Sensorimotor Control of Behavior:

MovementLecture 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

Ventral

+

-

+

+ Alpha Motor neurons

+

Upper 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 & musclesinertia, 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 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

M1

SMA

S1

PM

PPC

The Descending Spinal Tracts

Brain to Spinal Cord

Upper motor neurons communication with lower () 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 ~

Dorsal

Ventral

Spinal Cord: Lateral Pathway

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 ~

Dorsal

Ventral

Spinal Cord: Ventromedial Pathway

Tectospinal tract

Pontine Reticulospinal tract

Medullary Reticulospinal tract

Vestibulospinal tract

Major Descending Spinal Tracts

Motor Cortex

Spinal cord

Red Nucleus

Reticular Nuclei

Superior Colliculusvestibular nuclei

VentromedialLateral

Primary Motor Cortex

Somatotopic organization neurons have preferred direction

of movement Motor homunculus ~

Cortical Control of Movement

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 ~

M1 S1

PMA

SMA

Spinal cord

PPC

Planning Movements

Targeting vs trigger stimulusrecording 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

+

+

++

+

+

+

+

Tonic input

Flexor

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 ~