Ascending and Descending Tracts Flashcards
What does the Meissner Corpuscle detect?
Adaptation?
Discriminative touch
Rapid
What does the Pacinian Corpuscle detect?
Adaptation?
Deep pressure
Vibrations
Rapid ++
What does the Ruffini Ending detect?
Adaptation?
Touch
Slow
What does the Merkel Disc detect?
Adaptation?
Light, sustained touch
(slowly adapting)
Slow
What do the Free Nerve Endings detect?
Adaptation?
Pain and temperature
Rapid
Where do the sensory cortexes sit?
Parietal lobes
Left and right
Where is the Primary Somatosensory Cortex?
Post-Central Gyrus in the Parietal lobe
What information does the Primary Somatosensory Cortex receive on each side?
Contralateral sensory input from the body (including taste)
Behind what sulcus does the primary somatosensory cortex sit?
Central sulcus
Why is the Superior Parietal Lobe responsible for?
Integration of sensory inputs
Sensory memory
Perception of contralateral self/world
How does somatotopic mapping differ between the sensory and motor cortex?
Sensory cortex is responsible for genitals
What is the internal capsule?
Dense collection of white matter (myelinated axons) that carry sensory and motor tracts to and from the cortex
What are the 3 types of interconnection fibres in the cortex?
Projection - to brainstem
Association - within hemisphere
Commissural - between hemisphere
What type of projection is the internal capsule?
Commisural
What type of nerves does the internal capsule carry?
Sensory and motor nerves
What does white matter contain?
(Myelinated) axons
What does grey matter contain?
Cell bodies
Where is white matter found?
Deep inside the cortex
Outer portion of the spinal column
Where is grey matter found?
On the surface of the cortex
Inner portion of the spinal column
Name the 4 regions of the internal capsule
Anterior limb
Genu
Posterior limb
Retrolenticular
What type of nerves pass through the anterior limb of the internal capsule?
Motor fibres from the frontal lobe to the pons or thalamus
What type of nerves pass through the Genu of the internal capsule?
Motor fibres from the head
What type of nerves pass through the posterior limb?
Motor to arm and leg
All sensory fibres
In which portion of the internal capsule would you see both sensory and motor damage?
Posterior portion
What type of nerves pass through the Retrolenticular portion of the internal capsule?
Special fibres for vision and hearing
The Putamen and Globus Pallidus basal ganglia (together form the lentiform nucleus)
How many chains are there in a sensory neuron tract?
3 neuron chain
Do 1st order neurons of the sensory pathway from the limbs ascend the spinal cord ipsilaterally or contralaterally?
Ascend ipsilaterally
Where do 1st order neurons from the limbs synapse with 2nd order neurons?
Lower medulla
Where do ascending sensory pathways from the limbs decussate?
Lower medulla
2nd order neuron decussates after synapse with 1st order neuron (see handout)
Where do 2nd order neurons of the ascending sensory pathway from the BODY synapse with 3rd order neurons?
Ventral Posterior Lateral (VPL) nucleus of the Thalamus
Where does a 1st order sensory neuron from the face enter the CNS?
Lateral body of the pons - along CN V
Where does a 1st order sensory neuron from the face synapse with a 2nd order neuron?
Pons
Where does a sensory neuron from the face decussate?
In the pons
2nd order neuron decussates after synapse with 1st order neuron (see handout)
Where do 2nd order neurons of the ascending sensory pathway from the FACE synapse with 3rd order neurons?
Ventral Posterior Medial (VPM) nucleus of the thalamus
Which area of the thalamus contains the nuclei for somatosensory input?
Ventral Posterior Nucleus
What are the two areas of the ventral posterior nucleus?
What do they receive input from?
Ventral Posterior Lateral nucleus - sensory from body
(lateral = limbs)
Ventral Posterior Medial nucleus - sensory from the face
(face is medial)
Describe the basic structure of the spinal cord (white and grey matter)
Grey matter (cell bodies) surrounded by white matter (axons)
What are the names of the organised zones of the spinal cord?
Rexed Lamina
Why are the Rexed Lamina areas important?
Where ascending or descending tracts may synapse with other neurons - therefore contain cell bodies
Which Rexed Lamina zone contains nerve bodies associated with pain, temperature and touch?
Zone II
Dorsal horn of spinal cord
Which Rexed Lamina zone contains nerve bodies associated with pain transmission?
Zone II
Dorsal horn of spinal cord
Which Rexed Lamina zones contain lower motor neurons?
Zones VIII and IX
Ventral horn of spinal cord
How can white matter in the spinal cord be grouped?
How many groups are there?
Into Funiculi (bundles of more than one tract)
3 groups
What are the 3 funiculi called?
Dorsal funiculus
Lateral funiculus
Ventral funiculus
What type of nerves does the lateral funiculus carry?
Sensory and motor nerves
Why does the cervical region of the spinal cord have a large grey horn?
Grey horn = cell bodies
Large number of neurons required to innervate upper limbs
Why does the thoracic region of the spinal cord have a small grey horn and large white lateral horns?
Small grey horns - few neurons needed for segmental thoracic innervation
Large lateral horns - lateral funiculus carrying motor and sensory to the thorax
Why does the lumbar region have large grey horns?
Large number of cells bodies to innervate lower limbs
Which portion of the spinal cord will have a large portion of white matter compared to grey matter?
Cervical region
Large population of neurons needed to innervate upper limbs
Which portion of the spinal cord has a small grey horn but large white matter parts?
Thoracic region
Small grey horn - segmental thoracic
DIAGRAM:
Draw out the different portions of the spinal cord (e.g. cervical, thoracic) in terms of white and grey matter
Look at slide 16 of ascending tracts lecture
What are the 3 major ascending sensory pathways?
Dorsal column pathway
Spinothalamic tract
Spinocerebellar tracts
Which type of sensors send information through the dorsal column pathway?
Meissner’s
Merkel’s
Pacinian
Ruffini
What senses does the dorsal column pathway send?
Discriminative touch
Vibration
Conscious proprioception
What are the 2 areas of the dorsal column pathway called?
Fasciculus Cuneatus (lateral)
Fasciculus Gracilis (medial)
Where does the Fasciculus Cuneatus receive information from?
From above T6
Discriminative touch, vibration and conscious proprioception
Where does the Fasciculus Gracilis receive information from?
From below T6
(graceful legs)
Discriminative touch, vibration and conscious proprioception
Does the dorsal column pathway ascend contralaterally or ipsilaterally in the spinal cord?
1st order neuron ascends ipsilaterally to the medulla where it decussates
Describe the pathway of a neuron carrying sensation of discriminative touch, vibration and conscious proprioception from the body - 1st order neuron to the cortex
Stimulated by either Meissner’s receptor or Merkel’s receptor.
Carries sensation via 1st order neuron
Neurons below T6 enter the spinal cord at the medial Fasciculus Gracilis of the dorsal column.
Neurons above T6 enter the spinal cord at the lateral Fasiculus Cuneatus of the dorsal column
1st order neurons ascend the spinal cord ipsilaterally
Synapse with 2nd order neuron in the nucleus gracilis (below T6) or nucleus cuneatus (above T6) of the lower medulla, where they decussate
Travel to thalamus where they synapse with 3rd order neurons that travel to the primary sensory cortex
Which somatosensory pathway carries discriminative touch, vibration and conscious proprioception?
Dorsal columns pathway
Describe the dorsal column pathway carrying sensory innervation from the face
1st order trigeminal nerve sensory neurons enter PONS and synapse in CN V nucleus where they decussate
2nd order neurons ascend in trigeminal lemniscus to ventral posterior medial (VPM) nucleus of the thalamus, where it synapses with 3rd order neurons
3rd order neurons pass to primary sensory cortex
SLIDE REVIEW
Slide 24 - dorsal column damage
What senses does the spinothalamic tract carry?
Pain
Temperature
Simple touch
Which sensory pathway carries pain, temperature and simple touch?
Spinothalamic tract
Describe the spinothalamic pathway
1st order neurons from body ascend 1-2 vertebral levels (in the Tract of Lissauer) in dorsal grey matter before synapsing with 2nd order neurons
2nd order neurons decussate via (across) the anterior white commissure
2nd order neurons ascend in the spinothalamic tract to ventral posterior lateral (VPL) nucleus of thalamus
2nd order neurons synapse with 3rd order neurons in ventral posterior lateral (VPL) thalamic nucleus
3rd order neurons travel to primary sensory cortex
Do spinothalamic nerves from the body ascend the spinal cord ipsilaterally or contralaterally?
Ipsilaterally 1-2 vertebral levels then decussates with 2nd order neuron via the anterior white commissure
Which receptors carry sensation to the spinothalamic tract?
Free nerve endings
Where is the spinothalamic tract situated in the spinal cord?
Ventral portion of the spinal cord
Before they decussate, where do spinothalamic tract neurons ascend?
Ascend 1-2 vertebral levels via the Tract of Lissauer
What is special about spinothalamic tract organisation?
It is organised somatotopically
Lateral –> medial
Sacral
Leg
Arm/thorax
Head & neck
What sensation does the spinocerebellar tract carry?
Unconscious proprioception
Which sensory pathway carries unconscious proprioception?
Spinocerebellar tract
What are the 2 regions of the spinocerebellar tract called?
Dorsal spinocerebellar tract
Ventral spinocerebellar tract
What do dorsal spinocerebellar tract neurons monitor?
Muscle length
Speed of contraction
Tension
What is different about the DORSAL spinocerebellar tract compared to the spinothalamic and dorsal tract?
Dorsal spinocerebellar is a 2-neuron pathway
Ipsilateral pathway to the cerebellum
Describe the pathway of the DORSAL spinocerebellar tract from the lower limb
Sensory innervation enters the spinal cord and synapses in Clarke’s Dorsal Nucleus
Ascends as 2nd order neuron
What is a motor unit?
A lower motor neuron and the extrafusal muscle fibres it innervates
Where is the primary motor cortex?
In the pre-central gyrus, anterior to the central sulcus.
In the frontal lobe
Where is the pre-motor cortex?
Immediately anterior to the primary motor cortex
In the frontal lobe
What is the pre-motor cortex responsible for?
Motor programme retrieval
Where does the primary motor cortex receive input from?
Pre-motor cortex
Supplemental motor cortex
Cerebellum (via the ventrolateral nucleus of the thalamus)
Somatosensory nucleus
Which Brodmann area is the primary motor cortex in?
Brodmann Area 4
Which Brodmann area is the supplementary motor area (pre-motor cortex and supplementary motor area) in?
Brodmann Area 6
What does damage to the pre-frontal cortex usually result in?
Paresis (weakness) on contralateral postural muscles
Due to influence on the reticulospinal tract
Name the 3 types of ascending pathways
Dorsal column
Spinothalamic
Spinocerebellar (dorsal and ventral)
Name the 2 types of lower motor neurons
Alpha
Gamma
What does an alpha motor neuron innervate?
Motor units of extrafusal fibres
What does a gamma motor neuron innervate?
Intrafusal muscle fibres of muscle spindles
What does a muscle spindle measure?
Muscle strength and contraction.
Tells the brain how much strain the muscle is under.
Describe an alpha motor neuron
Large myelinated axons
Innervates muscle units, making muscle bellies contract
Describe a gamma motor neuron
Small diameter axons
Innervates intrafusal fibres of muscle spindles
Help to monitor stretch and contraction
What does the suffix ‘plegia’ mean?
Paralysis
What does the suffix ‘paresis’ mean?
Weakness
Describe the blood supply to the spinal cord
2 posterior spinal arteries
1 anterior spinal artery
Name the 3 descending pathways
Lateral Corticospinal Tract
Lateral Vestibulospinal Tract
Ventral Corticospinal Tract
Apart from the 3 main descending pathways, what is another descending pathway to be aware of?
What does it do?
Reticulospinal tracts
Helps inhibit lower motor neurons so you don’t have overactive spasticity
Where does the lateral corticospinal tract receive output from?
Primary motor cortex
Supplementary motor area
What is the lateral corticospinal tract responsible for?
Major voluntary, skilled motor movement
Which descending pathway is responsible for major voluntary, skilled motor movement
Lateral Corticospinal Tract
Where does the lateral vestibulospinal tract receive output from?
Vestibular nuclei of pons and medulla
What is the lateral vestibulospinal tract responsible for?
Extensor muscle control (anti-gravity action)
Which descending tract is responsible for extensor muscle control (anti-gravity action)
Lateral Vestibulospinal Tract
Where does the ventral corticospinal tract receive output from?
Primary motor cortex
What is the ventral corticospinal tract responsible for?
Minor motor movement
To a lesser extent than the lateral corticospinal tract
Which descending pathway is responsible for minor motor movement?
Ventral Corticospinal Tract
Where do descending neurons from the lateral corticospinal tract decussate?
In the medulla
Where do descending neurons from the ventral corticospinal tract decussate?
At the level of synapse with lower motor neuron (spinal level of exit)
Where do motor nerves exit the spinal cord from?
Ventral grey horn
What 4 symptoms would you see with an Upper Motor Neuron lesion?
Spastic paralysis
Hyper-reflexia
No muscle wasting
Extensor plantar response
What 4 symptoms would you see with an Lower Motor Neuron lesion?
Flaccid paralysis
Hypo-reflexia
Muscle atrophy (wasting)
Fasciculations
What would result from a hemi-section of the spinal cord?
Injury and symptoms
At the level of the injury the patient would experiences LMN symptoms (where the UMN synapses with a LMN)
Below the level of injury there would be UMN symptoms, because the UMN carries on past the first synapse
What is the corticobulbar tract?
Major motor pathway to the face
What is the major motor pathway to the face?
Corticobulbar tract
Where do motor nerves to the face decussate?
Higher in the medulla. Leave via CN V or CN VII
What effect would a brainstem lesion have on motor movement?
Ipsilateral LMN loss to the face.
Contralateral UMN loss to the body below the level of the lesion
Which descending pathway never decussates?
Lateral vestibulospinal tract
What has an important influence on LMN?
Reticulospinal tract - acting on Renshaw cells
Can influence cells that live in ventral horn of the spinal cord
What cells does the reticulospinal tract activate?
Renshaw Cells
Inhibitor cells of the LMN
