HNS19 Motor System II Flashcards
Vertebral column
Cervical: 7 Thoracic: 12 Lumbar: 5 Sacral: 5 (fused) Coccyx: 4 (fused)
Within vertebral column
- Spinal cord covered by Dura, Arachnoid, Pia mater
- dorsal/ventral **rootlets
—> dorsal/ventral spinal **nerve roots
—> joined to form ***spinal nerves (formed outside of vertebral column) - Dorsal root: Sensory (∴ have dorsal root ganglion: pseudounipolar neuron)
- Ventral root: Motor (carries LMN)
“Ascent” of spinal cord during development
Vertebral column grows faster than spinal cord
Results:
- Conus medullaris: terminal end of spinal cord, ~T12/L1 level
- Cauda equina: spinal nerve roots form a bundle —> descend and form spinal nerves —> exit at respective levels of spine lower down
- Filum terminale (~S2, end of dura): residual extension of spinal cord ***pia mater (modification of pia mater) running from conus to coccyx —> no function
Spinal nerves
- Total 31 pairs (8, 12, 5, 5, 1)
Cervical:
- 8 pairs
- C1 exit above C1 vertebra
- C8 exit between C7, T1 vertebrae
Thoracic, Lumbar:
- T1 exit below T1 vertebra
- L5 exit between L5, S1 vertebrae
Below T12, L1:
- Cauda equina of spinal nerve roots
Vertebral level vs Cord segmental level
Vertebral level =/ Cord segmental level
∵ spinal cord is much shorter than vertebral column
—> e.g. sacral spinal cord segment will be situated at a higher position within vertebral column (e.g. lumbar vertebral level)
Lumbar puncture
CSF withdrawn from Cauda equina region (usually L4/5 vertebral level) (Subarachnoid space)
—> needle will not hit spinal cord
Epidural and Spinal anaesthesia
Epidural: outside of Dura (between dura and ligament)
Spinal / Subdural: within Arachnoid mater
—> provide regional anaesthesia at + below the level
—> patient remains conscious!
Spinal cord cross-sectional anatomy
- Anterior / Posterior horn
- main nuclei / interneurons of spinal cord - Anterior / Lateral funiculus (funiculus group together —> fasciculus)
- long fibre tracts - Anterior / Posterior root of spinal nerve
(4. Spinal nerve)
Grey matter of spinal cord classification
Based on nuclei:
- Sensory nuclei (Dorsal)
- Autonomic function nuclei (Lateral + Medial)
- Motor nuclei (Ventral)
Based on laminae
***White matter of spinal cord
Motor and descending (efferent) pathways
1. Pyramidal tracts
—> Anterior corticospinal tract
—> Lateral corticospinal tract
- Extrapyramidal tracts
—> Rubrospinal tract (red nucleus: UL flexor predominant)
—> Reticulospinal tract (reticular nuclei: medial pontine —> stimulate extensor; lateral medullary —> inhibit extensor)
—> Olivospinal tract
—> Vestibulospinal tract (vestibular nuclei: extensor predominant)
—> Tectospinal tract
Sensory and ascending (afferent) pathways
1. Dorsal column-medial lemniscus pathway
—> Gracile fasciculus
—> Cuneate fasciculus
- Spinocerebellar pathway
—> Anterior spinocerebellar tract
—> Posterior spinocerebellar tract
(—> Cuneocerebellar tract (neurons originate from external cuneate nucleus at medulla)) - Anterolateral system
—> Spinothalamic tract (Anterior + Posterior)
—> Spinoreticular tract
—> Spinomesencephalic tract
Functional anatomy of descending and ascending tracts
Descending motor tracts
—> activate + modulate LMN within ventral horn
—> LMN then travel within ventral root
—> spinal nerve activate skeletal muscles
Sensory input
—> travel through dorsal root
—> ascend through spinal cord
—> provide feedback
***Corticospinal tract (CST)
- > 1 million of mostly myelinated axons
- mainly from Primary motor cortex (M1 / Area 4)
- Incompletely decussated at the pyramids of medulla
- 75-90% crossed
—> Lateral corticospinal tract (in Lateral funiculus)
—> synapse with interneurons + α motor neurons (at ventral horn)
—> supply ***distal musculature (hand, foot)
- Lateral CST
—> Cervical: medial
—> Sacral: lateral
—> Clinical significance: disease from outside (compression) / from inside (tumour)
- Uncrossed at medulla (may cross eventually to supply contralateral side)
—> Anterior corticospinal tract (in Anterior funiculus)
—> modulates **axial + **proximal musculature
- Uncrossed —> affect ***both side of Brainstem
- Bilateral **Axial + **Proximal muscle for posture
***Extrapyramidal tracts
- Originates from Brainstem nuclei (Reticular formation)
- Under influence of nigrostriatal system, cerebellum, sensory system etc.
- Modulates ***reflexes, posture, CST activities
Functions:
- ***Process information from
- Higher level special senses (smell, visual, auditory)
- Spinal cord (peripheral mechanoreceptors) - ***Modulate activities of:
- Corticobulbar tract to cranial nerve nuclei
- Corticospinal tract to spinal nerve nuclei - Send descending fibres in addition to the pyramidal tracts (i.e. extrapyramidal tracts)
- further ***modulate motor activities within spinal cord
- sometimes even though pyramidal tracts are diseased
—> can still have some degree of control on LMN - Critical for **postural adjustments + control of **axial + ***proximal musculature
Examples:
- Rubrospinal tract
- Tectospinal tract
- Pontine (medial) reticulospinal tract
- Medullary (lateral) reticulospinal tract
- Lateral vestibulospinal tract
Evolution of CNS
Lower animals:
- no distinct brain
- non-bilateral system
- net-like / radial system / ventral “spinal cord”
Higher animals:
- enlarged brain
- bilateral system
- dorsally placed spinal cord
Pyramidal and Extrapyramidal system
More “advanced” motor system
- Lateral descending system: Lateral CST + Rubrospinal tract (extrapyramidal)
- Crossed fibres (unknown reason)
- Interact with anterior horn cells for ***Distal muscles —> more complex, fine, “advanced” movements
More “primitive” motor system
- Ventral descending system —> Other extrapyramidal tracts
- Mostly uncrossed fibres —> ***Axial muscle, posture, reflex
- Interact with anterior horn cells for ***Proximal muscles —> more “basic” and for gross movement
Rubrospinal tract
- From magnocellular neurons of ***Red nucleus in midbrain
- ***Crossed fibres
- in ***Lateral descending system
- Control ***flexor muscles
- Significance in human unclear
Tectospinal tract
- From ***Superior colliculus (Tectum of midbrain): Vision
- ***Crossed fibres to contralateral cervical segments
- ***Reflex movement in response to Visual stimuli (auditory?)
Reticulospinal tract
- From less clearly defined nuclei within ***Reticular formation (brainstem)
- ***Uncrossed
- ***Influence voluntary movement
- Medial (Pontine) RST —> Excites ***extensors
- Lateral (Medullary) RST —> Inhibits ***extensors
Lateral Vestibulospinal tract
- From ***Lateral Vestibular nucleus (medulla)
- Input from Cerebellum and Vestibular apparatus
- ***Uncrossed
- Innervates ***ipsilateral trunk muscle (Extensor)
- Maintain ***posture and balance (Labyrinthine Righting Reflex: integrates head position balance signal with truncal muscle)
Summary of descending system
Lateral CST (crossed): executes conscious voluntary movement (distal musculature)
Ventral CST (uncrossed): controls balance + posture (axial + proximal musculature)
Extrapyramidal tracts serve ***additional roles:
- provide background condition and “readiness” for CST to act
- execute “unconscious” response to external stimuli (brainstem integration of sensory input) and sensory feedback from the body
- maintains a balance between flexion and extension motor activities as well as posture and balance
—> Diseases at different levels —> different clinical pictures + recovery potential
Clinical relevance of Extrapyramidal tracts
In unconscious patients, disease at different levels
—> different extents of loss of descending motor influence
—> different postures
- Decorticate (cortex cut off) posture:
Lesion above red nucleus
—> Rubrospinal tract intact
—> UL flexion remains - Decerebrate (cerebrum cut off) posture:
Lesion between Red nucleus (midbrain) and Vestibular nucleus (medulla) (More extensive involvement of brainstem)
—> Rubrospinal tract cut off
—> Un-opposed extensor activities of Reticular and Vestibular nuclei
—> UL extension (worse prognosis) - Post-stroke recovery:
E.g. Internal capsule infarction
- Lateral CST gone
—> poor recovery of fine movement - Extrapyramidal tracts intact (∵ infarction above brainstem)
—> some recovery of proximal muscles (trunk muscles, thigh) and gross movement (walking)
—> imbalance of flexion and extension influences to LMN
—> results in abnormal limb posturing (e.g. UL flexed and LL extended)
Disease of spine
Etiology:
- Trauma
- spinal fracture
- spinal epidural haematoma
- penetrating injury - Degeneration
- spondylosis - Tumour
- spinal tumour - Bleeding
- Inflammation —> multiple sclerosis - Loss of blood supply
- Congenital
Can affect:
- Spinal cord
- Cauda equina
- Individual nerve roots
- Combination of above
***UMN lesion vs LMN lesion
UMN lesion:
- Location
- CNS only - Structures involved
- Cortex
- CST / CBT - Distribution
- Groups of muscle - Paralysis
- **Spastic paralysis (esp. anti-gravity muscle)
- **Clasp-knife rigidity / spasticity - Reflexes
- ***Hyperactive - Muscle bulk
- ***Mild disuse atrophy - Classical signs
- ***Babinski’s response / Positive Babinski sign
- Clonus - Examples
- Stroke, head injury, spinal cord injury (e.g. CTS)
LMN lesion:
- Location
- PNS + CNS as well (Anterior horn cells: count as LMN but within spinal cord: CNS) - Structures involved
- α-motor neuron
- Motor fibres in spinal / cranial nerves - Distribution
- Segmental - Paralysis
- ***Flaccid paralysis - Reflexes
- ***Decreased / Absent - Muscle bulk
- ***Pronounced atrophy - Classical sign
- None - Examples
- Poliomyelitis affecting anterior horn cells
- nerve roots / spinal nerve / peripheral nerves (PNS)
Myelopathy vs Radiculopathy
Myelopathy - Spinal cord E.g. Central IV disc herniation - LMN lesion at that level (anterior horn cells) - UMN lesion below
Radiculopathy - Nerve root E.g. Lateral IV disc herniation - LMN lesion only - Cord itself unaffected
Spinal cord lesion
- At / above conus medullaris
- LMN lesion at that level
- UMN lesion below
- Lateral CST gone
—> no voluntary movement - Extrapyramidal tracts also gone
—> no descending modulation
—> hyperactive reflex arc (late) - Overall recovery poor: may not be even able to walk (worse than internal capsule infarct)
Levels of injury and extent of paralysis
C4 injury
—> Brachial plexus involved (C5-T1 spinal nerves to UL)
—> Quadriplegia
C6 injury
—> C5 of brachial plexus may be spared
—> still Quadriplegia (but some areas e.g. shoulders may be spared)
T6 / L1 injury
—> brachial plexus spared
—> Paraplegia
Hemitransection - Brown-Sequard syndrome
- **Ipsilateral motor symptoms
1. LMN lesion at the level - Flaccid paralysis
2. UMN lesion at level below - Spastic paralysis
- pyramidal weakness
Ipsilateral impaired joint position, sense and accurate touch localisation
Contralateral impaired pain and temperature sensation
Patterns of Radiculopathy
- Clinical assessment can locate / pinpoint the lesion
- Disease of specific nerve root —> specific pain distribution, weakness, sensory loss + reflex loss
- Peripheral nerves (distal to spine, several spinal nerves joined together) =/ Spinal nerves
- Peripheral nerve lesions —> can involve several spinal nerve roots
Conus medullaris and Cauda equina lesions
Cord: UMN lesion (except anterior horn cells)
Conus medullaris: Mixed LMN and UMN lesions
Cauda equina: LMN lesion
Cauda equina syndrome
- LMN lesions
- affect Lumbosacral nerves
- acute / subacute onset
Symptoms:
1. LL weakness
- Sensory loss
- e.g. Saddle anaesthesia (+/- sensory loss in one of both lower limbs) - Sphincter dysfunction (bladder, urethral sphincter, anal sphincter supplied by complex system of sensorimotor nerves for emptying and maintaining continence)
—> Bladder: painless acute urinary retention
—> Bowel: lax anal tone on PR exam, but constipation
—> BOTH irreversible unless very early intervention! - Sexual dysfunction
- Back pain
Case study 1
- Complaint: Weak right arm
- RLL weak
- Speech deficit
- Facial weakness
Left cerebral hemisphere stroke
Case study 2
- Complaint: Weak right arm
- RLL weak
- Right limbs hyperactive reflexes
- Left limbs normal motor function
- Cranial nerves normal
Tumour affecting mainly right side of cervical spinal cord
Cranial nerves normal —> Brain normal
Case study 3
- Complaint: Weak right arm
- Weakness and numbness in proximal UL
- Distal UL normal
Problem mainly at proximal UL
Compatible with C5/6 degeneration on MRI scan
—> rmb myotome
—> more proximal joint innervated by higher centres in spinal cord (C5-6)
Case study 4
- Complaint: Weak right arm
- Weakness in wrist extension
- Wrist flexion ok
- Proximal UL normal
Problem mainly at distal UL
NOT compatible with C5/6 degeneration on MRI scan
—> rmb myotome
—> more distal joint innervated by lower centres in spinal cord (C6-T1)
Summary
Spinal cord: final common pathway for limb and trunk motor function
Lateral CST: fine + complex movement
Extrapyramidal tracts: trunk + proximal movement
UMN vs LMN lesions: clinically distinct
Myelopathy vs Radiculopathy
Spinal cord lesions and Cauda equina syndromes have different clinical presentations
Clinical relevance: Apply neuroanatomy to: 1. Locate the disease - brain / spine - where - which side 2. Assess severity and urgency 3. Order the right investigation 4. Interpret investigation - culprit / incidental findings 5. Decide on treatment
