Ascending Tracts Flashcards
Describe the organisation of the white matter of the spinal cord.
Split into three funiculi
Dorsal funiculus is found between the midline and medial edge of the dorsal root
Lateral funiculus found on the lateral edge of the grey matter of the spinal cord
Ventral funiculus is found between the midline and medial edge of the ventral root
How is the grey matter of the spinal cord organised?
Its cell bodies are arranged into laminae
Rexed Laminae
There are X (10) rexed laminae
Each laminae is equivalent to a neuronal nucleus
Some have discreet names.
How is somatosensory information conveyed from the body top the brain?
Spinal sensory system:
Sensation arising from the top of the neck down to the toes
Sensory signals are carried by sensory division of spinal (or segmental) nerves
There are 31 pairs of segmental nerves carrying sensory signals from the body surface to the brain
Cranial sensory system:
Sensations arising from cranial structures
Sensory signals are carried by sensory roots of cranial nerves
(The trigeminal nerve carries the majority of somatosensory signals)
What are ascending tracts?
They are fibres by which sensory information is conveyed to the brain
Divisible by two broad categories
What are the two categories of sensation do the sensory signals of the body give rise to?
Conscious sensation (We are directly aware of this information)
- Pain, temp., crude touch
- Conscious proprioception, fine touch
Non-conscious sensation (We are not directly aware of this information)
- Tactile strength, muscle length, muscle tension, joint
position, joint angle
What are the routes the ascending tracts can take?
And what is the difference between them?
Spinothalamic tract
(conscious sensation - Pain, temp and crude touch)
Dorsal column
(Conscious proprioception and fine touch)
Spinocerebellar tract
(Unconscious sensation - proprioception etc….)
Describe the route of the primary afferent neurone of the spinothalamic nerves.
They enter the spinal cord via the dorsal root
It enters the spinal cord and divides, sending branches to adjacent spinal segments
Unbranched main axon enters the grey matter of its respective dorsal horn
It then terminates in the dorsal horn
(The laminae it terminates in determines its modality, it terminates in laminae I to IV)
(laminae III & IV are known as Nucleus Proprius)
What does the primary afferent conscious neurone do just after entering spinal cord?
They enter the spinal cord and then divide, sending branches to adjacent spinal segments
The branches will go 3 to 4 spinal segments in both directions, rostrally and caudally
Axonal branches travelling to adjacent spinal segment rostral and caudal to the neural level of entry travel in the posterolateral tract of Lissauer
(Specific for pain & temp. ascending tracts)
Describe the route of the secondary neurone of the spinothalamic tract.
It sends its axon towards the central canal, then ducks under and crosses the midline
It then joins the lateral funiculus of the white matter
And then joins the fibres of the lateral spinothalamic tract
These fibres then travel to the thalamus
To the ventral posterolateral (VPL) nucleus of the thalamus
There they terminate onto 3rd order sensory neurones.
What is meant by spinal decussation?
When spinal neurones cross the midline
What are the spinal decussation fibres of the spinothalamic tract known as?
Anterior commissures
Spinal Arcuate fibres
What are the consequences of spinal decussation in the spinothalamic tract?
Sensory signals arising from the right side of the body are carried by secondary sensory fibres of the spinothalamic fibres of the left side of the body
And vice versa
Describe the route of third order sensory neurones of the spinothalamic tract.
They ascend from the VPL nucleus to terminate in the post-central gyrus of the cerebral cortex.
They travel via the internal capsule
What are the different spinothalamic tracts?
Anterior spinothalamic tract
(Crude touch pressure)
Lateral spinothalamic tract
(Pain, Temp)
What is syringomyelia?
A chronic progressive disease in which longitudinal cavities form in the cervical region of the spinal cord. This characteristically results in wasting of the muscles in the hands and a loss of sensation
Due to damage at the central canal of the spinal cord, which results in scar tissue, this leads to central cavitation, and if the cyst expands ventrally the patient can suffer anaesthesia of upper limbs bilaterally
What sensory information is carried by the dorsal column?
Conscious proprioception and fine touch
Describe the route of the primary afferent neurone of the dorsal column.
Comes into the spinal cord via the dorsal root ganglion
Stays on same side
Immediately ascends in the dorsal funiculus still as a primary sensory neurones (without terminating)
It ascends in the dorsal columns
In either:
- Fasciculus cuneatus (lateral) - Signal from above T6
- Fasciculus gracilis (medial) - Signal from below T6
They then terminate in the medulla on the same side they arose from
Fasciculus cuneatus terminates in the nucleus cuneatus of the medulla
Fasciculus gracilis terminates in the nucleus gracilis of the medulla
Describe the route of the secondary neurones of the dorsal column.
The secondary sensory neurone in the nucleus gracilis or nucleus cuneatus sends its axon cranially
They travel ventromedially as internal Arcuate fibres
And cross the midline to become the medial lemniscus
The crossed fibres then ascend the medulla, pons and midbrain towards thalamus
All fibres terminate in the ventral posterolateral (VPL) nucleus of the thalamus
Describe the route of the 3rd order sensory neurones of the dorsal column.
From the VPL they ascend further
Travelling through the internal capsule
To finally terminate on the sensori-motor cerebral cortex
What is Brown-Sequard Syndrome?
damage to hemi-section of the spinal cord will lead to a sensory dissociation syndrome known as:
Brown-Sequard Syndrome
For example if the right half of the spinal cord was damaged at the level of T10
Then the right half below T10 would have no proprioception and fine touch,
Whereas the left side would have no pain and temperature sense.
How does the cerebellum receive proprioceptive information?
Non-conscious proprioceptive sensory information from muscle receptors and joint receptors
It receives the information through:
- Dorsal Spinocerebellar Tract (DSCT)
- Ventral Spinocerebellar Tract (VSCT)
- Cuneo-cerebellar Tract (CCT)
What happens when a proprioceptive sensory neurone enters the spinal cord?
It divides into two branches:
1st branch - hardly enters the dorsal horn before joining
the dorsal column
2nd branch - Terminates in laminae VII of the dorsal horn
also known as nucleus of Clarke
Tell me about Clarke’s column.
Successive nuclei of Clarke from all neural levels form a continuous column of cells in the rostro-caudal axis
This is Clarke’s column
Clarke’s column neurones act locally as interneurons and also send there axons to the cerebellum
Describe the route of the dorsal Spinocerebellar tract.
Derived from neurones of Clarke’s column
Axonal processes of neurones in Clarke’s column leave the grey matter to enter the white matter in the lateral funiculus
Here they join the fibres of the dorsal spinocerebellar tract
DSCT fibres ascend ipsilaterally without decussating
They pass through the medulla to terminate in the Vermis & anterior lobe of the Cerebellum
Briefly describe the route of the ventral spinocerebellar tract.
Parallel to DSCT
Primary afferent neurone has cell body in dorsal root ganglion
It enters the dorsal horn of the grey matter and synapses with a second neurone
This neurone then crosses the midline via the anterior white commissure and joins the VSCT in the lateral funiculus
It then travels to the cerebellum where it crosses back over to the same side it originated on.
What is the clinical significance of the cerebellar projections?
A patient with unilateral cerebellar damage will tend to sway, falling on the side in which the lesion is located
(If the Vermis of the cerebellum is damaged then the patient will fall backwards.)
