Download Control of Movement

Sensorimotor
Control of Behavior:
Movement
Lecture 9
Motor Systems
Functions
 movement
 posture & balance
 communication
 Guided by sensory systems
 internal representation of world & self
 detect changes in environment

external & internal ~
Movement & Muscles
Movement occurs at joints
 Contraction & relaxation of of
opposing muscles
 agonists

prime movers

antagonists
counterbalance agonists
decelerate movement ~
Dorsal
Upper Motor
Neurons
+
Ventral
+
Alpha
Motor neurons
+
Movement & Muscles

Movement control more than contraction
& relaxation
 Accurately time control of many
muscles
 Make postural adjustment during
movement
 Adjust for mechanical properties of
joints & muscles
inertia, changing positions ~
3 Classes of Movement

Voluntary
 complex actions
reading, writing, playing piano
 purposeful, goal-oriented
 learned
improve with practice ~
3 Classes of Movement

Reflexes
 involuntary, rapid, stereotyped
eye-blink, coughing, knee jerk
 graded control by eliciting stimulus ~
3 Classes of Movement

Rhythmic motor patterns
 combines voluntary & reflexive acts
chewing, walking, running
 initiation & termination voluntary
 once initiated, repetitive & reflexive ~
Organization of Motor Control
Hierarchical & Parallel
 Parallel
 pathways active simultaneously
 e.g. moving arm
1. muscles producing movement
2. postural adjustments during
movement
 Recovery of function after lesion
 overlapping functions ~

Hierarchical Control of Movement
3 levels of control
 Cortex
 Brainstem
 Spinal cord (SC)
 Division of responsibility
 higher levels: general commands
 spinal cord: complex & specific
 Each receives sensory input
 relevant to levels function ~

Hierarchical Control of Movement
Association cortices & Basal Ganglia
 strategy : goals & planning
 based on integration of sensory info
 Motor cortex & cerebellum
 tactics: activation of motor programs
 Spinal cord
 execution: activates a motor neurons
 reflexes
 rhythmic pattern generators ~

Sensorimotor Cortical System
Integration of sensory information
and directed movements
 Anatomy
 Descending spinal tracts
 Lateral pathway

Pyramidal Motor System


Ventromedial pathway
Extrapyramidal pathway ~
Cortical Anatomy
S1 - postcentral gyrus
 PPC - Posterior Parietal Cortex
 M1 - Precentral Gyrus
 Frontal Lobe
somatotopic organization
 M2 - Secondary Motor Cortex
SMA - Supplementary Motor Area
PM - Premotor Cortex

SMA
M1 S1
PM
PPC
The Descending Spinal Tracts
Brain to Spinal Cord
Upper motor neurons
 communication with lower (a) motor
neurons
 Lateral pathway
 direct cortical control
 Ventromedial pathway
 brain stem control ~

The Lateral Pathway
Voluntary movement
 distal limbs
 2 tracts
 Corticospinal tract
about 1 million axons
 Rubrospinal tract
small part of pathway ~

Spinal Cord: Lateral Pathway
Dorsal
Ventral
Corticospinal
tract
Rubrospinal
tract
Corticospinal tract
Motor cortex ---> spinal cord
 uninterrupted axon
 2/3 of axons from motor cortex
 1/3 from somatosensory cortex
 Decussates at medulla
 Contralateral control movement ~

The Rubrospinal Tract
Motor Cortex ---> red nucleus
 Red nucleus ---> spinal cord
 inputs from motor cortex
 bigger role in other mammalian
species ~

Lateral Pathway Damage
Lesion both tracts
 no independent movement of distal
limbs
 voluntary movements slow & less
accurate
 Corticospinal only
 same deficits
 recovery over several months
 compensation by rubrospinal tract ~

The Ventromedial Pathway
Neurons originate in brainstem
 Vestibulospinal & tectospinal tracts
 head & posture posture
 orienting responses
 Pontine & medullary reticulospinal tracts
 originate in reticular formation
 trunk & antigravity leg muscles
 tracts are antagonistic ~

Spinal Cord: Ventromedial Pathway
Dorsal
Vestibulospinal
tract
Tectospinal
tract
Medullary
Reticulospinal
tract
Ventral
Pontine
Reticulospinal
tract
Major Descending Spinal Tracts
Motor Cortex
Lateral
Red
Nucleus
Ventromedial
Reticular
Nuclei
Spinal cord
Superior Colliculus
vestibular nuclei
Cortical Control
of Movement
Primary Motor Cortex
Somatotopic organization
 neurons have preferred direction
of movement
 Motor homunculus ~

M1: Coding Movement
Movement for limbs
 Neuron most active
 Preferred direction
 but active at 45 from preferred
 How is direction determined?
 Populations of M1 neurons
 Net activity of neurons with
different preferred directions
 vectors ~

M1: Coding Movement
Implications
1. Most M1 active for every
movement
2. Activity of each neuron
1 “vote”
3. direction determined by
averaging all votes ~

Motor Association Cortex
Motor area other than M1
 secondary motor cortex (M2)
 Premotor & Supplemental Motor Areas
 Active during preparation for movement
 Planning of movements
 Stimulation - complex movements
 motor programs ~

Supplementary Motor Area - SMA
Primarily midline cortex
 Inputs from …
 PPC
 S1
 Bilateral output to M1
 to distal limbs ~

Premotor Area - PMA
Anterior to M1
 Input
 primarily from PPC
 reciprocal connections with SMA
 Outputs to M1
 then proximal limbs ~

SMA
M1
S1
PPC
PMA
Spinal cord
Planning Movements

Targeting vs trigger stimulus
recording activity of neurons
active when movement planned
 for specific direction
 Different populations of neurons active
 during planning (targeting)
 & execution (trigger stimulus)
 PM active before movement ~


Simple finger flexion
 only M1 activation

Sequence of complex finger movements
 M1 + SMA activation ~

Mental rehearsal of finger movements
 only SMA activation ~
Sensorimotor Integration
Perceptual development
 Active interaction required
 environmental feedback important
 Held & Hein (1950s)
 kittens passively moved
 depth perception deficits
& related responses, blinking,
looming ~

Sensorimotor Integration

Sensory inputs guide movement
visual, auditory, tactile
location of objects in space
 Proprioceptive & vestibular
position of our body
 Critical for planning & refining
movements ~

Generation of
Rhythmic Motor Patterns
Central Pattern Generators
Half-center Model
 alternating activity in flexor & extensor
 Step-cycle has 2 phases
 swing phase

foot off ground & flexing forward

stance phase
foot planted & leg extending

Each limb has own pattern generator ~
Half-center Model
Flexor
a
+
Tonic
input
+
+
+
+
a
+
+
Extensor
+
Rhythmic Patterns: Sensory Feedback
Not necessary for locomotion
 but slower, less coordinated
 Stumble correction reaction
 during swing phase
 tactile stimulus on dorsal foot ---> flexion
 Reflex reversal
 override during extension
 flexion would cause collapse ~

Goal-directed Locomotion
Requires intact supraspinal systems
 Coordination of antigravity muscles
Brainstem motor nuclei
 modulation of contraction strength
Reticulo-, rubro-, corticospinal
 Balance
Vestibulospinal tract (Ventromedial)
 Also cortical & subcortical areas ~
