The Motor Systems
What’s the motor system?
• Parts of CNS and PNS specialized for control
of limb, trunk, and eye movements
• Also holds us together
• From simple reflexes (knee jerk) to voluntary
movements (96mph fast ball)
• Remarkable: Muscles only contract
Plan
• Components of the motor systems
• Basic principle of movement control
– What is helpful for understanding basic motor
system organization
• Motor programs
• Descending motor pathways
Motor
Systems
Cortical motor areas
Basal ganglia
Descending
cortical motor
paths
Spinal cord:
Intermediate zone
Ventral horn
Muscle
Cerebellum
Descending
brain stem
paths
Functional Hierarchy of Motor Paths
Motor execution: force &
direction
Parallel Organization
Basal ganglia
Cerebellum
Motor
Cortical
areas
Internal capsule
from
Basal ganglia
from
Cerebellum
Organization of Movements
• Diverse & adaptive
• Hierarchical: 3 major types
– Reflexes
– Postural adjustments
– Voluntary movements
Organization of Movements
• Hierarchical: 3 major types
– Reflexes
– Postural adjustments
– Voluntary movements
Spinal cord circuits
Spinal & Brain stem
Spinal cord, Brain
stem, and cortex
Postural adjustments & voluntary movements
depend more on cerebellar and basal ganglia
function than reflexes
Reflexes
• Stimulus-evoked involuntary muscle
contraction
• Monosynaptic (+) reflex
– Knee-jerk
– Jaw-jerk
Knee Jerk
From muscle
stretch receptors
to muscle
Reflexes
• Stimulus-evoked involuntary motor
muscle contraction
• Monosynaptic (+) reflex
– Knee-jerk
– Jaw-jerk
• Disynaptic reflex (+)
– withdrawal
Why Disynaptic?
• Greater control (gate)
– Very simple context
• More complex response
Greater control:
from periphery
from
higher
centers
to muscle
Response inhibited
by inhibition
Greater control:
from periphery
from
higher
centers
to muscle
Response blocked
by inhibition
Motor I/O
Knee-jerk
Automatic
postural
adjustments
Flexible than
reflexes
Constrained
than voluntary
• Balance
• Limb support
Postural adjustments
• Context important
– Maintain balance--supported v/s
unsupported
• Feedback control-reactive
– Error correction
– Response lags stimulus; sometimes too late
• Feed-forward control-predictive
– Response anticipates stimulus
– More timely, but depends on internal models
– Practice, learning
Voluntary movements
• Organized around purposeful acts
• Flexible input-output relationships
– Limitless
– Price to pay: whole brain
Motor I/O
Knee-jerk
Automatic
postural
adjustments
Voluntary
movement
Voluntary movements
• Organized around purposeful acts
• Flexible input-output relationships
– Limitless
– Price to pay: whole brain
Discuss:
• Goal representation
• Motor program
The goal of voluntary movements is
represented… somewhere
• Motor equivalence
– Individual motor actions share important
characteristics even when performed in different
ways
• Abstract representation; effector independent
– Hand writing
– Soccer
• Goal representation
Voluntary movements are
organized by motor programs
• Translate goal into action
– Formation of a movement representation, or
motor program
• Program
– To produce the desired goal, which muscles should contract
and when
• 2 Key movement characteristics
– Spatial (hand path; joint angles)
– Forces/loads
Kinematic plan
Dynamic plan
• All accomplished by contracting muscles
Kinematic & Dynamic Plans
• Reach to target
– Straight hand path
• Reach up
– Against gravity
– More force to achieve goal
• Reach down
– Gravity assists
– Less force to achieve goal
• Kinematic and dynamic representations of upcoming
movement
Summary
• Motor behavior hierarchy
– Reflexes
– Postural adjustments
– Voluntary movements
• Internal representations
– Reflexes and postural adjustments--simple; invariant
– Voluntary movements--complex; flexible
• Voluntary movements
– Kinematic and dynamic representations
– Goal representation
• Motor hierarchy
– spinal, brain stem, cortical motor circuits
– Descending motor pathways
Motor
Systems
Cortical motor areas
1° motor cortex
Premotor cortex
Descending
cortical motor
paths
Spinal cord:
Intermediate zone
Ventral horn
Muscle
Red nucleus
Reticular formation
Vestibular nuclei
Superior colliculus
Descending
brain stem
paths
Premotor
areas
Direct
Motor Pathways Hierarchy
Indirect
Spinal Motor Columns
From brain
Segmental
interneuron
Motor neuron
Short
Long
Motor columns
(motor neurons)
Propriospinal-Intersegmental-neurons
Lateral pathways:
limb control
Medial pathways:
trunk control
Ventral Horn
Organization:
Proximal - distal
rule
Brain Stem Motor Paths
Medial
Lateral
Tectum
Vestibular
nuclei
Vestibulospinal
tracts
Reticular
formation
Tectospinal
tract
Reticulospinal
tracts
Red nucleus
Rubrospinal tract
Brain Stem Pathways
• Lateral
– Rubrospinal tract: distal limb control; crude
• Medial
– Tectospinal tract: eye-head coordination
– Reticulospinal tract: automatic postural
adjustments and movements (hip; shoulder)
– Vestibulospinal tract: balance (axial muscles);
automatic postural adjustments
Brain stem nuclei
Superior colliculus
Tectospinal tract
Red nucleus
Rubrospinal tract
Vestibular nuclei
Vestibulospinal tracts
Reticular formation
Reticulospinal tracts
Cortical
Motor
Paths
Medial
Lateral
Vestibular &
Reticular
nuclei
Medial
brain stem
paths
Ventral
corticospinal
tract
Red nucleus
Rubrospinal
tract
Lateral
Corticospinal
tract
Pyramidal X
Cortical Motor Areas
Primary motor
PMC
Lateral and ventral CST
Corticobulbar tract
Primary motor
SMA
CMA
Lateral and ventral CSTs
Corticobulbar tract
Why bother study the motor
pathways?
• Anatomical substrates: How it works
• Multiple parallel paths & diversity of spinal
connections
– Damage to 1° motor cortex and pre-motor cortex
projections recover some lost functions
– Damage to cortex and brain stem paths recover
some lost functions
– Loss of direct connections and alternate paths can
recover some lost functions