed.
A-alpha and A-beta (myelinated, large diameter, light touch, proprioception 30-75 m/s), A-delta (thinly myelinated, medium diameter, light touch, temperature, nociception, 5-30 m/s), and C fibres (unmyelinated, small diameter, temperature, nociception 0.5-2 m/s).
What is special about nociceptors in the periphery?
They have free nerve endings.
Name 4 types of specialised A-Beta fibres and uses.
Meissner’s (stroke/fluttering), Pacinian (vibration), Merkel discs (pressure), Ruffini (Stretch)
What kind of pain do c fibres produce? Describe pain transduction.
Slow dull ache, burning pain, poorly localised
What kind of pain do Adelta fibres produce? Describe pain transduction.
sharp pricking pain, well localised, acitvation of reflex arcs
What is the significance of having polymodal nociceptors?
Pain can result from pressure, temperature, and chemical responses, distinguished in CNS
Discuss Pressure Transduction Mechanism
Not yet identified in eukaryotic cells, mechanosensitive channels (Potentially acid sensing channels, or transient receptor potential TRP channels)
Discuss Temperature Transduction Mechanism
via TRP channels, TRPV1 (transience receptor potential vannilloid 1) used for Hot or capsacin. TRM for cold/menthol. TRPA1 for very cold or cinammon.
Describe pain pathway among first order neurons
Spinothalamic tract: Enter dorsal horn, form tract of lissauer, synapse in substantia gelatinosa, glutamate and substance p excites second order neurons
Describe pain pathway among second order neurons
Cross in dorsal horn at each level, ascend in anterolateral column to thalamus, synapse on third order neuron
What causes referred pain?
Convergence of visceral and cutaneous nociceptors on same second order neurons in spinal cord. Brain perceives this as cutaneous pain (eg angina perceived in upper chest wall and left arm)
Describe pain pathway among third order neurons
Ascend to primary somatosensory cortex (lower body to medial cortex, upper body to lateral cortex). Project to insula and cingulate cortex to encode emotional components of pain.
Why do some battle victims report no pain?
Stress-induced analgesia, activated by higher cortical pathways
What are the two important areas of descending regulation of pain? Describe path and purpose.
Periaqueductal gray matter (PAG) and Raphe Nucleus Magnus (in rostral ventromedial medulla). PAG to Raphe Nucleus to dorsal horn, modulates activity of spinothalamic tract.
How can the Raphe nucleus inhibit the spinothalamic tract?
Serotonergic projections excite inhibitory interneurons, which inhibit second order spinothalamic neurons
How do opioids inhibit pain?
Act on inhibitory metabotropic receptors.
Where are opioids released?
From interneurons, multiple sites: midbrain (PAG), medulla (raphe nucleus), dorsal horn
What activates nociceptors?
ATP (purinergic receptors P2X), H+ (acid sensing ion channels), Serotonin (5HT3 receptors)
What does the activation of one branch of a nociceptor axon trigger?
Release substance p and CGRP from another, causing vasodilation, increased permeability, activation of mast cells (releasing histamine = more inflammation)
What impact does inflammation have on pain?
Pain hypersensitivy, feels more painful!
Name the two types of pain hypersensitivity
allodynia- non-noxious stimuli produce pain, hyperalgesia- noxious stimuli produce exaggerated response
What are the mechanisms of pain hypersensitivity?
peripheral sensitization in primary hyperalgesia, and central sensitization in allodynia
What makes peripheral ends of nociceptors more sensitive?
Bradykinin and NGF reduce threshold of heat activated channels (bind to TRPV1 receptor, activates protein kinase which phosphorylates TRPV1), prostaglandin reduces threshold of sodium channels.
How is pain categorised by time?
Less than 3 months acute, more is chronic
Name one type of local anaesthetic and describe its mechanism of action
Lidocaine, lignocaine, topically applied, sodium channel blocker (prevent nociceptor firing)
What are NSAIDs? Give two examples explain and how they work.
Non-steroidal ant-inflammatory drugs (aspirin, ibuprofen), reduce inflammatory response by inhibiting prostaglandin synthesis, preventing peripheral sensitization. cyclooxeganase (COX) inhibited= prostaglandind synthesis reduced = prevented decrease in NA+ channel threshold
How does paracetamol work?
NOT NSAID. Acts centrally: inhibits COX enzymes in CNS, acts on descending serotonergic pathways, exact mechanism unknown
Why is capsaicin cream useful?
TRPV1 channel agonist, repeated application causes peripheral terminals to die back (calcium overload = mitochondrial dysfunction)
What is the idea of modulating pain at the level of the spinal cord called? Describe how this might work.
Gate control theory: pain evoked by nociceptors can be reduced by simultaneous activation of low threshold mechanoreceptors (A-beta fibres). eg rubbing/blowing on painful area to reduce pain sensation
What does stimulating A-Beta fibres do to activated c fibres?
activates interneurons in dorsal horn, which inhibit spinothalamic neurons (including c fibres)
Describe the mechanism of action of opioids.
Agonists of the endogenous opioid system. Sites of action in brain stem (disinhibition), spinal cord, and periphery (inhibit channels on nociceptors)
Name 6 mechanisms that could cause neuropathic pain.
compression, traction, sever, hypoxia, demyelination, tumour
How much of the population suffers from neuropathic pain?
~8%
Describe a neuroma and its implication.
At injury site, a neuronal axon will grow many small sprouts, meaning many more ion channels, leading to a more excitable axon (can lead to phantom limb pain)
What are the two central mechanisms of neuropathic pain?
central sensitization and cortical remapping
What are the underlying mechanisms of central sensitization?
reduced threshold for activation of 2nd order neurons, synaptic reorganisation, reduced inhibition
What are the steps leading to reduced threshold for activation of the CNS?
long-term potentiation: constant firing of axons from periphery following injury, sustained release of glutamate, prolonged depolarisation of post synaptic membrane, massive influx of Ca2+, activation of kinases, phosphorylation of NMDA/AMPA receptors, phenotype change leading to more ion channel production
What happens after central sensitization?
A-Beta fibres synapse onto 2nd order spinothalamic neurons, normally non-functional but now activated, leading to allodynia
What happens in synaptic reorganisation?
A-Beta fibres form new sprouts that synapse onto spinothalamic tract neurons, causing allodynia
What is important to know about treating chronic pain patients.
Acute tx does not work– need individual plans, especially management of primary condition and other associated symptoms. 80% are depressed. Sleep disturbances, fatigue.
What drugs help with chronic pain?
Tricyclic antidepressants, anticonvulsants, NMDA antagonists,
What non-drug treatments help with chronic pain?
Physiotherapy, psychological therapies, surgery
Name a tricyclic antidepressant and its MOA
Amitryptyline, unclear MOA, act on descending inhibitory pathways, inhibits reuptake of serotonin (SSRI activity)
Name two anticonvulsants and their MOA
Gabapentin (and pregabalin) and carbamazepine. MOA unclear, act in spinal cord to reduce excitability, blocks calcium (gabapentin) and sodium (carbomazepine) channels. Gabapentin does NOT work on GABAergic interneurons, blocks presynaptic voltage-gated Ca2+ channels, prevents release of glutamate from nociceptors
Name one NMDA antagonist and describe mechanism of action.
Ketamine, NMDA receptor antagonist reduces glutamate influx, prevents depolarization of second order neuron
What are the first, second and third line tx for neuropathic pain?
Amitriptyline or pregabalin, then switch or combine, then refer to pain specialist and consider oral opioid or topical lidocaine.
What non-drug treatments are there for neuropathic pain?
placebo, acupuncture, massage therapy, homeopathy, herbal medicine, hypnosis
What are the anatomical features of the basal ganglia?
Gray matter nuclei deep within the brain, most superior is caudate nucleus split into head and tail (cat’s tail), lateral and inferior to caudate is putamen, lateral and deep to putamen is globus pallidus, thalamus is posterior (technically separate but has key interactions w basal ganglia).
What are the two structures related to the basal ganglia located underneath the thalamus?
subthalamic nucleus and substantia nigra
what is the pallidum?
internal and external global pallidus
What is the putamen and globus pallidus called?
lentiform nucleus
What is the caudate + putamen + pallidum?
Corpus striatum, white matter of ascending and descending tracts, divided by internal capsule
what is the caudate + putamen?
neostriatum (caudate = dorsal striatum)
What is one of the roles of the basal ganglia?
interface with cortex to filter movement options
What are 3 other pathways of the basal ganglia besides motor coordination?
oculomotor loop: coordinates eye movement, prefrontal loop: executive functions eg working memory, limbic loop: reward processing/behaviour, related to addiction
Describe the direct pathway of the basal ganglia.
Cortex to Striatum to Globus Pallidus Internal to Thalamus back to cortex. Double inhibition link, GPi inhibits thalamus but can be inhibited by striatum. Allows movement.
Describe the indirect pathway of the basal ganglia.
Prevents movement. Cortex to striatum to Globus Pallidus External to Subthalamic nucleus to Globus Pallidus Internal to thalamus back to cortex. Double inhibition before and after GPe (inhibits STN but inhibited by striatum) .
Describe the hyperdirect pathway of the basal ganglia.
Emergency stop. Cortex to subthalamic nucleus to globus pallidus internal to thalamus back to cortex.
How does striatum know if a motor plan is to be promoted or prevented?
2 populations of striatum, Direct D1 and Indirect D2. D1 receptor activates by dopamine, D2 dampened down by dopamine release. Dopamine promotes movement.
what releases dopamine?
substantia nigra
How is dopamine activity/release to basal ganglia opposed?
Acetylcholine opposes dopamine, dampens D1 direct pathway, activates indirect D2 pathway. Ach prevents movement.
What is characterised by a high amplitude unilateral flailing of the limbs? Describe the pathway and cause.
Ballismus, hemiballismus if one half the body. Defect in subthalamic nucleus in indirect pathway, leading to greater excitation in thalamus, caused by stroke.