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Neuro > Somatic Sensation > Flashcards

Somatic Sensation Flashcards

1
Q

Somatosensory system involves

A

cutaneous sensation
proprioception
kinesthesis

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2
Q

proprioception

A

the sense of limb position

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3
Q

kinesthesis

A

the sense of limb movement

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4
Q

skin

A

epidermis ( dead layer of cells )
dermis ( living layer beneath epidermis )

in both layers are the mechanoreceptors located

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5
Q

glabrous

A

hairless

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6
Q

four principal mechanoreceptive afferent systems

A

slowly adapting type 1 ( SAI) affarents that end in Merkel cells

rapidly adapting ( RAI) affronts ending in Meissner’s corpuscles

rapidly adapting Pacinian corpuscles ( PC ) type II

and slowly adapting type 2 ( SAII) affarents that end in Ruffini endings

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7
Q

mechanoreceptive affarent systems

A
  • each serve a distinctive perceptual function , type of stimulation responded to best determined by accessory structures
  • tactile perception: sum of activity of four mechanoreceptive systems
  • respond to mechanical stimulation by producing a depolarising receptor potential
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8
Q

Pacinian corpuscle ( PC )

A

composed of:
concentric layers of cellular membranes alternating with fluid filled spaces
distributed widely
extremely sensitive - responding to 10 nm of skin motion at 200Hz

–> role in perception of events through an object held in hand

receptive fields: central zone of max sensitivity surrounded by large continuous surface

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9
Q

distribution of Pacinian corpuscle

A

wide

eg skin,
connective tissues in muscles,
periosteum of bones
mesentery of the abdomen

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10
Q

Meissner’s corpuscles

A

small receptive fields (averaging 3-5 mm), up to 20 receptors per neuron

150/cm^2

rapidly adapting structure

respond to low frequency vibration i.e 10-15 Hz

initial contact and motion

enhanced sensitivity and poorer spatial resolution ( like scotopic vision )

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11
Q

Merkel cells

A

dense innervation of the skin- small epithelial cell found under fingerprint ridges

pressure: firing frequency proportional to pressure applied

small receptive field --> high spatial resolution, 
decreased sensitivity ( like photopic vision )

10 times more sensitive to dynamic stimuli than static

sensitive to points, edges and curvature

spike discharge invariant ie very good at discrimination

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12
Q

rapidly adapting affarents respond to

A

change in stimulus

ie burst of APs at onset / offset but non if maintained

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13
Q

slowly adapting afferent fibres ( activation )

A

tonic activation if stimulus is continuous

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14
Q

phase locking in rapidly adapting affarents

A

if stimulus intensity ( ie. skin indentation) is sinusoidal, phase locking occurs with AP around peak stimulus as response has then decayed enough by next peak to detect a “change”
–> RA useful for sensing vibration

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15
Q

accessory structure

A

not directly involved in transduction but aids sensory process eg through protection, conduction, concentration, analysis, sensitisation, inhibition

eg pacinian corpuscle lamellae , cornea + sense of eye , basilar membrane cochlea, intrafusal muscle fibres

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16
Q

overall sensitivity to vibration determined by

A

combination of Meissner’s and Pacinian corpuscles - can be altered by changing the responsiveness of the two receptors

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17
Q

intensity of sinusoidal stimulus

A

encoded by the number of sensory fibres that are active
( not the frequency of firing )

numb of active fibres linearly related to amplitude of vibration

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18
Q

Pacinian corpuscle rapidly adapting

A

onset of step pulse/ turning off the stimulus

receptor potential rises + decays ( adapts ) rapidly

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19
Q

desheathed pacinian corpuscle adapting

A

lamellae removed –>increase in Receptor potential with increase in stimulus intensity, become slowly adapting

as receptor potential plateaus before repolarising

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20
Q

how to reduce sensitivity in Meissner’s corpuscle/ raise threshold

A 
local anasthetics ( lie close to skin) 
preadapting to low frequency stimulus ( 30 Hz) m
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21
Q

how to reduce sensitivity in Pacinian corpuscle/ raise threshold

A

pre-adapting receptor to 250 Hz stimulation ( high frequency)

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22
Q

Merkel cells linear dynamic range

A

AP firing rate and. perceived stimulus intensity both increase linearly with stimulus intensity from 200-2000 nanometers

Ouput of neuron and psychophyisical intensity match

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23
Q

dynamic range comparison Merkel cell vs visual auditoy systems

A

Merkel: less than one order of magnitude

vision/audition: many orders of magnitude

Vision hearing:
Non-linear –> saturated sigmodial input output functions, in vision: rods and cones ( light adaptation shift input output function )

Merkel: linear

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24
Q

Ruffini ending

A

little known
thought to contribute to:

motion perception ( respond to skin stretch )
information about hand shape and finger position
relatively high threshold : deep in skin

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25
Q

Ruffini ending transduction mechanism

A

tension applied to collagen tightens axon spirals

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26
Q

spatial event plots show

A

AP responses of fibres in response to spatial stimuli

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27
Q

reading Braille

A

Spatial even plots used to this
SAI ( attached to Merkel cells ) fibres responsible for reading Braille as most similar spatial event plot to the actual Braille pattern

  • also makes sense because smallest receptor field and only receptor in epidermis i.e most superficial
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28
Q

tactile acuity determined by

A

two-point limen : smallest discriminable distance between two points of contact

increases with higher mobility of body parts eg 20 fold from shoulder ( 40 mm ) to finger ( 2 mm )

small receptive fields : if two points contacting skin touch same receptive field cannot differentiate

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29
Q

fingertips

A

highest density of RAI and SAI fibres with small receptive fields to high tactile acuity

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30
Q

labelled lines theory

A

individual receptors and individual afferent fibres give information about a single type of stimulus

evidence provided by warm + cold spots
evidence through nociceptors
microneugraphy experiments in human subjects

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31
Q

warm vs cold spots

A

many more cold than warm with the relative proportion varying across the body

both few mm in diameter

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32
Q

spatial summation warm spots

A

if only warm spots convey information about warmth then large proportions of the body should be insensitive to heat

however, this is not the case :
hypothesis: many more warm receptors exists than spots ,
requires simultaneous activation of many receptors to elicit the sensation of warmth –> spatial summation

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33
Q

Trpv1 channels

A

active ingredient of child peppers, capsaicin and painful increase in temp above 43 degrees

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34
Q

Trpm 8

A

menthol

non-painful decreases in temp below 25

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35
Q

cold receptors connected to

A

A-delta

C-fibres

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36
Q

warm receptors connected to

A

C-fibres ( subpopulation )

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37
Q

TRP

A

transient receptor potential

Na+ and Ca+ channels

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38
Q

skin thermoreceptors are

A

free nerve endings - no accessory organs

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39
Q

paradoxical cold

A

sensory illusion
when heat stimulus over 45 degrees
applied to cold spot –> perceived as cold
applied to diffuse area of skin –> perceived as painful

activity of cold fibre experienced as cold irrespective of physical nature of stimulus

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40
Q

pain mediated by

A

nocireceptors

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41
Q

nociceptors

A

free nerve endings with no accessory organs (specialised endings)

two different affarent fibres ( A-delta and C ) respond to different components of pain :
early ( first ) sharp pain
second , dull, burning pain

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42
Q

A-delta fibre nociceptor

A

first pain, initial sharp pain

myelinated

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43
Q

free nerve endings nociceptors consequence

A

particularly sensitive to :

chemicals produced or released at site of injury

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44
Q

C-fibre nociceptor

A

second pain i.e throbbing
unmyelinated
polymodal ie one can respond to noxious hot, cold, mechanical ( strong ) , chemical stimuli ( Chilli peppers acid )

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45
Q

C-fibre tactile affarents ( CT )

A

fibres respond to light touch, low-velocity stroking –> pleasant stimulation

low conduction velocity ( 1m/s)

found only in hairy skin

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46
Q

microneurography

A

how does it work:

what are we trying to measure:

eg response of myelinated A-delta affarent shorter latency than CT affarent

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47
Q

CT - emotional feeling not touch?

A

there is a
non-linear relationship between velocity and action potential firing
non linear relationship between pleasantness of stimulation and stroke velocity

but a linear relationship between CT output ( mean impulse rate ) and rating of pleasantness

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48
Q

peripheral nerves ( bundles of fibres ) grouped by

A

fibre diameter

49
Q

large myelinated axons

A

A-alpha , A-beta

50
Q

small unmyelinated axons

A

C-fibres

51
Q

intermediate thinly myelinated axons

A

A-delta

52
Q

compound action potential

A

if stimulating median nerve action potentials are summed

53
Q

fibre diameter influences

A

conduction velocity

54
Q

pain evoked with

A

stimulation of A-delta and C-fibres

If these are removed with anasthetic , no pain felt even if A-beta fibres are stimulated strongly mechanically

55
Q

diabetes, multiple sclerosis

A

the myelin sheath of large diameter fibres can degenerate resulting in a slowing of nerve conduction or failure of impulse transmission

56
Q

role spinal chord and bilateral spinal nerves

A

receive affarent fibres from sensory receptors of the trunk and limbs

control movement of trunk and limbs

provide autonomic innervation for the viscera

57
Q

the spinal chord has two enlargements

A

wide portions due to many nerve fibres to and from limbs :

lumbar
cervical

58
Q

lumbar

A

CSF can be removed in lumbar puncture for diagnosis

anaesthetics can be injected into epidural space to induce epidural block

does not contain actual spinal chord but elongated spinal roots

59
Q

Cervical

A

may fill with fluid , resulting in syringomyelia

–> loss of fibres which cross to contralateral side die A-delta and C but ipsilateral A beta fibres retained

60
Q

dermatome

A

area of skin innervated by a single dorsal root

dermatomal boundaries overlap by mixing fibres from several dorsal roots in the peripheral nerve

61
Q

spinal chord anatomy

A

two symmetrical halves divided into by:
dorsal median sulcus
ventral median fissure

around central canal : H-shaped grey matter

  • -> nerve cell bodies
  • -> divided into functionally distinct laminae ( Rexed’s laminae)

white matter:
affarent and efferent axons
3 regions: dorsal, lateral, ventral

62
Q

dorsal column- medial lemniscal ( DC-ML ) system

A

main pathway for information about touch and proprioception : tactile, vibratory, proprioceptive sensations

consists of large diameter myelinated fibres ( A-alpha, A-beta)

63
Q

primary affarent entering spinal chord

A 
bifurcates into:
dorsal horn ( short branch )
dorsal columns ( long branch )
64
Q

long branch axons ( spinal chord )

A

enter spinal chord below the mid-thoracic level

ascend in the fascicles gracilis
terminate in fragile nucleus

65
Q

entering above mid-thoracic level

A

enter fascicles cuneatus

terminate in cuneate nucleus

66
Q

dorsal column nuclei

A

cuneate and gracile nuclei

organised according to somatic origin :
leg medially
arm laterally

67
Q

pathway is organised

A

somatopically

68
Q

leaving dorsal column nuclei

A

axons cross brainstem and ascend to the thalamus int the medial lemniscus

69
Q

spinothamalic tract

A

major ascending nociceptor pathway in spinal chord

axons from neurons in layers I and V-VII of dorsal horn

70
Q

anterolateral system

A

spinothalamic tract as the axons ascend in contralateral, anertolateral white matter

71
Q

lesions of anterolateral system

A

reduce pain sensations for contralateral side of body

however, pain relief often only temporarily

72
Q

spinoreticular tract

A

projects from laminae VII and VIII , terminates in reticular formation and thalamus

some axons travel ipsilaterally ( do not cross mid-line)

73
Q

spinomesencephalic tract

A

projects from laminae I and V , via anterolateral quadrant of the spinal chord to the mesencephalic reticular formation and the periaqueductal gray

74
Q

sensation in face

A

cranial nerves not spinal nerves

75
Q

periphery

A

skin, joint, muscles

76
Q

sensory fibres

A

dorsal side of spinal chord

77
Q

lesion info used to deduce location of

A

A-delta and C vs A-beta fibres in spinal chord

78
Q

Hemisection: Brown Sequard

A

effects below site of lesion:

loss of pain and temperature sensation contralaterally ( A-delta and C
loss of fine touch and proprioception ipsilaterally ( A-beta mechanoreceptors )

79
Q

Syringomyelia

A

caused by fluid filled cavity within the spinal chord

disrupts anterolateral system ( a-delta and C fibres )

  • -> loss of pain and temperature sensation in upper limbs and trunk
  • -> preservation of touch and pressure sensation
80
Q

posterior column syndrome

A

dosal lesion:

bilateral abscence of touch below lesion

81
Q

complete transection

A

impairment of all sensory modalities below level of transection

82
Q

trigeminal nerve

A

cranial nerve V
branches intracranially into three divisions : ophthalmic, maxillary ( purely sensory) , mandibular ( mixed sensory and motor )
provides general sensory innervation
providing motor fibres for muscles of mastication + smaller muscles

83
Q

Tic doulourex

A

trigeminal neuralgia

gentle stroking of face or mouth –> massive stabbing pain

example of allodynia

84
Q

allodynia

A

ophthalmic and maximiliary branches

85
Q

lateral inhibition

A

receptive fields show centre surround lateral inhibition ( analogous to that of the visual system )
–> enhances contrast between fibres hence point-point discrimination ( excitation of neurons between two points surpassed )

86
Q

thalamus

A

contains synapses for both the touch sensitive pathway ( ventral posterior nucleus ) and pain pathway ( ventral medial nucleus )

87
Q

receptive fields and gate control

A

receptive fields can change for a cell depending on gate control

eg dogs:

neurons may have a receptive field on a foot but this is only due to descending inhibition

if the inhibition is removed ( by cooling the relevant are of the spinal chord ) the receptive field moves to the flank

88
Q

Location of primary somatosensory cortex

A

S1 in post-central gyrus - which is located posterior of the central sulcus in parietal cortex

Brodman map : area 3, 1, 2

89
Q

cortex organised into

A

series of vertical columns

layers

90
Q

columns in somatosensory cortex preserve stimulus location

modality

A

stimulus location
modality

all neurons receive input from same are of skin about a particular modality eg touch, temperature

91
Q

where do thalamic affarents terminate

A

Layer Iv

92
Q

Which layer projects back to the thalamus

A

Layer VI

93
Q

Layer II and III project to

A

cortical regions

94
Q

Layer V projects to

A

subcortical structures: basal ganglia, brainstem, spinal chord

95
Q

Homunculus

A

somatotopic map of sensory inputs to cortex with exaggeration of regions according to how much of the cortex they occupy. Face big for humans, nose for star-nosed mole

96
Q

Direction sensitivity

A

some cortical neurons are direction sensitive by combining several lower receptive fields, just as occurs in the visual system

97
Q

What explain direction sensitivity

A

spatial arrangement of excitatory and inhibitory inputs –> whether they overlap and in what direction the motion is: are the excitatory and inhibitory areas stimulated simultaneously ( weak output ) , what is stimulated primarily

98
Q

Attention- cortex

A

in S2, firing rate of neurons is modulated by attention i.e reduced if the person is distracted by visual task

99
Q

cortex plasticity

A

receptive fields not fixed- can be modified by experience or injury ( amputations )

eg if monkey trained to keep finger on rotating disk ( receives reward ) expansion of representation of finger

eg homunculus of guitar players has unusually large representation of fingers of left hand

100
Q

phantom limb pain

A

plasticity of cortex responsible for this phenomenon

area of cortex receives ascending inputs from a different area of skin

often leads to perception of pain and doesn’t appear to be useful

101
Q

Phantom limb not restricted to somatosensory cortex

A

tinitus in audition

102
Q

Regions in cortex responding to pain

A

S-I , the anterior cingulate cortex, the insula

103
Q

Anterior cingulate cortex ( ACC )

A

Part of limbic system

Emotional element of pain

104
Q

Insular cortex

A

Processes information of internal state of body  autonomic component of overall pain response

105
Q

ACC experiment

A

ACC neuron responded with more vigour to the experience of pain (receiving) than by simply watching the delivery of painful stimuli to the examiner (watching)

106
Q

pain

A

“an unpleasant sensory and emotional experience associated with actual or potential tissue damage or described in terms of such damage”

107
Q

Projection neurons in lamina I

A

narrow dynamic range ie only tespond to noxious stimuli ( specificity theory of pain )

input from myelinated Aδ nociceptive fibres and both direct and indirect input from ‘C’ fibres

108
Q

Projection neurons in lamina V

A

receive input from both large diameter fibres (Aβ) from mechanoreceptors as well as input from other nociceptors.
 respond to innocuous stimuli at low intensity
 And noxious stimuli at high intensities
 wide dynamic range

109
Q

Wide dynamic range neurons

A

can signal changes in stimulus intensity by increases in spike discharge rate over a wide range of intensities

110
Q

Single neuron with narrow dynamic range

A

only signal changes in intensity over a limited range of amplitudes

111
Q

Gate control theory

A

A-delta and C fibres stimulated by injury and directly excite transmission cells in the spinal chord
Transmission cell also receives input from los-threshold myelinated affarents ( L )  A-beta fibres  whether this inhibits or excites depends on the intensity of the stimulus

112
Q

Decending systems

A

Pain, a heavily modulated sensation:

 integrated with other body systems eg skin reflexes, emotion, attention, autonomic regulation

113
Q

Location of descending systems

A

The periaqueductal gray ( PAG ) in mid-brain
The raphe nucleu
+ other nuclei in medulla

114
Q

Electric stimulation of PAG- effects

A

produce sufficient analgesia

115
Q

PAG respnsibilites

A

control the ‘nociceptive’ gate in the dorsal horn by integrating inputs from the cortex, thalamus and hypothalamus

116
Q

How can morphine induced analgesia be blocked

A

injection of naloxone (an opiate antagonist) into the PAG

Bilateral transection of the dorsolateral funiculus blocks both this stimulation- and morphine-induced analgesia

117
Q

Placebo analgesia

A

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

118
Q

Where does the placebo effect work

A

asthma, cough, diabetes, ulcers, multiple sclerosis and Parkinsonism

119
Q

Neural analgesic placebo

A

Is locally induced and opiate dependent as blocked by naloxone ( an opiod antagonist)
Only works for areas to which topical cream is applied  seems to involve attentional mechanisms

Can be measured experimentally when measuring pain threshold when capsacin is infused into distal limbs

Neuro (13 decks)

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