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Motor systems

Chris Thomson

BVSc(Hons), Dip ACVIM (Neurol), Dip ECVN, PhD

Associate Professor Neurobiology,

Dept. of Vet. Med.,

University of Alaska, Fairbanks,

Alaska.

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Quadrupedal Motor Systems

What are their functions?

1. Antigravity support

2. Postural platform for

movement

3. Movement initiation,

maintenance and

terminationFig 5.3 Thomson and Hahn

Motor hierarchy• Motor unit – LMN and NMJ

• Reflexes

• Central pattern generators

(CPG)

• UMN

– Semiautomatic function

– brainstem

– Skilled/learned function

– forebrain

• Motor planning centres

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EMG study Kiwi chick

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Neuromuscular junction

Fig 1.4 Thomson and Hahn

Motor unit = MN + innervated

muscle cells

Size determines degree of fine

control

Examples

A

B

A

B

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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

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Motor systems

Picture of ‘Stephie’ By Catie, aged 6

LMN also in CNN and visceral efferents (autonomic)

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Reflexes

• What is their

physiological role in

posture and locomotion?

– Agonist-antagonist

muscle interaction

– Antigravity

– Gait switch between

retraction and

protractionFig 4.3 Thomson and Hahn

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Appendicular muscle

reflexes– Agonist-antagonist

muscle interaction• Intersegmental connection

propriospinal tract

– Antigravity

– Gait switch between

retraction and protraction

Fig 5.3 Thomson and Hahn

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Axial muscle myotatic reflex

Fig 5.2 Thomson and Hahn

Effect on posture?

http://www.vcahospitals.com/

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Locomotion and reflexes

Fig 9.1 Thomson and Hahn

• Reflex wiring

– Basis of locomotion in quadrupeds

– Muscle stretch induces reflex

contraction

– Extensor postural thrust reflex

– Crossed extension reflex

– Diagonal stepping reflex

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Movement facilitates movement

Using reflex circuits

Gait initiationMovement 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

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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

– Oscillator neurons (spinal cord)

• Alternating support (extensor) and swing (flexor) phase

• Alternating limbs

– Influence LMN

http://almirah.deviantart.com/art/Moki-Run-Cycle

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How can this dog still walk?

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Spinal reflexes and

amplification

• UMN Connection to LMN

– UMN

-> interneuron

-> g MN (most UMN)

-> stretches muscle spindle

Amplification stage

-> reflex a MN firing

• Clinical significance1. Few UMN required to trigger oscillator neurons in

intumescence

2. Amplification of signal via ɣ motor neurons

What about Spinal Walking?Fig 5.2 Thomson and Hahn

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UMN centres

• Brainstem – origin of semi-automatic movements

• Forebrain – skilled/learned movements

Fig 4.15 Thomson and HahnFig 9.4 Thomson and Hahn

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Divisions of Motor Systems

• Extrapyramidal

– Most important in

quadrupeds

• Pyramidal

– Highly important in

humans

Fig 8.50, Dyce, Sack and Wensing, 4th ed.

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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

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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

– Rubrospinal tractFig 4.2 Thomson and Hahn

Extensor spasticity after TL lesion

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Spinal cord motor tracts

Fig 4-5 Thomson and Hahn

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

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Extrapyramidal Tract Function

• Rubrospinal• Important in dogs and cats

• Semiskilled and postural

(flexor) activity

• Reticulospinal– Medullary

• Suppresses antigravity muscle activity

– Pontine • Standing posture

http://www.releasethehounds.com/media

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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

• 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

Fig 8.5 Thomson and Hahn

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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’

https://s-media-cache-ak0

24Thomson and Hahn Fig A7

Corpora quadrigemina

• Rostral colliculus

• Caudal colliculus

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Extrapyramidal system

• Red lines are facilitatory

• Black lines are inhibitory

• NOTE: vestibulospinal tract is

facilitatory to ipsilateral side

Fig. 13.2 King

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Why do we get spasticity with of

UMN spinal cord lesions?

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Pyramidal Motor System• Mammals only

– Output from motor cortex:

– Via crus cerebri (A)

– longitudinal fibres of the pons (B)

• Corticopontine

– To cerebellum and back to motor cortex

• Corticonuclear

– e.g. to CNN nuclei of brainstem

• Corticospinal tract,

– via medullary pyramids (C,D), to spinal cord

– Tracts decussate

– Spinal cord

• 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

A

A

B

C

D

D

C

B

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Motor cortex output• Function

– Skilled /learned movement

• Humans/primates

– 30% spinal cord WM

• Quadrupeds

– Dogs 10% spinal cord WM

– c/w ungulates

– Note

» horses, camelids

» raccoons

http://www.horsenation.com/

Fig 8.50, Dyce, Sack and Wensing, 4th ed.

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Pyramidal Motor

System

– Clinical significance

of pyramidal lesions

• Humans vs

quadrupeds

Human Pons XS: 30 years post-stroke

Where is the Lesion?

Ovine pons, Thomson and Hahn Fig A30Fig 8.50, Dyce, Sack and Wensing, 4th ed.

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Why the difference with a forebrain

lesion?

http://graphics8.nytimes.com

Fig 4-10, Thomson and Hahn

33Fig 8-14 deLahunta and Glass

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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

CEF

Basal nuclei –

function

• Humans

– disease affecting BN?

• Feedback circuits

– modify motor output

– Ritual movements?

• ‘Ordering the component parts of

complex movement’ (Jenkins)

– Lesions

• putamen – propulsive activity

• globus pallidus – hypoactive

• caudate n. – athetoid movements

• Effect of unilateral lesion?

Forebrain neoplasia

Photo courtesy Kate Hill

What’s this dog’s presenting sign?

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NeuroRATReflexesAtrophy Tone

Differentiating disease in UMN versus LMN

Fig 5.6 Thomson and Hahn

Sign UMN – central MN dz

(damaged UMN)

LMN – peripheral

MN dz (damaged

LMN)

Reflexes Intact (increased) Decreased/absent

Atrophy Disuse: mild

generalised

Neurogenic: severe,

specific muscles

Tone Intact (increased) Decreased/absent

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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 hypoactivity)

– Schiff-Sherrington – acute thoracolumbar lesion

Fig 9.6 Thomson and Hahn

LMN lesions

• ↓/0 Reflexes

• neurogenic atrophy

• ↓/0 tone

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Henry 7 yo MN Corgi,

Hx 1 mo progressive RPL lameness

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Coordination of movement

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

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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

• Forebrain motor centres

Postural platform

Fig 7.9 Thomson and Hahn

• Planned motor activity

– Inform cerebellum

– Cerebellum checks body posture

(SCP)

– 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• Cerebellovestibular

• Cerebelloreticular

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Evans, 18-4 and 18-41

Cerebellar peduncles – bilaterally paired

Name for their position of attachment to the brainstem

• Rostral – 12 (upper image); 6 – lower image

• Middle 10

• Caudal 11; 3

Sequential images

Lateral aperture Caudal and rostral CP (?) Rostral CP Middle CP (?)Caudal CP (?)

Pontine nuclei

Transverse fibres of the pons

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Cerebellar

Efferents

Purkinje (pyramidal) cells

– May be stimulated or inhibited

– They are inhibitory

• to vestibular nuclei

– direct inhibition

• to excitatory cerebellar nuclei

– Decrease their facilitation of motor activity

– indirect inhibition

– Lateral cerebellar nucleus

» To forebrain

– Interposital nucleus

» To red nucleus and reticular formation

– Fastigial nucleus

» To vestibular nucleus and reticular formation

Fig 7.6c Thomson and Hahn

http://image.slidesharecdn.com/

histologyofnervesystem

Lateral CN

Fastigial CN

Interposital CN

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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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