Motor systems
Chris Thomson BVSc(Hons), Dip ACVIM (Neurol), Dip ECVN, PhD Associate Professor Neurobiology, Dept. of Vet. Med., University of Alaska, Fairbanks, 1 Alaska. Quadrupedal Motor Systems
What are their functions?
1. Antigravity support 2. Postural platform for movement 3. Movement initiation, maintenance and
termination Fig 5.3 Thomson and Hahn
2 Motor hierarchy
• Motor unit – LMN and NMJ • Reflexes • Central pattern generators (CPG) • UMN – Semiautomatic function – brainstem – Skilled/learned function – forebrain EMG study Kiwi chick • Motor planning centres
3 Neuromuscular junction Motor unit = MN + innervated muscle cells Size determines degree of fine control Examples A B
B
Fig 1.4 Thomson and Hahn
A
4 UMN and LMN: the confusing couplet
Upper motor neurons (UMN) – central MN • Location: confined to brain and spinal cord – ‘Management’ – Control motor activity » Initiate, regulate, terminate – Lower motor neurons (LMN) – peripheral MN • Location – nerve cell body in CNS, axon in PNS – ‘Workers’ – Connect to muscle of body, limb or head – Key part of the reflex – Spinal and cranial nerves » Cause muscle to contract
5 Motor systems
LMN also in CNN and visceral efferents (autonomic)
Picture of ‘Stephie’ By Catie, aged 6 6 Reflexes
• What is their physiological role in posture and locomotion? – Agonist-antagonist muscle interaction – Antigravity – Gait switch between retraction and protraction Fig 4.3 Thomson and Hahn 7 Fig 5.3 Thomson and Hahn
Appendicular muscle reflexes – Agonist-antagonist muscle interaction • Intersegmental connection propriospinal tract – Antigravity – Gait switch between retraction and protraction
8 Axial muscle myotatic reflex Effect on posture?
Fig 5.2 Thomson and Hahn
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http://www.vcahospitals.com/ Locomotion and reflexes
• Reflex wiring – Basis of locomotion in quadrupeds – Muscle stretch induces reflex contraction – Extensor postural thrust reflex – Crossed extension reflex
– Diagonal stepping reflex Fig 9.1 Thomson and Hahn
10 Movement facilitates movement Using reflex circuits Gait initiation Movement changes sensory (proprioceptive) input
motor neuron excitation • Sensory input stimulates reflex function • Same limb e.g. hip extension reflex hip flexion • Other limbs e.g. limb flexion crossed extension, diagonal stepping • Spinal reflexes are the basis of movement
• Used by central pattern generators 11 Central Programme/pattern Generators • Basic motor control for rhythmical movement – Alternating contraction / relaxation – Locomotion, flying, scratching, breathing, chewing, micturition • CPG – Trigger neurons (midbrain) • Affect timing, amplitude and pattern • Efferents via reticular formation, reticulospinal tract to oscillator neurons http://almirah.deviantart.com/art/Moki-Run-Cycle – Oscillator neurons (spinal cord) • Alternating support (extensor) and swing (flexor) phase • Alternating limbs – Influence LMN
12 How can this dog still walk?
13 Spinal reflexes and amplification • UMN Connection to LMN – UMN -> interneuron -> g MN (most UMN) -> stretches muscle spindle Amplification stage -> reflex a MN firing
• Clinical significance 1. Few UMN required to trigger oscillator neurons in intumescence 2. Amplification of signal via ɣ motor neurons
14 What about Spinal Walking? Fig 5.2 Thomson and Hahn 15 UMN centres
• Brainstem – origin of semi-automatic movements • Forebrain – skilled/learned movements
Fig 9.4 Thomson and Hahn Fig 4.15 Thomson and Hahn
16 Divisions of Motor Systems
• Extrapyramidal – Most important in quadrupeds • Pyramidal – Highly important in humans
Fig 8.50, Dyce, Sack and Wensing, 4th ed.
17 Extrapyramidal System
• Origin – All brain divisions • e.g. basal nuclei, red nucleus, pontine and medullary reticulospinal, vestibulospinal, tectospinal tracts • Multisynaptic • Ipsi- and contralateral projection • Termination – a and g MN brainstem and spinal cord
18 Extrapyramidal System
• Function – Posture and locomotion – Synapses primarily onto g-MN – Inhibitory • Medullary reticulospinal tract – Loss -> UMN spasticity – Excitatory • Extensor muscle facilitation – Vestibulospinal, tectospinal, pontine reticulospinal tracts
• Flexor muscle facilitation Fig 4.2 Thomson and Hahn – Rubrospinal tract Extensor spasticity after TL lesion
19 Spinal cord motor tracts
ID Name
A Propriospinal (spino-spinal)
H Rubrospinal
I Lateral corticospinal
J Lateral tectotegmentospinal
K Medullary (lateral) reticulospinal
L Pontine (ventral) reticulospinal
M Lateral vestibulospinal
N Tectospinal
O Ventral corticospinal
P Medial vestibulospinal and medial longitudinal fasciculus
Fig 4-5 Thomson and Hahn
20 Extrapyramidal Tract Function
• Rubrospinal • Important in dogs and cats • Semiskilled and postural (flexor) activity
• Reticulospinal – Medullary • Suppresses antigravity muscle activity – Pontine • Standing posture
21 http://www.releasethehounds.com/media Extrapyramidal Tract Function
• Vestibulospinal tracts (VST) • Lateral VST – From lateral vestibular nuclei (VN) – Stimulated by static head position – Ipsilateral antigravity muscles whole body • Medial VST – From medial, rostral and caudal VN – Stimulated by head acceleration – Output to neck/shoulder muscles » Maintains head posture Fig 8.5 Thomson and Hahn • Medial longitudinal fasciculus – Medial VN (and other brainstem nuclei) – VF – neck and cranial thoracic cord – Brainstem to CN III, IV, VI nuclei – Coordination eye, neck and TL posture during head movement 22 Extrapyramidal Tract Function
• Tectospinal tracts – Lateral (tectotegmentospinal) • UMN for sympathetic output to eyes – To T1/T2 spinal cord segments • Active pupillary dilation in response to dim light
– Medial • From the corpora quadrigemina – Rostral and caudal colliculi • Function – head/neck movement in response visual/auditory stimuli » ‘Visual grasp’, ‘auditory grasp’ 23
https://s-media-cache-ak0 Corpora quadrigemina • Rostral colliculus • Caudal colliculus
Thomson and Hahn Fig A7 24 Extrapyramidal system • Red lines are facilitatory • Black lines are inhibitory
• NOTE: vestibulospinal tract is facilitatory to ipsilateral side
Fig. 13.2 King
25 Why do we get spasticity with of UMN spinal cord lesions?
26 Pyramidal Motor System • Mammals only – Output from motor cortex: – Via crus cerebri (A)
– longitudinal fibres of the pons (B) A • Corticopontine – To cerebellum and back to motor cortex • Corticonuclear A
– e.g. to CNN nuclei of B brainstem B • Corticospinal tract, C – via medullary pyramids D (C,D), to spinal cord – Tracts decussate – Spinal cord C • 75% decussate at C1-2 into lateral funiculus (LF) • rest in VF, decussate just b/4 termination Fig A3 Thomson and Hahn Dog brain, ventral aspect 27 D Motor cortex output
• Function http://www.horsenation.com/ – Skilled /learned movement • Humans/primates – 30% spinal cord WM • Quadrupeds – Dogs 10% spinal cord WM – c/w ungulates – Note » horses, camelids » raccoons
28 Fig 8.50, Dyce, Sack and Wensing, 4th ed. Human Pons XS: 30 years post-stroke Pyramidal Motor Where is the Lesion? System – Clinical significance of pyramidal lesions • Humans vs quadrupeds
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th Fig 8.50, Dyce, Sack and Wensing, 4 ed. Ovine pons, Thomson and Hahn Fig A30 30 31 Why the difference with a forebrain lesion?
http://graphics8.nytimes.com 32 Fig 4-10, Thomson and Hahn Fig 8-14 deLahunta and Glass 33 Basal nuclei - components
Note red = grey matter, blue = white matter.
A. Caudate nucleus B. Globus pallidus C. Internal capsule D. Putamen E. External capsule F. Claustrum G. Extreme capsule
Corpus striatum = basal nuclei and intervening WM
FE C
34 Basal nuclei – What’s this dog’s presenting sign? function
• Humans – disease affecting BN? • Feedback circuits – modify motor output – Ritual movements? • ‘Ordering the component parts of Photo courtesy Kate Hill complex movement’ (Jenkins) – Lesions • putamen – propulsive activity • globus pallidus – hypoactive • caudate n. – athetoid movements • Effect of unilateral lesion?
Forebrain neoplasia Differentiating disease in UMN versus LMN
NeuroRAT Reflexes Atrophy Tone
Fig 5.6 Thomson and Hahn Sign UMN – central MN dz LMN – peripheral (damaged UMN) MN dz (damaged LMN)
Reflexes Intact (increased) Decreased/absent
Atrophy Disuse: mild Neurogenic: severe, generalised specific muscles
Tone Intact (increased) Decreased/absent
36 UMN lesions
• Clinical effect of UMN lesions – Depends on lesion location • Rigidity/spasticity – Loss of inhibitory input » From forebrain – decerebrate rigidity » From medullary reticular formation – limb and trunk hypertonus • Paresis/paralysis – Loss of movement initiation – Decreased facilitation LMN – Loss of skilled motor activity/control » Motor cortex (visual placing is a good test) • Postural abnormalities – Decerebrate rigidity – mesencephalic lesion – Pleurothotonus – mesencephalic lesion – Decerebellate rigidity – rostral cerebellum – Head tilt – vestibular dysfunction – Torticollis – forebrain or neck LMN (hyper or Fig 9.6 Thomson and Hahn hypoactivity) – Schiff-Sherrington – acute thoracolumbar lesion
37 Henry 7 yo MN Corgi, LMN lesions Hx 1 mo progressive RPL lameness
• ↓/0 Reflexes • neurogenic atrophy • ↓/0 tone
38 Coordination of movement
39 Cerebellar Function
– To coordinate posture and movement • Receives input information about – Position and movement of body parts » Spinocerebellar » Vestibulocerebellar tracts – Planned motor activity » Forebrain » Extrapyramidal system
• Send output to – Brainstem UMN centres – Forebrain » motor planning centres Cerebellar function
• Continual input from – Muscles and joints (SCP) – head, neck, trunk, limbs, tail – Vestibular system – head position – Motor planning centres • Modifies output from UMN centres – Forebrain (skilled) and brainstem (semi-automatic) – Coordinate agonist-antagonist muscle function – At rest and during locomotion • Sets the postural platform – On which motor activity can occur Functional connections of the cerebellum Cerebellar afferents: •Proprioceptive information from trunk, limbs and head •Motor planning • Motor cortex (voluntary) – via cortiocpontocerebellar • Extrapyramidal from forebrain and midbrain, via olivary nucleus •Cerebellar efferents • Brainstem UMN nuclei 42 • Forebrain motor centres Postural platform
• Planned motor activity – Inform cerebellum – Cerebellum checks body posture
(SCP) Fig 7.9 Thomson and Hahn – Cerebellar efferents to UMN centres sets postural platform (UMN coordination) – New SCP to cerebellum – Cerebellum informs motor cortex – Motor activity occurs
• Cerebellar dysfunction Postural paralysis Cerebellar peduncles
• Rostral CP – Afferent • ventral spinocerebellar tract – Efferent • cerebellar nuclei to midbrain and forebrain UMN centres • Middle CP – Afferent only • corticopontocerebellar tract • Caudal CP – (restiform and juxtarestiform bodies) – Afferent • Spinocerebellar (dorsal, cranial, cuneocerebllar), • vestibulocerebellar, • olivocerebellar, • reticulocerebellar (pontine and medullary)
– Efferent Evans, 18-4 and 18-41 • Cerebellovestibular Cerebellar peduncles – bilaterally paired • Cerebelloreticular Name for their position of attachment to the brainstem • Rostral – 12 (upper image); 6 – lower image • Middle 10 • Caudal 11; 3 44 Sequential images
Caudal CP (?) Lateral aperture Caudal and rostral CP (?) Rostral CP Middle CP (?) Pontine nuclei Transverse fibres of the pons Cerebellar Efferents
Purkinje (pyramidal) cells – May be stimulated or inhibited
http://image.slidesharecdn.com/ – They are inhibitory histologyofnervesystem • to vestibular nuclei – direct inhibition • to excitatory cerebellar nuclei – Decrease their facilitation of motor activity
– indirect inhibition Fastigial CN – Lateral cerebellar nucleus Interposital CN
» To forebrain Lateral CN – Interposital nucleus » To red nucleus and reticular formation – Fastigial nucleus » To vestibular nucleus and reticular formation
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Fig 7.6c Thomson and Hahn 47 Locomotion Summary • Spinal reflexes and CPG – Foundation of movement • Supraspinal input – Initiates – Terminates – Coordinates – Modulates spinal reflexes -> many, varied patterns of movement
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