Flashcards in Somatosensory system Deck (79):
1
What does the somatosensory system mediate?
Fine discriminatory touch (light touch, pressure, vibration, flutter)
Stretch (mechanosensation)
Proprioception
Thermosensation
Nociception (pain)
Pruriception
2
What are the 3 broad categories of the somatosensory system?
Exteroceptive division
Proprioceptive
Enteroceptive
3
What is the exteroceptive division?
Registers information from the surface of the body by numerous receptor types
4
What is the propceptive division?
Monitors posture and movement with sensors in the muscles, tendons and joints
5
What is the enteroceptive division?
Reports upon the internal state of the body and is closely related to autonomic function
6
What are the neurones involved in the somatosensory pathway?
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
7
What will stimulus (mechanical, thermal or chemical) do to ion channels?
Opens cation selective ion channels in the peripheral terminal of primary sensory afferents eliciting a depolarizing receptor potential
8
What determines the amplitude of the receptor potential
Stimulus intensity
9
What are important properties of sensory units?
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
10
What is the sensory unit for touch, pressure and vibration modality?
Skin mechanoreceptors
11
What is the sensory unit for proprioception modality?
Joint and muscle mechanoreceptord
12
What is the sensory unit for temperature modality?
Cold and warm thermoreceptors
13
What is the sensory unit for pain modality?
Mechanical, thermal and polymodal nociceptors
14
What is the sensory unit for itch modality?
Itch receptors
15
What does threshold relate to?
The intensity of a stimulus required to excite a sensory unit
16
What is a low threshold unir?
Responds to low intensity (non-damaging) stimuli
17
What is a low threshold mechanoreceptor?
Mediates fine discriminatory touch
18
What is a low threshold thermoreceptor?
Mediates cold through to hot
19
What is an example of a high threshold unit?
Nociceptors; responds to high (noxious) but not low intensity stimuli
20
What is a high threshold mechanoreceptor?
Mechanical nociceptors that respond to high intensity mechanical stimuli
21
What is a thermal nociceptor?
Responds to extreme degrees of heat ; >45 or <10
22
What do chemical nociceptors respond to?
Substances in tissue;
Inflammation
Prostaglandins
Bradykinin
Serotonin
Histamine
K+
H+ ATP
23
What will polymodal nociceptors respond to?
At least 2 of:
High threshold mechanoreceptors
Thermal nociceptors
Chemical nocicpeotrs
24
What is adaptation in the somatosensory context?
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
25
What does a slowly adapting/ tonic response convey?
Continuous information to CNS while terminal deformed
Provides information about position, degree of stretch or force e.g. stretch receptors
26
What does a fast adapting/ dynamic response convey?
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
27
What does a very fast adapting/ very dynamic response convey?
Responds only to very fast movement, such as rapid vibration
E.g. pacinian corpuscle
28
What differentiates the different types of primary sensory afferent fibres?
Axon diameter
Extent of myelination
Conduction velocity
Assoc sensory receptor
29
What are the different types of primary sensory afferent fibres?
Alpha - group 1
Beta - group 2
Delta - group 3
C - group 4
30
Describe an alpha fibre?
Thickest diameter
Thick myelination
Fastest conduction velocity at 80-120 m/s
Sensory receptor for proprioceptors of skeletal muscle
31
Describe a beta fibre
Medium diameter
Moderate myelination
Conduction velocity of 35-75 m/s
Sensory receptor for mechanoreceptors of skin
32
Describe a delta fibre
Small diameter
Thin myelination
Conduction velocity of 5-20 m/s
Sensory receptor for pain and temp
33
Describe a C fibre
Smallest diameter
No myelination
Conduction velocity of 0/5-2.0
Sensory receptor for temp, pain and itch
34
What is the receptive field?
Target territory from which a sensory unit can be excited
35
What is sensory acuity?
Fineness of discrimination
36
How does sensory acuity relate with RF size?
Correlates inversely
37
What is two point discrimination ?
An important measure of somatosensory function
38
How is two point discrimination clinically tested?
Applying simultaneously two sharp point stimuli, separated by a variable distance at different sites on the body surface
39
What is the relationship between 2 point threshold and the diameter of corresponding RF?
Regions with the highest discriminative capacity have the smallest RFs
40
What are the different types of cutaneous receptors?
Free nerve endings
Meissner's corpuscle
Merkel's disc
Hair end organs
Krause end bulbs
Ruffini endings
Pacinian corpuscles
41
Where can free nerve endings be found and what is the function?
Ubiquitous distribution
Pain, heat, cold
42
Where can meissner's corpuscles be found and what is the function?
Abundant in skin locations where two point discrimination is highest, not present in hairy skin
Touch
43
Where can merkel's discs be found and what is the function?
Same in meissner's corpuscle but present in moderate numbers in hairy skin
Touch
44
Where can krause end bulbs be found and what is the function?
At border of dry skin and mucous membranes
Touch
45
Where can ruffini endings be found and what is the function?
Within dermis and joint capsules
Pressure
46
Where can pacinian corpuscles be found and what is the function?
Within dermins and fascia
Pressure
47
What are the subdivisions of skin LTM?
Rate of adaptation: Fast or slow
Size of receptive field: small (type 1) or wide (type 2)
48
What are the receptors and parent fibres of free nerve endings?
Parent nerve fibre: delta of C
No receptor
49
What are the receptors and parent fibres of merkel discs?
Parent fibre: beta
Slow adapting type 1 unit receptor
50
What are the receptors and the parent fibre type of meissner's corpuscles?
Parent fibre: beta
Fast adapting type 1 unit
51
What are the receptors and the parent fibre type of ruffini endings?
Parent fibre: beta
Slow adapting type 2 unit
52
What are the receptors and the parent fibre type of pacinian corpuscles?
Parent fibre: betea
Fast adapting type 2 unit
53
What is the maximal vibration of human detection?
150 Hz; this is why a 128 tuning fork is utilised
54
What fibre class and termination in the spinal cord does nociceptors utilise?
Fibre: delta/C
Termination: laminae 1 and 2
55
What fibre class and termination in the spinal cord does LTMs utilise?
Fibre: beta
Termination: laminae 3-6
56
What fibre class and termination in the spinal cord does proprioceptors utilise?
Fibre: alpha
Termination: laminae 7-9
57
What is the laminae of rexed?
Subdivisions of grey matter
58
What sensory information does the dorsal column medial lemniscal pathway process?
Discriminatory touch
Pressure
Vibration
Proprioception
59
Where will the dorsal column fibres decussate?
Medulla at the great sensory decussation to then ascend in the medial lemniscus to the ventral posterior lateral nucleus of the thalamus
60
What sensory information does the spinothalamic tract process?
Pain, thermosensation, crude touch, itch, tickle
61
What is the difference between the medial gracile tract and the lateral cuneate tract of the dorsal column?
Gracile nuclei: lower limbs and lower trunk - below T6
Cuneate tract: upper limbs and upper trunk - above T6
62
How will the neurones of the DCML pathway reach the primary somatosensory cortex of the post central gyrus from the thalamus?
Posterior internal capsule
63
What is stereognosis?
Ability to recognise an object by feeling it
64
What is contrast enhancement?
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
What is lateral inhibition?
When one neurone is active, it will inhibit the activity of its neighbours via inhibitory interneurons
66
What will the trigeminothalamic pathway process?
General somatic information from the anterior head, oral and nasal cavities, sinuses, intracranial dura and cerebral arteries
67
Where are the soma of sensory neurones of the trigeminal nerve found?
Trigeminal sensory ganglion
68
What will synapse in the spinal nucleus?
Pain and temp from face
69
How will information from the trigeminal nuclei reach the thalamus?
Via trigeminal lemniscus to the ventroposteriomedial nucleus
70
How will information from the VPM nucleus of the thalamus reach the cortex?
Via thalamocortical neurones
71
Which brodmann areas is the somatosensory cortex in the parietal lobe associated with?
1,2,3a and 3b
72
Describe the sensory homunculus
Toes at top
Tongue at bottom
Hand separates the head from the face
73
How many layers are there to the somatosensory cortex?
6 cell layers
74
Which layer of the somatosensory cortex receives the majority of neurones from the thalamus?
4
75
What are the layers of the somatosensory cortex?
1 = molecular
2 = external granular
3 = external pyramidal
4 = internal granular
5 = internal pyramidal
6 = multiform
76
What does the posterior parietal cortex receive?
Information from the somatosensory cortex and other cortical areas such as visual and auditory and subcortical areas such as the thalamus
77
What does the posterior parietal cortex do?
Deciphers the deeper meaning of information from the somatosensory cortex
78
What can damage to the posterior parietal cortex result in?
Bizarre neurological disorders; agnosia, astereognosis, hemispatial neglect syndrome
79