3: Anatomy - Ascending and descending tracts Flashcards
How many spinal nerves are given off at each of the five spinal levels?
C - 8
T - 12
L - 5
S - 5
C - 1
At which level does the spinal cord terminate?
L1/2
What are the two large swellings found in the spinal cord?
Cervical and lumbar enlargements
What are the swellings found where the posterior rootlets enter the spinal cord?
Posterior root ganglia
What are groups of nerve bodies called when they are found in the
a) CNS
b) PNS?
a) nucleus
b) ganglion
What nerve fibres are found in
a) posterior rootlets
b) anterior rootlets?
a) Sensory
b) Motor (including autonomic)
What is the end of the spinal cord called?
Conus medullaris
What are the three layers of the meninges?
Dura mater
Arachnoid mater
Pia mater
Does the spinal cord have meninges?
Yes
Continuous with the brain
Which ligaments attach each spinal level to the arachnoid mater?
Denticulate ligaments
In the spinal cord, what is white matter arranged into?
Anterior, posterior and lateral columns
What connects the fourth ventricle to the spinal cord?
Central canal
What is found in
a) white matter
b) grey matter?
a) Axons
b) Nerve bodies, glial cells and blood vessels
What is found at the
a) Posterior horn
b) Anterior horn
of the spinal cord?
a) Sensory nerves
b) Motor nerves
T1 - L2 are special in that they have lateral horns of white matter.
What is found in the lateral horns of the spinal cord?
Pre-synaptic sympathetic fibres
At which spinal nerves are sympathetic nerves given off?
T1 - L2
What are the three main types of blood vessel supplying the spine?
Longitudinal arteries (run vertically)
Segmental arteries (branches of arteries like the intercostal and lumbar, pass through intervertebral foramen and supply spinal cord)
Radicular arteries (branches of the segmental arteries which supply the individual anterior and posterior roots)
How many arteries run vertically on the
a) anterior
b) posterior spinal cord?
Which arteries do they arise from?
a) 1
b) 2
Vertebral arteries
What is a notable segmental spinal artery which arises from a posterior intercostal artery between the 9th and 12th ribs?
Artery of Adamkiewicz
Which space exists between the spinal cord and dura mater posteriorly and allows for injection of anaesthetics?
Epidural space
Where is the primary somatosensory cortex i.e the first place where information is received by the brain?
Post-central gyrus of the parietal lobe, posterior to central sulcus
What happens to the proportions of white and grey matter as you go down the spinal cord?
White matter decreases
Grey matter increases
What sensations are carried by the Dorsal Column / Medial Lemniscus pathway?
Fine touch
Vibration
Proprioception (knowing where your limbs are in space without being able to see them)
What side of the brain processes information from the right?
Left side
and vice versa
In which specific part of the brainstem do nerve fibres from the Dorsal Column / Medial Lemniscus pathway cross over?
Medial lemniscus of the medulla
Once they have arrived at the medulla and crossed over, fibres from the DC/ML system synapse at nuclei (named for arm/leg).
Where do they go next?
Thalamus
The DC/ML pathway decussates (crosses over) in the medulla.
At which structures do fibres from the
a) lower body
b) upper body
synapse before crossing over?
a) Lower body > Nucleus gracilis (medial structure)
b) Upper body > Nucleus cuneatus (lateral structure)
What really important thing do nerve fibres from the DC/ML system do before ascending from the medulla to the thalamus?
Cross over - “decussate”
At the medial lemniscus of the medulla
Through which tract of white matter do fibres from the DC/ML system travel from the thalamus to the primary somatosensory cortex?
Internal capsule
Which sensations is the Spinothalamic tract responsible for?
Pain
Temperature
Deep pressure
Where do sensory fibres from the spinothalamic tract synapse first?
Spinal cord
happens quick
What happens after fibres from the spinothalamic tract synapse in the spinal cord?
They immediately decussate (cross over)
Compare this to the DC/ML fibres, which ascend to medulla on the ipsilateral side before crossing over
The main sensory pathways (DC/ML pathway and spinothalamic tract) are both described as having first, second and third order neurons.
Where do these neurons synapse in each pathway?
DC/ML system - dorsal root to medulla; medulla to thalamus; thalamus to post-central gyrus
Spinothalamic tract - dorsal root to spinal cord; spinal cord to thalamus; thalamus to post-central gyrus
Where do the sensory tracts decussate in the
a) DC/ML pathway
b) spinothalamic tract?
a) Medial lemniscus of the medulla
b) Spinal cord
Where exactly is the pre-central gyrus, or motor strip, found?
Frontal lobe, anterior to the central sulcus
Sensory neurons ___ from the spinal cord to the brain.
ascend
Motor neurons ___ from the brain to the spinal cord.
descend
What are the
a) sensory
b) motor
pathways we need to know about?
a) Dorsal column / medial lemnsicus pathway and spinothalamic tract
b) Corticospinal tract (or Pyramidal tract)
Where exactly in the brain do motor fibres descend to form the corticospinal tract?
Primary somatomotor cortex, found in the pre-central gyrus of the frontal lobe
What is the function of the corticospinal tract?
VOLUNTARY control of FINE MOTOR movement
What are the functions of the
a) DC/ML pathway
b) spinothalamic tract
c) corticospinal tract?
a) Fine touch, vibration, proprioception
b) Firm touch, temperature, pain
c) Fine movement
Motor fibres from the corticospinal tract also decussate to supply the contralateral muscles.
Where does this happen?
Pyramids of the medulla
What percentage of motor fibres decussate at the pyramids of the medulla?
Which tract do they form?
85%
LATERAL corticospinal tract
What percentage of motor fibres don’t decussate at the pyramids of the medulla?
Which tract do they form?
15%
Ventral / Anterior corticospinal tract
After they (mostly) decussate at the pyramids of the medulla, the corticospinal tracts (ascend / descend).
descend to supply the muscles on the contralateral side
What happens to the ventral corticospinal tract (i.e the motor fibres which DON’T decussate at the pyramids of the medulla)?
They descend to the spinal cord segment and decussate there
So the corticospinal tract still supplies the contralateral muscles, like the sensory pathways
Is the corticospinal tract the only example of a motor pathway?
No, there are accessory pathways e.g tectospinal tract, reticulospinal tract, vestibulospinal tract…
Which descending pathway contains the upper motor neurons for a number of the cranial nerves?
Corticobulbar pathway
Is the corticobulbar pathway involved in voluntary or involuntary control?
Voluntary control
So it’s a pyramidal tract
The UMNs of which cranial nerves are contained in the corticobulbar tract?
CN V
CN VII
CN XI
CN XII
What is special about the innervation of most of the UMNs in the corticobulbar tract?
Bilateral innervation
Meaning if an UMN is compromised on one side, the other side should take over
The corticobulbar tract contains the UMNs of CN VII.
What is the motor function of CN VII?
Muscles of facial expression
Some misc muscles like stapedius
The corticobulbar tract carries the UMNs of CN VII, which innervate the muscles of facial expression bilaterally.
Which muscle of facial expression is found in the forehead area?
Frontalis
Which pathologies can cause UMN damage re: the corticobulbar tract?
Stroke
Subdural haematoma (usually secondary to a head injury)
Tumour
Which pathologies can cause damage to the LMNs of CN VII?
What is the name of the presentation caused by LMN damage to CN VII?
Infection
Trauma
Bell’s palsy
What is the presentation of Bell’s palsy?
What specifically causes it?
Facial paralysis WITHOUT FOREHEAD SPARING
Reversible damage to the LMNs of CN VII; thought to be viral in origin
What is the presentation of damage to the UMNs of CN VII?
Facial paralysis WITH FOREHEAD SPARING
Why does an UMN CN VII lesion e.g stroke cause forehead sparing while a LMN (Bell’s palsy) does not?
UMN innervation is bilateral
LMN innervation is unilateral
This gets asked about all the time in exams
Name two important extra-pyramidal tracts and their functions.
What is meant by extra-pyramidal?
Rubrospinal tract - excites upper body flexors and inhibits upper body extensors
Reticulospinal tract - excites upper body extensors
Not passing through the pyramids of the medulla; involuntary movements
Where do the
a) corticospinal
b) rubrospinal
c) reticulospinal tracts originate?
a) Pre-central gyrus of cerebral cortex
b) Red nucleus of midbrain
c) Pons/medulla
Sort of like a tower
The corticospinal tract controls voluntary fine movements.
If it is compromised, which descending tract takes over?
What posture does this result in?
Rubrospinal tract
Decorticate posture - legs adducted and extended; upper limbs, wrists and fingers flexed against the chest
“Decorticate” because the cerebral cortex isn’t involved
If the corticospinal tract is compromised, the rubrospinal tract takes over and the patient adopts a decorticate posture.
What happens if the rubrospinal tract is compromised?
Reticulospinal tract (bottom of the tower) has to take over
Patient adopts a decerebrate posture - legs extended and adducted; arms extended
“Decerebrate” because the cerebrum itself is no longer involved
Describe
a) decorticate
b) decerebrate posturing.
Which descending motor tracts are responsible for these postures?
a) Decorticate posturing - legs extended and adducted; arms, wrists and fingers flexed against the chest; rubrospinal tract
b) Decerebrate posturing - legs extended and adducted; arms extended; reticulospinal tract
What are some causes of decorticate and decerebrate posturing?
Serious brain / brainstem pathology
