pain

Question 1

allodynia

Answer 1

sensitisation to normally innocuous stimulu

Question 2

specificity theory

Answer 2

pain is a distinct sensation

detected and transited by specific recpetors and pathways to distinct pain areas of the brain

Question 3

convergence theory

Answer 3

pain is an integrated plastic state represented by a pattern of convergent somatosensory activity within a distributed network (neuromatrix)

Question 4

A delta fibres

Answer 4

lightly myelinated Fast (relative) about 20 m/s
mechano-sensitive
mechano-thermal sensitive

Question 5

C fibres

Answer 5

unmyelinated SLOW about 2 m/s

polymodal: mechanical, thermal and chemical

Question 6

types of free nerve endings

Answer 6

Although mechanoreceptors, thermoreceptors, and nociceptors are all examples of free endings, nociceptors are the most common type.

Question 7

free nerve endings location and modality

Answer 7

location: widepsread in epithelia and connective tissues
modality: pain, heat, cold

Question 8

thermoreceptor response to pain

Answer 8

is specific
i.e. can find afferents whose activity correlates with pain perception

it can be clearly shown with heat response that thermo-receptor activation has already started before pain is perceived by nociceptor

Question 9

fast or ‘first’ pain

Answer 9

sharp and immediate

can be mimicked by direct stimulation of A delta fibre nociceptors

Question 10

slow or ‘second’ pain

Answer 10

more delayed, diffuse and longer lasting

mimicked by stimulation of C fibre nociceptors

Question 11

activation of which types of receptors never elicits pain sensation

Answer 11

stimulation of A alpha and A beta (proprioceptive and mechanoceptive) fibres

distinct set of A delta and C fibres specifically associated with pain detection

Question 12

molecular pain receptors activated in nociceptive A delta and C fibres at 45 oC

Answer 12

the capsaicin receptors (TRPV1)

the vanilliod, capsaicin is also active component in chillies

Question 13

TRP

Answer 13

Transient receptor potential proteins

Question 14

nociceptors can detect release of chemicals

Answer 14

capsaicin is thought to mimic endogenous vanilloids released by stressed tissues so nociceptors may also work by detecting release of chemicals from stressed cells

Question 15

sensory discrimation pathway of nocipetive info to the brain

Answer 15

signals location, intensity and type of stimulus

used the spinothamalmic tract

Question 16

affective-motivational pathwah of nociceptive info to the brain

Answer 16

singals unpleasantness and enables autonomic activation, flight or fight response

Question 17

spinothalamic tract//anterolateral system

Answer 17

major skin to thalamus sensory pathway

transmits signals from receptors ending via dorsal root ganglion cells via spinal cord, medulla and midbrain to thalamus

Question 18

MRIs can show representation of pain

Answer 18

activation of C fibres or A beta fibres shows same area of somatosensory cortex activation for both non painful mechanical stimuli and painful ones

but pain also activates a distinct response that includes other regions

e.g. insula and cingulate cortex

Question 19

activation of insula and cingulate cortex

Answer 19

connected to the limbic system and regulate emotional responses
so part of the affective-motivational response to pain

Question 20

affective motivational pathways

Answer 20

• shares some paths with spinthalamic pathway
• little or no topographic mapping (neutrons from parabrachial nucleus can respond to painful stimuli from anywhere on the body’s surface)
• number of points of input to the emotional (limbic) and homeostatic (hypothalamic) system
• strong correlation of painful experience (unpleasantness) with activation in cingulate cortex

Question 21

affective motivational pathway points

Answer 21

information from upper body
via spinal cord
caudal medulla
middle medulla-- reticular formation-- hypothalamus and limbic system
mid pons
parabrachial nucleus 
projections to the amygdala and hypothalamus 
thalamus 
insula cingulate cortex

Question 22

phenomena that don’t fit with the theories of specificity

Answer 22

pain perceived not always proportional to intensity of stimulus

modulation of pain by other stimulus (acupuncture)

perception of pain in severed limbs (phantom)

referral of pain from viscera to skin (heart attack)

placebo effect

Question 23

hyperalgesia (sensitisation)

Answer 23

increased response to a painful stimulus
e.g.hypersensitivity of damaged skin to a normally tolerable painful stimulus (e.g. light skin prick)

a result of lowered nociceptor thresholds which heightens pain response

Question 24

allodynia (sensitisation)

Answer 24

painful response to a normally innocuous stimulus

e.g. painful sensitivity of sunburnt skin to gentle mechanical stimulus (e.g. light brushing) or mild temp

Question 25

inflammatory response peripheral effects of tissue damage

Answer 25

tissue damage releases inflammatory substances which affect nerve function, recruit mast cells and neutrophils and increase local blood flow

e.g. bradykinin directly affects the function of nociceptive molecular receptors such as TRPV1

Question 26

pain relief

Answer 26

aspirin and ibuprofen act on cyclooxygenase (COX) an enzyme for prostaglandin biosynthesis

Question 27

dorsal horn

Answer 27

The posterior grey column (posterior cornu, dorsal horn, spinal dorsal horn posterior horn) of the spinal cord is one of the three grey columns of the spinal cord. It receives several types of sensory information from the body, including fine touch, proprioception, and vibration.

Question 28

release of prostaglandins from nociceptive dorsal horn neurons results in what

Answer 28

a lowering of the thresholds for action potential generation for neurones relaying nococpetive info – giving rise to hyperalgesia

another consequence is that these neutrons become sensitive to non nociceptive inputs- allodynia (non painful perceived as painful)
because release of prostaglandins in dorsal horn affect thermal and mechanic receptors (the non pain fibres)

Question 29

hyperpathia

Answer 29

a variant of hyperalgesia and allodynia (different underlying causes)

when there is axonal loss (centrally or peripherally) results in raising of the detection threshold (i.e. need greater stimulation to detect stimulus)
but when the detection is felt the pain is explosive!!

Question 30

central sensitisation in diseases

Answer 30

can occur when the central pathways themselves are damaged e.g. in diabetes, shingles, MS and after a stroke

the virus of shingles sits latent in nerves

Question 31

what is suggested by the phenomenon of phatom limb pain

Answer 31

indicates central representation of the body is not passive
it persists in the absence of peripheral input

children born without limbs still get phantoms suggesting the brain’s map of the body may be pre- formed

phatom limb pain is hard to control suggesting that the pain is also centrally represented

NOT fitting with specificity theory

Question 32

referred pain

Answer 32

pain due to damage in the viscera (gut) is often perceived as coming from specific locations in the skin according to what organ is affected e.g. heart attack- pain in left shoulder and arm

(not easily explained by specificity theory)

Question 33

why do we get referred pain

Answer 33

thought to reflect convergence of visceral afferent onto the asme pathways as cutaneous afferents in CNS

useful in aiding clinical diagnosis of organ dysfunction!

Question 34

central modulation of pain

Answer 34

mind of matter - when walking through fire

Henry Beecher - soldifers in WW2 with severe wounds often felt no or little pain

mere suggestion that pain will be controlled by pain relief creates ‘placebo effect’

Question 35

physiological basis of pain modulation

Answer 35

experiemnts stimlating regions of the midbrain produced pain relief

e.g. the periaquecductal grey activates brainstem nuclei that modulate activity of dorsal horn neutrons

in the dorsal horn, descending inputs activate enkephalin-relseaing interneurons which presynaptically inhibit nociceptive fibres

Question 36

enkephalins

Answer 36

members of a family of endogenous opioid peptides that also include endorphins and dynorphins

interneurons that release encephalon presynaptically inhibit nociceptive fibres

Question 37

local modulation of pain

Answer 37

rubbing an injury often reeves pain

thought to be due due local inhibit by mechanoreceptors (Adelta fibres) of conceptive (C fibres) inputs in the spinal cord

Melzack and Wall (1965)– proposed ‘gate’ or ‘sensory interaction’ theory of pain

suggested pain perception to be the result of integration of convergent sensory info

again challenges the assumptions of a straight through pain input of specificity theory

Question 38

we know that pain is not due to ramping up of normal thermo-receors in part because…?

Answer 38

thermoreceptor firing is saturated by the time pain is perceived in response to heat