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Neurology > Somatosensory system > Flashcards

Somatosensory system Flashcards

1
Q

What does the somatosensory system mediate?

A 
Fine discriminatory touch (light touch, pressure, vibration, flutter) 
Stretch (mechanosensation) 
Proprioception 
Thermosensation 
Nociception (pain) 
Pruriception
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2
Q

What are the 3 broad categories of the somatosensory system?

A

Exteroceptive division
Proprioceptive
Enteroceptive

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3
Q

What is the exteroceptive division?

A

Registers information from the surface of the body by numerous receptor types

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4
Q

What is the propceptive division?

A

Monitors posture and movement with sensors in the muscles, tendons and joints

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5
Q

What is the enteroceptive division?

A

Reports upon the internal state of the body and is closely related to autonomic function

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6
Q

What are the neurones involved in the somatosensory pathway?

A 
1st order (PNS): cell body location in the dorsal root ganglion or cranial ganglion 
2nd order (CNS): cell body location in dorsal horn of spinal cord or brainstem nuclei 
3rd order (CNS): cell body in thalamic nuclei
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7
Q

What will stimulus (mechanical, thermal or chemical) do to ion channels?

A

Opens cation selective ion channels in the peripheral terminal of primary sensory afferents eliciting a depolarizing receptor potential

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8
Q

What determines the amplitude of the receptor potential

A

Stimulus intensity

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9
Q

What are important properties of sensory units?

A

Modality - what type of stimulus excites the sensory receptor
Threshold - what intensity of the stimulus is required for excitation of the sensory receptor
Adaptation rate - does the sensory unit discharge action potentials continuously during the stimulus or does it respond preferentially to a changing stimulus
Conduction velocity - how rapidly does the sensory unit conduct action potentials along its axon
Site and extent of peripheral termination

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10
Q

What is the sensory unit for touch, pressure and vibration modality?

A

Skin mechanoreceptors

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11
Q

What is the sensory unit for proprioception modality?

A

Joint and muscle mechanoreceptord

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12
Q

What is the sensory unit for temperature modality?

A

Cold and warm thermoreceptors

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13
Q

What is the sensory unit for pain modality?

A

Mechanical, thermal and polymodal nociceptors

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14
Q

What is the sensory unit for itch modality?

A

Itch receptors

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15
Q

What does threshold relate to?

A

The intensity of a stimulus required to excite a sensory unit

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16
Q

What is a low threshold unir?

A

Responds to low intensity (non-damaging) stimuli

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17
Q

What is a low threshold mechanoreceptor?

A

Mediates fine discriminatory touch

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18
Q

What is a low threshold thermoreceptor?

A

Mediates cold through to hot

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19
Q

What is an example of a high threshold unit?

A

Nociceptors; responds to high (noxious) but not low intensity stimuli

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20
Q

What is a high threshold mechanoreceptor?

A

Mechanical nociceptors that respond to high intensity mechanical stimuli

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21
Q

What is a thermal nociceptor?

A

Responds to extreme degrees of heat ; >45 or <10

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22
Q

What do chemical nociceptors respond to?

A 
Substances in tissue; 
Inflammation 
Prostaglandins
Bradykinin
Serotonin
Histamine
K+ 
H+ ATP
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23
Q

What will polymodal nociceptors respond to?

A

At least 2 of:
High threshold mechanoreceptors
Thermal nociceptors
Chemical nocicpeotrs

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24
Q

What is adaptation in the somatosensory context?

A

Feature of sensory units that determines whether they change their firing rate only in response to a stimulus of changing intensity, or fire continuously throughout a constant stimulus

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25
Q

What does a slowly adapting/ tonic response convey?

A

Continuous information to CNS while terminal deformed

Provides information about position, degree of stretch or force e.g. stretch receptors

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26
Q

What does a fast adapting/ dynamic response convey?

A

Detects changes in stimulus strength (rate of movement)
Number of impulses is proportional to the rate change of stimulus
Includes muscle spindles, hair follicle afferents

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27
Q

What does a very fast adapting/ very dynamic response convey?

A

Responds only to very fast movement, such as rapid vibration
E.g. pacinian corpuscle

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28
Q

What differentiates the different types of primary sensory afferent fibres?

A

Axon diameter
Extent of myelination
Conduction velocity
Assoc sensory receptor

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29
Q

What are the different types of primary sensory afferent fibres?

A

Alpha - group 1
Beta - group 2
Delta - group 3
C - group 4

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30
Q

Describe an alpha fibre?

A

Thickest diameter
Thick myelination
Fastest conduction velocity at 80-120 m/s
Sensory receptor for proprioceptors of skeletal muscle

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31
Q

Describe a beta fibre

A

Medium diameter
Moderate myelination
Conduction velocity of 35-75 m/s
Sensory receptor for mechanoreceptors of skin

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32
Q

Describe a delta fibre

A

Small diameter
Thin myelination
Conduction velocity of 5-20 m/s
Sensory receptor for pain and temp

33
Q

Describe a C fibre

A

Smallest diameter
No myelination
Conduction velocity of 0/5-2.0
Sensory receptor for temp, pain and itch

34
Q

What is the receptive field?

A

Target territory from which a sensory unit can be excited

35
Q

What is sensory acuity?

A

Fineness of discrimination

36
Q

How does sensory acuity relate with RF size?

A

Correlates inversely

37
Q

What is two point discrimination ?

A

An important measure of somatosensory function

38
Q

How is two point discrimination clinically tested?

A

Applying simultaneously two sharp point stimuli, separated by a variable distance at different sites on the body surface

39
Q

What is the relationship between 2 point threshold and the diameter of corresponding RF?

A

Regions with the highest discriminative capacity have the smallest RFs

40
Q

What are the different types of cutaneous receptors?

A 
Free nerve endings
Meissner's corpuscle
Merkel's disc
Hair end organs
Krause end bulbs
Ruffini endings
Pacinian corpuscles
41
Q

Where can free nerve endings be found and what is the function?

A

Ubiquitous distribution

Pain, heat, cold

42
Q

Where can meissner’s corpuscles be found and what is the function?

A

Abundant in skin locations where two point discrimination is highest, not present in hairy skin
Touch

43
Q

Where can merkel’s discs be found and what is the function?

A

Same in meissner’s corpuscle but present in moderate numbers in hairy skin
Touch

44
Q

Where can krause end bulbs be found and what is the function?

A

At border of dry skin and mucous membranes

Touch

45
Q

Where can ruffini endings be found and what is the function?

A

Within dermis and joint capsules

Pressure

46
Q

Where can pacinian corpuscles be found and what is the function?

A

Within dermins and fascia

Pressure

47
Q

What are the subdivisions of skin LTM?

A

Rate of adaptation: Fast or slow

Size of receptive field: small (type 1) or wide (type 2)

48
Q

What are the receptors and parent fibres of free nerve endings?

A

Parent nerve fibre: delta of C

No receptor

49
Q

What are the receptors and parent fibres of merkel discs?

A

Parent fibre: beta

Slow adapting type 1 unit receptor

50
Q

What are the receptors and the parent fibre type of meissner’s corpuscles?

A

Parent fibre: beta

Fast adapting type 1 unit

51
Q

What are the receptors and the parent fibre type of ruffini endings?

A

Parent fibre: beta

Slow adapting type 2 unit

52
Q

What are the receptors and the parent fibre type of pacinian corpuscles?

A

Parent fibre: betea

Fast adapting type 2 unit

53
Q

What is the maximal vibration of human detection?

A

150 Hz; this is why a 128 tuning fork is utilised

54
Q

What fibre class and termination in the spinal cord does nociceptors utilise?

A

Fibre: delta/C
Termination: laminae 1 and 2

55
Q

What fibre class and termination in the spinal cord does LTMs utilise?

A

Fibre: beta
Termination: laminae 3-6

56
Q

What fibre class and termination in the spinal cord does proprioceptors utilise?

A

Fibre: alpha
Termination: laminae 7-9

57
Q

What is the laminae of rexed?

A

Subdivisions of grey matter

58
Q

What sensory information does the dorsal column medial lemniscal pathway process?

A

Discriminatory touch
Pressure
Vibration
Proprioception

59
Q

Where will the dorsal column fibres decussate?

A

Medulla at the great sensory decussation to then ascend in the medial lemniscus to the ventral posterior lateral nucleus of the thalamus

60
Q

What sensory information does the spinothalamic tract process?

A

Pain, thermosensation, crude touch, itch, tickle

61
Q

What is the difference between the medial gracile tract and the lateral cuneate tract of the dorsal column?

A

Gracile nuclei: lower limbs and lower trunk - below T6

Cuneate tract: upper limbs and upper trunk - above T6

62
Q

How will the neurones of the DCML pathway reach the primary somatosensory cortex of the post central gyrus from the thalamus?

A

Posterior internal capsule

63
Q

What is stereognosis?

A

Ability to recognise an object by feeling it

64
Q

What is contrast enhancement?

A

As information is conveyed from one neurone to the next in a sensory pathway, differences in the activity of adjacent neurones are amplified producing a contrast enhancement

65
Q

What is lateral inhibition?

A

When one neurone is active, it will inhibit the activity of its neighbours via inhibitory interneurons

66
Q

What will the trigeminothalamic pathway process?

A

General somatic information from the anterior head, oral and nasal cavities, sinuses, intracranial dura and cerebral arteries

67
Q

Where are the soma of sensory neurones of the trigeminal nerve found?

A

Trigeminal sensory ganglion

68
Q

What will synapse in the spinal nucleus?

A

Pain and temp from face

69
Q

How will information from the trigeminal nuclei reach the thalamus?

A

Via trigeminal lemniscus to the ventroposteriomedial nucleus

70
Q

How will information from the VPM nucleus of the thalamus reach the cortex?

A

Via thalamocortical neurones

71
Q

Which brodmann areas is the somatosensory cortex in the parietal lobe associated with?

A

1,2,3a and 3b

72
Q

Describe the sensory homunculus

A

Toes at top
Tongue at bottom
Hand separates the head from the face

73
Q

How many layers are there to the somatosensory cortex?

A

6 cell layers

74
Q

Which layer of the somatosensory cortex receives the majority of neurones from the thalamus?

A

4

75
Q

What are the layers of the somatosensory cortex?

A 
1 = molecular
2 = external granular 
3 = external pyramidal 
4 = internal granular 
5 = internal pyramidal 
6 = multiform
76
Q

What does the posterior parietal cortex receive?

A

Information from the somatosensory cortex and other cortical areas such as visual and auditory and subcortical areas such as the thalamus

77
Q

What does the posterior parietal cortex do?

A

Deciphers the deeper meaning of information from the somatosensory cortex

78
Q

What can damage to the posterior parietal cortex result in?

A

Bizarre neurological disorders; agnosia, astereognosis, hemispatial neglect syndrome

79
Q

What is hemispatial neglect syndrome?

A

Damage to right parietal cortex

Patients believe that the left side of the world does not exist

Neurology (48 decks)

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