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Neuroscience (Yeo) > Somatosensory System (Peripheral & Central Pathways) > Flashcards

Somatosensory System (Peripheral & Central Pathways) Flashcards

1
Q

What is the sensory map sharper for?

  • Nociception
  • Light Discriminative Touch
A

Nociceptive Pain Inputs –> SHARPER –> than Light Discriminative Touch

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2
Q

After childhood chickenpox infections, where does the herpes zoster virus lie?

A
  • Dormant in the Primary Sensory Neurone
  • In the Dorsal Root Ganglia
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3
Q

What sensation can the reactivated virus cause?

A
  • Severe Burning Pain
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4
Q

What happens in shingles?

A

Reactivated virus –> travels down the afferent axons –> causing blistering of the skin

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5
Q

How are the effects of shingles presented?

A
  • Often restricted to a single dorsal root –> therefore a single defined dermatome (e.g. T2)
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6
Q

What virus causes shingles?

A

Herpes-Zoster Virus

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7
Q

What can cause reactivation?

A
  • Stress
  • Immunosupression
  • Fatigue
    • Other States of Poor Health
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8
Q

What are the effects of shingles?

How does it cause its symptoms?

A
  • Produces inflammation in all the nerve fibres (starting at dorsal root ganglia itself)
  • Virus travels down the axons to the periphery –> producing skin blisters where there are cutaneous somatosensory endings

(too late to treat with anti-virals at this stage)

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9
Q

What can be a strong indicator of Herpes-Zoster Virus?

A
  • Severe Pain
  • Unilateral
  • Highly Regional Dermatomal Pattern
  • (Even if you do not see the blisters)
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10
Q

When can HZV be very dangerous?

Why?

A
  • In the Trigeminal System
  • Can affect vision quite badly
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11
Q

What are the laminae of Rexed?

A

Vertical Laminations of the Dorsal & Ventral Horns of the Grey Matter

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12
Q

What is found in Lamina II?

A

Substantia Gelatinosa

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13
Q

What is found in lamina III & IV?

A

Sensory Neurons

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14
Q

What is found in laminas V-VIII?

A

Interneurones

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15
Q

What is found in lamina IX?

A

Motor Neurones

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16
Q

What are the dorsal columns?

A
  • Direct & Uncrossed in the Spinal Cord
  • Carry discriminated touch & propioception
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17
Q

What is the passage for afferents which send fibres up the dorsal column?

A
  • Afferent fibres pass through the dorsal column –> before turning towards laminae (V-VIII) to synapse onto interneurones
  • One Branch –> runs up the dorsal column
  • One Branch –> synapses with interneurones at the same level

(it is one afferent fibre that does this - it does not synapse –> the same afferent fibre runs from receptor up)

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18
Q

What are the two divisons of the dorsal column?

A
  • Cuneate Tract
  • Gracile Tract
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19
Q

Where is the gracile tract found?

A
  • Starts at the Lowest Levels of the cord
  • Fibres run forward to the brain (from lower limb & lower trunk)
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20
Q

Where is the cuneate found?

A
  • Fibres are added into the dorsal column as you ascend the spinal cord
  • This builds up and forms the cuneate tract
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21
Q

What is the spinothalamic tract?

A
  • Indirect Tract
  • Crosses the spinal cord
  • Conveys pain & temperature
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22
Q

Where do afferents involved in the spinothalamic tract route terminate?

A
  • Substantia Gelatinosa (lamina II)
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23
Q

What first order afferent types convery nociception, temperature & light touch?

A
  • C-Fibres
  • A-delta fibres
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24
Q

Describe the route of the first & second order neurons in the spinothalamic tract route.

A
  • First order neuron comes in from receptor
  • Synapses with the second order neuron at the substantia gelatinosa
  • Neurone projects across the spinal cord
  • Crosses anterior to the central canal to the antero-lateral pathway (spinothalamic tract)
  • Second order neuron ascends to the thalamus
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25
Q

What is the difference between dorsal columns & spinothalamic tracts in terms of crossing?

A
  • Dorsal Column –> first order neurone itself goes up on the ipsilateral side to receptor
  • Spinothalamic Tract –> second order neurone crosses over then ascends
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26
Q

Where does the second order neurone of the spinothalamic tract cross?

A

In front of the central canal (anterior to it)

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27
Q

What condition can affect these crossing spinothalamic fibres?

A
  • Syrinx
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28
Q

What branches off the first order neurone and where does it go?

A
  • Branch from the first order neurone –> comes off to ascend a local tract
  • This tract is called Lissauer’s Tract (run up a few segments)
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29
Q

What is the spinocerebellar tract?

A
  • Indirect
  • Mainly propioception
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30
Q

Describe how muscle spindle afferents from the legs & lower trunk join spinocerebellar tracts.

A
  • Muscle spindle afferents ascending the gracile tract –> exit at the thoracic level
  • They synapse with neurons of the nucleus dorsalis (Clarke’s column, T1-L3)
  • Axons from Clarke’s column form the dorsal spinocerebellar tract (DSCT) ending in the cerebellum ipsilaterally
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31
Q

Name 2 other spinocerebellar tracts.

A
  • Ventral Spinocerebellar Tracts
  • Cuneocerebellar Tract
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32
Q

What does the ventral spinocerebellar tract do?

A
  • Supplies Golgi Tendon Organ information
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33
Q

What does the cuneocerebellar tract do?

A
  • Similar to the DSCT
  • Equivalent of DSCT but for the upper body
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34
Q

Why are muscle spindle afferents important?

A
  • Supply cerebellum with vital movement control information
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35
Q

What is the Dorsal Spinal Cerebellar Tract (DSCT)?

A
  • This receives afferents which go through the Dorsal Column
  • They then come out to the nuclei in Clark’s Column
  • They then run up to the cerebellum
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36
Q

Describe the route of the first order neuron in the dorsal column tract.

A
  • Enters the dorsal column
  • Branch turns up & ascends to the brain (ipsilateral side)
  • Arrives at the dorsal column nuclei where the tract terminates (in the medulla)
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37
Q

Where do the first order neurons of the fasciculus gracilis tract terminate?

What level is this?

A
  • Nucleus Gracilis
  • Medial Nucleus in the Medulla
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38
Q

What takes place in the nucleus gracilis & nucleus cuneatus?

A
  • First order neurone synapses with the second order neurone
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39
Q

Where does the dorsal column decussate?

A
  • Level of the Dorsal Column Nuclei in the Medulla
  • Synapses with the second order neurone
  • Projects down & ventrally (diagram) –> goes to other side via central decussation
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40
Q

What is the central decussation of the dorsal column called?

A
  • Decussation of the Medial Lemniscus
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41
Q

Describe the pathway of the second order neurons of the medial lemniscus.

Where do they terminate?

A
  • They run up the medial lemnsicus (ascending tract)
  • They terminate in the ventral-posterior nucleus (thalamus)
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42
Q

Where in the thalamus are neurons associated with the motor system found?

A
  • Ventral-Lateral Thalamus
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43
Q

Where in the thalamus are neurons associated with the somatosensory system found?

A

Lateral Ventral-Posterior Thalamus (also medial division)

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44
Q

Where does the third order neurone go from and to in the medial lemniscal pathway?

A
  • From the thalamus to the somatosensory cortex (S1)
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45
Q

Describe briefly the overall dorsal column - medial lemniscal pathway.

Include modalities

A
  • Discriminative touch & propioception –> enter the dorsal column and ascend on ipsilateral side
  • They synapse at the dorsal column nucleus –> then the projection decussates (second order)
  • This goes to the lateral ventral-posterior thalamus
  • This synapses onto the third order neuron which projects to the somatosensory cortex (S1)
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46
Q

What is the trigeminal touch pathway the equivalent of?

A
  • Dorsal Column for the Face
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47
Q

Where does touch information from the face go to in terms of nucleus?

A
  • Principal Sensory Nucleus of the Trigeminal Nerve
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48
Q

Where does touch information project to in the thalamus?

A
  • Medial Ventral-Posterior Nucleus

(this projects S1)

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49
Q

Describe the pathway of a touch/sensory fibre in the face.

A
  • Sensory Fibres (first order neurones) in the trigeminal nerve –> going to the principal sensory trigeminal nucleus
  • Second order nucleus –> goes from the principal nucleus –> where it crosses then ascends –> with the medial lemniscal fibres –> targetting the medial ventral-posterior nucleus of the thalamus
  • This then projects to S1
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50
Q

Roughly where is the face respresented in terms of the somatotopic map in S1?

A
  • Lateral on the Hemisphere
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51
Q

What is the Ventral-Posterior Nucleus of the Thalamus responsible for?

A
  • Somatosensation
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52
Q

What are the two parts of the Ventral-Posterior Nucleus of the Thalamus and what are they responsible for?

A
  • Medial Division –> somatosensation from the face
  • Lateral Division –> somatosensation from the upper body & lower body
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53
Q

What two modalities is the spinothalamic system responsible for?

A
  • Nociception
  • Temperature
54
Q

Describe the route of the spinothalamic neurones.

A
  • First order neurones synapses with second order neurone –> which crosses the midline at level of entering spinal cord
  • Ascend through the medulla (contralateral side) –> to the ventral-posterior nucleus of the thalamus
  • Somatosensation from dorsal column & spinothalamic tract merge here –> thus thalamus is a convergence point
  • Third order neurone from thalamus takes it to primary somatosensory cortex (S1)
55
Q

Where does the dorsal column cross?

A
  • Dorsal Column Nuclei in the Medulla
56
Q

Where does somatosenstion of the dorsal column & spinothalamic tract meet?

A
  • Ventral-Posterior Nucleus of the Thalamus
57
Q

Pathways of somatosensation.

A
58
Q

What criteria is needed before either pathway can cross?

A
  • Both pathways cross after first synapse
59
Q

What sensory modalities is the dorsal column responsible for?

A
  • Touch
  • Propioception
60
Q

Name three dorsal column syndromes.

A
  1. Tabes Dorsalis
  2. Friedreich’s Ataxia
  3. Brown-Sequard Syndrome
61
Q

What is Tabes Dorsalis a result of?

A
  • Most common form of neurosyphilis

(problems with myelination causing loss of propioception making walking difficult showing typical stamping gate - due to unable to determine feedback in muscle during locomotion)

62
Q

What is Friedreich’s Ataxia?

A
  • Genetic degenerative disease of dorsal columns & spinocerebellar tract
63
Q

What causes Brown-Sequard Syndrome?

A
  • Spinal Cord Hemisection

(can be due to physical damage such as knige to spinal cord)

(can also cause problems with spinothalamic tract –> causing spinothalamic syndrome)

64
Q

Name 2 spinothalamic syndromes.

A
  1. Anterior Spinal Artery Syndrome
  2. Syringomyelia
65
Q

What two main modalities is the spinothalamic tract responsible for?

A
  • Pain
  • Temperature
66
Q

What does the anterior spinal artery supply?

A
  • Spinothalamic Tract (decussating fibres)
  • Anterior part of the spinal cord
67
Q

What two things cause anterior spinal artery syndrome?

A
  1. Atherosclerosis
  2. Spinal Injury
68
Q

What is syringomyelia?

A
  • Occlusion of crossing pathways by degenerative expansion of the spinal central canal

(tends to be exclusively seen at cervical level)

69
Q

What causes Brown-Sequard syndrome?

A
  • Cord Hemisection (cut in half)
70
Q

What three effects are seen in Brown-Sequard Syndrome?

A
  1. Loss of Touch & Propioception Ipsilaterally
  2. Loss of Pain & Temperature sensations Contralaterally
  3. Complete loss of sensation at the level of hemisection (on side of hemisection)
71
Q

What injury types cause hemisections of the spinal cord?

A
  • Stabbing Injuries
  • Twisting of Spinal Cord
  • Bending of Spinal Cord
72
Q

What occurs in a partial hemi-section?

A
  • Cut the Afferent (Ascending) Tract on the Same Side

(does not copeltely cut the descending motor tract as it is not complete)

73
Q

Why does complete loss of sensation occur at level of injury?

A
  • Complete loss due to injury to the dorsal root
74
Q

Where does lateral inhibition take place?

A
  • Synaptic actions take place at relay stations such as the dorsal column nuclei
75
Q

What determines the level of acuity in information from sensory receptors?

A
  • Density of Area of Surface

(acuity of the system to be able to resolve between 2 points - e.g. vision)

  • Resolve high levels of detail (e.g. retina)
76
Q

What is important to do to get the best acuity & best version of a sensory event?

A
  • Supress Noise Levels
  • Increase Clarity of Information & Intensity

This is done by signal processing –> to sharpen the image –> give the best version of the sensory events (e.g. photoshop)

77
Q

Why is it important to contrast between two points and ‘distort’ reality?

A
  • Sharpening information is important as information straight from sensory neurones may not be able to distinguish (due to low signal on noise background)
  • Needs amplification (& supression of local noise)
78
Q

How is noise amplification (increase signal) done?

A
  • Via Inhibitory Networks
  • In a system called ‘lateral inhibition’
  • This is a fundamental construct of the sensory nervous system
79
Q

How does lateral inhibition work?

(briefly)

A
  • Input drive –> passed onto both the output neurone + inhibitory interneurone
  • This signals to adjacent signalling stream

This causes it to push adjacent signal down (supress it) –> allowing stronger signals to push down weaker adjacent signals

(some but comparatively less inhibition from the weaker side)

80
Q

What is the aim of lateral inhibition?

A
  • Distortion of the Truth
  • Higher signal-noise ratio for the signals we are interested in
  • Improves detection of incoming events from receptors

(gives an enhanced version of the original sensory information coming in –> allowing you to see differences more clearly)

81
Q

Where does lateral inhibition take place?

A
  • Thalamus
  • Cortex
  • Dorsal Column Nuclei
82
Q

What is the homunculus?

A
  • Topographic map of the contralateral body surface
    (e. g. in post-central gyrus)
83
Q

What is the cost of using lateral inhibition?

A
  • Slight Inaccuracy
    e. g. the points between two activated areas (where it is supressed more than it should be)
84
Q

What does the lateral side of the homonculus map for?

A
  • Face
85
Q

What is the ventro-basal nucleus of the thalamus connected to in the cortex?

A
  • Somatosensory Cortex
86
Q

What nucleus in the thalamus is responsible for auditory part of the cortex?

A
  • Medial Geniculate Nucleus
87
Q

What part of the thalamus is connected to the frontal cortex?

A
  • Medial Dorsal
88
Q

What part of the thalamus is connected to the temporal cortex?

A
  • Lateral Posterior Nucleus
89
Q

What part of the thalamus is connected to the Parietal cortex?

A
  • Pulvinar
90
Q

What nucleus in the thalamus is responsible for visual part of the cortex?

A
  • Lateral Geniculate Nucleus
91
Q

What are the lateral & medial part of the somatosensory ventral-posterior lateral thalamus responsible for?

A
  • VPL (lateral) –> Lower & Upper Body
  • VPL (medial) –> Face

(reverse of homunculus)

92
Q

How many divisions is the post-central gyrus divided into?

A
  • 3 Broddmann Areas

(divided on the basis of pathology)

93
Q

Roughly how are the divisons of the post-central gyrus distributed?

A
  • Top of Post-Central Gyrus –> brodmann area 1
  • Banks either side –> brodmann area 2 & 3

(brodmann area 3 is further divided into 3a & 3b)

94
Q

How is the post-central gyrus divided up?

A
  • Area 3b –> individual digits of the limbs
  • Twin map in area 1 & 3b –> but not exactly the same
95
Q

What is the neuronal architecture in S1 like?

A
  • More similar than different from the rest of the cortex
  • Functionally specialises into mosaic of tiny areas
96
Q

How many layers are there in the cerebral cortex?

A

6 layers

97
Q

Which layer do output neurones come from?

What type of cell are these?

A
  • Layer 5
  • Pyramidal Neurones (output neurones)

(layer 3 are also output neurones)

98
Q

Which layer do input neurons go into?

A
  • Layer 4
99
Q

What is seen in brodmann areas 3b to 5?

A
  • 4 separate maps of the body map are seen in S1
100
Q

What happens to the receptive field as you progress from brodmann area 3b to 5?

A
  • Receptive fields increase in size & complexity
101
Q

What does area 3b cover?

A
  • Smallest Receptor Field
  • Limited to a single fingertip
102
Q

What does area 1?

A

Cells which respond to touch on any of the 4 digits

Receives information from multiple fingers (join up)

103
Q

What does area 2 do?

A
  • Larger area of all the fingers on one hand
    (i. e. any touch sensation on any part of any finger triggers it
104
Q

What does area 5 do?

A
  • Covers both hands
  • All the area of the fingers on either hand
105
Q

What things are the receptive fields selective for?

A
  1. Orientation Selective
  2. Direction Selective
106
Q

How is a receptive field orientation selective?

A
  • Set of all the smaller receptor neurones –> come together –> make a whole field –> which lines up along the axis (that it wishes to measure)
107
Q

How is direction selectivity set up?

A
  • Receptive fields are lined up
  • Inhibitory connections between the receptive fields (not balanced - inhibit in only one direction)
  • Therefore movement of object on skin in certain direction will inhibit the cells in front

(in the other direction it will not work –> will be inhibiting the wrong side)

108
Q

What is area 3b divided up into?

A
  • Divided up into microdomains
109
Q

What do the microdomains in area 3b represent?

A
  • Different classes of skin receptors
    (e. g. Merkel cells & Meissner corpuscles)
110
Q

What does area 3b contain?

A
  • Region for each digit (e.g. D3)
  • Coded in separate parts of the cortex
111
Q

How is location on the body surface represented?

A
  • By columns in the cerebral cortex
112
Q

What happens if you stimulate a specific part of S1?

A
  • Depending on the place stimulated –> you feel sensation in that part
  • Region activity is quite fixed (i.e. will feel hand sensation in han area in S1)
113
Q

When is phantom limb sensation seen?

A
  • After the loss of parts of a limb
114
Q

What is a common complaint of those who have lost a limb?

A
  • Report somatosensation (usually pain) in parts of the limbs they have lost
    (e. g. feel pain in their hand but they have amputated it)
115
Q

Why does phantom limb sensation take place?

A
  • Due to nerve input into somatosensory cortex –> which used to respond to sensation in those amputated parts
  • These are still active (continued activity) –> thus give perception of somatosensation
  • This is because you have a representation of the amputated limb in the cortex set up
116
Q

What is referred phantom limb?

A
  • Occurs in high level amputations
  • Patients report sensation of distal limb in proximal limb (like fingers on the top of arm)

(e.g. fingers perceived when you touch the face)

117
Q

What causes referred phantom limb pain?

A

Referred Phantom Pain = sensation of fingers on the residual part of the arm

  • Due to adjacent territories in the homonculus lost for the part of the territory where somatosensation is lost (e.g. no finger sensation neurones - but homonculus is still there)

This is because somatosensory region in S1 is stimulated by input from a signal that is not where it originally came from (e.g. from face now instead of fingers)

118
Q

What two things can cause referred phantom limb?

A
  • Existing but previously silent connections
  • Sprouting of old connections

(thus get activation of centres causing finger sensation when touching the face)

119
Q

What causes referred phantom sensation?

A
  • Hand area activated by sensory input from alternative source
  • However it causes activation of hand region of the cortex
120
Q

How is visceral pain usually interpreted?

A
  • Somatic sensation
121
Q

What does visceral pain usually relate to?

A
  • Usually relates to the myotome from which it originally developed from embryologically

(viscera migrate considerable distanced from embryonic origin –> thus referred pain can be quite distant from real source)

122
Q

Where does the somatic projection of visceral pain go to from the appendix?

A
  • T10
123
Q

Where is the somatic projection of visceral pain from the OVARIES or UTERUS go to?

A
  • T10-T12
124
Q

Where is the somatic projection of visceral pain from the testes or prostate go to?

A
  • T10-T12
125
Q

Where is the somatic projection of visceral pain from the diaphragm?

A
  • C3-C4 (scapular area)
126
Q

What receptors do viscera have and when are they activated?

A
  • They have silent nociceptors
  • These are not constantly activated
  • Only felt during inflammation (e.g. stomach ache)
127
Q

What part of the cortex is devoted to visceral pain?

A
  • Small part as they are usually not active (needed)
128
Q

What is visceral pain?

A
  • Inflammation in visceral region
  • Pain is reflected/represented elsewhere on the cortex
  • These have been set up to represent somatic inputs (therefore pain there shows something else is hurting - e.g. stomach)
129
Q

Why does referred pain occur?

A
  • Body has no percept of deep internal pain
  • They are reflected onto somatic regions which reflect early embryological origin (in terms of nerve supply & migration)
130
Q

Why is referred pain important clinically?

A
  • Patients report pain on superficial structures
  • Diangosis will be deep & visceral

Neuroscience (Yeo) (8 decks)

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