Somatosensation Flashcards
1
Q
What is proprioception and kinesthesis
A
proprioception - the sense of limb
position)
kinesthesis - the sense of limb movement
2
Q
How many principal mechanoreceptors innervate the glabrous skin of the hand?
What are they?
How do they all respond to mechanical stimulation
A
4
Slow adapting type 1 (SAI) Rapidly adapting (RAI) Rapidly adapting Pacinian corpuscles (PC) Slow adapting type 2 (SAII)
by depolarising
3
Q
What do slowly adapting type 1 fibres end in
What about type II
A
Merkel cells
Ruffini cells
4
Q
What is a Pacinian corpuscle composed of
A
concentric layers of cellular membranes alternating with fluid filled spaces
5
Q
Where are Pacinian corpuscles found?
A
distributed widely, including connective tissue in muscles,
periosteum of bones and mesentery of the abdomem
6
Q
How are Pacinian corpuscles distributed in the hand
A
finger has 350
800 in the palm
7
Q
How sensitive are Pacinian corpuscles
A
responding to 10 nm of skin motion at 200 Hz
8
Q
Describe the receptive field of Pacinian corpuscles
A
central zone of maximal sensitivity surrounded by a large continuous surface on the fingers or palm.
9
Q
What are Meissner’s corpuscles attached to
A
RAI afferents
10
Q
What is the receptive field of Meissner’s corpuscles?
A
3-5 mm in diameter and they respond best to low frequency vibration
11
Q
What is the density of Meissner’s corpuscles
A
150/cm^2
12
Q
What can Meissner’s corpsucles be compared to in the visual system
A
analogous to the receptors used in scotopic vision—they show enhanced sensitivity and poorer spatial resolution whereas the SAI afferents (Merkel cells) correspond more closely to receptors in photopic conditions with their higher spatial resolution and decreased sensitivi
13
Q
Broadly, when do rapidly and slowly adapting receptors respond during stimulation
A
rapid- onset of stimulus
slow- tonic response to a steady stimulus
14
Q
How does a rapidly adapting receptor respond to stimuli
A
phase locking
responds to low frequency sinusoidal mechanical stimuli with a single action potential for each phase of the stimulus
effectively treats each period of the waveform as a new stimulus
15
Q
True or false
the intensity of the sinusoidal stimulus must be encoded by the firing rate
A
false
encoded by number of sensory fibres active
16
Q
How is number of rapidly adapting touch fibres active related to vibrations
A
The number of active fibres is linearly related to the amplitude of vibration.
17
Q
True or false
the Pacinian corpuscle is rapidly adapting
A
true
it is RAII
18
Q
How can we examine the influence of the lamallae in Pacinian corpuscles adaptation
A
by peeling them away
19
Q
Describe an experiment assessing the purpose of PC lamallae in adaptation by peeling them away
A
In response to the onset of step pulse the receptor potential usually rises and then decays (adapts) quite rapidly. A similar response is seen to the turning off the stimulus. In contrast, when recorded direct from the nerve ending (i.e. de-sheathed), the receptor potential produced in response to a step pulse was slowly adapting.
20
Q
What are accessory structures
A
Structural components of sense organs
which may play an important role in protection, conduction,
concentration, analysis, sensitization or inhibition; but they are
not directly involved in the transduction process
21
Q
Give 4 examples of accessory structures
A
e.g. lamellae of the Pacinian corpuscle,
the intrafusal fibres of the muscle spindle,
eye structures,
basilar membrane.
22
Q
How can human vibration sensitivity be altered
A
by changing the responsiveness of
RA I and RA II
23
Q
How does local anesthetic affect detection of vibration if it is applied to superficial layers
A
reduces the effectiveness of the Meissner’s corpuscles as they lie close to the surface
24
Q
How can you use vibrations to desensitise Pacinian and Meissner’s corpuscles
A
effectiveness of the
Pacinian corpuscle can be reduced by pre-adapting the
receptor to stimulation around 250 Hz.
Meissner’s: preadapting the skin with a low frequency stimulus
25
What are Merkel cells attached to
SAI afferents
26
Describe the recepetive field of Merkel cells
small, highly localised receptive fields
27
Where do Merkel cells innervate and how do they respond/ what do they respond to
innervate the skin
densely
respond to indentation with a linear response to 1500μm.
sensitive to points, edges and curvature and can resolve spatial detail of 0.5 mm.
28
True or false
| Merkel cell spike discharge is largely invariant
true
| they are very good at discrimination
29
Describe the dynamic range of Merkel cells
usually between 200micrometer indentation to 1500 micrometers
less than 1 order of magnitude (much smaller than visual and auditory systems)
30
What happens if you knock out Piezo2
Merkel cells will not produce an inward current in response to touch
31
Compare stimuation of Merkel cells in a skin-nerve preparation for wild type and Piezo2 KO mice
strong neural response from SA1 fibres in wild-type mice (WT) whereas the response from the same fibres in which Piezo2 had been knocked out failed to demonstrate a sustained response.
32
Which nerves has Piezo2 been expressed in
dorsal root ganglion, Aβ, Aδ and C-fibre low threshold mechanoreceptors
33
How does the role of SA1 nerve terminals differ from the role of the attached Merkel cells
The SA1 nerve terminals are responsible for the rapid and dynamic response to mechanical stimuli while the Merkel cells are responsible for the sustained response.
34
What are Ruffini thought to play a role in
How is this supported
perception of object motion and in providing information about hand shape and finger position.
this role is reflected in Ruffini endings residing deep
35
What are Ruffini endings attached to
SAII afferents
36
Which afferents are most accurate for reading Braille
Describe an experiment to show this
SAI
The Braille patterns were scanned (60 mm/s) repeatedly over the afferent fibres’ receptive fields, which were located on the distal finger pads. The patterns were shifted vertically after each scan. Each black dot represents an action potential
evoked by the Braille pattern. These spatial event plots show that the response of SAI fibres looks rather similar to the original dot pattern.
37
Is tactile acuity equal across the body
no
| better in the hands than back for instance
38
What is the compass test
What are the results
to determine the smallest discriminable distance between two points of contact, the so called two-point limen.
In general, the two-point limen improves up to twenty-fold from the shoulder (40mm) to the fingers (2mm).
39
What is tactile acuity correlated with (2)
acuity increases with mobility (less true for lower extremities)
areas of high acuity have small receptive fields
40
What happens if two points contacting the skin stimulate just one receptive field?
we have no information that two points on the skin were stimulated.
41
Acuity appears to increase with increase in mobility. Areas with high tactile acuity have small receptive fields. What can we therefore predict about RAI and SAI fibres
have small receptive fields
| and the highest density on the fingertips.
42
What is our perception of hand held objects determined by
the overall pattern of activity
| produced by all these receptors.
43
What was the first evidence that warm and cold should be considered as different modalities
maps of warmth and cold spots
44
How does the number of warmth spots compare to cold spots on a map
concentration of cold spots can far exceed that of warmth spots.
can be at least ~30 times as many cold as warmth spots.
Different body areas have different proportions of cold
and warmth spots i.e. the lip has six times as many cold spots as the sole
45
Does 1 warmth spot equate to 1 warmth receptor?
no - then large areas of the body should be insensitive to warmth
46
What is spatial summation in the somatosensory system
more receptors may exist than there are spots and that it usually requires the simultaneous activation of many receptors to elicit the sensation of warmth.
47
Give 2 things TRPV1 responds to
capsaicin, and also to painful increases in temperature above 43°C
48
Name 2 things TRPM8 responds to
menthol and are activated by non-painful decreases in temperature below 28°C
49
What are warm and cold receptors attached to
‘cold’ receptors are connected to A-delta and C-fibres
‘warm’ receptors by a sub-population of C fibre
50
What happens when a heat stimulus of >45 degrees is applied to a cold spot on the skin
paradoxical cold
51
What is paradoxical cold
What is this an example of
high temperature (>45 degrees) is usually perceived as painful when applied to a diffuse area of the skin, however, when applied to a single cold spot it is perceived as cold.
Therefore, activity in the cold fibre is experienced as cold
irrespective of the physical nature of the stimulus
example of labelled line coding
52
What are the specialised endings to nociceptor axons
do not possess any specialised endings and hence referred to as bare or free nerve endings
53
What does the bare nerve endings of nociceptors provide
makes them particularly sensitive to chemicals produced or released at a site of injury.
54
How is pain often separated
into an early (first), sharp pain and a second, dull, burning pain.
55
How does abolishing A-delta fibres affect pain
What about blocking C fibres
removes early (first) sharp pain
abolishes second pain
56
Are c fibres myelinated
what do they respond to
no
polymodal: respond to thermal (< 15°C and >43°C), mechanical (strong not mild) and chemical stimuli (e.g. chilli peppers, acid)
57
What encodes pleasant touch
C fibre tactile afferents (CT)
58
What do C fibre tactile afferents respond to
light touch, low-velocity stroking. Such stimulation is usually regarded as pleasant
59
What characterises pleasant touch fibres (2)
how was this confirmed
a low conduction velocity (~ 1m/s) and are only found in hairy skin.
This was confirmed using the technique of microneurography.
60
How do the responses of A-delta and CT afferents to a small tap stimulus compare
The response of the
myelinated Aβ afferent has a much shorter latency than the
response of the CT afferent
61
What is the velocity of long latency CT responses
conduction velocity of around 1 m/s.
62
How does stroke velocity correspond with rate of AP firing
As stroke velocity increases there is a non-linear relationship with action potential firing
63
How does pleasantness correspond with stroke velocity
What has this discoveruy led to speculation of
non-linear relationship is seen when subjects are asked to rate the pleasantness of
the stimulation as stroke velocity increases
similar to AP firing v stroke velocity
leads to a positive
correlation between CT output with the pleasantness of the stimulation.
CT afferents afferents are more concerned with emotional feeling than touch.
64
What do sensory peripheral nerves tend to consist of
bundles of axons grouped by fibre
| diameter
65
What are the different types of somatosensory fibres
Give them in size order and state whether they have myeline
largest, myelinated:
Aα and Aβ;
intermediate, thinly myelinated group: Aδ.
the smallest, unmyelinated: C fibres.
66
Order the sensory afferent types according to length of delay
Aβ
| afferents have the shortest delay whereas the unmyelinated Cfibres conduct slowly producing a late peak.
67
Which sensory afferent fibres are affected by anoxia
the large Aβ fibres
68
Which sensory fibres are primarily affected by local anaesthetic
small ‘C’ and Aδ axons
69
Describe the pain evoked when Aβ afferents are stimulated alone
no pain, , even at intensities sufficient to
mimic an intense mechanical stimulus.
Pain is only evoked by stimulation of the Aδ and C fibres
70
Which fibres are required for pain to be felt
stimulation of the Aδ and C fibres
71
Give 2 conditions when the myelin sheath of large diameter fibres is broken down
what does this lead to
diabetes
MS
slowing of nerve conduction or
failure of impulse transmission.
72
How many spinal nerves
31
73
Is the diameter of the spinal cord constant
varies considerably at different levels and it has two enlargements, cervical and lumbar
74
Where is the cauda equina
lies below approximately L2
75
What is a lumbar puncture and where can it be done
| which space
below L2
| hollow needles can be inserted into the subarachnoid space to remove CSF for diagnostic purposes
76
Which space can you insert anaesthetics into for surgery
epidural space
77
Are epidural blocks and lumbar punctures performed in the same meningeal space
no
| lumbar puncture: subarachnoid space
78
How do dermatomes overlap
dermatomal boundaries overlap by mixing fibres from several dorsal roots in the peripheral nerve.
79
Describe a general cross-section of the spinal cord
divided into two symmetrical halves by the dorsal median sulcus and the ventral median fissure.
Surrounding the central canal is the
‘H’-shaped grey matter consisting of nerve cell bodies.
Afferent and efferent axons run in the white matter.
80
How is the white and grey matter divided in the spinal cord?
The white matter can be divided into three regions, defined relative to the grey matter: dorsal, lateral
and ventral.
The grey matter is divided into functionally distinct laminae (aka Rexed’s laminae).
81
What is the main pathway for proprioception and touch
What kind of fibres convey which information?
dorsal column – medial lemniscal
(DC-ML) system
consists of large-diameter myelinated fibres that convey tactile, vibratory and proprioceptive sensations.
82
What happens when tactile fibres enter the spinal cord?
primary afferent bifurcates:
A short branch
enters the dorsal horn and a long branch enters the dorsal columns
83
What happens to the axons that form the long branch of the primary tactile afferents
a) below mid thoracic level
b) above mid thorax
a) ascend in the fasciculus gracilis and terminate in gracile nucleus.
b) enter the fasciculus cuneatus and terminate in the cuneate nucleus
84
How are the cells in the dorsal column nuclei arranged
What is this arrangement called and is it common?
according to their somatic origin with the leg located medially and the arm laterally
somatotopic organisation is preserved at all levels of the pathway
85
What do the axons leaving the dorsal column nuclei do generally?
cross the brainstem and then ascend to the thalamus in the medial lemniscus.
86
What is the major nociceptive tract in the spinal cord
what does it comprise
spinothalamic tract
axons from neurons in layers I and V-VII of the dorsal horn.
87
What is the spinothalamic tract also called?
Why?
anterolateral system
The axons ascend in the contralateral,
anterolateral, white matter
88
Does cutting the spine carefully reduce pain
yes
Lesions of the spinothalamic tract (usually achieved surgically by an anterolateral cordotomy) reduce pain sensations from the contralateral side of the body. Unfortunately, pain relief is often only temporary
89
Where does the spinoreticular tract extend from to
t projects from laminae VII and VIII and terminates in the reticular formation and thalamus
90
Do axons of the spinoreticular tract cross the midline in the spine?
no they travel ipsilaterally
91
Where does the spinomesencephalic tract begin and end
projects from laminae I and V, via the anterolateral quadrant of the spinal cord to the mesencephalic reticular formation and the periaqueductal gray
92
What does Brown Sequard syndrome describe
consequence of a hemi-section through the spinal column.
affects both the spinothalamic tract and DC-ML on the same side of the cord resulting in a loss of pain and
temperature sensation below and contralateral to the site of the lesion and loss of fine touch and proprioception below and ipsilateral to the site of the lesion
93
What is syringomyelia: cause and symptoms
caused by a fluid-filled cavity within the spinal (usually cervical) cord.
cavitation usually disrupts the decussating fibres of the anterolateral system but not the ascending fibres of the DCML system
resulting in the characteristic "cape-like" distribution of loss of pain and temperature sensation in the upper limbs and trunk but preservation of touch and pressure sensation.
94
What is the characteristic symptom of syringomyelia
"cape-like" distribution of loss of pain and temperature sensation in the upper limbs and trunk but preservation of touch and pressure sensation.
95
What causes posterior column syndrome
tabes dorsalis – tertiary syphillis
96
What is the symptoms of posterior column syndrome
a bilateral absence of touch below the level of the lesion. There is also a loss of
proprioceptive feedback below the site of the lesion leading to a characteristic stamping gait.
97
How would sensory modalities be affected by a complete transection of the spinal cord
complete transection leads to the impairment of all sensory modalities below the level of the transection
98
what is Tic douloureux?
trigeminal neuralgia
| a syndrome whereby gentle stroking of the face or mouth provokes a massive stabbing pain.
99
What is Tic douloureux an example of
allodynia
100
what are the fibres of the CN V branches
ophthalmic and maxilliary branches are purely sensory while the mandibular is a mixed sensory and motor branch
101
Does the somatosensory system have lateral inhibition
yes
| This allows the amplification of differences in the output of neighbouring neurons and effectively enhances contrast
102
What does stimulation of a single point of skin produce
What happens when stimulating 2 adjacent points
What can we see based on this
an excitatory
response surrounded by a small inhibitory response
lateral inhibition suppresses excitation of the neurons between the points thus sharpening the focus or spatial clarity of the two points.
importance of inhibitory interactions in fine tactile discriminations such as reading Braille.
103
Where does the DCML system terminate in the thalamus
in a group of cells known as the ventral posterior nucleus
104
Is the thalamus involved in pain?
identified the posterior part of the ventral medial nucleus (VMPO) as a specific nucleus for pain and temperature sensation in both monkeys and humans.
105
Can a CNS cell's receptive field change?
yes:
| Blocking of normal input to cells in the CNS may be followed by immediate unmasking of previously ineffective inputs
106
Give an example of immediate unmasking of a previously ineffective sensory input to the CNS following blockage of the original input
What is this an example of
Initially the cell responded only to pressure on its toes (the no block condition).
When all input to the leg was temporarily removed by cooling of the lumbar spinal cord the cell responded to touch on an area of the flank (during block).
Removal of the cold block returned the receptive field
to the area of the toes
gate control
107
How can gate control be described?
whereby the failure of one input unmasks the presence of inputs which are ordinarily suppressed by ‘inhibitory’ mechanisms
108
Where do tactile signals from the thalamus go?
to the primary somatosensory cortex (S-I) in the post central gyrus
109
Where is the post central gyrus located
immediately posterior to the central sulcus
110
What are particularly pronounced sulci in the brain referred to as
fissures
111
Which of Brodman's areas does S-I occupy
areas 3, 1 and 2
112
How is the cortex organised
in a series of vertical columns, 300-600 μm wide, spanning all six layers from the cortical surface to the white matter
113
How is stimulus location preserved in the brain
r. All neurons within a
| column receive input from the same area of skin
114
How do the columns in the cortex preserve touch modality
In addition to sharing a common location, all neurons also only respond to a single modality e.g. touch, temperature etc
115
Where do somatosensory afferents from the thalamus terminate
Which layer projects back to the thalamus
layer IV
layer VI
116
Where do layers II, III and V in the cortex project to
II, III: other cortical regions
| V: to subcortical structures
117
What is the homunculus
What does it represent
The map of somatosensory inputs to the cortex
does not represent the topography of the skin exactly but rather exaggerates certain body regions. Each part of the body is represented in the brain according to its importance to the organism
118
In humans, where is much of the cortical map of somatosensation (homunculus)
what about in rodents
hands and face
whiskers
119
Can you get direction sensitive somatosensory neurons?
yes
a neuron in area 2 of S-I could respond strongly eg to movement from the ulnar side of the wrist to the radial fingers
Moving the stimulus in the opposite direction
produced the smallest response
120
How can we explain the direction sensitivity of tactile neurons
The spatial arrangement of excitatory and inhibitory inputs
121
How can you predict the responses of S-I neurons to somatosensory stimuli
Is this the same as the responses in S-II
What does this mean
by the bottom-up input from the periphery
responses of neurons in
area S-II are influenced by top-down processes such as attention
by decreasing attention on the stimulus, response to the stimulus is decreased
122
Do somatosensory neurons have memory?
yes S-II can understand analyse the value of a stimulus - ie uses reward pathway
123
What is passive touch
What is the problem with this
the experimenter
presents a stimulus to a static receptive field.
This is a relatively unusual situation
124
What is the difference on the respective fMRIs in an experiment where activity of cortical areas was
examined following passive stroking of the right hand
with a sponge, or active touching of the sponge?
In both conditions, areas 3b and 1 were activated in the left hemisphere.
In active touch, however, areas M1 (primary motor cortex) and ACC (anterior cingulate
cortex) were also engaged.
125
How does the ability for neuronal
activity to predict behavioural performance change as we progress along a pathway from primary somatosensory cortex to parietal cortex and finally to the premotor cortex?
graded increase
eg . Rapidly adapting neurons in area 3b/1 in the primary somatosensory cortex had a neural detection threshold that matched the perceptual/ behavioural
threshold. While these neurons carry the information necessary to detect a stimulus, they could only predict behavioural performance at chance level
126
Can activity in M1 predict behavioural performance?
Which study is this
no
Luo, 2016
127
Are cortical receptive fields fixed
no
| can be modified by experience or injury
128
How did scientists examine the effect of differential stimulation of restricted skin surfaces of the hand on the representation of these surfaces in 3b?
What was the result?
b a monkey was trained to maintain contact with a rotating disc in
order to get a reward.
The cortical hand representation of the same monkey, following 20 weeks of daily training, showed a marked expansion of the representations of the distal aspects of digits 2 and 3.
129
Give evidence that supports the results of the monkey touching a rotating disc experiment
In support of this finding, musicians who play stringed instruments have a greater than normal cortical
representation for the highly stimulated fingers on their left hand.
(cortical representation increases with use - plasticity)
130
What is the theory to explain phantom limbs
use an example of a someone with left arm amputation
There is a complete
map of the fingers on both the face and upper arm. The hypothesis is that the
sensory input from these areas is now innervating the hand area of the
somatosensory thalamus or cortex.
131
Is reorganisation of the sensory input from amputated regions beneficial
can be - without phantom limb it can be impossible to use a prosthetic
but greater the reorganisation the
greater the amount of pain felt!
132
Are phantom sensations unique to the somatosensory system?
no
| tinnitus is a phantom auditory sensation with many similarities to phantom limbs.
133
How can pain be described
an unpleasant sensory and emotional experience associated with actual or potential tissue damage or described in terms of such damage
134
What does lamina I receive input from in the dorsal horn
lamina V?
myelinated Aδ nociceptive fibres and both direct
and indirect input from ‘C’ fibres
input from both large diameter fibres (Aβ) from mechanoreceptors as well as input from other nociceptors. Lamina V neurons
therefore respond to innocuous stimuli at low intensity and noxious stimuli at high intensities and they are known as wide-dynamic range neuron
135
Describe the basis of referred pain
Signals from an inflamed visceral organ converge on projection neurons at the dorsal horn of the spinal cord. Sensory input from a distant somatic
structure converges on the same neurones. The CNS
cannot distinguish between superficial and deep pain and
this failure results in the incorrect assignment of pain to the healthy somatic area.
136
What is a well known referred pain
from angina pectoris
This is triggered by an inadequate supply of blood to the heart and is experienced as referred pain in the chest and left arm.
137
Which areas of the cerebral cortex respond to pain
The main areas are
| located in S-I, the anterior cingulate cortex (ACC) and insula
138
What are the ACC and insula and what is their role in pain>
The anterior cingulate is part
of the limbic system and is thought to be
responsible for the emotional element of pain
The insular cortex processes information on the internal state of the body and thus contributes to the autonomic component of the overall pain
response.
139
why might you get a cingulotomy
to treat psychiatric disease
140
What did the original simplified gate control hypothesis lead to the development of
transcutaneous electrical nerve stimulation (TENS)
| for pain relief
141
Describe the neuronal circuitry of the gate control system
there is are 3 connections to the projection neuron:
Aβ fibers (tactile), inhibitory interneuron, and C fibre (nociceptor)
Both Aβ fibers and C fibres connect to both inhibitory interneuron and projection neuron
Aβ fibers excite both interneuron and projection neuron. C fibre inhibits interneuron and excites projection neuron
C fibres activating therefore activate projection neuron
Aβ fibers firing activates interneuron, decreasing projection neuron firing
142
Where does gate control happen
substantia gelitanosa
143
What is the influence of descending systems on gate control
diverting attention away from painful stimulus decreases pain felt
The PAG is thought to control
the ‘nociceptive’ gate in the dorsal horn by integrating inputs from the
cortex, thalamus and hypothalamus.
144
What characterises the projection or transmission neurons in gate control
a wide dynamic range (WDR), i.e. they can
signal changes in stimulus intensity by increases in spike discharge rate over a wide range of intensities - the intensity theory of pain
(cf. a neuron with a narrow dynamic range can only signal changes in
intensity over a limited range of amplitudes.)
145
What does the specifity theory of pain require
How is this seen in the body
neurons that respond selectively to touch (the mechanoreceptors) and other neurons that respond selectively to high amplitude stimuli that produce pain (the nociceptors).
both types of neural response are observed in the spinal cord; nociceptors are prominent in lamina I while WDR
neurons are in lamina V
146
Why is pain so moodulated
enables it to be integrated
with many other body systems including skin reflexes, autonomic
regulation, emotion, and attention
147
Where are the descending systems that modulate pain located (3)
in the periaqueductal gray (PAG) matter of the mid-brain, the raphe nuclei, and other nuclei of the rostral medulla.
148
How can the PAG be used in surgery
Electrical stimulation of the PAG can produce sufficient analgesia to
perform abdominal surgery without the need for anaesthesia. Other,
non-painful, sensations were left intact
149
How is morphine induced analgesia blocked
by injection of naloxone (an opiate antagonist) into the PAG.
150
Bilateral transection of the dorsolateral funiculus leads to...
Bilateral transection of the dorsolateral funiculus blocks both descending gate control
stimulation- and morphine-induced analgesia.
151
What does placebo analgesia refer to
the situation where administration of a substance known to be non-analgesic
produces an analgesic response when the subject is told that it is a pain killer.
NB the placebo effect also works for asthma, cough, diabetes, ulcers, multiple sclerosis and Parkinsonism
152
How can we test the neural mechanisms of pain placebo
Results?
capsaicin applied to the distal region of all four limbs and were then asked to report the magnitude of pain felt at each site.
Following subcutaneous injection of capsaicin, specific expectations were
induced by the topical application of a placebo cream and by telling the subjects
that it was a powerful local anaesthetic.
placebo analgesia only occurred in treated part
153
Can you abolish spatially specific placebo effect?
abolished with the
intravenous infusion of naloxone (an opioid antagonist) suggesting that it was mediated by one of the endogenous opioid systems
154
Does non-specific release of endogenous opioids
| throughout the nervous system underlie placeo?
no
rather that attentional
mechanisms, via expectation of analgesia, can be directed to specific parts of the body. It is argued that
spatial specific expectations have their own organization that is maintained at the level of the endogenous
opioid systems
155
It is argued that spatial specific expectations have their own organization that is maintained at the level of the endogenous opioid systems. Give evidence from a rat model to support this
a somatotopic organization of the PAG (in rats) has been found such that stimulation of different areas produced analgesia in different cutaneous regions.
