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
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
3
Q
What sensation can the reactivated virus cause?
A
- Severe Burning Pain
4
Q
What happens in shingles?
A
Reactivated virus –> travels down the afferent axons –> causing blistering of the skin
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)
6
Q
What virus causes shingles?
A
Herpes-Zoster Virus
7
Q
What can cause reactivation?
A
- Stress
- Immunosupression
- Fatigue
- Other States of Poor Health
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)
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)
10
Q
When can HZV be very dangerous?
Why?
A
- In the Trigeminal System
- Can affect vision quite badly
11
Q
What are the laminae of Rexed?
A
Vertical Laminations of the Dorsal & Ventral Horns of the Grey Matter
12
Q
What is found in Lamina II?
A
Substantia Gelatinosa
13
Q
What is found in lamina III & IV?
A
Sensory Neurons
14
Q
What is found in laminas V-VIII?
A
Interneurones
15
Q
What is found in lamina IX?
A
Motor Neurones
16
Q
What are the dorsal columns?
A
- Direct & Uncrossed in the Spinal Cord
- Carry discriminated touch & propioception
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)
18
Q
What are the two divisons of the dorsal column?
A
- Cuneate Tract
- Gracile Tract
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)
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
21
Q
What is the spinothalamic tract?
A
- Indirect Tract
- Crosses the spinal cord
- Conveys pain & temperature
22
Q
Where do afferents involved in the spinothalamic tract route terminate?
A
- Substantia Gelatinosa (lamina II)
23
Q
What first order afferent types convery nociception, temperature & light touch?
A
- C-Fibres
- A-delta fibres
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
25
What is the difference between **dorsal** **columns** & **spinothalamic** **tracts** in terms of crossing?
* **Dorsal Column** --\> **first order neurone** itself goes up on the **ipsilateral** **side** to **receptor**
* **Spinothalamic Tract** --\> **second** **order** **neurone** **crosses over** then **ascends**
26
Where does the second order neurone of the spinothalamic tract cross?
In front of the central canal (anterior to it)
27
What condition can affect these **crossing** **spinothalamic** **fibres**?
* Syrinx
28
What branches off the first order neurone and where does it go?
* 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)
29
What is the **spinocerebellar** **tract**?
* Indirect
* Mainly propioception
30
Describe how **muscle** **spindle** **afferents** from the **legs** & **lower** **trunk** join **spinocerebellar** **tracts.**
* **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**
31
Name 2 other spinocerebellar tracts.
* Ventral Spinocerebellar Tracts
* Cuneocerebellar Tract
32
What does the **ventral** **spinocerebellar** **tract** do?
* Supplies **Golgi Tendon Organ** information
33
What does the **cuneocerebellar** **tract** do?
* Similar to the DSCT
* **Equivalent** of **DSCT** but for the **upper** **body**
34
Why are muscle spindle afferents important?
* Supply **cerebellum** with **vital** **movement** **control** **information**
35
What is the Dorsal Spinal Cerebellar Tract (DSCT)?
* 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**
36
Describe the route of the first order neuron in the dorsal column tract.
* 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)
37
Where do the first order neurons of the fasciculus gracilis tract terminate?
What level is this?
* Nucleus Gracilis
* Medial Nucleus in the Medulla
38
What takes place in the nucleus gracilis & nucleus cuneatus?
* **First order neurone** synapses with the **second order neurone**
39
Where does the dorsal column decussate?
* 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**
40
What is the **central** **decussation** of the **dorsal** **column** called?
* Decussation of the Medial Lemniscus
41
Describe the pathway of the second order neurons of the medial lemniscus.
Where do they terminate?
* They run up the **medial** **lemnsicus** (ascending tract)
* They terminate in the **ventral-posterior nucleus (thalamus)**
42
Where in the **thalamus** are neurons associated with the **motor system found**?
* Ventral-Lateral Thalamus
43
Where in the **thalamus** are neurons associated with the **somatosensory** system found?
**Lateral Ventral-Posterior Thalamus** (also medial division)
44
Where does the third order neurone go from and to in the medial lemniscal pathway?
* From the **thalamus** to the **somatosensory cortex (S1)**
45
Describe briefly the overall dorsal column - medial lemniscal pathway.
Include modalities
* **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)**
46
What is the **trigeminal** **touch** **pathway** the **equivalent** of?
* Dorsal Column for the Face
47
Where does **touch** **information** from the face go to in terms of **nucleus**?
* **Principal** **Sensory** **Nucleus** of the **Trigeminal** **Nerve**
48
Where does touch information project to in the thalamus?
* Medial Ventral-Posterior Nucleus
(this projects S1)
49
Describe the **pathway** of a **touch/sensory fibre** in the face**.**
* **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**
50
Roughly where is the face respresented in terms of the somatotopic map in S1?
* Lateral on the Hemisphere
51
What is the **Ventral**-**Posterior** **Nucleus** of the **Thalamus** responsible for?
* Somatosensation
52
What are the two parts of the **Ventral-Posterior Nucleus** of the **Thalamus** and what are they responsible for?
* **_Medial_ Division** --\> somatosensation from the **face**
* **_Lateral_ Division** --\> somatosensation from the **upper body & lower body**
53
What two modalities is the **spinothalamic** **system** responsible for?
* Nociception
* Temperature
54
Describe the **route** of the **spinothalamic** **neurones**.
* 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
Where does the dorsal column cross?
* **Dorsal** **Column** **Nuclei** in the **Medulla**
56
Where does somatosenstion of the **dorsal** **column** & **spinothalamic** **tract** meet?
* **Ventral**-**Posterior** **Nucleus** of the **Thalamus**
57
Pathways of somatosensation.
58
What criteria is needed before either pathway can cross?
* Both pathways **cross** **_after_** **first** **synapse**
59
What **sensory** **modalities** is the **dorsal** **column** responsible for?
* Touch
* Propioception
60
Name three dorsal column syndromes.
1. Tabes Dorsalis
2. Friedreich's Ataxia
3. Brown-Sequard Syndrome
61
What is **Tabes** **Dorsalis** a result of?
* **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
What is **Friedreich's** **Ataxia**?
* **Genetic degenerative** **disease** of **_dorsal columns_** & **_spinocerebellar_** tract
63
What causes **Brown**-**Sequard** **Syndrome**?
* 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
Name 2 spinothalamic syndromes.
1. Anterior Spinal Artery Syndrome
2. Syringomyelia
65
What **two** **main** **modalities** is the **spinothalamic** **tract** responsible for?
* Pain
* Temperature
66
What does the **anterior** **spinal** **artery** supply?
* **Spinothalamic Tract (decussating fibres)**
* **Anterior** **part** of the **spinal** **cord**
67
What two things cause **anterior** **spinal** **artery** **syndrome**?
1. Atherosclerosis
2. Spinal Injury
68
What is **syringomyelia**?
* Occlusion of **crossing** **pathways** by **degenerative** **expansion** of the **spinal** **central** **canal**
(tends to be exclusively seen at cervical level)
69
What causes Brown-Sequard syndrome?
* Cord Hemisection (cut in half)
70
What three effects are seen in **Brown-Sequard Syndrome**?
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
What injury types cause **hemisections** of the **spinal** **cord**?
* Stabbing Injuries
* Twisting of Spinal Cord
* Bending of Spinal Cord
72
What occurs in a **partial** **hemi**-**section**?
* Cut the **Afferent (Ascending) Tract** on the **Same Side**
(does not copeltely cut the descending motor tract as it is not complete)
73
Why does **complete** **loss** of **sensation** occur at **level** of **injury**?
* **Complete** **loss** due to **injury** to the **dorsal** **root**
74
Where does lateral inhibition take place?
* Synaptic actions take place at **relay stations** such as the **dorsal column nuclei**
75
What determines the level of acuity in information from sensory receptors?
* 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
What is important to do to get the best acuity & best version of a sensory event?
* 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
Why is it important to contrast between two points and 'distort' reality?
* **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
How is **noise** **amplification** (increase signal) done?
* Via **Inhibitory Networks**
* In a system called **_'lateral inhibition'_**
* This is a fundamental construct of the **sensory nervous system**
79
How does lateral inhibition work?
| (briefly)
* **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
What is the aim of lateral inhibition?
* 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
Where does lateral inhibition take place?
* Thalamus
* Cortex
* Dorsal Column Nuclei
82
What is the **homunculus**?
* **Topographic** **map** of the **contralateral** **body** **surface**
(e. g. in post-central gyrus)
83
What is the cost of using lateral inhibition?
* Slight Inaccuracy
e. g. the points between two activated areas (where it is supressed more than it should be)
84
What does the **lateral** **side** of the **homonculus** map for?
* Face
85
What is the **ventro-basal nucleus** of the thalamus connected to in the cortex?
* Somatosensory Cortex
86
What nucleus in the thalamus is responsible for **auditory** **part** of the cortex?
* Medial Geniculate Nucleus
87
What part of the **thalamus** is connected to the **frontal cortex**?
* Medial Dorsal
88
What part of the **thalamus** is connected to the **temporal cortex**?
* Lateral Posterior Nucleus
89
What part of the **thalamus** is connected to the **Parietal cortex**?
* Pulvinar
90
What nucleus in the thalamus is responsible for **visual** **part** of the cortex?
* Lateral Geniculate Nucleus
91
What are the lateral & medial part of the somatosensory **ventral-posterior lateral thalamus** responsible for?
* VPL (lateral) --\> Lower & Upper Body
* VPL (medial) --\> Face
(reverse of homunculus)
92
How many divisions is the post-central gyrus divided into?
* **3 Broddmann Areas**
(divided on the basis of pathology)
93
Roughly how are the divisons of the post-central gyrus distributed?
* **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
How is the post-central gyrus divided up?
* Area 3b --\> individual digits of the limbs
* Twin map in area 1 & 3b --\> but not exactly the same
95
What is the neuronal architecture in S1 like?
* **More** **similar** than **different** from the **rest of the cortex**
* **Functionally** **specialises** into **mosaic** of **tiny** **areas**
96
How many layers are there in the cerebral cortex?
6 layers
97
Which layer do output neurones come from?
What type of cell are these?
* Layer 5
* Pyramidal Neurones (output neurones)
(layer 3 are also output neurones)
98
Which layer do input neurons go into?
* Layer 4
99
What is seen in brodmann areas 3b to 5?
* 4 separate maps of the body map are seen in S1
100
What happens to the receptive field as you progress from brodmann area 3b to 5?
* **Receptive** **fields** increase in **size** & **complexity**
101
What does area 3b cover?
* Smallest Receptor Field
* Limited to a single fingertip
102
What does area 1?
Cells which respond to **touch** on **_any_** of the **4 digits**
Receives information from multiple fingers (join up)
103
What does area 2 do?
* **Larger area** of **_all_ the fingers** on **_one_** **hand**
(i. e. any touch sensation on any part of any finger triggers it
104
What does area 5 do?
* Covers **_both_** **hands**
* **All the area of the fingers** on **either hand**
105
What things are the receptive fields selective for?
1. Orientation Selective
2. Direction Selective
106
How is a receptive field **orientation selective**?
* 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
How is direction selectivity set up?
* 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
What is area 3b divided up into?
* Divided up into **microdomains**
109
What do the microdomains in area 3b represent?
* Different classes of skin receptors
(e. g. Merkel cells & Meissner corpuscles)
110
What does area 3b contain?
* Region for each digit (e.g. D3)
* Coded in separate parts of the cortex
111
How is location on the body surface represented?
* By columns in the cerebral cortex
112
What happens if you stimulate a specific part of S1?
* 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
When is phantom limb sensation seen?
* After the **loss** of **parts of a limb**
114
What is a common complaint of those who have lost a limb?
* 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
Why does phantom limb sensation take place?
* 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
What is **referred** **phantom** **limb**?
* 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
What causes **referred phantom limb pain**?
**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
What two things can cause referred phantom limb?
* Existing but previously silent connections
* Sprouting of old connections
(thus get activation of centres causing finger sensation when touching the face)
119
What causes referred phantom sensation?
* **Hand** **area** activated by **sensory** **input** from **alternative** **source**
* However it causes **activation** of **hand** **region** of the **cortex**
120
How is visceral pain usually interpreted?
* Somatic sensation
121
What does visceral pain usually relate to?
* 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
Where does the somatic projection of visceral pain go to from the appendix?
* T10
123
Where is the somatic projection of visceral pain from the **OVARIES** or **UTERUS** go to?
* T10-T12
124
Where is the somatic projection of visceral pain from the **testes** or **prostate** go to?
* T10-T12
125
Where is the somatic projection of visceral pain from the diaphragm?
* C3-C4 (scapular area)
126
What receptors do viscera have and when are they activated?
* They have **silent** **nociceptors**
* These are **_not_** **constantly** **activated**
* **Only** **felt** during **inflammation** (e.g. stomach ache)
127
What part of the cortex is devoted to visceral pain?
* Small part as they are usually not active (needed)
128
What is visceral pain?
* **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
Why does referred pain occur?
* 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
Why is referred pain important clinically?
* Patients report pain on superficial structures
* Diangosis will be deep & visceral
