23: Sensory Phys And Pain Flashcards
What determines how much a fiber contributes to a compound AP?
Conduction velocity
Two types of skin (thick and thin)
Thick: glabrous
Thin: hairy
Receptor adaptation
When a stimulus persists unchanged for a period of time, the neural response diminishes and sensation is lost
Slowly vs rapidly adapting receptors
Slow: respond to prolonged and constant stimulus - you feel it the whole time
Rapid: respond only at the beginning or end of a stimulus
Receptive field
Area of innervation where individual mechanoreceptors fibers convey info from a limited area of skin
What does 2-point discrimination test?
Tactile acuity
Where is tactile acuity highest?
Fingertips, lips (small receptive fields)
What principle describes why phantom limb pain exists?
Law of Projection
Law of Projections
No matter where along the afferent pathway a stimulation is applied, the perceived sensation arises from the origin of sensation
Pain vs nociception
Pain: unpleasant sensory and emotional experience associated with actual or potential tissue damage
Nociception: neural process encoding noxious stimuli, without pain necessarily being implied
Consequences of nociception
Can be autonomic (ex: elevate BP) or behavioral (ex: motor withdrawal reflex)
Hypersensitivity
Increased responsiveness of nociceptive neurons to their normal input
Hyperaesthesia
Increased sensitivity to stimulation, excluding special senses
Hyperalgesia
Increased pain from a stimulus that normally provokes pain
Allodynia
Pain due to stimulus that does not normally provoke pain (ex: laying on sheets while sunburnt)
Myelination of C fibers vs A(delta) fibers
C fibers: unmyelinated
A(delta): myelinated
Three types of pain characterization/nociceptor modalities
- Mechanical
- Chemical
- Thermal
Two reasons we have referred pain
- Brain requires experience to localize pain - visceral pain is not typically experienced early in development to train the brain to localize it
- Afferents converge in dorsal horn
What does TRP stand for?
Transient receptor potential
What type of receptors are TRPs?
Ligand gated non-selective cation channels (for Ca, Na, K)
What happens when a TRP V1 is activated?
AP occurs + neuropeptides are released -> further signaling occurs -> vasodilation and immune cell recruitment -> inflammation
Two fiber types in nociceptors
C fibers and A(delta) fibers
What type of receptor is a nociceptor
Free nerve endings
Two types of free nerve endings
Peptidergic: expresses neuropeptides
Non-peptidergic: does not express neuropeptides
Two neuropeptides expressed by peptidergic free nerve endings
Substance P, CGRP
Where are peptidergic free nerve endings found vs non-peptidergic?
Peptidergic: most visceral afferents + 1/2 of cutaneous afferents
Non-peptidergic: few visceral + 1/2 of cutaneous afferents
Peptidergic free nerve endings are involved in what?
Chronic inflammation, visceral pain
What are non-peptidergic free nerve endings involved in?
Somatic chronic pain states (ex: diabetic neuropathy)
Neuropeptides released by C fibers and A(delta) fibers
A(delta): EAA
C fibers: EAA, SP/CGRP
Two ways nociception is modulated
- Local system: gate control theory
2. Descending inhibition: dampens on way up to cortex
Most powerful form of inhibitory control of all primary afferent fibers
Pre-synaptic inhibition
What happens in presynaptic inhibition?
- GABAergic associated influx of Cl into axon
- Hyperpolarization
- Less Ca enters cytosol
4 .less NT release
Six steps of descending inhibition
- PAG activated by opiates, EAA, and cannabinoids
- Descending projections -> locus ceruleus + raphe nucleus
- Serotonin and NE released into dorsal horn -> activate inhibitory interneurons
- Local inhibitory interneurons release opiates
- Opiates activate mu receptors on pre-synaptic terminals of C-fiber
- Reduction of SP from C-fiber, reduces nociception
Central vs peripheral sensitization
Central: post-injury hypersensitivity due to neuronal plasticity in the CNS
Peripheral: neuroplastic chnages in function, chemical profile, or structure of the PNS involving the receptors, ion-channels, and nt expression levels
Central inflammation
Pro-inflammatory signals from glial cells that contribute to central sensitization
What causes peripheral sensitization
Inflammatory soup due to injured tissues - sensitized nociceptors
What factors of sensitization play a role in chronic pain
Both peripheral and central sensitization
Insular cortex functions
- Interpreting nociception
- Processing info about internal state of body
- Autonomic response to pain
- Integrates all signals related to pain
What does damage of insular cortex cause?
Asymbolia
Asymbolia
Altered experience of pain
What is the amygdala important for with pain?
Emotional component of pain
