Spinal cord, motor & Sensory Pathways II Flashcards
What are upper motor neurons (UMNs)?
Primary pyramidal cells that sit in the primary motor cortex and brainstem (in several nuclei) that descend the spinal cord and synapse with lower motor neurons that convey motor functions.
Where do Lower Motor Neurons (LMN) arise from?
From the ventral horn (grey matter), ventral nerve roots, cranial nerve nuclei (brainstem), and craineal nerves
What are the types of motor neurons?
Alpha, beta, and gamma depending on types of fibers they innervate
What do pyramidal tracts control?
Voluntary movements
What are the types of pyramidal tracts?
Corticospinal tract (L & V) - Musculature of the body
Corticobulbar tract - Musculature of the head and neck
What do extrapyramidal tracts control?
Involuntary and autonomic control
What are the 4 extrapyramidal tracts?
Vestibulospinal tract (divided into lateral and medial)
Reticulospinal tract (divided into lateral and medial)
Rubrospinal tracts
Tactospinal tracts
What does the vestibulospinal tract control?
Balance and posture, innervates “anti-gravity” flexor and extensor muscles
What is the difference between lateral and medial vestibulospinal tracts?
Medial controls head position
Lateral controls muscle tone, posture and balance
What do the reticulospinal tracts control?
They influence/coordinate movement, reflex, posture
What is the difference between medial and lateral reticulospinal tracts?
Medial facilitates voluntary movement and increases muscle tone
Lateral inhibits them
What do the rubrospinal tracts control?
Mediate voluntary movement of hand and limb flexor muscles
What do tectospinal tracts control?
Reflexive head/neck movement in response to visual and auditory stimuli
Describe the pathway of the lateral corticospinal tract:
UMN -> Pyramids -> decussation -> Spinal Cord -> Ventral horn -> LMN
Describe the pathway of the Ventral corticospinal tract:
UMN -> Pyramids -> Spinal cord -> Decussation -> Ventral Horn -> LMN
What is the difference betweent lateral and ventral corticospinal tract pathways?
Decussation in lateral tract happens in the brain and in the ventral tract happens in the spinal cord
What is the corona radiata?
The most prominent projection fibers are the corona radiata, which radiate out from the cortex and then come together in the brain stem.
How do the ventral corticospinal neurones decussate?
They pass through the ventral white commissure
Which tract is the longest continuous white matter tract in the CNS?
The corticospinal tract
Are upper motor neurons of the corticospinal tract ipsilateral to the nerves they innervate?
No they decussate in spinal cord
What percentage of corticospinal tract neurons are ventral?
80 - 90%
what percentageof corticospinal tract are lateral?
10 - 20%
What percentage of UMN (CST) terminate at the cervical level?
55% (we have lots of control of hand muscles and arms)
What percentage of UMN (CST) terminate at the thoracic level?
20%
What percentage of UMN (CST) terminate at the lumbosacral level?
25%
What pathway do corticobulbar tract neurons go?
Fibres from UMNs of lateral primary motor cortex descend through corona radiata and internal capsule
Fibres continue to descend as the corticobulbar tract and synapse with motor nuclei of the cranial nerves in the brain stem (Trigeminal, facial, glossopharyngeal, vagus, and hypoglossal)
Does corticobulbar tract use interneurons?
Not very often but sometimes it does
Where do vestibulospinal tract neurons typically initiate?
Lateral vestibular nucleus in the pons
Where do vestibular nuclei get their input?
They receive input from the vestibular nerve (Part of vestibulochlear nerve or VIII)
What is the path of the vestibulspinal tract when acting on the lumbar level (the lateral vestibular nucleus)?
Lateral vestibular nucleus in the pons -> Spinal cord ipsilaterally as lateral vestibulospinal tract -> synapse on interneurons in intermediate grey area or LMNs in the ventral horn at the lumbar level.
What is the path of the vestibulospinal tract when acting on the cervical level (medial vestibular nucleus)?
Medial vestibular nucleus (medulla) -> spinal cord ipsilaterally as medial vestibulospinal tract (ventral column) -> to synapse on LMNs in the ventral horn at the cervical level
Where do the fibers of the reticulospinal tract originate?
From the pontine reticular formation
What pathway do fibers of the reticulospinal tract follow?
Fibers from pontine reticular formation descend ipsilaterally as the medial reticulospinal tract (ventral column) to synapse on interneurons in the intermediate grey area or LMNs in the ventral horn
Fibers from the medullary reticular formation (medulla) descend ipsilaterally as the lateral reticulospinal tract (ventral column) to synapse on interneurons in the intermediate grey area or LMNs in the ventral horn
What pathway do fibers from the rubrospinal tract follow?
Fibres from the red nucleus in the midbrain immediately decussate via the ventral tegmentum then they descend the spinal cord as the rubrospinal tract (lateral column; adjacent to the lateral CST) to synapse on LMNs in the ventral horn
Where does the rubrospinal tract receive input?
Red nucleus receives input from the primary motor cortex and cerebellum
Where does the tactospinal tract receive input?
Superior colliculi receive input from the optic nerves (CN II)
What pathway do tectospinal tract fibers follow?
Fibres from the superior colliculus in the midbrain decussate via the dorsal tegmentum and descend the spinal cord contralaterally as the tectospinal tract (ventral column) then they Synapse on LMNs in the intermediate grey area and ventral horn at the cervical level
Summary
Corticospinal tract – voluntary motor control of the body and limbs
Corticobulbar tract – voluntary motor control of the head and neck
Vestibulospinal tracts – maintain balance and posture
Reticulospinal tracts – influences and coordinates movement, reflex, posture
Rubrospinal tracts – mediation of voluntary movement, control of limb flexor muscles
Tectospinal tracts – reflexive head and neck movements in response to visual and auditory stimuli
What are the types of reflex arcs?
Monosynaptic
Polysynaptic
Where do inferior tectospinal colliculi recieve their input?
From the Ears (vestibulocochlear nerve)
Where do fibers of the tectospinal tract decussate?
Dorsal tegumentum (within the brain)
What is the lowest level at which the tectospinal tract reaches?
The cervical level only
Why do we feel pain way after the withdrawal of our hand from fire?
Reflex arcs are much faster in conducting nerve impulses than sensory nerves
What causes spinal cord lesions?
Vitamin deficiency, infection, tumour, degenerative disease, disruption of blood flow, traumatic spinal cord injury
What do spinal cord lesions do?
They can disrupt sensory and/or motor function
What can be used to detect where injury is occurring?
Sensory and motor function
How can dorsal column lesion location affect the side on which there is sensory loss?
The location of the decussation is the deciding factor.
If lesion is below decussation then sensory loss is ipsilateral. If lesion is above decussation then sensory loss is contralateral.
What happens when dorsal column lesion is above T6?
Lower and upper extremities are affected
What happens when dorsal column lesion is below T6?
Only the lower limbs are affected
What happens to proprioception of someone with a dorsal column lesion?
It is abolished with some cutaneous sensation retained
What happens to movement in people with dorsal column lesion?
Ataxia and lack of coordination is common due to unaffected voluntary control but no proprioceptive input to the brain
What sensory system can assist people who are ataxic due to dorsal column lesion?
Visual system
How can a patient be tested for dorsal column lesion?
Vibration can be sensed so it can be applied.
Proprioception testing
Discriminative touch
How can proprioception be investigated in dorsal column lesions?
Romberg’s test (can patient stand straight with eyes closed?)
Flex/Extend joints (Can the patient tell you what you’ve done?)
How can discriminative touch be investigated in patients with dorsal column lesions?
Place cotton wool on the skin, can patient localise the sensation?
Apply two sharp points to the skin, can patient identify one or two points?
What side are symptoms seen in anterolateral spinothalamic tract lesions?
Decussation at all levels of spinal cord results in contralateral sensory loss
Why is contralateral anaesthesia done 1 - 2 segments below an anterolateral spinothalamic tract lesion?
Because of passage through lissaurs fasciculus
How can anterolateral spinothalamic tract lesions be tested?
Pain testing using sharp and dull objects on skin and asking patient to report the correct sensation.
Temperature testing by placing hot or cold probes on the skin
Where is motor loss seen with corticospinal tract lesions?
Below decussation = ipsilateral motor loss
Above decussation = contralateral motor loss
What kind of paralysis results from corticospinal tract lesion?
Spastic paralysis (due to increased muscle tone and hyperactive reflexes)
How many traumatic spinal cord injuries do we see in Australia every year?
300 new cases each year
What causes spinal cord injuries?
45% motor vehicle accidents
32% falls
10% water related accidents
5% sports
What age group commonly gets spinal cord injuries?
30% 15 - 24 year olds
83% males and 17% females
What percentage of spinal cord injuries are complete?
35% complete
65% incomplete
What is the most common level of the spine in which we see spinal cord injuries?
64% are cervical
What are the types of injury responses seen in spinal cord injuries?
Primary injury events directly caused by mechanical injury
Secondary injury events caused by the inflammatory/immune response
What happens in secondary injury events to the spinal cord?
Inflammatory cascade
Immune cell infiltration
Oxidative damage
Excitotoxic events
Demyelination
Cyst formation
Glial scarring
Which direct does damage spread in traumatic spinal cord injury?
Along the rostro-caudal axis
What are the types of spinal cord injury?
Contusion (Resulting from blunt trauma)
Transection injury is very very rare. (due to gun fights)
What does contusion injury to the spinal cord typically affect?
Midline contusions affect the dorsal columns and horns of the grey matter.
If the contusion is unilateral damage could be on one side only
What does anterior cord syndrome affect?
Affects front 2/3rds of the spinal cord and blocks both spinothalamic tracts on both sides, ventral horn, and lateral corticospinal tract
What does posterior cord syndrome affect?
Bilateral DCML
Dorsal horn
What does Brown-sequard syndrome affect?
Unilateral DCML
Lateral corticospinal tract
Spinothalamic tract
What can cause posterior cord syndrome?
Posterior spinal artery occlusion
What is Brown-Sequard syndrome?
Damage to one side of the spinal cord resulting in proprioception + vibration and motor contol problems on one side of the body and pain + temperature sensations on the other side
What system is used to grade spinal cord injuries?
ASIA impairment scale (AIS)
How does the ASIA impairment scale grade injuries?
A = Complete (no loss of sensory or motor function in S4-S5)
B = Sensory incomplete (but not motor function)
C = Motor incomplete (Muscle grade below NLI <3)
D = Motor incomplete (Muscle grade below NLI > 3) [NLI >3 means the most caudal segment has intact sensation and antigravity muscle function strength]
E = Normal
