Block 3: Spinal cord and ascending tracts Flashcards
How can we detect different stimuli in the same patch of skin?
Through different sensory nerve endings contained within the epidermis and dermis.
What are the five types of sensory receptors? Are they fast or slowly adapting?
1) Meissener’s corpuscle- discriminative touch, rapid adaptation
2) Pacinian Corpuscle- Deep Pressure and vibration, rapid adaptation
3) Ruffini ending- touch/sheer stress/ forces, slowly adapting
4) Merkels cells/disc- light, sustained touch- slowly adapting
5) Free nerve endings- pain (slowly adapting) and temperature (fast adapting).
What are the functional areas of the cortex that relate to sensory perception?
- Brodmann area 1 = the primary somatosensory cortex which receives sensory information from the contralateral side of the body.
- Sits behind the Central sulcus
- Superior parietal lobe - brodmann areas 5/19: Integrates different sensory inputs, sensory memory, perception of the contralateral self/ world.
What is somatotopic mapping? What is a homunculus?
How does the map relate to the function of different body parts?
- Somatotopic mapping is the mapping of specific body regions to particular areas of the cortex- in this case the somatosensory cortex.
- A homunculus is a visual representation of the cortex, showing the proportion of cortex dedicated to a particular body part.
- The somatotopic map relates to function as the larger the proportion of cortex devoted to a particular body region the more highly innervated that body region is e.g. hands are large on homunculus as are highly sensitive vs the knees.
Define somatotopy
- Point for point correspondance of an area of the body to a specific point on the CNS
What are the three types of interconnection fibres in the cortex?
- Association fibres- between regions of within one hemisphere.
- Commisural fibres- between one hemisphere to the other
- Projecting fibres- between the cortex and spinal cord and vice versa.
What is the internal capsule?
What type of fibre is it?
What is the corona radiata?
What is the corpus callosum?
- The internal capsule is formed by the ascending (Sensory) and descending (motor) white matter tracts between the cortex and spinal cord.
- It is a projection fibre
- The internal capsule becomes the corona radiata, a continuation of the sensory/ ascending white matter tracts and motor/descending tracts.
- The Corpus callosum is a commisural fibre that connects the R and L hemispheres.
What are the 4 parts of the internal capsule?
- Anterior limb
- Genu (angle/ knee)
- Posterior limb
- Retrolenticular nucleus
What does the anterior limb of the internal capsule contain?
- Anterior limb of the internal capsule carries connections between the Thalamus (medial dorsal nucleus) to the prefrontal cortex and between the pontine nuclei and prefrontal cortex.
What does the posterior limb/ genu of the internal capsule contain?
What is special about the arrangement of fibres in the internal capsule?
what does this mean if there is a focal lesion in the internal capsule?
- The posterior limb/ genu contains more anteriorly:
- Motor tracts between cortex and brainstem (corticobulbar) first (genu)
- Motor tracts between the cortex and spinal cord (corticospinal tracts) after, arranged arm, trunk, leg.
- The posterior limb contains more posteriorly:
- Sensory tracts between the ventro posterior lateral (VPL) and ventro posterior medial (VPM) nuclei of the thalamus and the primary somatosensory cortex.
- Arrangement of fibres is special as both motor and sensory are somatotopically arranged with the head more anterior, followed by arms and legs.
- If there is a focal lesion in the internal capsule this means there will be contralateral loss/ paralysis to the side of the brain affected.
What does the retrolenticular nucleus of the internal capsule contain?
- Retrolenticular nucleus of the internal capsule contains fibres that carry:
- Auditory information to the auditory cortex (from the MGN of the thalamus)
- Visual information to the primary visual cortex (from the LGN of the thalamus)
Describe the general structure of the ascending sensory tracts from the body to the primary somatosensory cortex
How does the P/W for sensory information to the face differ?
- Generally sensory information from the body is carried into the body via 1st order neurone that travels ipsilaterally up the spinal cord to the lower medulla.
- At the medulla, this 1st order neurone synapses with a 2nd order neurone
- This 2nd order neurone then decussates and travels up the spinal cord to reach the thalamus via a tract known as a lemniscus.
- At the thalamus the 2nd order neurone synapses with a 3rd order neurone.
- This 3rd order neurone carries information from the thalamus to the primary somatosensory cortex via internal capsule.
- Sensory information from the face differs:
- 1st order neurone from face (in skin/ mucus/ muscles/ joints) predominantly from the trigeminal nerve (also some from facial/ glossopharyngeal and vagus nerves).
- Trigeminal afferents enters at the lateral body of the pons and synapses with a 2nd order neuron in the trigeminal sensory nucleus.
- The 2nd order neuron in trigeminal sensory nucleus decussates immediately
- joins the contralateral ventral trigeminal lemniscus and travels up to the VPM of the thalamus to synapses with a 3rd order neuron.
- third order neurone travels in posterior limb of internal capsul to the primary somatosensory cortex (Brodmann 1).
What are the two thalami?
what are two key nuclei for sensory input into the thalamus?
Which nucleus receives what type of information?
Where does the thalamus project to?
- The two thalami (L/R) contain a collection of subcortical relay neurons
- There are two nuceli specialised to receive sensory information:
- Ventro posterior medial nucleus (VPM)
- Ventro posterior lateral nucelus (VPL)
- VPM- receives information from the face
- VPL receives information from the rest of the body.
- The thalamus projects to the primary somatosensory cortex.
In the spinal cord where does grey/white matter lie?
What is special about the way the spinal cord is viewed?
- In the spinal cord white matter tracts surround central grey matter with a central spinal canal.
- The grey matter is arranged into a dorsal horn (Sensory), lateral horn (autonomic) and ventral horn (motor)
Fill out the arrows
Describe the organisation of the grey matter in the dorsal and ventral horns of the spinal cord
- The grey matter within the spinal cord is organised into distinct zones with different functions
- These distinct zones are called Rexed Lamina- meaning a layered structure.
- In these zones there are cell bodies of neurones where descending (motor) or ascending (sensory) tracts may synapse.
- The dorsal (sensory horn) is organised into specific zones:
- Zone 1 - pain, temperature and touch
- Zone 2- pain transmission neurones
- The ventral (motor horn) is also organised into these specific zones
- Zones 8 and 9 = lower motor neurones.
Fill in the labels
What is the meaning of the last word on the right three labels?
Funiculus= bundle of nerve fibres enclosed in a sheath of connective tissue forming the main tracts of the spinal cord
Describe the differences in the appearance of axial sections of spinal cord as you descend from the cervical region–> thoracic–> lumbar–> sacral
Why are there these differences?
- Cervical: Larger grey horns as there are a large proportion of sensory nerves and motor nerves needed to innervate the upper limbs
- Thoracic: small grey horns as innervation to the segmental thoracic wall requires few neurons. Note larger lateral horns which contain autonomic nerve cell bodies.
- Lumbar: large grey matter horns due to large proportion of motor and sensory nerves needed to innervate the lower limbs
- Sacral: small white matter tracts given the few white matter tracts at this level.
- There are differences in the proportions of white matter/ grey matter at different levels of the spinal cord depending on the proportion of innervation at different levels. (i.e the amount of fibres coming in and leaving spinal cord).
- N.B remember on pictures below grey matter stains WHITE , white matter stains GREY.
Describe the dorsal column pathway:
1) inputs
2) what sensation this allows us to feel
Inputs: Dorsal column P/W
- Meissener’s corpuscles- Discriminitive touch
- Merkels cells- soft touch
- Pacinian corpuscles- Deep pressure and vibration
- Ruffini endings- touch
- Joint Proprioceptors
Allows:
- Discriminitive touch
- Vibration sensing
- concious proprioception
Describe the spinothalamic pathway:
1) Inputs
2) sensation detected
- Input: Free nerve endings
- Sensations: Pain/ temperature/ simple touch
Describe the spinocerebellar p/w:
Inputs: ?
Sensation this allows: ?
- Spinocerebellar inputs:
- Muscle spindles
- Golgi tendon organs
- Sensation: Unconcious proprioception
Fill the blanks
Describe the functions of the ascending spinal tracts shown on the image
- Dorsal column pathway generally carries discriminitive touch, vibration and concious proprioception.
- Dorsal column pathway split into two further portions:
- Fasciculus gracilis- carries sensory information inferior to T6 (legs)
- Fasciculus cuneatus- carries sensory information superior to T6 (arms)
- The spinothalamic tract (light blue) carries sensation from free nerve endings allowing pain, temperature and simple touch sensations.
- Spinothalamic tract divided into anterior and lateral parts
- The spinocerebellar tract carries information from muscle spindle and glogi tendon organs allowing unconcious proprioception.
What are the two parts of the image shown?
The two parts are part of the columns of the dorsal column pathway that carries discriminitive touch, vibration and concious proprioception.
Formed of the lateral (yellow) fasciculus cuneatus - carries sensations from above T6- arms and the medial (blue) fasciculus gracilis - carries sensation from below T6 (graceful legs).
Describe the dorsal column pathway:
How does the sensory information from peripheral sensory receptors get to the primary somatosensory cortex?
- 1st order neurons from the peripheral sensory receptors enter the spinal cord and ascend in the two columns of the dorsal column pathway:
- Either in the fasciculus gracilis- from legs and trunk (below T6) or in the fasciculus cuneatus- from arms and trunk (Above T6).
- These 1st order neurons travel up to the lower medulla where they synapse with second order neurons in the cuneate nucleus (for above T6) or the gracile Nucleus (for below T6).
- These 2nd order neurons decussate and travel via medial lemniscus to synapse with thalamic nuclei in the VPL - ventro posterior lateral nucleus.
- These 3rd order neurons in the VPL travel up to the primary somatosensory cortex via the posterior limb of the internal capsule.
What do the two columns of the dorsal column pathway synapse with in the medulla?
- The fasciculus gracilis synapses with the nucelus gracilis in the medulla (contains the 2nd order neurones)
- The fasciculus cuneatus synapses with the nucleus cuneatus in the medulla (contains the 2nd order neurons).
Describe how sensory information from the face is carried in the dorsal column pathway and how this differs from information from legs/arms/trunk?
- Sensory information from the face is carried by the Trigeminal nerve
- It enters the spinal cord at the level of the lateral pons and synapses with the Trigeminal (CN V) nucleus.
- These 2nd order neurons from the trigeminal nucleus forms the trigeminal lemniscus which decussates before it ascends.
- After decussation, the trigeminal lemnicus ascends to the VPM- ventro posterior medial nucleus of the thalamus.
- 3rd order neurons pass from the VPM to the primary sensory cortex.
What senses would be lost in this patient?
what side would the loss be on?
How might the patient present?
The lesion is before the point of decussation and in the dorsal column pathway (see both fasciculus cuneatus and fasciculus gracilis). Therefore would lose discriminiting touch, vibration and concious proprioception on the ipsilateral side (Left side) below the level of the lesion.
Patient may present with inability to maintain balance when eyes are closed and loss of sensation on affected side.
Patient will present with sensory ataxia: abnormal stomping gait due to loss of proprioception, +ve Romberg sign.
Patient will present with pseudoathetosis - writhing of digits/hands/feet
What can affect the dorsal columns?
How does dorsal column damage present?
- Dorsal columns can be affected by compression, infarction, infection or B12 deficiency.
- Sensory ataxia- stomping gait- + Romberg sign
- Pseudoathetosis- writhing of hands/digits/ feet
What senses would be lost?
What side would be affected?
How might a patient present?
Senses lost: Discriminating touch, vibrations, proprioception would be lost on the contralateral side to the lesion (on the left side) and below the level of the lesion. Lesion is in the cortex- after the point of decussation hence loss on contralateral side.
Patient may again present with sensory ataxia, stomping gait, +ve Rhomberg sign
Pseudoathetosis- writhing of hands/digits/feet
What sensations does the spinothalamic pathway carry?
Describe the STT pathway
- Spinothalamic pathway carries Pain, temperature and simple touch
- Sensory neurons enter the spinal cord and ascend 1-2 spinal levels in the tract of lissauer in dorsal grey matter until they synapse with 2nd order neurons.
- These 2nd order neurones decussate at this level via the anterior commisure
- 2nd order neurones ascend in the spinothalamic tract, carry the information up to the VPL nucleus of the thalamus, synapsing with a 3rd order neurone
- This 3rd order neurone carries the impulse to the primary somatosensory cortex.
In the spinothalamic tract:
2nd order neurones decussate via the ________ _______ __________.
After they have ascended __ to ___ levels in the _____ ___ __________.
2nd order neurones are located in ________ ________ ______ of the dorsal horn.
In the spinothalamic tract:
2nd order neurones decussate via the anterior white commisure.
After they have ascended 1-2 levels in the tract of lissauer.
2nd order neurones are located in the rexed 1 area of the dorsal horn.
What is syringomyelia?
- Syringomelia is cavitation/ expansion of the spinal canal in the cervical region due to development of a cyst.
- Initial symptoms arise from obliteration of spinothalamic fibres decussating in the anterior white commisure at this region.
- Fibres that have already decussated, i.e the spinothalamic tracts themselves will not be damaged.
How does syringomelia in the cervical region present and why?
Note syringomelia refers to development of a cyst within the spinal cord and can happen in any region (not just cervical region).
- “vested” presentation, with loss of pain, temperature and simple touch sensations in the upper limbs, shoulder and clavicular region.
- Due to compression of the decussating 2nd order neurones of the spinothalamic pathway in the cervical region
- Symptoms present 1-2 spinal levels lower than the cervical region as 1st order neurons ascend 1-2 spinal levels before synapsing with the 2nd order neurone in the rexed area 1 of the dorsal horn of grey matter.
- Chiari malformation = developmental defect where cerebellum protrudes out of foramen magnum and into upper portion of spinal canal due to mishapen or small skull.
Describe the organisation of the spinothalamic tract
- The spinothalamic tract is somatotopically organised, with the sacrum at the most dorsal point, followed by the leg, arm/ thorax, head and neck as you move more ventrally along the STT.
When there is damage in the spinothalamic tract:
what is sacral sparing?
- Sacral sparing refers to the somatotopic organisation of the spinothalamic tract:
- The sacral region of the STT that receives pain/temp/simple touch sensations from sacral sensory nerves is most dorsally located
- The leg/ arm/ trunk/ head regions are then more ventral.
- If there is a ventral grey horn tumour it will compress the ventral portions of the spinothalamic tract (i.e head and body sensations) but will not affect the sacral region due to its more dorsal position.
Describe the dorsal spinocerebellar tract pathway:
How does muscle spindle info from the lower limb reach the cerebellum?
How does muscle spindle info from the upper limb reach the cerebellum?
- From the lower limb:
- enters the spinal cord and synapses with 2nd order neurones in Clarke’s dorsal nucleus
- From clarke’s dorsal nucleus these 2nd order neurons ascend ipsilaterally in the dorsal spino cerebellar tract to the cerebellum.
- From the upper limb:
- enters the spinal cord and ascends with the fasciculus cuneatus (of the dorsal column pathway) and synapses at the pons.
- At the pons it continues to the cerebellum via the cuneocerebellar tract.
What types of sensation are lost if there is a lesion here?
What level are we? what pathways decussate where?
Will a patient present as number 1 or 2?
- Will lose the dorsal column pathway on the right side of the spinal cord which carries concious proprioception, vibration and discriminitive touch. The dorsal column decussates at the level of the medulla. As this a lower level than the medulla, fibres have yet to decussate and therefore sensation will be lost on the ipsilateral side.
- Will also lose the spinothalamic pathway in the right side of the spinal cord which carries pain, temperature and simple touch sensation. Spinothalamic neurones enter the spinal cord and ascend 1-2 levels via tract of lissauer before synapsing with 2nd order neurones (in area 1 of the dorsal grey horn) which decussate. Therefore sensation will be lost from the contralateral side of the body 2 spinal levels lower than the lesion.
- Therefore image 2 is the correct patient presentation.
In a brainstem lesion on the left side of the pons:
How would sensory information from the face and body be affected?
- Sensory information from the body carried in dorsal column pathway and the spinothalamic tract will have already decussated, therefore the loss would be presented on the contralateral side of the body (right side of the body)
- Sensory information from the face carried by the trigeminal nerve synapses at the pons and decussates here. Therefore as the neurones have yet to decussate the sensory loss would be on the ipsilateral side of the face - left side.
- Therefore image 1 is correct patient presentation.
