Supraspinal Motor Control

Alpha motor

Neuron

Muscle fibers

+

-

Renshaw cell

The Motor Unit

Flexor-Crossed Extensor Reflex

Long Spinal Reflexes

Inputs: vestibular, neck proprioceptors

Organized by circuits of propriospinal neurons

Muscle

Forebrain

Brain

Stem

Alpha Motor Neuron

"lower motor neuron"

Spinal cord circuitry

Multisegmental

Long spinal

Flexor/crossed extensor

Segmental

Reciprocal innervation

Myotatic

Motor unit

M1S1Posterior

ParietalPrefrontal

Thalamus

VLc VLo

Cerebellum

Spin

ocere

bella

r Tr.

Inf.

Olive

Vest.

Nuc.

Retic.

Form.

Red

Nuc.

Vest.

Spinal

Tr.

Retic.

Spinal

Tr.

Rubro

Spinal

Tr.

Cortico-

Spinal

Tr.

Cere

bella

r Loop

Basal G

anglia

Loop

Pons

Sup. & Inf.

Colliculi

Tecto-

Spinal

Tr.

SMA

PMA

Caudate

Putamen

N. Accumbens

Globus pallidus

Substantia nigra

Subthalamic nuc.

Primary motor andPremotor cortex

Brain stem

Motor systems of spinal cord

and brain stem

Interneurons

Motor neurons

Basal ganglia

Cerebellum

Thalamus

Skeletal muscle

Limbs and Trunk Head

Intermediate zone Reticular formation

Ventral hornCranial nerve

motor nuclei

Spinal cord Brainstem

Lawrence and Kuypers

The Three Major Descending Pathways

“Upper motor neurons” “Long motor tracts”

Vestib.Nuclei

Reticularformation

RedNuc.

Cerebral cortex

MidbrainTectumVentromedial

brainstempathway

Lateral brainstempathway

Corticospinalpathway

Tectospinal Vestibulospinal Reticulospinal Rubrospinal Corticospinal

Corticobulbar

(Pyramidal)

Tracts

Targeted muscle

groups

Axial Girdle/proximal

(upper limb)

All

The Three Major Descending Pathways

“Upper motor neurons”

“Long motor tracts”

Spinal termination of

the red nucleus

Tracer injected in contralateral

RN (midbrain) of the cat

Lateral brainstem

pathway

(rubrospinal tract)

DescendingAscending

Ventromedial

brainstem

pathway

Control of trunk,

axial muscles impaired

(↓ anti-gravity posture,

difficulty righting)

Bilateral pyramidotomy

(eliminate CST input)

Left lateral brainstem lesion

(eliminate rubrospinal

influence on left)

Lateral brainstem intact Eliminate lateral brainstem

Cerebral cortex

Red nucleus

Medulla

Pons

Midbrain

Thalamus

Superior colliculus

Vestibular nuclei

Reticular nuclei

Vestibulospinal Tr.

Reticulospinal Tr.

Corticospinal Tr. Rubrospinal Tr.

Tectospinal Tr.

MidlineLeft Right

Descending Motor Pathways

Decorticate

rigidity

Cut here

Decerebrate

rigidityCut hereVestib.Nuclei

Reticularformation

RedNuc.

+ Extensors + Flexors

Cerebralcortex

Spinal cord

MidbrainTectum

Ventromedialbrainstempathway

Lateral brainstempathway

Corticospinalpathway

Hemiplegia Paraplegia Quadriplegia

-plegia = paralysis -paresis = weakness

Disease of muscle, lower motor neuron Flaccid Paralysis

Damage to long motor tracts (UMN) Spastic paralysis

Long Motor Tract Signs

Spasticity

Weakness

Clonus

Babinski sign

Hypertonia

Hyperreflexia

Positive Babinski sign(following lesions to the pyramidal tract)

Thought to reflect a flexor withdrawal reflex that is normally

suppressed by the corticospinal tract

1

sec

Stimulation of site

A

Stimulation of site

B

Ia firing rate

Medial

reticular

formation

Lateral

reticular

formation

Normally under cortical inhibition

Gamma

motor

neurons

Gamma

motor

neurons

+ +

Clonus

Hyperreflexia due to increased excitability of and/or MN

Alpha motor

Neuron

Muscle fibers

+

-

Renshaw cell

The Motor Unit

Tract Tracing With Chemical Tracers

Horseradish peroxidase (and its enzymatic reaction products)

travel in both the retrograde and anterograde directions

Inject retrograde tracer

near (injured) terminals

Retrograde axonal

transport

Labeled cell soma

Inject anterograde

tracer near (or in) somata

Anterograde axonal

transport

Labeled synaptic

terminals

Electrophysiological Tract Tracing

Orthodromic Conduction

Antidromic and Orthodromic Spikes

?Possible Target

Nucleus

Stimulus PulseRecord

1

ms

Shock

Latency 1 ms

Short, constant latency Follows high frequency stimulation

Antidromic Invasion

B

Soma

Axon

Terminals

OrthodromicAntidromic

Conduction Direction

A Shock

Tract Tracing by A/D Invasion

Collision

The Critical Interval Test

Collision Test

?

Stimulus PulseRecord

Orthodromic Antidromic

Stimulator

Out

A/D stim

A/D latency

Orthodromic

spike

Response intermittent

Collision occuring

No A/D spike

Collision every time

Critical interval

1

2

3

4 TimeSpontaneous or evoked orthodromic spike triggers oscilloscope

and, after a variable delay, triggers the A/D stimulator

Variable

Delay

Oscilloscope

A/D spike

Measurement of Conduction Velocity

Soma Axon Terminals

Record

S1

L

A/D Latency 1 A/D Latency 2

T = Lat. 2 - Lat. 1 Conduction Velocity = T

L

S2