Unit 2 PNS and Pain Flashcards
Describe somatic sensation
- function: provides awareness of body sensations (touch, temp, limb position, pain)
- composition: sensory receptors consist of discrete nerve endings in the skin and other body tissues
Described the sensory afferent system in the PNS.
- somatic (body sense): sensation from body surface and proprioception
- visceral afferent: incoming pathway for info from internal viscera (originates in body cavities)
- special senses: vision, hearing, taste, smell
Describe how sensory information is coded.
Three levels:
1) Type or stimulus modality is distinguished by:
i. type of receptor
ii. specific pathway
iii. area of cerebral cortex
2) Location of stimulus
i. location of activated receptive field
ii. pathway of transmitted information
iii. area of somatosensory cortex responsible for that location
3) Intensity or stimulus strength: frequency of action potentials and the number of receptors activated
What are the type of somatosensory receptors?
- photoreceptors: respond to visible wavelengths of light
- mechanoreceptors: sense mechanical energy
- thermoreceptors: hot and cold
- asinoreceptors: changes in concentrations of solutes in body fluid and changes in osmotic activity
- chemoreceptors: specific chemical and smells, taste and oxygen or CO2 levels in blood, chemical concentrations in digestive tract
- nociceptors: sense pain, tissue damage, or distortion of tissue
Describe the aspects of a stimuli that affect its modality (somatosensory pathways).
- lateral spinothalamic tract: ends at the post-central gyrus, pain/temp.
- anterior spinothalamic tract: crude touch and pressure
- posterior column tract: fine touch (discriminative), pressure, vibration, body position, equilibrium
http://upload.wikimedia.org/wikipedia/commons/0/09/Spinal_cord_tracts_-_English.png
How does location of stimulus impact sense?
- by the receptive field or pathway (dermatomes)
- receptive field size (smaller field = greater discrimination ability)
Describe how stimulus strength impacts sensation.
By frequency of action potential (see p. 1178-1184….Honey don’t be lazy, look at the text)
Describe pain
- unpleasant sensory and emotional experience associated with actual or potential tissue damage
- subjective
- protective mechanism
- unpleasant sensory or emotional experience
What are the categories of pain receptors?
- mechanical nociceptors: cutting, crushing, pinching
- thermal nociceptors: extreme temp.
- polymodal: respond equally to all kinds of damaging stimuli including irritating chemicals from injured tissues
What are the four theories that explain the neurophysiology of pain?
1) Specificity theory: separate sensory modalities evoked by activity of specific receptors
- amount of pain related to amount of tissue damage
- accounts for many types of injuries but does not explain psychological contribution
2) Pattern theory: pain receptors share endings/pathways with other sensory modalities
- but different patterns of activity in the same neurons can be used to signal painful or nonpainful stimuli
3) Gate control theory: proposed by Melzack/Wall 1965
- pain transmitted by large A and small C fibers
- terminate in substantia gelatinosa (gate)
- stimulation of larger fibers (A-beta) close the gate
- stimulation of small fibers opens the gate
4) Neuromatrix theory (recent)
- brain has widely distributed neural network
- somatosensory, limbic, thalamocortical comp.
- multiple input sources
Describe how somatosensory information is processed centrally.
- perception involves awareness of:
i) stimuli (type)
ii) localization
iii) discrimination of characteristics
Describe the spinal cord and brain centers that transmit and receive pain stimuli.
Two tracts in the spinal cord:
i) Neospinothalamic: fast conducting fibers interconnect with somatosensory cortex, pain precision and discmination
ii) Paleospinothalamic: slow conducting fibers (multi-synaptic), connections with larger areas of limbic cortex, association with hurtfulness, mood altering effect of pain
Describe the characteristics of fast and slow pain.
i) FAST pain: occurs on stimulation of mech/therm nociceptors, carried by small mylinated A-delta fibers, produces sharp prickling sensation, easily localized, occurs first
ii) SLOW pain: occurs at stimulation of polymodal nociceptors, carried by small unmyleinated C fibers, produces dull aching burning sensations, poorly localized, occurs 2nd (persists for longer time; more unpleasant)
Describe the neurotransmitter associated with pain sensation.
1) Two best known:
- substance P: activates ascending path that transmits nociceptive signals to high levels for further processing
- glutamate: major excitatory neurotransmitted
2) Brain has built in analgesic system
- supressed transmitted in pathways as they enter the spinal cord
- depends on presence of opiate receptors (endogenous opiates = endorphins, enkephalins, dynorphin
How is pain neuromodulated?
- advanced gate control theory
- specific descending pathways
- discovery of opioid receptors in brain
