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somatosensory pathways Flashcards

1
Q

what is somatosensory function

A

the ability to interpret bodily sensations - mechanical, thermal, proprioceptive, nociceptive (encoding of noxious stimuli, not necessarily painful)

gives lots of info about touch - pressure, where, speed, of it is a threat, know when stimulus ends

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2
Q

what does the somatosensory system compose of

A

sensory receptors in skin, tissues and joints

nerve cells and tracts in the spinal cord

brain centres that process and modulate sensory info

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3
Q

list the somatosensory modalities *

A

touch

thermoception

nociception

proprioception

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4
Q

define the different somatosensory modalities *

A

touch - sense light mechanical stim, pressure and vibration

thermoception - temperature

nociception - noxious/potential damaging stimulation - much more susceptible to contextual influences than other modalities

proprioception - mechanical displacement of muscles or joints

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5
Q

what sensory modality uses free nerve endings *

A

thermoreeptors and nociceptors

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6
Q

what sensory modality uses enclosed nerve endings *

A

mechanoreceptors

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7
Q

how do receptors differ within a sensory modality *

A

structure

location

size of receptor feild

the tactile info that they encode for

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8
Q

describe the classification of sensory neurons *

A

AB fibres - large and fast, because big diameter and high myelin - for innocuous mechanical stimulation

Adelta - moderate speed, smaller but still myelinated - noxious mechanical and thermal stimulation

(therefore info about touch reaches cortex before info about pain)

C fibres - no myelin = slow - noxious mechanical, thermal and chemical stimulation

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9
Q

define the term receptor *

A

sensory receptors are transducers that convert energy from the environment into neuronal action potentials

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10
Q

describe thermoreceptors *

A

Ad and C fibres

free nerve endings

senstive to temperature because of membrane mechanisms, they can percieve small differences in temperature

temperature sensation is not uniformly distributed - some areas sensitive to cold and some to warm - depends on the type of ion channel:

TRPV1-4 - activated by heat 43degrees (temp that is painful)

TRPM8 and TRPA1 - aactivated by 15degrees which is painful cold

30degrees is neutral

cold is relayed by Ad, heat by C fibres

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11
Q

describe the effects of capsaicin *

A

bind to TRPV1 for a long time - cause painful heat response

because bonded for a long time, it decreases the ability of other stimuli to cause painful response

therefore used as a topical analegic

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12
Q

4 different types of mechanoreceptor and what they relay *

A

meissner’s corpuscle - fine discriminitive touch, low freq vibration

Merkel cells - light touch and superficial pressure

pacinian corpusle - detects deep pressure, high freq vibration and tickling

Ruffini endings - continuous pressure or touch and stretch

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13
Q

how can you clinically induce stretch

A

use monofilaments that deform the tissue underneath

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14
Q

what is the clinical benefit of testing mechanoreceptors

A

see the somatosensory function from receptor to interpretation `

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15
Q

define stimulus threshold *

A

a threshold is the point of intensity at which the person can detect the presence of a stimulus 50% of the time (absolute threshold)

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16
Q

define stimulus intensity *

A

encoded by how quickly the neuron fires

increased stimulus strength and duration = increased NT release = increased intensity of response to stimulus

usually firing freq is related to Log(stim intensity) ie stim increase 10 fold mean firing frequency doubles

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17
Q

describe adaption in relation to tonic receptors *

A

they detect a continuous stimulus strength

they continue to transmit impulses to the brain as long as the stimulus is present - keeping brain informed of the status of the body

Merkel cells - slowly adapt allowinhg for superficial pressure and fine touch to be percieved, burst at the start and end of the stimulus

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18
Q

describe adaption in terms of phasic receptors *

A

they detect a change in stimulus strength

transmit an impulse at the start and end of stimulus - when a change is taking place

pacinian receptor - sudden pressure excites the receptor, transmits a signal again when the signal is released

they adapt quickly to a constant stimulus and turn off, firing only when the stimulus is turned off

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19
Q

define receptive field *

A

the region on the skin which causes activation of a single sensory neuron when activated

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20
Q

describe the differences in size of the receptive field *

A

large receptive fields on the back allow fewer cells to detect changes over a wider area, but this is less precise

small receptive fields allow for detection of fine detail over small areas - precise but requires more cells

fingers have very densly packed mechanoreceptors with small receptive fields

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21
Q

define 2 point discrimination *

A

minimum distance that 2 points are percieved as separate

related to teh size of the receptive field

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22
Q

how can you use 2 point discrimination clinically

A

see if there is normal 2 point discrim and therefore if normal sensory perception in periphery

in hand can be used to check that the median, ulnar and radial nerves are in tact

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23
Q

describe somatosensory dermatomes

A

each spinal nerve has a specific dermatome on the skin

dermatomes provide mapping system to see where sensory info is percieved on the body

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24
Q

list the main dermatomes

A

C5 - clavical

C6 - thumb and index finger

T4 - nipple

T10 - umbilicus

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25
where are the cell bodies for the face and body
face - trigeminal ganglia body - dorsal root ganglia
26
describe the pathway from the limbs to the cortex \*
from lower limbs nerve enter lumbar spinal cord primary neurons travel in the cuneate and gracile fasiculus and synapse in the medulla in the gracile and cuneate nuclei the secondary neurons are called internal articulate fibres and cross at the medulla forming the medial lemniscus tract they synapse in the thalamus in the ventral nuclei tertiary neurons relay to the primary somatosensory cortex
27
describe the pathway from the face to the cortex \*
nerves enter in the pons synapse in the trigeminal nucleus enter thalamus - synapse 3rd order neurons go to the somatosensory cortex
28
describe the neurons in the dorsal horn \*
divided into projection neurons (axons project to the brain) and interneurons (axons stay in spinal cord) Ad and C fibres are superficial and are in lamina 1 and 2 of the dorsal horn AB fibres are deep, in lamina 3 4 and 5
29
define lateral inhibition \*
the difference between adjacent inputs is enhanced by lateral inhibition
30
describe lateral inhibition \*
a receptive field can overlap with another receptive field - so difficult to distinguish between 2 receptive fields lateral inhibition prevents this, it is mediated by inhibitory interneurons in the dorsal horn, and allows enhanced senssory perception and pinpoint accuracy in location of a stimulus
31
describe the ascending pathway for touch and proprioception \*
dorsal column, for innocuous mech stimuli (fine discriminitive touch, vibration) AB fibres enter the dorsal horn - enter ascending dorsal columns (gracile tract from lower limbs) travel ipsilaterally synapse in the gracile/cuneate nucleus 2nd order neurons decussate in the caudal medulla and form the contralateral medial leminiscus tract they terminate in the ventral posterior lateral nucleus of the thalamus 3rd order neurons from the VPL project to the somatosensory nucleus thee size of the somatotropic areas is proportional to the number of sensory receptors in that area - eg hands and lips are big
32
describe the acending pathway for pain temp and crude touch \*
spinothalamic pathway pain and temp in lateral spinothalamic tract, crude touch in anterior spinothalamic tract 1st order neurons terminate when enter spinal cord 2nd decussate immediately forming the spinothalamic tract, teminate in the ventral posterior lateral nucleus 3rd order go to the somatosensory cortex for analysis of localisation and intensity of the noxious sytimulus collatoral branches go to brainstem and convey info to forebrain structures fo teh pereption of pain - affective pathway branches to periaqueductal grey of the midbrain inhibit pain - central inhibition pathway
33
key differences between the spinothalamic and dorsal column tracts \*
spinothalamic decussate in spinal cord before medulla, dorsal synapse in medulla dorsal is for light touch, vibration and 2 point discrimination spinothalamic is for pain, temp and course touch
34
define crude touch
a sensory modality that allows teh subject to sense that ssomething has touched them, without being able to localise it
35
what is quantitative sensory testing \*
where the stimulus and pt response are quantified you detect small abnormalities in sensation adn compare these to normal values can test temperature pain sensation, light touch using glass monofilaments, sharp/blunt sensation using pin pricks, whether innocuous stim is painful usng a brush, normal and abnormal vibration stimuli using graded tuning fork, and normal/abnormal perception of pain using a graded pressure algometer psychophysical - going to depend on mental capacity of pt oe if they are tired it will affect how they interpret pain
36
clinical signigicance of 2 point discrimination test to assess posterior column patency \*
you can evaluate loss or gain to normal thermal or mechanical stimuli test pain and temp in different areas to test the integrity of the spinothalamic tract test discriminitive touch to see patency of the dorsal column
37
what would be the effect of an anterior spinal cord lesion on 2 point discrimination \*
a blocked anterior spinal artery = ischemic damage ot anterior part of spinal cord this will damage the spinothalamic tract = pain and temp loss below the level of the lesion however you will still have touch and vibration 2 point discrimination becasue the dorsal columns are in tact - therefore ensure you test different stimuli so you know what part of the spinal cord is affected
38
what is pain \*
an unpleasant sensory and emotional experience assoictated witha ctual or potential tissue damage, or described in terms of such damage emotional pain has a similar inprint to physical pain
39
describe the different types of nociceptors \*
Ad mediate sharp, intense or 1st pain - type 1 = noxious mechanical, type 2 = noxious heat C fibres - medaite dull, aching or second pain, noxious chemical (bradykinin/histamine), mechanical or thermal stimuli - polymodal
40
how can we test teh nociceptors \*
with pin prick, heat or pressure - see their patency and response to 1st pain and 2nd pain
41
describe teh 1st synapse in the pain pathway \*
glutamate synapse- excitatory NT - released from sensoruy afferents in response to acute or persistant painful stimuli. this is in the superficial dorsal horn, in lamina 1/2
42
describe pain as a multidimentional experience \*
sensory, emotional and learning experience lateral spinothalamic tract is the sensory component, the spinoreticular tract is the emotional component
43
describe the pain matrix \*
fMRI has shown there is a cerebral signiture for pain in the cortex it involves: S1, SII, insula cortex, anterior cingulate cortex, prefrontal cortex amygdala cerebellum brainstem however these areas are also involved in other functions
44
what are the 5 tenants of pain \*
it is subjective - only self-report can represent its existance pain threshold is variable - failure to detect pain on fMRI is not an absence of pain there is pain regardless of intesnity or complexity no exclusive, specific dedicated pain spots in brain salience and pain are intertwined and share neural representation
45
describe the gate control theory \*
peripheral inhibition by rubbing a painful area you activate the AB fibres which through a projection fibre blocks input from the C fibres so it doesnt procede to the somatosensory cortex and reduces the pain happens in the superficial levels of the spinal horn - substantia gelatinosa
46
how can descending control pathways be altered to change pain \*
strong emotions can inhibit pain electrical analgesia reduces pain by affecting the descending pain pathway from the cortex the descending pathways can be affected by fascilitation or inhibition monoamines ie serotonin/NA inhibit the spinothalamic tract and reduce function in the C fibres opiods sihbit stimulation at the periaqueductal grey (PAG) and the RVM placebos also activates descending inhibition in humans - there is activation of the opiodergic descending pain control system and spinal cord involvement in placebo analgesia
47
what is chronic pain
pain for 3 or more months
48
describe the categories of pain \*
nociceptive - noxious stimulation of a nocieptor (somatic or viscera) affecting skin, mucles, ligaments, joints, bones or viscera eg arthritis, fractures, burns or headache neuropathic - lesion or disease of thee somatosensory system - sciatica, diabetic neuropathy, trauma, chemo, post-surgical, infection mixed - osteoarthritis/lower back pain
49
describe peripheral sensitisation \*
inflammatory soup eg NT (Glutamate), peptides, lipids, cytokines, proteases, peptide C and bradykinin make nociceptors more sensitive - reduce the threshold to peripheral stimuli at site of injury - therefore have primry hyperalgesia here and around this econdary hyperalgesia
50
describe central sensitisation \*
reduce the threshold to peripheral stimuli at an adjacent site to the injury hypersensitivity of C fibres in the periphery which causes plasticty in the proijection neurons in the dorsal horn affecting near cells - adjacent Ad fibre becomes sensitised in the dorsal horn expansion of the receptive field spontaneous pain
51
what is allodynia \*
pain due to a stimulus that doesnt normally invoke pain
52
what is hyperalgesia \*
increased pain from a stimulus that normally provokes pain - primary or secondary shift to the L in pain intenstity related to stimulus intensity
53
describe the diagnosis of neuropathic pain
need to have 3 things to classify as neuropathic pain: have to have underlying disease or injury to somatosensory system the symptoms have to be present in an area consistant with somatosensory involvement there needs to be sensory loss or gain (need to do questionnaires and somatosensory tests to make a diagnosis)
54
describe the variability in neuropathic pain sensory profiles \*
vary within condition - this is a driver for the low efficacy of pharmacology using quantitive sensory testing there are 3 neuropathic pain clusters - sensory loss, thermal hyperalgesia and mechanical hyperalgesia knowing this you can identify subsets of pts that respond to certain pharmacology
55
what are SSRIs and what is there effectiveness in treating pain
serotonin selective re-uptake inhibitor have low analgesic efficacy compared to TCAs or dual NA serotonin inhibitors pharm in general is not very efficace
56
describe the effect of NA and serotonin for treating pain
NA is protective because it is inhibitory serotonin is a fascilitator so is not as useful at treating pain - it is harmful
57
described conditioned pain modulation \*
free of cortical influence pain inhibits pain it is a psychophysial measure of endogenous pain inhibitory pathway - the pt has participate induce pain in pt and get them to rate it, then give different pain to different site, then give same pain to 1st sight and get them to rate it - it will have decreased becasue threshold will have increased or pain will have decreased. This info is being relayed through PAG and the RVM then can use this to see if pt groups have deficient conditioned pain modulation and if this is a driver for their pain eg in people with chronic pancreatitis CPM can be rescued pharmacologically
58
describe neuromodulation in chronic pain patients \*
non-invasive primary moror cortex stimulation - activates endogenous analgesic systems in the brain - PAG and the anterior cingulate cortex there is a drive for this because of ineffective and SE of pharm
59
describe mechanoreceptors \*
all receptors for touch and proprioception are mechanoreceptors they transduce mechanical stimulus into electrical ones modified terminals of the peripheral axons of primary sensory neurons
60
what does teh function of mechanoreceptors depend on \*
degree of specialisation - from free nerve endings to elaborate accessory structures location - eg in various layers of skin, around hari shaft, in muscles and in tendons physiological properties - activation threshold determines sensitivity - all touch and proprioception receptors are low threshold - may be slow or fast adapting
61
what is the axon type for sensory neurons \*
muscle spindles and tendon organs are innervated by type 1 axons most otehr mechanoreceptors are innervated by type II/A B axons all are fast conducting
62
describe somatosensation in the cortex \*
touch and proprioceptive stimuli only become a conscious localised sensation when it reaches the cortex response of neurons in somatosensory 1 (S1) varies - some respond to stimulus properties eg pressure, vibration and some to abstarcted properties eg movement and object shape S2 recieves intracortical signals from S1 posterior parietal cortex - necessary for the interpretation of spatial relationships
63
summaries sensory deficits \*
injury to the pathway anywhere form the cortex to periphery may cause anaethesia or parathesia some degenerative disorders affect the somatosensory system predominantly - eg peripheral neuropathies
64
describe the somatosensory path for visceral pain \*
carried peripherally by the ANS centrally in the spinothalamic and dorsal column pathways referred to skin at same spinal level
65
describe the effective pathway of the spinothalamic tract \*
axons send collatoral branches to the brainstem (reticular formation), thalamus (intralaminar nuclei), hypothalamus and some cortex (cingulate gyrus, insula) the pathway triggers increase in awareness and registers the unpleasantness of the stiumulus
66
describe nociceptive dysfunction \*
disruption of the pathway may reduce pain but predisposes to increased injury some changes may exacerbrate pain eg windup in dorsal horn, thalamic syndrome, phantom pain
67
sensory adaption definition \*
a change over time in the responsiveness of the sensory system to a constant stimulus.

neuro - yr 2 (24 decks)

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