Somatosensory System (Peripheral & Central Pathways) Flashcards
What is the sensory map sharper for?
- Nociception
- Light Discriminative Touch
Nociceptive Pain Inputs –> SHARPER –> than Light Discriminative Touch
After childhood chickenpox infections, where does the herpes zoster virus lie?
- Dormant in the Primary Sensory Neurone
- In the Dorsal Root Ganglia
What sensation can the reactivated virus cause?
- Severe Burning Pain
What happens in shingles?
Reactivated virus –> travels down the afferent axons –> causing blistering of the skin
How are the effects of shingles presented?
- Often restricted to a single dorsal root –> therefore a single defined dermatome (e.g. T2)
What virus causes shingles?
Herpes-Zoster Virus
What can cause reactivation?
- Stress
- Immunosupression
- Fatigue
- Other States of Poor Health
What are the effects of shingles?
How does it cause its symptoms?
- 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)
What can be a strong indicator of Herpes-Zoster Virus?
- Severe Pain
- Unilateral
- Highly Regional Dermatomal Pattern
- (Even if you do not see the blisters)
When can HZV be very dangerous?
Why?
- In the Trigeminal System
- Can affect vision quite badly
What are the laminae of Rexed?
Vertical Laminations of the Dorsal & Ventral Horns of the Grey Matter
What is found in Lamina II?
Substantia Gelatinosa
What is found in lamina III & IV?
Sensory Neurons
What is found in laminas V-VIII?
Interneurones
What is found in lamina IX?
Motor Neurones
What are the dorsal columns?
- Direct & Uncrossed in the Spinal Cord
- Carry discriminated touch & propioception
What is the passage for afferents which send fibres up the dorsal column?
- 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)
What are the two divisons of the dorsal column?
- Cuneate Tract
- Gracile Tract
Where is the gracile tract found?
- Starts at the Lowest Levels of the cord
- Fibres run forward to the brain (from lower limb & lower trunk)
Where is the cuneate found?
- Fibres are added into the dorsal column as you ascend the spinal cord
- This builds up and forms the cuneate tract
What is the spinothalamic tract?
- Indirect Tract
- Crosses the spinal cord
- Conveys pain & temperature
Where do afferents involved in the spinothalamic tract route terminate?
- Substantia Gelatinosa (lamina II)
What first order afferent types convery nociception, temperature & light touch?
- C-Fibres
- A-delta fibres
Describe the route of the first & second order neurons in the spinothalamic tract route.
- 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
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
Where does the second order neurone of the spinothalamic tract cross?
In front of the central canal (anterior to it)
What condition can affect these crossing spinothalamic fibres?
- Syrinx
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)
What is the spinocerebellar tract?
- Indirect
- Mainly propioception
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
Name 2 other spinocerebellar tracts.
- Ventral Spinocerebellar Tracts
- Cuneocerebellar Tract
What does the ventral spinocerebellar tract do?
- Supplies Golgi Tendon Organ information
What does the cuneocerebellar tract do?
- Similar to the DSCT
- Equivalent of DSCT but for the upper body
Why are muscle spindle afferents important?
- Supply cerebellum with vital movement control information
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
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)
Where do the first order neurons of the fasciculus gracilis tract terminate?
What level is this?
- Nucleus Gracilis
- Medial Nucleus in the Medulla
What takes place in the nucleus gracilis & nucleus cuneatus?
- First order neurone synapses with the second order neurone
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
What is the central decussation of the dorsal column called?
- Decussation of the Medial Lemniscus
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)
Where in the thalamus are neurons associated with the motor system found?
- Ventral-Lateral Thalamus
Where in the thalamus are neurons associated with the somatosensory system found?
Lateral Ventral-Posterior Thalamus (also medial division)
Where does the third order neurone go from and to in the medial lemniscal pathway?
- From the thalamus to the somatosensory cortex (S1)
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)
What is the trigeminal touch pathway the equivalent of?
- Dorsal Column for the Face
Where does touch information from the face go to in terms of nucleus?
- Principal Sensory Nucleus of the Trigeminal Nerve
Where does touch information project to in the thalamus?
- Medial Ventral-Posterior Nucleus
(this projects S1)
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
Roughly where is the face respresented in terms of the somatotopic map in S1?
- Lateral on the Hemisphere
What is the Ventral-Posterior Nucleus of the Thalamus responsible for?
- Somatosensation
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
What two modalities is the spinothalamic system responsible for?
- Nociception
- Temperature
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)
Where does the dorsal column cross?
- Dorsal Column Nuclei in the Medulla
Where does somatosenstion of the dorsal column & spinothalamic tract meet?
- Ventral-Posterior Nucleus of the Thalamus
Pathways of somatosensation.
What criteria is needed before either pathway can cross?
- Both pathways cross after first synapse
What sensory modalities is the dorsal column responsible for?
- Touch
- Propioception
Name three dorsal column syndromes.
- Tabes Dorsalis
- Friedreich’s Ataxia
- Brown-Sequard Syndrome
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)
What is Friedreich’s Ataxia?
- Genetic degenerative disease of dorsal columns & spinocerebellar tract
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)
Name 2 spinothalamic syndromes.
- Anterior Spinal Artery Syndrome
- Syringomyelia
What two main modalities is the spinothalamic tract responsible for?
- Pain
- Temperature
What does the anterior spinal artery supply?
- Spinothalamic Tract (decussating fibres)
- Anterior part of the spinal cord
What two things cause anterior spinal artery syndrome?
- Atherosclerosis
- Spinal Injury
What is syringomyelia?
- Occlusion of crossing pathways by degenerative expansion of the spinal central canal
(tends to be exclusively seen at cervical level)
What causes Brown-Sequard syndrome?
- Cord Hemisection (cut in half)
What three effects are seen in Brown-Sequard Syndrome?
- Loss of Touch & Propioception Ipsilaterally
- Loss of Pain & Temperature sensations Contralaterally
- Complete loss of sensation at the level of hemisection (on side of hemisection)
What injury types cause hemisections of the spinal cord?
- Stabbing Injuries
- Twisting of Spinal Cord
- Bending of Spinal Cord
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)
Why does complete loss of sensation occur at level of injury?
- Complete loss due to injury to the dorsal root
Where does lateral inhibition take place?
- Synaptic actions take place at relay stations such as the dorsal column nuclei
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)
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)
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)
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
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)
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)
Where does lateral inhibition take place?
- Thalamus
- Cortex
- Dorsal Column Nuclei
What is the homunculus?
-
Topographic map of the contralateral body surface
(e. g. in post-central gyrus)
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)
What does the lateral side of the homonculus map for?
- Face
What is the ventro-basal nucleus of the thalamus connected to in the cortex?
- Somatosensory Cortex
What nucleus in the thalamus is responsible for auditory part of the cortex?
- Medial Geniculate Nucleus
What part of the thalamus is connected to the frontal cortex?
- Medial Dorsal
What part of the thalamus is connected to the temporal cortex?
- Lateral Posterior Nucleus
What part of the thalamus is connected to the Parietal cortex?
- Pulvinar
What nucleus in the thalamus is responsible for visual part of the cortex?
- Lateral Geniculate Nucleus
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)
How many divisions is the post-central gyrus divided into?
- 3 Broddmann Areas
(divided on the basis of pathology)
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)
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
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
How many layers are there in the cerebral cortex?
6 layers
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)
Which layer do input neurons go into?
- Layer 4
What is seen in brodmann areas 3b to 5?
- 4 separate maps of the body map are seen in S1
What happens to the receptive field as you progress from brodmann area 3b to 5?
- Receptive fields increase in size & complexity
What does area 3b cover?
- Smallest Receptor Field
- Limited to a single fingertip
What does area 1?
Cells which respond to touch on any of the 4 digits
Receives information from multiple fingers (join up)
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
What does area 5 do?
- Covers both hands
- All the area of the fingers on either hand
What things are the receptive fields selective for?
- Orientation Selective
- Direction Selective
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)
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)
What is area 3b divided up into?
- Divided up into microdomains
What do the microdomains in area 3b represent?
- Different classes of skin receptors
(e. g. Merkel cells & Meissner corpuscles)
What does area 3b contain?
- Region for each digit (e.g. D3)
- Coded in separate parts of the cortex
How is location on the body surface represented?
- By columns in the cerebral cortex
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)
When is phantom limb sensation seen?
- After the loss of parts of a limb
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)
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
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)
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)
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)
What causes referred phantom sensation?
- Hand area activated by sensory input from alternative source
- However it causes activation of hand region of the cortex
How is visceral pain usually interpreted?
- Somatic sensation
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)
Where does the somatic projection of visceral pain go to from the appendix?
- T10
Where is the somatic projection of visceral pain from the OVARIES or UTERUS go to?
- T10-T12
Where is the somatic projection of visceral pain from the testes or prostate go to?
- T10-T12
Where is the somatic projection of visceral pain from the diaphragm?
- C3-C4 (scapular area)
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)
What part of the cortex is devoted to visceral pain?
- Small part as they are usually not active (needed)
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)
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)
Why is referred pain important clinically?
- Patients report pain on superficial structures
- Diangosis will be deep & visceral
