The Motor Systems Whats the motor system Parts

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 (96 mph 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 ØFlexible than reflexes ØConstrained 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) Kinematic plan – Forces/loads 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